Testing gyros for buntovers potential

[gyrogreg]

The thread ”Future of Gyros” (http://www.rotaryforum.com/forum/showthread.php?t=16558&goto=newpost) prompts me to start this discussion on flight testing of gyros. An ASTM Task Team is working on some refinements to the stability portion of the standard. In this investigation, I believe a major light bulb has gone off – at least for me.

People worry most about getting into PIO or a buntover (PPO in terms of a HTL). One intent of the ASTM standard was to provide a way to easily determine by flight test if any particular gyro would be susceptible to buntover or PIO. A simple flight test, performed and reported on many gyro configurations, could be most informative. As you have all probably read, our ASTM standard focused on the static G-Load stability (actually this is the AOA static stability) of a gyro. The static G-Load stability flight test, a banking constant airspeed turn, was intended to be the test that could say whether a gyro could buntover or not. The presumption was that there was a symmetry from the +G-Load condition of a banking turn to the -G-Load condition of a buntover. A buntover is really an AOA static instability that causes a decreasing G-Load to continue and accelerate in a worsening reducing G-Load – nose down pitching.

It turns out that the banking turn flight test really only works for some severely AOA unstable gyros – such as an old-style Air Command with no HS. For less severe AOA unstable gyros, and especially for gyros with strong offset gimbal trim spring arrangements, this test did not define a buntover susceptible gyro very well. In fact, the revealing fact was when Jerry Tiahrt reported that his RAF 2000 passed the static G-Load test and nearly passed the static Power stability test! So, back to the drawing board we went!!! Apparently, the Static Stability flight tests were not truly definitive – by that I mean we know the RAF 2000 is capable of a buntover, and yet Jerry’s static flight testing did not identify it as such. The major problem with the G-Load test is that the offset gimbal provides the overwhelming aft stick pressure in a banking turn – whether the gyro is AOA stable or not. This test works for a FW airplane, but not for a gimbaled gyro rotorhead!

There may be different technical opinions on this, but what I think we discovered with Jerry’s testing is a new, “back door” way to determine if (or when) a gyro is statically AOA unstable and therefore capable of a buntover! The Static Stability tests are important, but they do not provide a definitive indication of buntover susceptibility, or not. What Jerry noted was that in performing the Fixed Stick static flight testing in his RAF 2000, he found that above a certain airspeed, the gyro responded to moderate turbulence with divergent pitch and airspeed oscillations. This got Jerry’s attention! This is an indication of DYNAMIC INSTABILITY. Jerry reported this was disquieting, and he subsequently lowered his personal Vne for this aircraft. (Jerry, if you are listening, please correct anything I might be relating differently than you reported – I did not go back and research our emails and your postings recently.)

I believe this has pointed us to a new determination of actual static AOA instability – ability to buntover. Please try to follow this: If an aircraft is able to oscillate in pitch, whether convergent, divergent or constant amplitude oscillations, it must be statically AOA stable. The mere fact that it demonstrates Phugoid pitch oscillations (slow rate, constant AOA oscillations), that gyro has static AOA stability. If it was statically AOA unstable, it could not oscillate – it would simply diverge exponentially nose-up or nose-down (buntover)! A phugoid oscillation occurs because there is static AOA stability – it is the small AOA divergences from steady state that provide the negative feedback restoring moment that causes the gyro to oscillate rather than statically diverge.

Fixed Stick flight testing was the key! With a fixed stick, you are evaluating the pure aircraft response and the airframe pitching, whether in the restoring or divergent direction. With a fixed stick, the airframe pitch response to a disturbance couples directly to the rotor (wing) and causes the whole gyro to react. If the airframe pitching response is in the unstable direction, the aircraft is statically AOA unstable. The key is that it is the Fixed Stick condition that really reveals the ability to oscillate in turbulence or if disturbed by a pilot input. This is actually the standard DYNAMIC fixed stick flight test that has always been part of the gyroplane ASTM standard – and that Jerry inadvertently flight tested when his gyro developed diverging oscillations in moderate turbulence!.

What I think we have discovered is that most gyros, such as the RAF 2000, at moderate airspeeds near their Minimum Power Required Speed (MPRS), are statically AOA stable and will exhibit slow rate phugoid pitch oscillations. The existence of oscillations indicate the AOA static stability! However, as in the case of the RAF, some gyros might become statically AOA unstable at higher airspeeds (airframe aerodynamic nose-down moments) and at higher power levels – both of which are moving the “effective” RTV forward of the CG – to the Chuck B. “classic” definition of AOA unstable and able to buntover!

If a gyro approaches static AOA instability (with a worsening power/airspeed condition), the static restoring AOA pitch moment becomes weaker and weaker. As it does so, the phugoid dynamic oscillations become less convergent, neutral, and eventually divergent oscillations. The standard requires that a gyro not be dynamically unstable – in other words, still able to oscillate, but with diminishing oscillations. But, even if the dynamic phugoid oscillations become neutral or divergent, they are still oscillations, and there is still static AOA stability – however weakening! If we require a gyro to be dynamically stable – excited phugoid oscillations eventually diminish to zero - that gyro still has a margin of static AOA stability – it is assuredly statically AOA stable and cannot buntover in that flight (power and airspeed) condition!

So, then how do we test for the ability or not for a gyro to buntover? We conduct the standard Fixed Stick Dynamic Test. The key to this is that the stick must be truly mechanically fixed. It is not good enough to just try to hold the stick steady! An experienced pilot just cannot avoid stabilizing inputs! With the stick fixed, fly at MPRS in afternoon turbulence to observe that the pitch oscillations that are excited do not start getting worse and worse – divergent dynamic response – dynamically unstable and weak static AOA stability. (This test can also be done in calm air with a pilot “step” input on the stick – like professionals do it – but just flying in turbulence will work for informal testing just as well!) Then, increase the Fixed Stick airspeed by about 5 mph – change the fixed position of the stick - and watch for divergent oscillations in moderate turbulence again! Keep increasing airspeed and power to see if there is any airspeed where the pitch oscillations start getting worse and you, the pilot, release the fixed stick to stabilize the oscillations. This is the point, beyond which, that gyro will have meager AOA static stability, or beyond which it will be actually statically AOA unstable and able to buntover! That determines the flight condition (power and airspeed) beyond which that gyro can buntover or PPO. That should identify your Vne condition. Gyros that are capable of bunting over will only do so at statically unstable combinations of power and airspeed – normally at higher airspeed because airframe aerodynamic moments are normally statically destabilizing! This flight testing will determine your “safe operating flight area”. A really good thing to know – especially since buntovers are so insidiously surprising otherwise!

This test should be conducted at higher airspeeds with lower power settings as well – the prop thrust moment or lack of moment can contribute to AOA instability on certain configurations!

This Phugoid dynamic stability test is simple and safe to be conducted by any gyro pilot that is proficient in flying that particular gyro in moderate (afternoon thermal) turbulence conditions. The second post below will describe two ways to safely “fix the stick” for this testing. Jerry Tiahrt helped develop the “chain” or rope method. This is how he identified the buntover susceptible flight conditions for his RAF 2000.

I would appreciate any comments or flight testing feedback on these concepts. I have been evaluating these concepts for some time, and I think it is time to start sharing all this with others to hopefully provide some flight test feedback and validation. I have become convinced that this simple flight test will truly indicate if or when a gyro can buntover (or PPO). But what about PIO? To pass this test, a gyro must have a dynamic damper – a HS! It turns out that a HS, besides providing static stability directly, actually provides dynamic damping that also extends the static AOA stable margins and range. The dynamic damper is also the mechanism that prevents rapid PIO pitch oscillations (the rapid, constant airspeed, varying AOA oscillations) that promote PIO. I don’t have complete theoretical reasoning for this, but it can be expected that any gyro that is shown to be buntover proof from the above testing, will also be highly resistant to PIO due to its strong dynamic damping..

There are many aspects of the above theory that I haven’t discussed here – such as susceptibility to buntover in a push over the top of a zoom! But, the above are the basics. I hope some of you will provide some flight testing – and I hope this flight testing really opens some eyes – as it did for Jerry Tiahrt!

If you do some testing, please share your results.

- Greg Gremminger

[gyrogreg]

The “Fixed Stick” testing must be done with a truly mechanically fixed stick – it is not good enough to just hold the stick as still as you can! Both of these methods of “fixing the stick” lend themselves to quick release for pilot recovery from uncomfortable flight conditions. Both of these methods allow for lateral stick movements for roll control while fixing the stick in pitch:

Method 1 – Chain: A small chain attached to a secure point forward of the cyclic, with an electrical conduit clamp on the cyclic, works well. Route the small chain through the conduit clamp hole and hold it in your hand on the cyclic. Be sure that releasing the chain releases the restriction on the cyclic. To conduct the test, fly at the test airspeed, pull the slack out of the chain through the hole and grip the chain under your hand. Then, pull aft enough to hold the chain tight throughout the test. Adjust the length of the chain to fly at different airspeeds. (You can use a braided nylon rope in place of the chain if you assure taht the rope cannot stretch and the knows will not allow the length to tretch under tension - a small cahin from a hardware store may be better!)

Method 2 – “Jam Stick”: Fashion an adjustable stick or several stick lengths to jam between the cyclic stick and a solid point forward of the stick – instrument panel! Adjust the length of the stick for different airspeeds. Hold the forward pressure on the “jam stick” to hold it into position against the stick. I coated the rounded tips of the stick with a bit of rubber to help hold it in place, while still allowing some amount of lateral movement for roll control. To regain full control, simply relax forward pressure on the stick to let the “jam stick” fall out.

See photos attached. Both of these methods require that the solid point forward of teh stick be truly solid - if the instrument panel can flex, I fashioned a metal bracket attach point to temporarily stabilze the attach point.

- Greg Gremminger

[C. Beaty]

The chain and conduit clamp will cull out the egregious stability offenders but the mildly unstable machines will slip through as a result of control flexibility. Particularly if trim springs are located at the rotorhead, allowing control backlash to play a role.

But good enough for home testing. It is important the chain be as pictured, made from sheet metal stampings and not the kind with twisted wire links that can snag and hang up.

Any meaningful stability testing for certification purposes must provide a breakaway lock at the rotorhead.

[donshoebridge]

“safe operating flight area” = SOFA, a nice compfy place to be.

Greg,

Are you going to update your testing documents for everyone to download?

[gyrogreg]

“safe operating flight area” = SOFA, a nice compfy place to be.

Greg,

Are you going to update your testing documents for everyone to download?

Don,

Eventually, I hope to provide such guidance - detailed PRA articles. But, I would prefer to vet these ideas deeper, and it would be nice to get more correlating flight test feedback on this concept.

The ultimate validation for any theory is to correlate flight test data with real world history - in this case of buntover history. I fully expect that gyros that indicate buntover susceptibility per the dynamic stability flight test, within their published operating area, would also be those that have a history of buntovers and PIO.

Having been slightly stung by the lack of correlating flight test reports on the Static Stability flight testing recommendations, I would prefer a bit more flight feedback from a wider range of gyro models. (It took a couple years before we got any data from an RAF - Jerry Tiahrt's reports - to identify an issue with this guidance!) I would prefer to get flight test correlating feedback before making any declarations based on pure theory!

One problem, I suspect, with getting flight test feedback is that perhaps when someone's favorite configuration actually identifies problems per the flight tests, they are not so willing to admit that. Jerry's report on his RAF 2000 was one of the few reports we have ever received – thanks Jerry, you may have done the whole sport a real service! I think it is important to at least get some flight test feedback for the more common and popular gyro models – not just Magni, SH and RAF – specifically confirming that phugoid pitch oscillations are damped – dynamically stable – or not! I’m confident that all of the buntover free gyros will demonstrate dynamic stability throughout their published operating area. I am confident that gyros that do have buntover histories will have areas of their published flight envelope where they are not dynamically stable!

I do intend soon to distribute our proposed ASTM standard changes - which includes test method suggestions for this Dynamic Stability criteria as well. I will be distributing this to manufacturers and old ASTM gyroplane committee members, and others who might provide valuable comments and flight test reports.

But, this particular Dynamic Stability flight test is simple. Here it is in more words than you probably want!:

- Prepare and install one of the suggested methods to "fix the stick"

- Fly in the middle of a sunny day where there are thermals you can feel in the gyro - thermals you regularly have flown in!

- Start at a comfortable airspeed - MPRS (45-55 mph?)

- Set the stick position, using the chain or "jam stick" for that airspeed

- Let the gyro ride the thermals and verify the pitch disturbances do not just get worse and worse. - use lateral stick movement to keep the gyro level, but keep pressure on the chain or "jam stick" to maintain that stick pitch position. (Do not lean or sway your body to try to steady the aircraft!!!! - experienced pilots might do this naturally!)

- Repeat this test for 5 mph higher airspeed - set the new stick position.

- Repeat at 5 mph incrementally increased airspeeds.

- When pitch oscillations start getting worse instead of steadying out on their own, stop the test - beyond this airspeed/power point that gyro will likely be susceptible to buntovers! This is your Vne - at least for that power condition!

- Strictly speaking, this test should be done at all combinations of power and airspeeds. So, for all the higher airspeeds, the test should be done at normal power, idle power and full power - allow the gyro to climb or glide with the different power settings. Typically, stability issues arise at higher airspeeds! But, they may be more prominent with higher power or with lower power (at high airspeeds), depending on prop thrustline configuration. You don’t really need to worry about or measure the prop thrustline, just test the gyro at higher and higher airspeeds for both high power and low power!

One caveat: If the fixed stick positions do not result in a steady airspeed, that gyro has a more basic problem – Static Airspeed Instability. All aircraft should have a specific airspeed that results from a specific stick position. This airspeed may be different at different power settings, but airspeed should at least be steady at any particular fixed stick position. If, when the stick is fixed, the airspeed keeps getting faster and faster, or slower and slower – never settles out – then that gyro has other issues and will definitely have controllability issues.

- Thanks, Greg Gremminger


- Greg

[WaspAir]

Yikes!
Good luck guys -- I hope you get some useful data without incident.
Personally, I won't be caught above 0 kts IAS and/or 0 ft AGL without the "full, free, and correct" motion of all controls that's been on every checklist I've ever used. I just can't get past the cringing that comes from even hearing the term "jam stick".

[Not Yet]

Either one of these devices can be instantly disabled to allow full movement of the stick. What is the problem?

[Resasi]

The often thankless and unremarked upon test flying regime often does call for doing procedures that a lot of pilots would not consider doing.

That someone is prepared to look into the areas where we should not be for the benefit of all of us is a time for us to be very grateful. In this particular instance it does seem to be safe enough.

[Wannabe1]

I'm with Waspair...

I've only thing I fly is the R22, and other than a ride in a gyro, I can't speak to them. But I can't imagine rigging MY stick to no back cyclic even with a quick release. Bad things can happen pretty fast. Maybe a good CFI that is used to fixing problems that students have created. IMHO mortal pilots like myself shouldn't be messing with 'jambing sticks.

Tom

[Dean_Dolph]

I'm thinkin' (always dangerous!) that those that are concerned about the safety of these tests aren't visualizing how the test is conducted.

In the method of using the adjustable stick, 'Jamming' doesn't mean wedged as normally thought of. In this case it only means held in place by the controlling hand where it can't move. Quit holding it in place and the device disengages (drops from position) instantly; well it will as long as we have gravity!

I can't think of, but may be wrong, any scenario where an emergency would cause the test pilot to want or have to push the stick forward. In fact, in a gyro, this reaction is kinda a no, no anyhow!

I see the chain method as possibly being a little more problematic but if designed right where there is no possibility of the chain/string getting snagged then it appears safe also.

I do think that the 'stick' method might be better from the stand point that unconscious movement of the cyclic would be easier with the chain/string.

Mr. Beaty has mentioned several times how difficult it is for a pilot to conduct fixed stick tests (no devices used) because of learned responses to outside forces. Unless the pilot is extremely well calibrated, it is unlikely that he/she will be aware of subtle inputs. Thus the need for 'training wheel' type of devices that doesn't require a calibrated pilot.

I'm would think that eventually these test methods would evolve to include ways to quantify or document the results. It would appear to me that manufacturers in particular could benefit from this.

But 1st things 1st; the tests need to be performed by several pilots and their data reported. The data reported would include any problems in conducting the tests using these devices/methods.


Edit to add: It suddenly dawned on me that the 'stick' method might not work too well in an open frame gyro! "Whoops, didn't mean to bonk you on the head Farmer Brown!" Sticks are cheap but head bonks might not be! Haven't thought this thru this long enough to suggest a restraint that doesn't present problems of its own.

[magilla]

I'll test 'em! This is what I love about being a test pilot.

Ooooops, gotta get my gyro rating add-on first, then I will be more than glad.

Seriously, these tests, if conducted properly in a regimented fashion should prove no more dangerous than regular maneuvers.

The tests that get me on edge are checking max performance of the engine, operating for minutes at a time at max throttle/performance.

Fixed stick flight control checks shouldn't be a problem if you plan the maneuver, practice it dead stick (to include simulated emergencies) then go execute in a controlled environment (traffic pattern).

Good Luck,

And great work, Greg! Thanks for the update.

PS, I did not read into this thread ANY negativity about RAF2000s. The argument was well presented and well explained, and I cannot see anyone finding bias or fault with the test as described.

[C. Beaty]

What’s the big deal? Both Dick DeGraw and his wife Carroll have been flying gyros with stick locks for years. Admittedly, spring detent locks.

Paul Bruty in Australia flew his RAF-2000 with the telescoping stick lock and found he had to drop it after the first excursion from trim. That’s been several years ago.

Roger Wood in Cincinnati Flew his Bensen with a stick lock back in the 70s and learned he had to take over after about 3 cycles of oscillation.

There is simply no other way of determining dynamic stability of a gyro due to the masking effect of the offset gimbal rotorhead without expensive instrumentation.

At least one helicopter had a spring detent stick lock, the Hiller Hornet. There was a heel button the floor to engage/disengage it.

[Chuck_Ellsworth]

The tests that get me on edge are checking max performance of the engine, operating for minutes at a time at max throttle/performance.

.

That also used to make me nervous and it took a while to get used to it when I first started doing unlimited aerobatics in the Pitts S2B.....but we just set full RPM and full throttle and lock it there during the whole routine.....amazing how they hold together. :plane:

[mark treidel]

The only thing that comes to mind is - what if the jam stick dropped out and got stuck in the control mechanism or rudder assembly somehow and the pilot was not able to reach it?? We lost a fellow here in an ultralight a few years ago for that very reason. The stick would have to be secured from 'dropping' in advance of the test in my opinion (unless someone has a third hand). Am I missing something here??

[gyrogreg]

The only thing that comes to mind is - what if the jam stick dropped out and got stuck in the control mechanism or rudder assembly somehow and the pilot was not able to reach it?? We lost a fellow here in an ultralight a few years ago for that very reason. The stick would have to be secured from 'dropping' in advance of the test in my opinion (unless someone has a third hand). Am I missing something here??

Tie a line on it or tie one end loosely to the instriment panel. - Greg

[gyrogreg]

Yikes!
Good luck guys -- I hope you get some useful data without incident.
Personally, I won't be caught above 0 kts IAS and/or 0 ft AGL without the "full, free, and correct" motion of all controls that's been on every checklist I've ever used. I just can't get past the cringing that comes from even hearing the term "jam stick".

If a gyro requires such constant attention on the stick, you already have your answer! That gyro is statically unstable, like constantly having to work at balancing a yard stick upright in the palm of your hand. Stable gyroplanes and aircraft do not require constant attention on the stick! A stable gyro will fly indefinitely at a near constant airpseed with a fixed stick - even in turbulence! The stick fixed position should determine a constant mean airspeed - determined by power and stick position. Any deviation from that attitude or airspeed should tend to immediately correct for that deviation! That is because a stable aircraft has a restoring pitch moment when there is any deviation from steady state pitch and airspeed - the definition of static stability.

A stable gyro requires much less attention to the stick than a helicopter - a helicopter, without stability augmentation is basically statically unstable* - but this does not need to apply to gyroplanes. The examples Chuck quotes above, where the locked stick must be immediately released, are perfect examples of dynamically iunstable unstable gyros, and they can very likely buntover if pushed a little harder with the right disturbance and an inattentive pilot! - fixed stick or not! If they actually still oscillate before the pilot has to take over, with just a little more power or airspeed, that gyro will be statically unstable and primed to buntover!

A well stabilized gyro requires much less attention to the stick than similarly sized airplanes in the same turbulence. This is partly because the rotor is less sensitive to wind deviations. But it is also because the stable gyro forces immediate restoring cyclic into the rotor - and the rotor very powerfully starts restoring the steady state. This is one of the most remarkable features of a stable gyroplane, one that astonishes every doubting Thomas airplane pilot until they actually experience it themselves. That is one of the most fun things to demonstrate to FW pilots in a gyro - find the most turbulence you can find and have THEM fly in it! I have sold a few gyros (Magnis) to FW pilots for just that reason!

It might be difficult to design a gyro that would not have phugoid oscillations, and therefore be AOA statically stable, at MPRS or less. That's why I say to start at that airspeed - then increase airspeed in small increments. At the point where oscillations start growing instead of settling out, you do still have static AOA stability - you are still safe from buntover, but stop there and don't go any higher airspeed because you are losing your static stability safety margin.

Thanks, Greg

* Ironically, a helicopter with an effective HS on the long tail moment arm - lots of dynamic and static stability - becomes more and more AOA and airspeed statically stable at higher airspeeds. That alone should be enough hint about what to do with gyros. Where do you think the thrustline is on a helicopter? It is at the rotor level - very HTL! And yet, they can fly stable at higher airspeeds if they employ a good HS. And, a helicopter in those conditions, also does not require a lot of constant attention to the stick. (Cobra and Apache pilots have lots more things to do than to just fly the helicopter.) In all aircraft types, the only attention the joystick should require is for the pilot to command a deviation from the steady state condition.

- Now is about the time someone complains that the stable gyro won't be any fun then! It will be a "lead sled"! Come on, where are you? Speak up! Let's see if you know the difference between a fixed wing and a non-fixed wing! Anyone want to explain the difference as it relates to maneuverability and controllability?

[PW_Plack]

... -use lateral stick movement to keep the gyro level, but keep pressure on the chain or "jam stick" to maintain that stick pitch position...

Either a stick or a chain will cause any movement of the stick in the roll axis to create a change in stick pitch position, since the locks will hold the stick in an arc, not in a straight lateral line. Is this significant?

[gyrogreg]

... -use lateral stick movement to keep the gyro level, but keep pressure on the chain or "jam stick" to maintain that stick pitch position...

Either a stick or a chain will cause any movement of the stick in the roll axis to create a change in stick pitch position, since the locks will hold the stick in an arc, not in a straight lateral line. Is this significant?

Paul, Not really! The gyro is still flying without pilot stabilizing pitch inputs. It would be very difficult for the pilot to learn how to put in stabilizing pitch inputs with this lateral stick action only. In moderate turbulence, once it is level at the power and trimmed (stick position) airspeed, there are norrmally not a lot of lateral inputs required anyway.

If you really wanted to be precise with the "jam stick", you could square the base of the "jam stick" - against the flat instrument panel, so lateral movements on the corner of the stick would compensate the pitch input! Do the geometry. But, this is not necessary. Just do it the simple way - it will be very revealing! The difference between a well stabilized gyro and a gyro that starts losing its dynamic stability is very convincing! Remember, at the power/airspeed point where it starts losing its dynamic stability, it is still oscillating and therefore still AOA statically stable - and incapable of a buntover at that apoint. But, don't press beyond that loss of dynamic stability point! You found your Vne!

- Greg

[C. Beaty]

* Ironically, a helicopter with an effective HS on the long tail moment arm - lots of dynamic and static stability - becomes more and more AOA and airspeed statically stable at higher airspeeds. That alone should be enough hint about what to do with gyros. Where do you think the thrustline is on a helicopter? It is at the rotor level - very HTL! And yet, they can fly stable at higher airspeeds if they employ a good HS. And, a helicopter in those conditions, also does not require a lot of constant attention to the stick. (Cobra and Apache pilots have lots more things to do than to just fly the helicopter.) In all aircraft types, the only attention the joystick should require is for the pilot to command a deviation from the steady state condition.

A helicopter rotor, like a propeller produces a line of thrust, not thrust applied to a point.

The thrust produced by a helicopter rotor acts in a straight line that is very nearly coincident with the tip plane axis. The horizontal component of the forward tilted thrust line supplies the propulsive force.

A helicopter can be either nose heavy or tail heavy, depending upon how the projected thrust line passes relative the aircraft’s CG. The only difference is that it doesn’t have propeller thrust to contend with.

Generally, helicopters without horizontal stabilizers are always tail heavy at forward airspeed. The aerodynamic drag of the fuselage swings the CG aft of the rotor thrust line, the rotor thrust line being fixed in space by operating conditions.

An exception is the synchopter, with the outboard blade tips moving aft. A synchopter has an unbalanced component of rotor torque acting about the pitch axis that provides a nose up moment on the airframe. Think of a dragster with cambered wheels doing wheelies.

[jvitable]

How you guys have knoted and twisted this great sport with all this bull amazes me.I had no idea we had so many distinguished aeronautical enginners in the group.Since (it seems to me) that 90+% of all gyro accidents are caused by lack of training or bad judgment we could spend our time more wisely in this area.Why don't all you geniuses sell me your death machines cheap(that is if you own one).Problem solved!!! LC PS-then you will have the money to buy a nice Sofa with airbags---be safe-Joe PSS-all joking aside I believe most crashes are caused by the angle of the dangle being out of synchronization with those two small nuts located just behind the control stick-this problem can be corrected by replacement of said small nuts with a larger size--!!!!

[WaspAir]

Either one of these devices can be instantly disabled to allow full movement of the stick. What is the problem?

I, for one, won't fly when there is any foreign object loose in the cockpit that could possibly jam any control (cyclic, pedals, or whatever, accidentally or on purpose), ESPECIALLY when one is intentionally flying into turbulence, as explicitly suggested here. If/when your stick falls out in rough air, it sounds like just the beginning of a potential new problem to me. Perhaps my CFI attitude is too conservative for some of you, but I'll leave the macho to others, and plan to live a long time.


I can't think of, but may be wrong, any scenario where an emergency would cause the test pilot to want or have to push the stick forward. In fact, in a gyro, this reaction is kinda a no, no anyhow!
. . .
Haven't thought this thru this long enough to suggest a restraint that doesn't present problems of its own.

There's a reason that there is forward travel on the cyclic -- if it was not needed for any flight regime, we could just put in a permanent stop. I want it available when I fly. I agree on the restraint problems, for the reasons I mentioned above.

What’s the big deal? Both Dick DeGraw and his wife Carroll have been flying gyros with stick locks for years. . .

Stories of survival are not enough to convince me of safety, and shouldn't be for you, either. People survive lots of stupid actions and bad design choices, as much of the discussion on this forum confirms.

Can you imagine what the NTSB report will say if something goes wrong, after you intentionally introduce a control jamming device in your aircraft? "Probable cause: pilot intentionally fouled his own controls; Contributing factors: overconfidence, poor planning, and stupidity."

What's principally bugging me here is the implicit call to everybody to go out and try this and report the results (i.e., "I would appreciate any comments or flight testing feedback on these concepts.", along with directions/pictures on how to do it). If you've got Society of Experimental Test Pilots credentials, a carefully designed protocol, data capture arrangements that will survive any accident, good insurance, and back-ups for your back-ups (anybody here wear a 'chute?), be my guest. If you're just going to play Test Pilot For A Day, I wish you and your family well, but won't be joining you.

Lack (so far) of a better test regimen doesn't make this a good one for the community at large to be trying out. I'd put this in the "professional on closed course; don't try this at home" category. Let's not have people out there getting themselves hurt in the name of safety.

[Chuck_Ellsworth]

all joking aside I believe most crashes are caused by the angle of the dangle being out of synchronization with those two small nuts located just behind the control stick-this problem can be corrected by replacement of said small nuts with a larger size--!!!!

That opinion sounds like it comes from someone who has a brain 1/10 th. the size of his nuts. :focus:

[C. Beaty]

A spring detent mechanism produces a snap back to center action when the controls are released, the same as the centering mechanism on swinging saloon doors, Waspair.

Yet, without the foggiest idea of the device under discussion, you conclude that anyone discussing anything connected with gyros is by definition, feebleminded.

You are an inspiration. But don’t you believe your brilliance is wasted those of us that are less gifted than you?

[jvitable]

That opinion sounds like it comes from someone who has a brain 1/10 th. the size of his nuts. :focus:

Upchuck---very poor comeback---as usual--maybe my brain was affected when preforming unlimited aerobatics in a 727 locked at full RPM and full throttle--If they ever have a Biggest Puke on the Planet contest you are destined for fame .You have already saved more lives than "penicillin"-seems you would be happy!!!There is not another sport out there that is tearing itself apart as we are.No one begs a person to fly a gyro.No one forces a person to fly a gyro.It is personal choice-if I had overwhelming concerns about their safety I would get out of the sport!You ignored the main portion of my text and focused on "nuts"We have gone from CLT to onboard flight recorders to telling flyers to experiment with locking down the control stick--We should be focused on picking the correct gyro for the individual-plenty of flight training which includes using good judgment(like don't take off on your first flight in a snowstorm)(you probably blamed that fatal crash on RAF also.Again your hate for RAF drives you to destroy the sport for everyone-only then will your failure be vindicated.The biggest shame is you are doing a good job of it. I am one of only a very few(obviously) crying out in the wildernest!!!!!

[WaspAir]

A spring detent mechanism produces a snap back to center action when the controls are released, the same as the centering mechanism on swinging saloon doors, Waspair.

Yet, without the foggiest idea of the device under discussion, you conclude that anyone discussing anything connected with gyros is by definition, feebleminded.

You are an inspiration. But don’t you believe your brilliance is wasted those of us that are less gifted than you?

You're awfully quick to switch from the subject to personal venom. I'll keep my poison pen in the drawer for now and give you a measured response.

First, the device under discussion is a jam stick as pictured in post #2. That's quite a bit more than a foggy idea. You raised the spring detent by analogy with your "what's the big deal?" comment and a list of people who have tried stick-fixing devices, but that isn't what we've all been talking about.

I quoted a snippet (to save space) from you to challenge the nature of your reasoning; just because some survive doesn't mean a practice should be recommended to others. After all the RAF brouhaha on this site, I would think you, of all people, would agree that survival by some doesn't establish the wisdom of a practice or a design.

I didn't accuse anybody of being feeble-minded, nor did I claim to be brilliant, nor did I suggest that anybody is less gifted than I.

What I did attack is the wisdom of encouraging the readers of this forum to fly their aircraft with devices that will certainly intentionally, and could also accidentally, restrict control movement while intentionally flying through turbulence. I would hate to see a desire for gathering stability data for safety purposes backfire and lead to accidents while that data is being gathered. I don't consider that a responsible way to pursue safety.

My advice to pilots is: (1) don't try it, and (2) always keep your cockpit free of anything that might snag a control at an inconvenient time.

[helipaddy]

I've avoided reading a threads like "Looping a Gyro" and this one, tryin to avoid a mention in the Darwin Awards. this is the realm of a cautious controlled test regime, not "anyone can do it with a bit of chain and a broom handle ". Frightens the bejaysus out of me.
Ill just keep flying plain vanilla
Paddy

[Doug Riley]

These are "experimental" aircraft. We -- supposedly -- are experimenting. Greg's post suggests how to do a CONTROLLED experiment (the only kind that yields any useful information).

Eyeballing a design and blasting off to see what happens is an UNcontrolled experiment. It does nothing to further anyone's knowledge.

[gyrogreg]

Hey guys, we're trying to advance knowledge and understanding for everyone here. Knowledge is paramount to making good decisions. If you are uncomfortable performing these tests, then please don't do them! As with anything in flying or life, approach it in little steps and keep it safe. But, if you want to help advance gyroplane safety and are comfortable with a little experimenting with your experimental aircraft, then we could use the data.

A point to make: The 40 hour Phase I flight test period that all Experimental Aircraft are supposed to do is intended to evaluate the aircraft's safe flight envelope and its limitations. These are actually evaluations you were supposed to have made during your phase I period. We're just trying to help you identify your aircraft's limitations safely and effectively. Wouldn't you prefer to know if or when your aircraft is a loaded mouse trap waiting to kill you? This Dynamic Stability flight test is safe and normal and standard to conduct on any aircraft. If you don't understand why it is safe and how to keep it safe, then please don't do it! And, by the method I described, you are not getting into flight realms that are more dangerous than the flying you normally do. You were supposed to have done this already - in Phase I! If you are worried about dropping a stick into your contols, I hope a pilot is smart enough to figure out how to not do that! If not, please don't do the test!

BTW, the "chain" method does not fall on the floor if you use a long enough chain. If you don't like the chain, use a (non-strechable) nylon chord - same place you can find the chain in a hardware store. That same hardware store will have different size electrical conduit clamps, so take one over to the chain reel and select a chain that fits and slips well in the hole in the clamp. But, I'm sure a pilot is smart enough to figure that stuff out themselves - but if not, PLEASE don't do this testing!

Military helicopter test pilots, doing similar flight tests, have a specially rigged electrical lock on the stick that is engaged while they hold a button on the cyclic stick, and disengages when they release it. What I am suggesting is the equivalent, "poor man's" version.

But, don't attempt this test if you are really uncomfortable with it!

- Thanks, Greg

[Fiesty]

Having worked as a helicopter test pilot for a number of years I perfer the the old ploting board method of cyclic position. Will not jam and will have a record of cyclic stick position. You may return any position later for varification. I started teaching test flight procedures to Army helicopter pilots in 1950. Worked as test pilot on the Army`s H-34 proram in 1955. I taught over 150 pilots test flight procedures, 1950 thru 1954.

[gyrogreg]

Having worked as a helicopter test pilot for a number of years I perfer the the old ploting board method of cyclic position. Will not jam and will have a record of cyclic stick position. You may return any position later for varification. I started teaching test flight procedures to Army helicopter pilots in 1950. Worked as test pilot on the Army`s H-34 proram in 1955. I taught over 150 pilots test flight procedures, 1950 thru 1954.

Feisty, could you please elaborate on the "plotting board" method? - Greg

[C. Beaty]

While fixed stick testing is about the only way of testing for dynamic stability without a truckload of instrumentation, there is a simpler way of evaluating bunt resistance.

From trimmed cruise flight, snap the throttle shut while holding the stick as stationary as possible. If the machine pitches noseup, you’ve got a potential bunter. If the machine stands on its tail and does a mid-air flare, you’ve got a ticking timebomb.

The correct response following throttle closure to pitch nosedown while maintaining trimmed airspeed with the stick held in the trim position.

[gyrogreg]

Hi Chuck and all. I’m afraid I must comment on this, and I really appreciate the opportunity to discuss these issues in depth – I think they are important to understand. And, perhaps my concepts are questionable. Pardon me for detailed explanation below – I’m not trying to patronize anyone who does not need the basic explanations, but I am trying to help everyone understand these basics and to further a constructive discussion.

---- there is a simpler way of evaluating bunt resistance.

From trimmed cruise flight, snap the throttle shut while holding the stick as stationary as possible. If the machine pitches noseup, you’ve got a potential bunter. If the machine stands on its tail and does a mid-air flare, you’ve got a ticking timebomb.

I do not recommend an average pilot “chopping” the. I believe this can be dangerous. First a bit of a definition:

“Unbalanced Propeller Thrustline”: This is when the HS, reacting to propwash, is not able to compensate for an offset propeller thrustline (from CG). On a HTL or a LTL (deviation from the actual CG), the static prop thrust pitch moment will change upon change of engine power, requiring the airframe attitude to adjust accordingly – until the RTV offset again adjusts to balance all the remaining moments on the airframe. It is possible to embed the HS in the propwash enough to compensate for some or all of HTL or LTL. However, perfection of this “balance” is also affected by the other (aerodynamic) pitching moments on the airframe, so there may only be one airspeed where the HS in the propwash can be arranged to perfectly “balance” a prop thrustline. The airframe reaction to a power change is therefore the result of the “effective prop thrustline” when the propwash on the HS is included in the “balance” of the prop thrustline.

Changing the power quickly, for an unbalanced prop thrustline, causes the airframe to pitch suddenly to the new balance point of the “sum of moments” on the airframe. For a badly unbalanced HTL, this could cause the airframe to pitch suddenly and severely nose-up – the reason for the old instructor adage to reduce power if you think you are getting into trouble. This would also indicate as you suggest, Chuck, the possibility that the gyro might be prone to PPO (buntover). But, for a badly unbalanced LTL, the pitch reaction to a “chop” in power will be suddenly nose down. A sudden nose down pitch reaction may not be so good:

Especially with the stick held “fixed” during such a radical airframe pitch reaction, the spindle will input a sudden cyclic action to the rotor. In either the nose-up reaction, or the nose-down reaction, the sudden and severe cyclic input could “precess stall” the rotor – suddenly stall more of one or both rotors enough that the teeter range is exceeded. In the nose up direction of an unbalanced HTL, a precession stall might only cause severe mast bumping. In the nose-down direction of an unbalanced LTL, a precession stall might not only cause a severe mast bumping, but the sudden reduced rotor loading could also suddenly slow the rotor at the same time. This is especially severe at higher airspeeds where the aerodynamic airframe moments can cause more severe nose-down pitch reaction. This is all what could happen if the stick is fixed or the pilot does not react quickly or adequately enough with opposite cyclic reaction to avoid severe cyclic input to the rotor.

But, also in the nose down reaction of a LTL, if the pilot does allow or commands a quick compensating cyclic input – not “fixed” stick - the nose-down airframe reaction cab cause the airframe CG to suddenly move to a new position aftward – statically AOA less stable or even unstable condition. (With the power at idle or off, there is no longer a LTL artificially holding the RTV aft of the CG, so the new steady state trimmed condition will likely be with the RTV forward of the CG because of the other nose-down aerodynamic moments on the airframe – especially at higher airspeeds! If at any time in this transient, the RTV becomes forward of the CG – AOA unstable - and if the cyclic input has not fully corrected the reducing G-Load on the rotor, that gyro is in danger of a buntover! It would not be a good situation to have the nose still dropping, the rotor load still reducing, at the same time the RTV is forward of the CG.

In a severe power “chop” on this LTL condition, without immediate and adequate cyclic input, the pilot could end up in an AOA unstable condition with the nose still dropping – a possible initiator of a buntover – rapid AOA static divergence in the nose-down direction! Especially with the gyro suddenly in a statically unstable situation, rapid pitching could easily excite the pilot into over-reaction – possibly inducing a PIO pilot reaction.

There are issues with the power “chop” on a severely unbalanced HTL also – but at least it is moving the CG in the more stable direction and gyros do not really have a “bunt-up” reaction – but the precession stall and inducement of a pilot PIO reaction is still possible.

I do not recommend a power “chop” as a test for other than a professional test pilot. I did this once in a standard 618 Dom ONCE! I was familiar with the power “chop” reaction of my original Air Command and with my High Command modification. During the 40 hour Phase I flight testing on the Dominator I had built, and before I understood much about gyro aerodynamics, I tried a power “chop” at 70 mph and 20 ft over the runway. Upon the “chop” in power, the airframe suddenly pitched down about 15 degrees and I was looking at eating the runway! My reaction was, of course, an immediate aft cyclic and I avoided contacting the runway! I over reacted somewhat with the stick and ended up PIOing a bit as I added power back in – I was totally unfamiliar with, and unexpecting of those reactions in that gyro! Later, after I though this over a lot, I realized that I could have precess stalled the rotor, that I had probably experienced a point of AOA instability accounting for the sudden handling sensitivity – realizing that perhaps my commanded cyclic input could have been less than ideal to save that situation!

Another indication that this normally docile-in-turbulence Dom could have conditions where it was not so AOA stable is when I would reduce power at high airspeed and attempt to continue that high speed in an idle power gliding descent. In turbulence, this was a very uncomfortable condition. I believe this is because, without the LTL artificially holding the CG well forward of the RTV, the nose-down aerodynamic moments actually presented a statically AOA unstable condition – the RTV was forward of the CG – at least it did not have the large margin of AOA stability one gets used to in that LTL at normal high speed cruise! Without the LTL to augment the static AOA stability, that gyro flew rather uncomfortably in turbulence! I had learned by then to not “chop” the power at higher airspeeds, but I also avoided high speed glides with reduced power simply because it felt very uncomfortable! I think I realize now that was because I was flying a marginally AOA stable gyro when LTL power was not augmenting AOA stability by holding the CG well forward of the RTV.

The ASTM standard has a section to check for this “unbalanced prop thrustline” – the “Static Power Stability” test. This “Power Stability” test was originally suggested by the FAA Rotorcraft Directorate for the standard – probably because they recognized the issue’s severe airframe pitching might cause pilot over reaction. This test does not suggest “chopping” the power for the above reasons. However, a slow power change will slowly adjust the pitch attitude and trimmed airspeed to indicate an unbalanced prop thrustline condition just as well as a “power chop”, but without the dangerous implications above.

IMHO, even if this test was safe to perform, it does not allow for a conclusive determination of static AOA stability or susceptibility to buntover – the judgment of the severity of the pitch reaction is subjective – we are looking for an objective determination that is not dependant on the pilot subjective evaluation or stick reaction. At a minimum, for an objective evaluation, the stick would have to be mechanically fixed, a situation in which I would certainly not suggest a sudden power change – for a non-professional test pilot!

Thanks, Greg

[gyrogreg]

The correct response following throttle closure to pitch nosedown while maintaining trimmed airspeed with the stick held in the trim position.

IMHO, this is an over simplistic conclusion. This response, and a reduced steady state airspeed if power is reduced slowly, instead of “chopped”, only identifies any “unbalanced” prop thrustline offset. But, prop thrustline offset is not the mechanism of a buntover – it may be an element or a worrisome indicator of a possible PPO (buntover) issue. But, the true mechanism of a buntover is the statically divergent condition of the “effective RTV” forward of the CG. This reduced power (or increased power) test identifies only the physical position of the “effective prop thrustline” above or below the vertical CG. But, this test does not fully identify if that condition truly makes the gyro statically AOA unstable. A definition:

“Effective RTV”: Although the RTV position relative the CG is an element of static AOA stability, it is not the determinate issue – not the whole picture! There are other static and dynamic elements that enter into the ultimate AOA stability picture – the actual position of the “effective” RTV relative to the CG. I believe it was Udi Ziegerson who originally opened our eyes to elements other than just RTV in the overall stability determination. The “effective” RTV accounts for or lumps in all other elements. The other elements that play a part in actual AOA static stability include: The lift slope differences between the HS and the rotor, the offset gimbal, rotor “blowback effects, dynamic damping, RRPM changes, and the inertial reactions of the airframe and rotor. These are very difficult parameters to quantify or factor into the determination of an “effective RTV”. The proposed “Power Chop Test” or the ASTM Static Power Stability test only identifies one factor in the determination of the “effective RTV”. It also does not factor in the other aerodynamic moments on the airframe that also determines the real RTV.

For these reasons, even a test that simply statically identifies that the real RTV is physically forward or aft, does not really determine the true static AOA stability condition - the relative position of the CG to the “effective RTV”. The “Power Chop” test, or the ASTM Static Power Stability test cannot tell us where the “effective RTV” is in relationship to the CG. It can only tell us if the prop thrustline is “balanced” or not! IMHO, the erroneous hypothesis is that the prop thrustline (really the “effective” prop thrustline), by itself determines the static AOA stability – it does not. The true determinate of static AOA stability is the relationship of the CG to the “effective RTV”.

Because it is composed of such complicating elements, it is probably futile to try to determine or quantify the other individual elements into a determination of the “effective RTV” – at least on paper! However, a flight test that determines if or when static AOA stability exists, indirectly verifies that the “effective RTV” is truly aft of the CG. This is what the proposed “Dynamic Stability” test does – verifies that the gyro is statically AOA stable! There is no need to try to isolate the other individual elements that factor into the ultimate result of static AOA stability – we just know that it is statically AOA stable and therefore incapable of a buntover in that flight condition. For most of us, that is all we need to know. For some of us, we would probably still like to understand these other elements better – for one reason, they suggest ways to further improve the stability of the gyroplane!! But, my intent with this thread is to just provide a way to identify the safe flight envelope in which there is little or no risk of a buntover.

Some might suggest that these other “elements” that factor into the “effective RTV” are not significant or important. I maintain otherwise. These must be real elements that provide true static AOA stability even for gyroplane configurations that have an unbalanced HTL! The Magni gyro is one example! The Static Power Stability test (and the “power chop” test) indicates the Magni certainly has an unbalanced HTL. If this nose-up reaction to a “power chop” or power reduction were the sole determination of the ability to buntover, we might expect to see examples of buntovers in a Magni. With over 450 examples flying worldwide, there are no reports of either PIO or PPO (buntovers). (Don’t try this at home, but) I have tried to buntover a Magni M16 by jamming the stick hard forward at high power and high airspeed (100 mph)! In the extreme “jab” to the point where the rotor actually bumps the teeter limits and stick, the nose drops hard and severe, and while holding the stick in that “fixed” position (against a “jam stick), it flies out of it back to trimmed airspeed after 2-3 oscillations! These other “elements” of static AOA stability are what is preventing a buntover in this gyroplane – the unbalanced HTL is not indicating it will buntover! Interestingly, and as partial validation of this “Dynamic Stability” flight test determination of ability to buntover or not, the Magni tests to be dynamically stable beyond its published Vne of 115 mph. This is the case at MPRS, idle or full power!

Sorry for the length of this – the concept of the “power chop”, IMHO, introduces a lot of issues. And, this is my thread, so I don’t feel so bad taking up a lot of its space.

- Thanks, Greg Gremminger

[C. Beaty]

Sometimes, Greg, in trying to avoid long winded, convoluted posts, I economize on verbiage too much.

Perhaps I should have stated that in no flight testing does one pull all stops on any initial test. The key to survival is a graduated and systematic approach.

Initially, testing for response following power reduction should be performed with a mild power reduction at a safe altitude until the particular behavior of the machine becomes familiar, not an abrupt power chop a few feet over the runway as a first test.

Finally, I make no excuses for having the propeller thrust line anywhere but on the CG with perhaps a ±2” tolerance.

I believe Cierva had it right:

[PW_Plack]

If you've got Society of Experimental Test Pilots credentials, a carefully designed protocol, data capture arrangements that will survive any accident, good insurance, and back-ups for your back-ups (anybody here wear a 'chute?), be my guest. If you're just going to play Test Pilot For A Day, I wish you and your family well, but won't be joining you.

If you're going to build and fly your own an experimental aircraft, you're a test pilot, one way or the other.

Now, convince me that fixed-stick testing is more dangerous than flying ignorant of your machine's stability profile.

[ferranrosello]

This is an interesting discussion. But once more I see than nearly everybody is looking at the stability problem like RTV position versus cg. I think this is right in a fixed wing aircraft, but rotary wing are different. Why? Because is the cg who moves to cancel the moments about it, but not the rotor. And this fact makes the control and stability mechanics of rotary wing aircrafts neatly different from its fixed wing brothers.

But now we can find a new issue: dynamic stability. Of course, it is much better to study this kind of stability than forget it. But, gentlemen, what you are testing by the fixed controls method is the long term dynamic stability. Why do you think that this is the important one for PPO’s?

Another question is that fixed controls do not imply jammed controls, only a condition “without pilot inputs”. The entirely fixed controls are used when performing manoeuvring stability test (another kind of dynamic stability test) and the significant ones when exploring gyro’s PPO characteristics (but this test is delicate and dangerous, and only should be performed by specifically trained pilots).

Chuck, helicopters are much less stable than gyros. In fact, a lot of them have automatic stabilization systems and flying tests are performed with these devices working. So the idea that is necessary to eliminate the rotor gimbals effect for testing our gyros is wrong. The idea than helicopters are more stable at higher airspeeds is wrong too. There is a range of airspeeds (typically between 40 and 50% of VNE ) where helicopters are less unstable. But believe me; helicopters need a pilot to fly (or a good AFCS). Gyros fly very well by themselves.

Ferràn

[C. Beaty]

A helicopter rotor responds very differently from a gyroplane rotor, Ferran.

A disturbance that produces a momentary increase of angle of attack causes an unstable response in a helicopter rotor.

An upward gust for instance, increases the angles of attack of both advancing and retreating blades by the same amount, but since the advancing blade moves at higher speed than the retreating blade, the increment of lift is greater on the advancing blade side. This causes the rotor disc to pitch noseup, magnifying the effect of the disturbance; i.e., unstable vs. angle of attack.

The opposite effect occurs with a gyro rotor. An upward gust causes an increase of rotor speed, which decreases the airspeed differential between advancing and retreating blades, causing a nosedown tilt of the rotor disc. An inertialess gyroplane rotor would have neutral angle of attack stability; a real rotor with inertia behaves somewhere between a helicopter rotor and an inertialess rotor.

A German helicopter pioneer, Kurt Hohenemser, received patents covering a pitch-cone coupled rotor. This is a rotor in which an increase of coning angle pulls collective out of the rotor, reversing the angle of attack slope. McDonnell Aircraft built several compound helicopters that used the Hohenemser patents and their helicopter was as stable as a gyroplane. These were built under military contract but the US military wasn’t interested since very effective SAS systems were beginning to be produced.
********
For any moving object to possess stability, the disturbing force acting upon it must trail the CG. This same rule applies universally to everything from birds to blimps.

[Steve McGowan]

Yall be sure to let me know what hapins,,

cause Yall Skeerin the Sheet outta me.. R Ya gonna install a parachute 2 ?

I certainly Hope SO

VooDoo ............My A&&....just fly the damned thang :D

[birdy]

Perhaps I should have stated that in no flight testing does one pull all stops on any initial test. The key to survival is a graduated and systematic approach.
Zactly CB.
Who jumps into the bath tub to test the temp? Only any idiot.
You start with your finger tip, and if theres no pain, your rite to go to the next step.

[gyrogreg]

Why do you think that this (dynamic stability) is the important one for PPO’s?

Ferràn


Ferran, It is my suggestion that DYNAMIC stability, or the existance of dynamic stability in gyro, is an indicator that static AOA stability exists. You cannot have dynamic stability unless static stability exists. In fact, you cannot have long term phugoid oscillations at all, damped or not, unless static stability exists - there is a static restoring moment or action.

So, if, at the flight condition you test (airspeed / power / loading), the gyro shows to have oscillations at all, and especially if it shows to have damped oscillations (dynamic stability), it must certainly have static AOA stability. Having actually damped oscillations (dynamically stable) verfies a bit more static stability margin than for the condtion where oscillations are existing, but no longer damped (neutral or negative dynamic stability).

Static AOA instabilty is the root of a buntover - a disturbance from the steady state condition causes a more divergent static moment or action, which causes a worse disturbance from steady state, which causes, a worse divergent moment or action, and so on and so on! If this disturbance and resulting static divergence is in the nose down direction, that is the mechanism of a buntover - Static AOA instability.

Static AOA stability is the protection from a buntover. When a gyro is statically AOA stable (in pitch) a restoring moment or action starts the pitch moving in the corrective direction. So, when there is a disturbance from steady state the pitch starts restoring back to steady state - this is much like the action of a spring - pull on it and it tries to go back to its normal state. It is the dynamic properties, inertia and damping, that determine if that restoring action will either oscillate or simply "slide" back to the steady state condition (steady AOA). These dynamic properties may also allow the pitch to overshoot and therefore oscillate about the steady state - before it actually settles down to the initial steady state condition (dynamic stability). (In any statically stable aircraft that has some mass, the response to a pitch disturbance will almost always be oscillatory - unless that HS is so powerful that it actually cannot even oscillate!) If there is no damping, or if there is external additional energy provided into the oscillation, the oscillation may remain the same or actually get larger oscillations (neutral or negative dynamic stability) - such as a child swinging her legs on a swing to make it keep swinging or go higher.

Phugoid pitch oscillations are actually slow pitch changes in airspeed - with essentially constant AOA. This essentially constant AOA is the result of the static AOA stability. The electronic analogy that helps me understand this is that that of a voltage follower Op Amp. The feedback (restoring action) from the output (disturbance) to the input causes the input to change the output in the correcting direction. If the "feedback" (restoring moment or action in a gyro) is weak, there is more ouput error allowed. If there is delay in the "loop", the response will be an oscillation. So, with weak "feedback", in a gyro phugoid pitch oscillation, there will be a small AOA error, but it will not be statically divergent - it will still be trying to get back to the initial steady state.

My point here is just to try to explain why the phugoid oscillatory action is an indicator of the strength of the static AOA stability - restoring force. If, at increasingly worse power and airspeed combination, the phugoid pitch oscillations become neutral or start getting worse on their own, that is an indication that the static AOA stabilty is getting weaker - it still exists, but it is weaker and will dissappear for worse condtions. Verfying that the gyro is dynamically stable (damped oscillations eventually settling out to no oscillations), verfies a margin of static AOA stability which also verifies a margin of buntover protection.

All our previous attempts from static stability flight testing cannot identify safely or definitively whether a gyro is statically AOA stable or not. This "back door" method of verifying static AOA stability is a way to do so while accounting for all other difficult-to-evaluate restoring moments or actions. This method doesn' care if it is the RTV, thh RRPM, the HS, or anything else that is providing the restoring moment or action, it only cares that the result of all in combination still provides a pitch oscillation and therfore verifies that all effects together still provide static AOA stability.

- Thanks, Greg

[Steve McGowan]

Gentlemen,

I truly appreciate all the mind aspects for whatever this is,,

But DAMN Yall complicating the Hell Outta the NOVICE Pilot..and me TOO


AnyWay,,,,,Yall's PARTY

[C. Beaty]

Ignorance kills but I don’t know of knowledge hurting anyone.

[Chuck_Ellsworth]

Or there is that saying.

Fools rush in where angels fear to tread.

[jvitable]

Is it just a coincidence that people with the first name of Chuck have a talent for taking the fun out of this sport!!! What garbage!!!Just put another deadbolt on my door knowing you strangeies are out there.I am sure all the Newbies(who we need in this sport) are really taking notes on this crap!!They now know they will need at least a masters degree to be successful in this sport--mine is only mail order but thank God for it!!!----on another note--Anyone out there have an extra" slide rule'-I lost mine and now can't start my gyro!------LC 281-489-2019

[birdy]

.........idiot

[Resasi]

One doesn't need a masters to fly anything.

It's a great advantage however to have available to us sources of information that go down to the deeper levels of the 'what' and 'how' of gyro aerodynamics for those who would like to understand but never had the formal education.

Yes there are newbies taking notes, probably those who don't want to trust to blind luck and, 'because he does I can.'

Every so often, perhaps too often, we read on this forum of someone who didn't make it. It is of course no guarantee, but a greater knowledge of exactly what they were doing may have prevented some of those statistics

[birdy]

Every so often, perhaps too often, we read on this forum of someone who didn't make it. It is of course no guarantee, but a greater knowledge of exactly what they were doing may have prevented some of those statistics
Zactly.
Those who have no interest in understanding are either too proud [ interprit as arrogant] to admit to themselves that they just mite learn sumthn, or have a blind faith that wen the sh1t hits the fan, sum miraculous intervention will save their ass,... or both.
Either way, they be fools to ignor freely available info.
It dont matter wot machine im sitn in, be it a car, grader, road train, bike, gyro or heli, i like to know all there is to know. Coz the better you know the beast, the more control you have, and the more you can get out of it, without over do'n it.

[Chuck_Ellsworth]

grader,

Birdy, good choice a grader is exceptionally stable until you try and operate it on too steep a slope.

Once the slope becomes too steep its center of gravity will change its stability.

You and I and most here understand that simple fact, the problem in this group is it seems to attract a few who are unable to grasp such basic principles.

Maybe gyros are natures new vehicle to cleanse the gene pool?

[jvitable]

.........idiot

" POT "-all of this theory is not worth one day of training!When you are trying to interest someone in this sport(any sport) the worst thing you can do is present to much heavy theory!!All of this is enforceing the point of many that gyros are very dangerous machines and bad for your health! ----That this "heavy" theory is necessary to be a good pilot is bull---I know many people who are great fliers without it---- with all of our differences one thing will always be true------men who fly planes thank God--when I fly a gyro I am a God!The only thing Superman has on me is tights and a better butt!----your friend " Kettle " 281-489-2019

[C. Beaty]

Some of these characters remind me of my NYC grown first wife.

At the NY State fair in Syracuse, she peered underneath a blue ribbon bull that was on display and said; “My, I’ll bet that cow gives a lot of milk!”

It ruined her whole day (and mine) when I spoiled her blissful ignorance and informed her that what she took to be an udder was in fact a scrotum and that while there might be a superficial resemblance, the functions were entirely different.

Many gyro people are the same way; they get very annoyed when informed their most cherished fallacies are just that.

[StanFoster]

Chuck- Your last post madd me laugh out loud so much so that the squirrels in my trees are looking at me like I am a big NUT ! There coming to take me away - got to love this forum. Have a good day there Chuck! Stan

[Vance]

Is it just a coincidence that people with the first name of Chuck have a talent for taking the fun out of this sport!!! What garbage!!!Just put another deadbolt on my door knowing you strangeies are out there.I am sure all the Newbies(who we need in this sport) are really taking notes on this crap!!They now know they will need at least a masters degree to be successful in this sport--mine is only mail order but thank God for it!!!----on another note--Anyone out there have an extra" slide rule'-I lost mine and now can't start my gyro!------LC 281-489-2019

Each year we seem to lose around four participants in our sport of flying experimental gyroplanes.

I feel that the dead participants limit the growth of the sport of flying gyroplanes more than the fear of reaching my personal intellectual limits because there are things to learn to make me safer and better able to access risks.

Thank you, Vance

[BUD ONEAL]

Joe'
When I first started to learn how to fly the gyro Chuck B was my saviour more than once. There were no two place trainors yet and the only way to learn and live was to listen to what the "Chucks" of this world that we live and play in said. I have been in gyros for close to twenty five years now. Chuck would make suggestions and I listened,
learned and lived. Not a bad strategy,worked for me!

[barnstorm2]

I am sure all the Newbies(who we need in this sport) are really taking notes on this crap!!They now know they will need at least a masters degree to be successful in this sport--

All the newbies need to know is:

1. Buy a STABLE gyro with a good safe reputation. Some examples are Dominator, Monarch, Magni, CLT AirCommand (only), SparrowHawk, SportCopter, Xenon and others.

2. Get training from a properly rated gyro CFI. Even if you are a rated high time pilot in everything but gyros you still NEED gyro specific training.

If you have 1 and 2 you can ignore 3 and 4 below...

3. All currently in business gyro kit manufacturers are selling safe stable machines. However, one company that did make unstable gyros with known design flaws might be starting business back up in South Africa. If the machine you are looking at does not have a Horizontal Stabilizer it is almost certainly NOT a safe stable machine. The test described in this thread fails on these machines past certain airspeeds.

4. Those that need to do the tests described in this thread are mostly experienced pilots that own a machine that is scratch-built or of a legacy design (high thrust line). If this test fails there are ways (described in other threads) to enhance the stability and safety of your gyro.

[jvitable]

Each year we seem to lose around four participants in our sport of flying experimental gyroplanes.

I feel that the dead participants limit the growth of the sport of flying gyroplanes more than the fear of reaching my personal intellectual limits because there are things to learn to make me safer and better able to access risks.

Thank you, Vance

Pushing yourself to intellectual limits on ridiculous theory and then using bad judgment in flying will lead to the same results. LC--If we attacked the real cause of 99% of these crashes with the same vigor as the Chucks approach this crap we would indeed save lives. LC 281-489-2019

[jvitable]

Joe'
When I first started to learn how to fly the gyro Chuck B was my saviour more than once. There were no two place trainors yet and the only way to learn and live was to listen to what the "Chucks" of this world that we live and play in said. I have been in gyros for close to twenty five years now. Chuck would make suggestions and I listened,
learned and lived. Not a bad strategy,worked for me!

Bud---helped one -scared off 100's---Thousands of potential gyro pilots read this forum---yet our sport is dying a not so slow death-I just recieved a call from a Newby who agrees!LC 281-489-2019

[jvitable]

All the newbies need to know is:

1. Buy a STABLE gyro with a good safe reputation. Some examples are Dominator, Monarch, Magni, CLT AirCommand (only), SparrowHawk, SportCopter, Xenon and others.

2. Get training from a properly rated gyro CFI. Even if you are a rated high time pilot in everything but gyros you still NEED gyro specific training.

If you have 1 and 2 you can ignore 3 and 4 below...

3. All currently in business gyro kit manufacturers are selling safe stable machines. However, one company that did make unstable gyros with known design flaws might be starting business back up in South Africa. If the machine you are looking at does not have a Horizontal Stabilizer it is almost certainly NOT a safe stable machine. The test described in this thread fails on these machines past certain airspeeds.

4. Those that need to do the tests described in this thread are mostly experienced pilots that own a machine that is scratch-built or of a legacy design (high thrust line). If this test fails there are ways (described in other threads) to enhance the stability and safety of your gyro.

Tim--you copied a previous thread of mine--All correct and I fully agree!!!! What I do not agree to is that to safely drive a car I not only should know engine theory and mechicanics but know and understand the automotive engineering than went in to the car design etc and that somehow this will save me from being killed because I am a bad driver--Bull-Bull _ bull-call me any time--LC--281-489-2019--still getting calls from people we scared off----

[brett s]

Flying a gyro with known design issues that regularly kills people certainly qualifies as bad judgment, especially given that there's no secret to fixing the flaws.

Flight training isn't a substitute for proper aircraft design.

People getting killed needlessly is part of why this sport is dying - better to perhaps "scare off" a few than watch them die because they choose an unsafe design. Even better, there are undoubtedly a bunch of folks that have learned & made better design choices. That's what will save this sport...

Sticking your head in the sand doesn't work, the physics don't change just because you choose not to believe.

[Vance]

Pushing yourself to intellectual limits on ridiculous theory and then using bad judgment in flying will lead to the same results. LC--If we attacked the real cause of 99% of these crashes with the same vigor as the Chucks approach this crap we would indeed save lives. LC 281-489-2019

What is the real cause of 99% of these crashes?

How would we attack it?

Thank you, Vance

[jvitable]

Flying a gyro with known design issues that regularly kills people certainly qualifies as bad judgment, especially given that there's no secret to fixing the flaws.

Flight training isn't a substitute for proper aircraft design.

People getting killed needlessly is part of why this sport is dying - better to perhaps "scare off" a few than watch them die because they choose an unsafe design. Even better, there are undoubtedly a bunch of folks that have learned & made better design choices. That's what will save this sport...

Sticking your head in the sand doesn't work, the physics don't change just because you choose not to believe.Brett--You are right on with this--(-included in one of my threads also-)--Who on this forum disagrees with you ---not me!!!But the truth is we are losing pilots flying your approved machines also----That is why I harp on accident causes and not just machine types---------AS far as all this theory talk--aeronautical engineers are still arguing about the theory on prop preformance etc.Lots of talk---I have no hidden agendas-have no need to want people to think I am special because I fly a gyro.I see you are a helo man(like to know my enemy)--( a joke!!)If the average person could afford one of these as easily as a gyro the death rate would blow your mind.-----WE do agree to pick one of the more forgiving machines---We do agree to get proper training---We do agree to use good judgment in flying---Do we at this point agree a gyro is then one of the safest machines flying????This should be the agenda of this forum!!!!!!!!!!!Let go of all the other crap between certain parties on this site---I will help anyone out there as much as I can--call 281-489-2019 or cell 713-302-0292--I am not that knowledgable but will stear you on to someone who is--This is the greatest sport in the world--follow your dreams and become a God like me---Fly safe LC

[jvitable]

What is the real cause of 99% of these crashes?

How would we attack it?

Thank you, Vance

Vance-----two words come to mine------"PILOT ERROR"----Not a put down to those who died---( I may well be next )-but we all make mistakes----call me at 281-489-2019 anytime---you seem fairly normal!!!------PS Anyone can afford a gyro but find training is as much or more expensive than the machine(and much more difficult to get)-Add to this people look upon them as a toy and add to this some fixed wingers think they are capable of flying one and you end up with bodies---Even after all the lessons and one flys well the God syndrome enters the picture and we get carried away with the power--more bodies---throw in bad judgment and you could fill a grave yard---BUT IT IS FUN!!! LC

[magilla]

An experimental aircraft is not a car, nor an ATV. If you can't understand that basic premise, then none of us on this forum can help you.

True, a thermodynamics degree won't help you when the engine quits, and an aeronautical degree won't help you once you start to fall from a bunt-over, but in both cases, the knowledge would have helped to prevent either occurrence.

Jvitable, you are free to believe whatever you want about flying, but please don't ruin an excellent thread on testing gyroplane dynamic stability based on your naive and limited understanding of aviation.

You don't need to be rocket scientist to fly a gyro - many here have proven that... but don't poo-poo a thread that is attempting to make us all more educated because you are either intimidated by the knowledge or don't understand it.

Spencer

[Vance]

Vance-----two words come to mine------"PILOT ERROR"----Not a put down to those who died---( I may well be next )-but we all make mistakes----call me at 281-489-2019 anytime---you seem fairly normal!!!------PS Anyone can afford a gyro but find training is as much or more expensive than the machine(and much more difficult to get)-Add to this people look upon them as a toy and add to this some fixed wingers think they are capable of flying one and you end up with bodies---Even after all the lessons and one flys well the God syndrome enters the picture and we get carried away with the power--more bodies---throw in bad judgment and you could fill a grave yard---BUT IT IS FUN!!! LC

Why are some aircraft designs over represented in fatal accidents if it is 99% pilot error?

Does a particular design encourage pilot error?

Does a particular design have less tolerance for pilot error?

For me, having some understanding of how to stay out of trouble encourages me to participate in gyroplane aviation. I feel that someone that clings to ignorance to overcome their fear may not be well suited to aviation as a hobby and is not likely to produce the growth in our sport that you desire.

I make mistakes and I hope that the way my gyroplane is designed will help me to manage the results of my errors. It is nice to have a way to quantify the tendency to have a power push over. I like not finding out the hard way that my aircraft needs work.

Thank you, Vance

[Chuck_Ellsworth]

The more people who get scared off and do not fly dangerous machines the less funerals there will be.

[jvitable]

An experimental aircraft is not a car, nor an ATV. If you can't understand that basic premise, then none of us on this forum can help you.

True, a thermodynamics degree won't help you when the engine quits, and an aeronautical degree won't help you once you start to fall from a bunt-over, but in both cases, the knowledge would have helped to prevent either occurrence.

Jvitable, you are free to believe whatever you want about flying, but please don't ruin an excellent thread on testing gyroplane dynamic stability based on your naive and limited understanding of aviation.

You don't need to be rocket scientist to fly a gyro - many here have proven that... but don't poo-poo a thread that is attempting to make us all more educated because you are either intimidated by the knowledge or don't understand it.

Spencer Spencer-you have a very condescending way about you.I knew enough about gyro theory to get my ticket add-on--You go from basic premise to the most detailed aeronautical analysis of the inner workings of gyroplane flight.I think your comment about me being naive and having a limited understanding of aviation is a very dumb thing for a person in your position to say.You know as well as I that anyone interested in becoming a gyro pilot would be confused and disheartened by this indepth stuff.Your group wants to be test pilots but have never studied the previous crash cases---thats stupid and naive---When I suggest a close look at the root causes I get zip because it does not fall into the groups agenda!!! PS-my engine quit and I am still here-lucky I guess-----Test all you want but please not over my house-----LC

[barnstorm2]

The reason people get scared off from gyros is our high accident rate. "Gyrocopters" are almost synonymous with "widow-maker" among older pilots.

The reason we have a high accident rate is because so many pilots and passengers have died in PPOs.

Remove the PPOs and we have a pretty good safety record.

We have PPOs because over that past 20 years we have had many machines sold that have design flaws.

The "Pilot Error" is not understanding how and why PPOs happen and how susceptible your machine is to PPOs and if it has the known flaws that lead many pilots into PPOing.

We know that MANY high time pilots and CFIs have died from PPOs so training while it may partially mitigate the problem does not solve the problem or even reduce it to reasonable levels.

We do know that a properly designed gyro can not PPO (CLT) or will be very difficult to PPO (Stable).

This thread describes a fantastic way to rationally and scientifically test your machine for these flaws and tendencies.

Therefore, this thread and methodology is DIRECTLY related to helping our sport and saving lives.

.

[Chuck_Ellsworth]

.........idiot

I notice not many people are challenging Birdy on his opinion, I wonder why?

[jvitable]

The reason people get scared off from gyros is our high accident rate. "Gyrocopters" are almost synonymous with "widow-maker" among older pilots.

The reason we have a high accident rate is because so many pilots and passengers have died in PPOs.

Remove the PPOs and we have a pretty good safety record.

We have PPOs because over that past 20 years we have had many machines sold that have design flaws.

The "Pilot Error" is not understanding how and why PPOs happen and how susceptible your machine is to PPOs and if it has the known flaws that lead many pilots into PPOing.

We know that MANY high time pilots and CFIs have died from PPOs so training while it may partially mitigate the problem does not solve the problem or even reduce it to reasonable levels.

We do know that a properly designed gyro can not PPO (CLT) or will be very difficult to PPO (Stable).

This thread describes a fantastic way to rationally and scientifically test your machine for these flaws and tendencies.

Therefore, this thread and methodology is DIRECTLY related to helping our sport and saving lives.

.Bstormer-remove the ground and you will have a very good safety record-da

[All_In]

I know I’m going to regret getting involved in this!
Buuuuuuuuuuut, being a newbie I have a perspective experienced gyro pilots and forum members probably don’t.

My hobby is actually learning, so threads like this are very helpful to gyro engineering wantabee’s like me.

However I have observed that even my closes friends and family members eyes glaze over when I try and share or discuss theory with them on any subject like this. Most newbie’s probably are paying much more attention to the squabble unfortunately than the substance of this thread.

As to scarring newbie’s out of the sport, can it get any worst?

Perhaps my current experience will shed some light on the question. All the FW pilot and my friends I have told about entering your sport have all told me, without exception so far, that “You’re going to kill yourself”. So I believe the evidence thus far has demonstrated that the general public even pilots are already convinced the sport is lethal.

Hard to imagine that if they have gotten this far that anything said here so far is really going to keep them out of the sky.

I really soaked up the information on which Gyro you consider as stable and made a note.

Now can anyone make a list of the gyros that are not stable, although maybe we better stick to list the ones you consider stable as this might start another even bigger battle, when you name someone’s favorite?

Cheers be happy,
John

[C. Beaty]

Here are a couple of articles you might find interesting. The first is a synopsis of research performed at the University of Glasgow on the subject of gyroplane stability. The second is an article by French aerospace engineer Jean Fourcade, based at least partially on research conducted at the University of Glasgow.

http://www-legacy.aero.gla.ac.uk/Research/Fd/Project5.htm

http://www.asra.org.au/L_Stability.htm

[All_In]

Here are a couple of articles you might find interesting. The first is a synopsis of research performed at the University of Glasgow on the subject of gyroplane stability. The second is an article by French aerospace engineer Jean Fourcade, based at least partially on research conducted at the University of Glasgow.

http://www-legacy.aero.gla.ac.uk/Research/Fd/Project5.htm

http://www.asra.org.au/L_Stability.htm

Oh those links look promising espically the 2nd, now reading…
Thanks,
John

[mceagle]

Your group wants to be test pilots but have never studied the previous crash cases---thats stupid and naive
Joe, I think you will find exactly the opposite. Those with the most interest in gyroplane technical safety are also those that have studied accident causation the most. What would be stupid and naive is for us to consider that we know it all and there is no more to learn.
I do not believe that anyone is trying to force you (or any low hour pilots) to do their own stability testing.
Make no mistake, there can be absolutely no doubt that the more knowledge a pilot has of gyroplane aerodynamics and handling, the better and safer pilot he will be. If it wasn't for people like the Chucks and Greg G, you would still be flying basic Bensen's with the ever reliable McCulloch engine.

[jvitable]

Joe, I think you will find exactly the opposite. Those with the most interest in gyroplane technical safety are also those that have studied accident causation the most. What would be stupid and naive is for us to consider that we know it all and there is no more to learn.
I do not believe that anyone is trying to force you (or any low hour pilots) to do their own stability testing.
Make no mistake, there can be absolutely no doubt that the more knowledge a pilot has of gyroplane aerodynamics and handling, the better and safer pilot he will be. If it wasn't for people like the Chucks and Greg G, you would still be flying basic Bensen's with the ever reliable McCulloch engine.

Tim-I don't remember seeing a Chuckbird gyro or a Gregtax engine before-sorry!---If you disected these crashes it would give prospective gyro pilots a chance to see these machines are not falling out of the air on their own accord with no warning.But then that would not fit in with C e's agenda.I believe in clt-great-lets make gyros as safe as possible and as forgiving as possible but to be constantly negative about our sport is not the way to move forward.Thrashing CFI's is another problem!Testing is great but making this sport seem so complicated to beginners with all the heavy aeronautical talk just adds to their fears. Our main goal is to eliminate PPO just as designers of fixed wing wanted to eliminate stalls. They finally did--they called the new craft a gyro----I am still getting calls from beginners or want-a-bes that are obviously intiminated by the direction of the forum.The Public dislikes us and we dislike each other--at least we are consistant! LC

[ckurz7000]

Just for the sake of some lurkers or newbies getting a bit behind the aerodynamics flung around in this thread:

There is static stability and dynamic stability. A nice mental image of a statically stable situation is a ball sitting in a bowl like the picture below.

46089

If you give the ball a bit of a nudge to either side, it will respond by returning to its original position. Now, if you were to turn the bowl upside down you'd have yourself an unstable situation:

46091

Here, the ball is precariously balanced ontop of the bowl and any slight nudge will make it roll into the setting sun, i.e., it won't return to its original position by itself.

Returning to the original example of static stability, you'll agree with me that after the nudge the ball will roll around a bit inside the bowl until it gradually comes to rest at its original position. This is what in aerodynamic-speak is so eloquently termed a "phugoid oscillation". The picture of the ball in bowl is an example of static stability (because the ball wants to return to its original position after the nudge) as well as dynamic stability (because the rolling around in the bowl after the nudge dies down after a while and the ball comes to rest at its original position).

In the next picture I have tried to separate static and dynamic stability by adding my hand to it, which jiggles the bowl back and forth.

46088

I had to add my hand to the picture because I wanted to create a situation that has the potential to be statically stable but dynamically unstable. Just imagine that I jiggle the bowl too much and the ball eventually jumps out of it and rolls toward the setting sun. Even though the ball always wanted to roll toward the bottom if left to its own devices, the jiggling motion makes it roll around more and more until it eventually jumps out of the bowl: statically stable, yet dynamically unstable ("divergent phugoid oscillations" is the proper term to drop at your next cocktail party).

The problem -- or shall we say "challenge" -- we face with our gyros is that the shape of the bowl is determined by a lot of parameters and even changes under different flight characteristics. You might have a deep narrow bowl (high static stability) when flying slowly with little engine power. Going really fast, however, your bowl might look more like a flat skillet. In the former, your well trained aeronautical engineer will be speaking of a "high static stability margin", whereas the skillet presents a situation of almost "neutral static stability".

So how are we to determine the shape of our flying stability bowl? One way is by looking at how vehemently the gyro wants to return to its originally trimmed attitude. This is like assessing the shape of the bowl by looking at the force with which the ball wants to roll down its sides: steep bowl -- strong returning force, shallow bowl -- weak returning force. Assessing this restoring force in actual flying conditions is not an easy task for a number of reasons I don't want to go into. But there is an easier way...

You can look at the dynamic stability to learn somthing about static stability. In other words, you can look at how much the ball rolls around in the bowl to learn how deep the bowl is. This is what Greg is suggesting. The good news is that if the ball rolls around at all, you can be sure that you're still holding the bowl right side up as can be seen in the picture below.

46090

Here, it becomes clear that with negative static stability (inverted bowl) you can't get the ball to roll around at all. The precondition, therefore, to observe "phugoid oscillations" (i.e., the ball rolling around) in your gyro is to have positive static pitch stability.

I hope that helps, -- Chris.

[birdy]

when I fly a gyro I am a God!
Like i said justin...... idiot. :) :)

Does a particular design encourage pilot error
Unfortunatly Vance, their first decision was they greatest error, just not their last.

[b.charlton]

I can trim my RAF to fly hands off for extended periods in nil to mild turulence up to 70mph, over 70 the oscillations become larger and over 80 more divergent, i find it difficult to fly over 80mph for more than 30sec without touching the controls whereas at 60-70mph i can fly all day using only power and rudder. In strong turbulence pucker factor prevents me from letting go of the stick anyway, no matter what the speed. My machine has had a stab on for the last 8 years.

[chuter]

jvitable:
I am still getting calls from beginners or want-a-bes that are obviously intiminated by the direction of the forum.

Well, they've made their first mistake. God help 'em if they're calling you for advice.

Newbies, look over this guy's past posts; you'll see he doesn't have much to add, just complains a lot.

[gyrogreg]

Chuck, I don't know where you get the patience! For as many years as I have been in this sport, you have been working tirelessly (and absorbing abuses) to try to improve the safey of this sport! I don't know how you keep doing this - year after year! Thank you Chuck. I wish we could really know how many lives you have certainly saved over all these years. I guess some people are jut not savable.

But, sometimes our toughest tasks are to undo the damage others seemingly just want to do! (Trying to convince people that knowledge doesn't matter!?!?) I hope you keep hanging in there! Thanks again!

Also, Chuck, thanks for your last "bowl" post. You have a way to describe these things that I really envy!

I started this thread to try to improve our collective knowledge about these things, and to help individuals be able to make better decisions what and how they fly. I don't intend that these investigations identify just two classes of gyroplanes - safe and unsafe! It is likely even the gyro with the worst accident history has safe areas of their flight envelope! It is the knowledge of what the safe operating range really is on that model that is really important! Exceeding this safe operating envelope is what kills people. We all have to know what that safe operating range is for each aircraft we fly! We are just suggesting a way to determine this safe operating envelope without having to discover it in the few seconds before we die!

What we don't know we don't know is what kills us! If people stray into areas they don't even know exist, they are going to get into trouble one day! But, for the more stubborn among us, the challenge seems to be helping them even accept such important truths as such "knowledge is safety".

I started this thread for two reasons:

1) To expand our overall knowledge

2) To help people grow their knowledge - at least better know what they don't know!

I am sorry that there are attempts to hijack this thread to try to convince people that knowledge is not important. For #2 above, we are just trying to help people make better decisions about what and how to fly. I'm sure most people following this thread can recognize this value. It is those few people however that the hijacker just might influence to ignore valuable knowldege that I do worry about!

- Greg Gremminger

[gyrogreg]

I can trim my RAF to fly hands off for extended periods in nil to mild turulence up to 70mph, over 70 the oscillations become larger and over 80 more divergent, i find it difficult to fly over 80mph for more than 30sec without touching the controls whereas at 60-70mph i can fly all day using only power and rudder. In strong turbulence pucker factor prevents me from letting go of the stick anyway, no matter what the speed. My machine has had a stab on for the last 8 years.

Ahh, real feedback! Thank you Barry. This somewhat echoes what Jerry Tiahrt first reported on his stock RAF 2000.

But, this report is for a Stick Free condition. A stick free condition can actually be somewhat forgiving! The best way to fly a gyro when it is in an unstable situation is to allow the stick to move. This somewhat prevents actual airframe movement from coupling into the rotor. When the rotor is forced to move in the divergent direction - as a result of the spindle moving with the airframe - fixed stick - the rotor (wing) further agravates the unstable situation by again changing the rotor AOA or loading which again forces the airframe to pitch further in the wrong direction!

The best way to fly a gyro when it is statically unstable is to allow the stick to move where it wants to move - restricting stick movement makes unstable airframe movements worse! The pilot that is proficient at flying an unstable gyro actually learns to move the stick to prevent any divergent motions of the airframe from coupling into the rotor. The worst situation when a gyro is statically unstable is when the pilot restricts the stick even more - or worse yet, reacts to misleading airframe pitch motions with the wrong stick input!

But, on most gyros, such as on your RAF, there is friction and the trim spring arragement that probably makes "stick free" very similar to "fixed stick". We know that when you release the stick in an RAF flying in turbulence, the stick can be observed to move around - this is at least preventing some of the airframe movement from coupling to the rotor! So, your stick free condition is probably very similar to, but likely a bit better than fixed stick. We propose fixed stick just to be sure the stick is not allowed to move at all - which makes the static instabilty more apparent. This also better simulates what a novice pilot might do on the stick - freeze it, or move it wrongly!

When a gyro is flying in the statically stable condition, the airframe movements are in the correct or restoring direction. In this case two beneficial things happen:

1) The airframe movements, from the pilot restricting the stick, or from friction or trim spring coupling these restoring movements back into the rotor, causes the rotor to respond in the restoring, or statically stable direction. A statically stable gyro flies more stably when the stick is fixed! Just the opposite of a gyro in an unstable condition.

2) The airframe moves (pitches) in the proper direction to indicate actual flight path or rotor load condition. This means that if the pilot responds to the pitch movement of the airframe (seat and nose movement), that response will be in the correct, restoring direction. For a gyro in a statically unstable condition, the airframe often pitches in the wrong direction, and the pilot is likely to be excited to exasperate this condition with wrong direction stick movement! In the statically stable case, the airframe coupling to the rotor mostly compensates the disturbance, the airframe barely pitches in the disturbance, and the pilot isn't even necessarily excited into a response - less likelihood of being excited into an over corrective response! A statically stable gyro flies even moe stably in teh fixed stick mode!

Anyway Barry, thanks for the raref eedback. This is good feedback for an RAF with a HS! I certainly hope we might get some fixed stick flight test feedback on this variant - and on other models too! But, if you are not comfortable with fixed stick, please don't do it! Or, at least approach it in small steps!

Thanks, Greg

[Steve Osborne]

Chuck does not have any problem dishing it out Greg. If you go back and read the majority of CB posts you will see that he has hi-jacked 90% of the RAF threads regardless of content. Welcome to the world of daily abuse and bashing.:)

[Steve Osborne]

Greg, watch this video of my RAF in flight. http://www.youtube.com/watch?v=4PQoDR9zYB8 Pay particular attention to the stick and also out the door at the ground. You will notice that the wind pitching it about and it is correcting itself. How much more should we expect from a gyroplane?

[LuftCarl]

While it is very true that one does not need to be an automotive engineer or race car driver to drive a car safely and effectively, it is also true that knowing how and why something works is always beneficial. When your machine (car, plane, gyro, whatever) has a problem while you are operating it, you are far more likely to save your butt if you understand how it works and what the limitations are from the beginning.

[Vance]

Also, Chuck, thanks for your last "bowl" post. You have a way to describe these things that I really envy!


- Greg Gremminger

Hello Greg,

I would like to point out that the “bowl” post was from an interesting fellow in Vienna named Chris Lang.

He often has interesting thoughts to share and a very elegant way of sharing them.

Thank you, Vance

[RockyMeLad]

I notice not many people are challenging Birdy on his opinion, I wonder why?

Maybe you can't have opinions about the truth ?

[amphib]

I can only speak for myself, since I don't have hordes of newbies calling me for advice.

I had considered gyros on and off for 10 years or so, but kept getting scared off by the fatality rate. When I found this forum, and read the posts (it took a few months going back over all the old posts) from the learned folks like Chuck, Greg, Doug, Udi, Raghu (where'd he go?), et al, I was convinced to finally go for it and get my gyro rating (and buy a gyro). Had they not shared their technical knowledge on the forum, I would not have become a gyro pilot.

Chalk me up as a newbie who relied on this forum to join the gyro movement. Even newbies can quickly weed out the folks who know what they're talking about from those who are just hot air. The difference? Those who know what they are talking about back it up with technical explanations, vector diagrams, free-body diagrams, calculations, etc.

My thanks to all those folks who share their technical knowledge on this forum!


Neil

[C. Beaty]

Civil engineer, eh? Move dirt from one pile to another.:D

Juan de la Cierva graduated from the Madrid Engineering College for Canals, Harbors, Roads, etc with a degree in civil engineering.

[RotoPlane]

My thanks to all those folks who share their technical knowledge on this forum! I completely agree with you, Neil. I am designing my own gyro cause I want something different....don't enjoy making a copy, and without the technical knowledge within this forum's threads, I would be trying to re-invent the wheel....maybe an unsafe one. This is one of those excellent threads.

I usually try to steer clear of threads with guys who add nothing constructive in their posts. The trouble is I find I miss some really good ones too. Sigh.....

[Udi]

With all the background noise around here (this forum tends to induce a poor signal-to-noise ratio), I think many people may have missed the significance of the information Greg is sharing with us in this thread.

Greg's efforts are at the forefront of getting gyros past the "shade tree-designed flying lawn chair" era (in the us - Europe is already past that stage) and into the new era of FAA/GA acceptance. This is done by convincing the FAA (and ultimately the GA community) that today we know how to build safe gyroplanes, and that we can write standards that can discriminate between the safe and the unsafe.

Those who've been involved in this process know this is not at all an easy task. Gyroplane aerodynamics is more complex than FW aerodynamics, and it is significantly different than helicopter aerodynamics. Although gyroplane aerodynamics is complex, gyroplane are actually very simple aircraft from a mechanical point of view. Gyros are, mechanically, much more simple machines than helicopters, and flying gyros is much easier than flying helicopters. When gyros are built correctly, it takes no special skills to fly them and learning to fly gyros should take no longer than learning to fly FW aircraft.

For these reasons, we believe that well-designed and built gyros should be allowed to be sold as LSA aircraft - AS LONG as we make sure that ONLY well-designed gyros become part of the deal.

In this thread, Greg is proposing a new approach for (flight) testing gyros for pitch stability. The reason we need a new approach is because we have found, thru flight testing of gyros with known stability issues, that the previously published procedures for testing pitch stability were not adequately discriminating between good and bad gyros. More work with the same gyros have shown that other tests (for dynamic stability), which were not part of the original battery of tests, could be better in discriminating between good gyros and bad gyros.

You may, or may not, be interested in the theoretical explanations of why the new tests are better than the old tests. Regardless -- Greg is asking for help from anyone capable, and willing, of performing some of these tests to share their findings. Having practical test results from as many pilots as possible will improve our confidence that these tests can really discriminate between the safe and the unsafe.

Asking for such help on a public forum can be problematic because when the average Joe is reporting results you can't always know for certain if the data is reliable or not. However, when people are willing to work with us, we can work with them off-line to make sure the tests are performed properly and the data is good.

As pointed out before, asking people to perform flight tests may give the impression that we are asking people to take unnecessary risks. If you feel performing these tests increases your level or risk, than please do not do them! For some people, performing these tests may be fun and educational. If done right, they are not dangerous. But I will admit that it takes a certain aptitude to be a test pilot. People should be aware of their own personal limits.

Udi

[barnstorm2]

I really soaked up the information on which Gyro you consider as stable and made a note.

Now can anyone make a list of the gyros that are not stable, although maybe we better stick to list the ones you consider stable as this might start another even bigger battle, when you name someone’s favorite?


Thank you for your input John, and welcome to the sport!

Well you touched on the issue when you said "battle, when you name someone’s favorite?" You will find that the 2 posters in this thread that are disruptive and advising against using physics, science and reason to evaluate gyroplane safety and use just happen to be owners of the model of gyroplane that has the greatest number of these flaws.

I try to have some compassion for them for a number reasons. I once owned an unstable high thrust line gyroplane and I was at the time looking for excuses to believe my gyroplane was safe to fly rather then looking for the facts and science from a rational perspective.

Eventually, Doug Smith, Ron Awad, the Chucks and the people on Norm's forum started to get the message through to me and perhaps saved my life.

Here is some interesting history for you. Two very large and popular gyro companies sold badly flawed unstable gyroplanes, Air Command and RAF.

AirCommand learned from their mistake, changed to a safe stable design and declared the older designs 'unairworthy'. Then they sold inexpensive upgrade kits to make the old models safe. Notice there are LOTS of owners of Air Command gyros around here and they do not try to make excuses for flying unsafe and unstable gyros. When someone comes on the forum and says they have one of the old 'unairworthy' designs Air Command owners and pilots are the first to step up and recommend the safety upgrades that remove the dangerous flaws in the original design.

RAF did not learn from their mistake. Rather than releasing a kit to fix the flaws in their machines they started a mis-information campaign through their dealers, CFI's, website and owners. It was up to the owners of the RAF machines to fix the flaws of these gyroplanes.

Eventually, a 3rd Party company released and sold an upgrade kit to make the Stock RAF gyroplane safe. This was so popular that they eventually released there own version of the RAF that had all of the flaws fixed from the start. This machine is called the SparrowHawk.

RAF still did not produce a kit to fix the flaws but continued their mis-information network until they finally went out of business.

Unfortunately, because of the reason you stated, 'owner pride' and legacy of misinformation from the manufacturer and it's factory CFI's many RAF owners are still looking for excuses to believe in to help them ignore the now well known physics of unstable gyroplanes, the flaws in the Stock RAF design, and the high death rate of experienced and trained RAF pilots.

You will see this behavior here on the forum.

Unlike the Air Command owners whos factory admitted the mistake and provided corrective measures, some RAF owners continue to look for and give non-scientifically backable excuses. Rather than talk physics they like to point out things like Mr. X has a bizzion hours in a Stock RAF and he is not dead, or 'my machine seems to fly just fine to me', everyone who dies of PPO was due to pilot error or lack of training.....

My particular favorite is "Ask (only) someone who flys them", as if only Stock RAF owners understand the physics-defying magic of these machines but everyone else in the world is deluded. What is actually happening is they want you to speak to someone else who as fallen for the company mis-information campaign and drinks what Chuck calls the "RAF Cult" koolaid.

Even though many RAF owners have testifed to the unstabliy of their machines and testifed how greatly improved these machines are when the stablity fixes are applied these testiments are wholly ignored by the 'RAF faithful'.

As far as a 'list' of the dangerously unstable machines to go with the list safer stable machines my personal list would be something like this:

(in general ANY gyroplane with out a Horizontal Stabilizer)

1. STOCK RAF*
2. Pre-Smith era Air Command gyroplanes*
3. Benson / Brock gyroplanes that have been modified from the original design
4. Brock KB3 Gyroplanes
5. Custom Designed gyroplanes.

*from an easy-to-bunt perspective many people put AirCommand's early machines first. I do not for these reasons: AC's are mostly single-place machines, many at least have a small Hstab, and there is owner-based & factory support for fixing the flaws in these machines.

The good news is that all of these machines can be made stable and safer cheaply and easily.

The procedures outlined by Greg in this thread will allow you to determine the current stability state of your machine and the effectiveness of any modifications you have made to it.

[All_In]

Most excellent information! The fog is lifting and I can see clearly now. I suspected but wasn’t sure without your background and summary.
This is one newbie who is very impressed with the quality of information of the thread and this site!
Thanks,
John

[ckurz7000]

My point here is just to try to explain why the phugoid oscillatory action is an indicator of the strength of the static AOA stability - restoring force. If, at increasingly worse power and airspeed combination, the phugoid pitch oscillations become neutral or start getting worse on their own, that is an indication that the static AOA stabilty is getting weaker - it still exists, but it is weaker and will dissappear for worse condtions. Verfying that the gyro is dynamically stable (damped oscillations eventually settling out to no oscillations), verfies a margin of static AOA stability which also verifies a margin of buntover protection.

Greg, I agree with you when you say that phugoid oscillations presuppose static stability. But I don't see how you can assess the static stability margin from the envelope of pitch oscillations since it's not just the "spring constant" that's changing but also the driving force and conditions for dynamic stability. Therefore, even if you see positive dynamic stability for one airspeed and neutral dynamic stability at another, you can't conclude that the static stability margin has decreased likewise.

-- Chris.

[ckurz7000]

Regarding the U of Glasgow stability study cited previously:


http://www-legacy.aero.gla.ac.uk/Research/Fd/Project5.htm

It is interesting to note their finding that a stability condition seems to be the vertical position of the CG not deviating by more than 5 cm (2 in) in either direction (i.e., up or down). That implies that a LTL gyro with more than 5 cm CG offset has its own set of stability problems.

-- Chris.

[gyrogreg]

Hi Chris, thanks for your constructive comments - that is the higher intent of these discussions:

Greg, I agree with you when you say that phugoid oscillations presuppose static stability. But I don't see how you can assess the static stability margin from the envelope of pitch oscillations since it's not just the "spring constant" that's changing but also the driving force and conditions for dynamic stability. Therefore, even if you see positive dynamic stability for one airspeed and neutral dynamic stability at another, you can't conclude that the static stability margin has decreased likewise.

-- Chris.

I agree, with changing airspeed conditions, elements other than the RTV are changing - the static effectiveness of the HS, for instance, is changing with the square of the airspeed! The dynamic effectiveness of the HS - dynamic damping - is changing even more with airspeed! The only way the real combination of all could be determine beyond the the point I am suggesting is safe to test - neutral dynamic stability - would be to push the airspeed/power combo futher - beyond the actual negative dynamic stability point. But, that is not safe for other than professional pilots to do.

For the increasing power airspeed condition, part of my confidence in this hypothesis is that we know that the HTL gyros that have an accident history do get more difficult to fly at higher power/airspeeds. Especially with no HS, and/or probably with a less effective HS, the benefits of that HS at higher airspeeds are questionable or nil! But, at the higher power settings, and with the drag and other statically destabilizing aerodynamic static moments on the airframe increasing with the square of higher airspeed, AND with the buntover accidents happening at higher airspeeds, I feel it is a reasonable assumption that if the static stability margin is deteriating for HTL configurations at higher power/airspeeds, it is reasonable to assume that a further worsening of the RTV relationship to the CG at higher airspeeds will result in true loss of static stability at worse power airspeed conditions.

All this suggests that this evaluation does need to be conducted with different combinations of power and airspeed - but for HTL configurations that are getting worse static stability margin at higher power/airspeed combos, I think it is a reasonable conclusion to assume the static stability margin that is getting worse, will continue to get worse when pushed further! But, to truly verify this, one would have to push beyond this power/airspeed point to the point where a buntover is likely possible - I would not suggest that for even a professional test pilot.


Regarding the U of Glasgow stability study cited previously: It is interesting to note their finding that a stability condition seems to be the vertical position of the CG not deviating by more than 5 cm (2 in) in either direction (i.e., up or down).
-- Chris.


I have said this before, but I do not have a lot of faith in the U of G determination or guidlines. I do not find that study to have appropriately evaluated the effect of either the static or dynamic effects of the HS. Their computer model does not appear to accurately match the flight testing they performed - on a HTL old style Magni that they claimed to have measured with a 2" HTL offset! That is the basis of the guidance that the thrustline should be within 2" - both high or low! First of all, the actual prop thrustline of the Magni since then is somewhat higher than 2" - still no buntover accidents with over 400 of the newer (HTL!) models flying!

We do know that the greatest improvement on gyroplane safety can be achieved with the installation of a HS - yet the guidance provided by the U of G to the British CAA does not even mention this. Based on the U of G guidance, contracted by the CAA after the rash of gyro fatalities years ago, the CAA focusses on the 2" offset only! A good, conservative guidance, but accident history does not support the presumption then that any exceedance of this is unacceptable! IMHO, the computer model they developed may be incomplete.

That implies that a LTL gyro with more than 5 cm CG offset has its own set of stability problems.

-- Chris.

Although I don't think the U of G report really supports this, IMO, the LTL CAN have its own set of problems - different from some of the HTL problems above. The RTV is repositioning toward the less statically stabilizing condition with reducing power condition. With reducing power, the effectiveness of an embedded HS canalso be reducing with power - propwash enhancement reducing. But, at higher airspeeds, the destabilizing aerodynamic moments can be dramatically increasing at higher airspeeds. The real question of buntover for a LTL configuration is whether the RTV repositioning with lesser power, in combination with the nose-down destabilizing aerodynamic static moments at higher airspeed might put that gyro in the suituation where the "effective" RTV might be forward of the CG at some point in the (lesser) power / aispeed combination. (To physically provide a CLT or LTL configuration, the landing gear needs to be lower - with associated more-nose-down destabilizing aerodynamic static moments, static stability might be also worse.

This suggests that an "effectively" LTL who's static stability is augmented by the nose up static moment provided when power is applied, is less often in a marginal static stability condition than an "effectively" HTL gyro - spends less time in a statically unstable condition than a HTL might! This certainly correlates with the fewer buntover accident history - some say no buntover history! (I'm not sure the Adler accident was not a buntover accident! This was certainly an extemely LTL gyro though!)

My experience with a LTL gyro - about 60 flight hours - demonstrated to me that that gyro was a lot more sensitive to turbulence and much more uncomfortable to fly at speeds above 80 mph with power at idle - in a glide. Anectodally, some people have correlated this. This is more my subjective opinion - "feels worse" - than an objective flight test - I don't have access to such a gyro now, but I would love to hear some flight test feedback for such a gyro now - low power and high airspeed corner of the envelope!

I think the more concerning situation with an "effectively" LTL gyro might be in the transient that occurs upon a sudden loss of power. With a sudden loss of the strong nose-up prop thrust moment, the nose will suddenly drop - CG moving aft - at the same time an embedded HS also has reduced propwash augmentation! With no nose up LTL static moment, and with an intitially up-lifting HS situation - balancing the LTL nose-up moment, -the nose can drop strongly and rapidly. Inertia can carry this pitch motion - CG - to and beyond the CG alignement on (and then aft) of the RTV - neutral or negative static stability. If the CG actually moves aft of the "effective" RTV, that gyro is at least momentarily statically unstable - with the nose dropping and accompanying radically different control response, the pilot might be in danger of a buntover and/or over control PIO! Again, this situation presents a risk of buntover or PIO much less often or on fewer occasions that the HTL pilot is exposed to - but, it is possible.

As far as the power "chop" on a LTL, I do have some scarry experience with this - chopped power on the LTL at 70 mph. Nose dropped suddenly at least15 degrees. If I had not responded with aft stick as quickly as I did, if the nose drop would have moved the spindle a sudden 15 degrees nose-down, the rotor would likely have precess stalled - too much cyclic input for the teeter stops. Needless to say, I never "chopped" power on that gyro again! But, what if this would not have been a pilot commanded power "chop"? What if I had been surprised by the sudden nose drop? What if the nose drop had exceeded possible cyclic range? All I'm saying is it is possible - I'm not saying the situation happens also. But, a pilot needs to understand all possible limits of their gyro in order to make safe decisions on how to fly that gyro.

The LTL buntover situation is a lot different than a HTL buntover situation or risk! But, I hope we can get some flight test feeback on all configurations - to see if this LTL hypothesis holds any water. My theoretical understanding of all this is not intended to condem LTL configurations. But, very few people can even determine, other than the flight testing I'm suggesting, whether their "CLT" gyro might actually be ("effectively") CLT, LTL, or HTL! I give Ernie and Chuck and all that had pioneered improved gyroplane stability with the credit for likely saving a lot of lives by opening our eyes to the important stability issues. Certainly even strongly LTL gyros improved and still improve real gyro safety as well as pilot understanding, appreciation and knowledge of these issues. I hope we can still build on this understanding and knowledge! Gyros have the true potential to be the safest form of sport aviation - I want to see us and the rest of sport aviation realize that goal!

- Thanks, Greg

[C. Beaty]

Regarding the U of Glasgow stability study cited previously:



It is interesting to note their finding that a stability condition seems to be the vertical position of the CG not deviating by more than 5 cm (2 in) in either direction (i.e., up or down). That implies that a LTL gyro with more than 5 cm CG offset has its own set of stability problems.

-- Chris.With a low thrustline, the slope of restoring moment Vs. angle of attack can become too steep, causing behavior similar to that of a nose heavy fixed wing.

[RotorTom]

Agree ... disagree ... rant ... rave ... opinionate ... question ... answer ... debate ... argue. IT ALL HAPPENS HERE!

But threads like this one show the real value of this forum. It is an exchange of information. No other aviation communiuty delves into their activity like gyro-heads.

I learn something everyday.

But one suggestion ... when writing ... keep non-technical people in mind and teach the masses and assume they do not always know all of the "terms" we use.

I know this forum has been criticized for the bickering ... but who cares? There is way more positive stuff than negative.

Just had to say it.

[barnstorm2]

But one suggestion ... when writing ... keep non-technical people in mind and teach the masses and assume they do not always know all of the "terms" we use.

.

Good point.

Here is a short list of Common Terms:

http://gyrowiki.com/GyroWiki/Common%20Terms.aspx

Here is a long list of most of the technical terms in gyrodom (by Greg G):

http://gyrowiki.com/GyroWiki/Glossary.aspx

(click on the letters at the top of the page to navagate)

.

[ckurz7000]

Hi Greg,

I wanted to "pm" you because I think we are honing in on pretty detailed stuff that might not be interesting to most other forum members. But on pressing the send button, I got the error message that you've exceeded your quota on pm's. So here is my reply as a post.

All this suggests that this evaluation does need to be conducted with different combinations of power and airspeed - but for HTL configurations that are getting worse static stability margin at higher power/airspeed combos, I think it is a reasonable conclusion to assume the static stability margin that is getting worse, will continue to get worse when pushed further! But, to truly verify this, one would have to push beyond this power/airspeed point to the point where a buntover is likely possible - I would not suggest that for even a professional test pilot.

Hi Greg,

thanks for the thorough reply to my messages. I appreciate your dedication to physics and data rather than belief and hearsay. The reason I chose to go "pm" instead of post in the thread is that I think my point may be perceived as nit picking and come across as detracting from the real issue.

Here is what I want to say:

I doubt that you can conclude anything but the presence of positive static stability based on the approaching of the dynamic stability margin. In other words, I don't follow your reasoning when you say -- and I paraphrase -- when we approach neutral dynamic stability, we are also closer to the edge of positive static stability.

The point is you can be violently dynamically unstable while your static stability has even gotten better! The only example I can think of (and it's not a good example at that) is control flutter in FW aircraft. If you look at, e.g., the aileron, it is always statically stable because of its CG lying behind its hinge. The static stability gets even better with higher airspeeds, yet so does the free energy available to excite flutter.

(There's no need to tear apart this example, I know it's bad and doesn't really apply to gyros. But the principle point still remains, doesn't it?)

The relative positions in a thrust/airspeed parameter space of static and dynamic stability margins are independent of each other. Knowledge of the dynamic stability margin does not convey (useful) information about the static stability margin.

Of course, knowing the point whence you travel into the land of negative dynamic stability is a very good thing! And never shalt thou trespass beyond it. But the reason for this has nothing to do with static stability, in my understanding. You KNOW you have positive static stability as long as you also have its dynamic sibling. Once you lose the latter doesn't necessarily mean you're any closer to losing the former. The only thing you've lost is one indication that you're still statically stable.

What say you?

-- Chris.

[All_In]

Good links barnsrorm2 I needed that!

You guys are full of it!

Good Information that is.
John

[Udi]

Hi Greg,

I wanted to "pm" you because I think we are honing in on pretty detailed stuff that might not be interesting to most other forum members. But on pressing the send button, I got the error message that you've exceeded your quota on pm's. So here is my reply as a post...
I vote that you keep going at it right here. Those who find this kind of discussion boring are free to move on.

Udi-

[gyromike]

I vote that you keep going at it right here. Those who find this kind of discussion boring are free to move on.

Udi-

I agree with Udi.

[All_In]

I vote that you keep going at it right here. Those who find this kind of discussion boring are free to move on.

Udi-Can't see how anyone could be hurt with this advice.
Everyones happy,
John

[RotoPlane]

I agree with Mike...who agrees with Udi. I may not fly a gyro yet, but I want to learn all I can.

[Not Yet]

I’ve been a member of the PRA for 3+ years so I don’t know if I’m still a newbie or not.
But.
I’ll voice my support, for continuing this discussion here. I find it very interesting, and insightful.

[RotorTom]

Preach to me ... preach to me.

No knowledge is boring!

If we never heard it before -- it's revelation.

If we heard it before -- it's review.

If we're not quite ready for it -- it's preparation.

If it fits into our lives -- it's education.

If we don't understand it -- it's incentive to learn.

[Resasi]

Nice one Tom.

Very interesting thread.

[Papa Smurf]

Oh Yes, Please keep this discussion going here.

The light is flickering.....I believe I can see it..

[gyrogreg]

Hi Greg, ------
Of course, knowing the point whence you travel into the land of negative dynamic stability is a very good thing! And never shalt thou trespass beyond it. But the reason for this has nothing to do with static stability, in my understanding. You KNOW you have positive static stability as long as you also have its dynamic sibling. Once you lose the latter doesn't necessarily mean you're any closer to losing the former. The only thing you've lost is one indication that you're still statically stable.

What say you?

-- Chris.

Hi Chris, I also agree these discussions should be kept on this forum. My intent on this thread is to vet this concept, and all thoughts need to be presented to all who might be able to contribute. I need to think about this a bit more, but I do think I agree with all you presented in this last post. True, all we know for sure, if any phugoid oscillation is still present, is that there is still static AOA stability existing. So stopping the test at the simple loss of dynamic stability, when the oscillation is no longer damped, does not insure that static stability will certainly be lost when pushed further. Safety prudence says we should not actually push the testing fully into the dynamic instability realm to find out!

But, I think we are seeing some correlation between accident histories and dynamic instability occurring within that gyro's common operating power/airspeed envelope. We don't have a lot of data, but we do have two reports of the RAF 2000 exhibiting dynamic instability well within the commonly accepted flight envelope - dynamic instability at 80 mph, vs. a published Vne of 125 mph. The SH and Magni are examples at the other extreme - no history of buntovers (or PIO), and no reported dynamic instability within their approved Vne/power operating envelope. Totally anecdotal, I know, but it is precisely this reason that I would like to have more data on more configurations - to validate or invalidate the concept with more correlation between accident histories and occurrences of dynamic instability.

However, I do think there is some reasoning that suggests that a diverging phugoid oscillation might be a valid indicator of weakening static restoring moment. Consider that a tendency for phugoid oscillations to grow in amplitude can be the result of two factors that might be changing with worsening power/airspeed combo:

1) Dynamic damping is getting weaker. The inherent (fixed stick) dynamic damping of a gyro comes mostly (or completely) from the HS. The common thread in all buntovers (and PIOs) is higher airspeeds - where dynamic damping is actually increasing at a square factor rate! It seems improbably that a phugoid oscillation would grow or become dynamically unstable (less damped?) at higher airspeeds when the damping mechanism is becoming stronger! I tend to dismiss decreasing dynamic damping to be the cause of this dynamic instability because damping is increasing at the higher airspeeds where buntovers – static AOA instability - more likely occur!

2) The second factor that can grow the amplitude of phugoid oscillations is a weakening of the restoring moment - static AOA stability margin. Like a weaker spring constant! We do know that the higher airspeed is most likely moving the CG aftward - reducing the static stability margin at higher airspeeds - for HTL, due to higher prop thrust and higher airspeed moments on the airframe; and for LTL due to reducing prop thrust and higher nose-down airspeed moments on the airframe.

The oscillations reported to be growing at this power/airspeed point are growing in amplitude, not in frequency! An increase in frequency, in my thinking, would be the result of an increasing spring constant, an increasing restoring force - increasing static stability. However, if the amplitude of the oscillation is what is growing - as reported so far - IMO this indicates a weakening restoring force - exactly what we expect to happen as a result of the reducing RTV / CG spread occurring under such worsening power/airspeed combination.

I suggest that, because the dynamic damper (HS) should be improving with higher airspeed, the actual cause of growing phugoid oscillation amplitudes would have to be a reducing restoring moment – weakening static AOA stability. And, if it is this weakening static AOA stability margin that is the cause of this dynamic instability onset, we know that is what does happen under the worsening power/airspeed conditions.

I admit I’m not smart enough to devise the equations that express this mathematically – and my intuition may not be technically correct. Maybe a dynamics expert or student can provide some mathematical explanation of what factors are actually indicated by our flight test results. Having been burned before by theory vs. flight test reports, I tend to rely on flight test results correlation with real accident history as the final validation – or at least the validation that would be better understood by most of us in the sport!

I also agree with you that control surface flutter is a poor analogy – control flutter is mostly a resonance with other structural components – a better analogy for PIO where the pilot provides the “resonant” input, than for fixed stick phugoid dynamic instability.

- Thanks, and please continue the conversation - Greg

[ckurz7000]

:typing:The oscillations reported to be growing at this power/airspeed point are growing in amplitude, not in frequency! An increase in frequency, in my thinking, would be the result of an increasing spring constant, an increasing restoring force - increasing static stability. However, if the amplitude of the oscillation is what is growing - as reported so far - IMO this indicates a weakening restoring force - exactly what we expect to happen as a result of the reducing RTV / CG spread occurring under such worsening power/airspeed combination.

Greg, what you say sounds compelling. But just from a physics point of view (I am a physicist with little to none prior education in aerodynamics) I have to take issue with the quoted statement above: changing the spring constant and changing the restoring force is one and the same and would, as you correctly point out, change the frequency of oscillation -- stronger restoring force/spring constant results in higher frequency. Conversely, lower restoring force/spring constant leads to lower frequency. No matter if we are talking about an "ideal" or "real" system, the oscillation frequency is always primarily affected by the spring constant.

There are mainly two things that would predominantly influence the amplitude:

(1) a change in the driving force amplitude (just an aside to the unsuspecting reader: in the bowl analogy posted previously, the driving force correponds to my hand jiggling the bowl whereas the restoring force corresponds to gravity's pull on the ball making it reluctant to roll up the sides of the bowl), and

(2) an approach to a resonance condition.

Returning to the bowl image: if you surmise that we are getting closer to the static stability margin (i.e., the bowl is getting shallower) then the frequency of oscillation would grow lower (the ball takes longer to roll from one end to the other).

But taking a step back from our discussion, isn't the important point that nobody should be flying a dynamically unstable aircraft, period. If it so happens that by increasing thrust/airspeed, we first encounter the dynamic stability margin than that is clearly the more stringent restriction on our operational envelope. Exactly how far away from static instability we might be at that instance is a moot point, in my opinion.

Your important contribution to the stability discussion, as I understand it, is to point out that if we don't have pitch oscillations we are already beyond static stability. So everybody check to see under which conditions they observe (fixed stick!!) phugoid oscillations and make sure only to fly under conditions where those oscillations damp out over time. I only added the observation that we shouldn't fixate on static stability alone. Static stability might still be going strong or weak at that point -- we simply don't know. But luckily that doesn't matter! We shouldn't be flying in a dynamically unstable region anyway.

Greetings, -- Chris.

[gyrogreg]

Chris,

Thanks for the clarification, and a Physicists understanding and explanation on such subjects will certainly be helpful here.

I agree that it is important that any aircraft not be dynamically unstable - should not be flown into conditions where it is dynamically unstable. However, I am not sure there is enough pilot appreciation of what risk this is to them:

Gyro pilots can easily learn to stop phugoid oscillations before they get going to bad. So, it feels easy to fly into realms of flight where the phugoid oscillations are actually divergent because they are slow enough that the pilot learns to automatically stop (stabilize) them themselves. So, the impression is "what's the big deal?" It may be just a small step to venture unknowingly even further - into the realm where osicllations no longer exist. Now, at that point, the pilot MUST stop a divergence before it gets too far - if not, it is no longer going to reverse itself on it's own. If let go too far, it statically diverges - a buntover if that static divergence is in the nose-down direction.

But, buntovers are insideous - they pop up when pilots least expect them! I think it is important for pilots to appreciate when the BIG ONE can happen - to appreciate and avoid the danger areas where a buntover is possible.

I also think it is important that this correlation be readily understandable and therfore widely accepted. If we could, or a reputable Physicist couid, put this concept into a believable and understandable context, we would influence more people to research what the real safe flight envelope of their gyro really is. Traditionally though, many peole in our sport have trouble understanding concepts like forces and moments, static and dynamic, stable and unstable, divergent and convergent, damping, etc. - eyes start to glaze over and other less scientific arguments start to hold much more weight than they deserve.

One way I hope will help people appreciate the correlation between this dynamic stability criteria is to collect enough data on various models so that the correlation between historically dangerous machines and historically safe machines (no history of buntovers) becomes a glaring truth anyone can appreciate! We already have two ancedotal reports on one model that is widely recognized to have a buntover history. We have examples on the other end of the scale with no buntovers reported. With real data, a bit more than just ancedotal, I think we might influence more people who just can't or won't accept a theoretical scientific explanation.

If I read your post correctly, I think you are saying that a weaker restoring force would result in slower but larger oscillations - damping factors remaining the same. And a stronger restoring force (more statically stable) would result in a faster and smaller oscillations. This might be a parameter to actually measure - slower oscillation rate, as well as larger oscillations, as static AOA stability margin deteriates - at faster airspeeds! But, we are also trying to keep this simple to measure - without expensive monitoring equipment and without requiring special Test Pilot skills. It is relatively easy for anyone to recognize that the gyro doesn't take care of turbulence itself -fixed stick - but requires the pilot to take over to "stabilize" the diverging oscillations. There are so many gyro configuration variations and variables, that I think it is important that the average Joe be able to make that assessment - safely! These turbulence inspired oscillations are something pilots naturally and safely learn to stabilize safely once they identify the gyro won't do it inherently itself fixed stick.

Are you also agreeing that the (HS) damping would not be getting worse, but actually getting better at higher airspeeds and would not likely be contributing to worsening phugoid oscillations at higher airspeeds?

I think you are mentiong another factor in the onset of divergent phugoid oscillations - approach to a resonance condition. But, I'm not sure what these phugoid oscillations might start resonating with when the gyro is flown fixed stick! Unlike flutter of a control surface, it is hard to imagine these stiff airframes actually resonating with anything. (In a PIO event, with actual very quick AOA oscillations, the pilot is the resonating partner!) So, I'm not sure that the onset of divergent phugoid oscillations could come from a responant condition. In my, non-Physicist mind, I still fall back on the one contributor to the onset of divergent oscillations - the worsening RTV/CG relationship at increasing airspeeds.

What do you think? My education in "dynamics" was 35 years ago - as part of an Electronics Engineering degree. So, if I'm off base or it can be explained better for me and the masses, please continue. The voice of a professional in these subjects carries a lot of weight - even if we don't understand everything!

- Thanks, Greg

[RotorTom]

OK ... May I have things clarified ... for both deep thinkers and newbies?

I think one of the problems with this thread is that we make assumptions that our phraseology means the same things to all of us. Yet, using certain terms can actually mean very different things when applied to aviation.

For example ... what do you mean by "Divergent Oscillations"?

Yes "Oscillations" (specifically Phugoid Oscillations) mean up and down pitch movements. But when you add the term "Divergent" you may think it means one thing, when actually it can mean something opposite (BTW, my object here is not to criticize but rather to clarify).

Divergent means the pulling apart or spreading apart of ideas, relationships, movements and in this case "Oscillations". So in a gyros, a divergence of oscillations can mean that they eventually get farther and farther apart until they stop or "smooth out" to reach equilibrium or "stability".

But it can also mean that they get farther and farther apart as in how far they go up and down which would eventually lead to disaster (either a stall or bunt over) or "instability".

Which one are you referring to?

Also ... the word "convergent" can have opposite meanings as it relates to oscillations. Convergent means to get "closer together". This could mean that they get closer together to achieve "stability" (as in a ball rolling into a bowl ... the movements get tighter and tighter until the ball stops in total stability).

But "Convergent" movement can also mean that the movements get closer together to cause almost a "flutter" which would be bad.

Which are you referring to in this discussion?

I think a basic explanation is in order.

[All_In]

OK ... May I have things clarified ... for both deep thinkers and newbies?
...

Which are you referring to in this discussion?

I think a basic explanation is in order.You hit the head on that Nail!
And Nailed It!!

[gyrogreg]

Tom, please refer to Glossary of Gyro Terms (http://www.pra.org/images/documents/gyroterms.pdf) to help on some of the terminology. I’ll try to briefly clarify some below:

OK ... May I have things clarified ... for both deep thinkers and newbies?------
For example ... what do you mean by "Divergent Oscillations"?

This means the amplitude of the oscillations is getting larger and larger (dynamic instability) – rather than “converging” to smaller and smaller amplitudes and eventually disappearing (dynamic stability)

Yes "Oscillations" (specifically Phugoid Oscillations) mean up and down pitch movements. But when you add the term "Divergent" you may think it means one thing, when actually it can mean something opposite (BTW, my object here is not to criticize but rather to clarify).

Phugoid oscillations are long-period (slower) oscillations – mostly in airspeed. This can be observed in pitch oscillations too – but the Angle of Attack (AOA) of the rotor and aircraft remains essentially the same during phugoid oscillations. Such oscillations are not the same as the dangerous “PIO” oscillations we often talk about. PIO oscillations are actually very short period (fast) oscillations of AOA – with relatively constant airspeed. PIO oscillations are true pitch oscillations as the AOA is going up and down quickly.

Both Phugoid and short period (PIO) oscillations can be “divergent” – growing in amplitude. The PIO divergent oscillations “diverge” to large amplitude so quickly that a pilot cannot stop them. Phugoid oscillations are normally slow enough that a pilot can readily stop or “stabilize” the oscillations.

Divergent means the pulling apart or spreading apart of ideas, relationships, movements and in this case "Oscillations". So in a gyros, a divergence of oscillations can mean that they eventually get farther and farther apart until they stop or "smooth out" to reach equilibrium or "stability".

“Divergent” oscillations generally means the amplitude of each cycle is getting worse. The oscillation frequency may change – faster or slower – but generally divergence refers to the amplitude of the oscillations.

“Divergence” can also apply to a STATIC divergence the condition deviates further and further from its initial condition – such as in a buntover.

Also ... the word "convergent" can have opposite meanings as it relates to oscillations. Convergent means to get "closer together". This could mean that they get closer together to achieve "stability" (as in a ball rolling into a bowl ... the movements get tighter and tighter until the ball stops in total stability).

“Convergence” can also refer to both static and dynamic stability. In the case of dynamic oscillations, “convergence” refers to the amplitudes getting smaller and smaller until they (hopefully) disappear. Dynamic stability is achieved with a dynamic “damper”.that creates a force or moment in the opposite direction of a movement - a force or moment that is also proportional to the velocity of that movement. and is created by the movement, not by the actual position (of the HS). The "damping" moment from a HS results from the upward or downward movement of the HS, not the actual AOA of the HS. This is different than the "restoring" force or moment, in timing and source, below.

“Convergence” in the static sense is that when a condition is disturbed from its statically stable initial condition, it tends to return back to that initial condition. Static stability is achieved with a “restoring” force or moment that is in the opposite direction of the disturbance – such as pulling on a spring. That "restoring" force is in the opposite direction of the position or angle of the deviation from the equilibrium point - initial steady state condition. The "restoring" force is proportional to how far the condition is away from its initial steady state condition. The "restoring" moment of a HS results from the deviation or AOA of the HS, not its movement.

These are the two distinct functions or benifits provided by the HS - the static "restoring" moment is different than the dynamic damping moment created purely by the movement (up or down) of the HS!

- Hope this helps. If not, there are others that often get these descriptions across better than maybe I have done!

- Greg Gremminger

[carlschneider]

My thurst line is 7 inches below the CG. Is that a bad thing?
I really have a problem with all the big words. Why not speak in plain english. That is the offical lanauge in this country.

Carl

[C. Beaty]

Horrors, Carl, you’re nose heavy. With 7” LTL and say 300 lb. propeller thrust and 500 lb. of rotor thrust, the rotor thrust line must pass 300/500 x 7” = 4.2” behind the CG unless altered by horizontal stabilizer lift. (300 x 7 = 500 x 4.2)

Being nose heavy means a stronger restoring force when the airframe is disturbed by a gust; in other words, perhaps too much longitudinal stiffness.

When discussions first began about the effect of propeller thrust line Vs CG; Carl did something about it. He showed up at the PRA National flyin in Hearne TX, I think the year was 1990, with a secondary keel that could be telescoped up and down. Carl reported the higher he went, the better it felt.

[carlschneider]

Chuck

I'm not trying to be funny but your gyro hung 7 inches LTL. Did you plan it that way?

There is CG and there is CP. The center of pressure is more important than CG over 70 MPH. The CG is just the pivoting point.

Carl

[C. Beaty]

Not quite, Carl. My gyro is 4” LTL and I didn’t plan it that way.

When first laid out, my intent was to use wood rotor blades and fiberglass hub that would have placed the prop thrust line exactly on the CG.

The only downside I’ve noticed is that with locked stick and throttle change, airspeed is overcompensated; throttle chop causes an airspeed increase of ~10 mph at 50 mph and wide open throttle causes a reduction of airspeed of ~10 mph, also at 50 mph.

[Mike Jackson]

Just a reminder for all, especially new folks to the sport, there are some pretty good places to aid understanding of autogiro terms and related stability discussions:

pra.org under gyroplanes - glossary of terms and

gyrowiki.com under shared documents on, the left pull down - specifically many of Greg's articles

Mike

[Bob Gregory]

Spreadsheet originally setup for HTL but can be useful for visualizing the location of LTL if reversing the down to up. The engine for it, courtesy of C Beaty is hidden far to the right.
Fun to balance the climb rate and drag to hp available.

Carl Schneider's gyro as I remember it.

[Redbaron]

good riddance, shouldn't the machine being extremely simple to fly be a summory of its stability! no comment!

[Doug Riley]

Regarding Chuck's Post #116 -- my tandem Dominator behaved just about as he describes, whether solo or two up, with light 618 or heavier 912, full gas or nearly empty. It had inflight electric trim, which reduced the annoyance of its over-compensation for throttle changes.

Without the trim, you could get tried of holding forward pressure to maintain airspeed on climbout, or back stick on power-off descent. If you were distracted, the stick would sneak out of position as you relaxed your grip. I never experienced any abrupt/wild pitching reactions -- over-compensation is the right term.

Interestingly, the Gyrobee with a 6-sq.-ft. HS in the propwash and -3 deg. of incidence behaves EXACTLY the same way. The tail more than compensates for the couple inches of HTL.

I would not want a gyro any more LTL than the Dom was. If I were designing a gyro from scratch, I'd shoot for CLT at the mid-range of (vertical) CG location, and put just a touch of down-load on the HS for starters. Then test, tweak and poke until done.

[MichaelBurton]

Phugoid oscillations are long-period (slower) oscillations – mostly in airspeed. This can be observed in pitch oscillations too – but the Angle of Attack (AOA) of the rotor and aircraft remains essentially the same during phugoid oscillations. Such oscillations are not the same as the dangerous “PIO” oscillations we often talk about. PIO oscillations are actually very short period (fast) oscillations of AOA – with relatively constant airspeed. PIO oscillations are true pitch oscillations as the AOA is going up and down quickly.


“Divergent” oscillations generally means the amplitude of each cycle is getting worse. The oscillation frequency may change – faster or slower – but generally divergence refers to the amplitude of the oscillations.

“Divergence” can also apply to a STATIC divergence the condition deviates further and further from its initial condition – such as in a buntover.



“Convergence” can also refer to both static and dynamic stability. In the case of dynamic oscillations, “convergence” refers to the amplitudes getting smaller and smaller until they (hopefully) disappear. Dynamic stability is achieved with a dynamic “damper”.that creates a force or moment in the opposite direction of a movement - a force or moment that is also proportional to the velocity of that movement. and is created by the movement, not by the actual position (of the HS). The "damping" moment from a HS results from the upward or downward movement of the HS, not the actual AOA of the HS. This is different than the "restoring" force or moment, in timing and source, below.

“Convergence” in the static sense is that when a condition is disturbed from its statically stable initial condition, it tends to return back to that initial condition. Static stability is achieved with a “restoring” force or moment that is in the opposite direction of the disturbance – such as pulling on a spring. That "restoring" force is in the opposite direction of the position or angle of the deviation from the equilibrium point - initial steady state condition. The "restoring" force is proportional to how far the condition is away from its initial steady state condition. The "restoring" moment of a HS results from the deviation or AOA of the HS, not its movement.

These are the two distinct functions or benifits provided by the HS - the static "restoring" moment is different than the dynamic damping moment created purely by the movement (up or down) of the HS!

- Hope this helps. If not, there are others that often get these descriptions across better than maybe I have done!

- Greg Gremminger

I agree with most of Gregs post. It is more of semantics than anything as the ideas are similar.

Static stability refers to the way the aircraft responds to a disturbance .

The common analogy is that of a pea in a dish. Push the pea away from the center and it will try to return when the force it removed. This is an example of positive static stability.

If the aircraft has positive static stability it will have a tendency to return to the undisturbed state. If the aircraft does not have the tenancy to return to the previous state but remains in the new state it is neutral in static stability. This is the case with many stunt aircraft. If the aircraft has the tendency to increase the state of disturbance than it has negative static stability.

An aircraft can not have dynamic stability unless it has positive static stability as there will be no oscillation.

The second area I feel needs addressing is that of Angle of attack (AOA)
If there is a change in air speed there is a change in AOA.

Now on to my own input on airspeed stability

I expect an aircraft to increase in speed (pitch down)when the throttle is reduced and to decrease in airspeed (pitch up) when the throttle is increased. All dynamically stable aircraft that I am aware of respond in this way. It is a good thing as long as it is not so large as to cause static instability.
If the aircraft does not behave in this way it will have a divergent pitch with power change .

[Doug Riley]

"If there is a change in airspeed, there is a change in angle of attack."

In practice, this is almost always true. But A.S. and AOA are not inherently linked. We can imagine an aircraft carefully designed so that an increase in airspeed does not result in either an increase or a decrease in AOA. In this perfect craft, an increase in airspeed would result in an increase in lift proportional to the square of the increase in airspeed. Therefore, the craft would climb when airspeed increased, but would not pitch up or down any more then necessary to maintain the same old AOA.

In practice, the water's muddier than that. Normal rotors (with correct blade CG's and reflex) will "blow back" as airspeed increases. This increases the rotor's AOA (with no pilot input). HTL or LTL will cause changes in AOA that may either add to, or wipe out, the effects of rotor blowback. Airframe centers of pressure well above or (more likely) below the CG can do the same.

[MichaelBurton]

Changing the airspeed changes the AOA even if the pitch relative to the ground did not change. If airspeed changes AOA changes.
There may be some theoretical way to eliminate this relationship between AOA and Airspeed however in aircraft flying in the real world it can not be avoided.

[Doug Riley]

Michael, picture a section of wing that spans the width of a wind tunnel. Bolt it rigidly to the right and left walls of the tunnel so that it can't pitch up or down, or move in any other way. Turn on the fan. Whether the breeze is 50 mph or 100 mph, the AOA is what it is. AOA and airspeed are independent things.

They can and often do interact in actual aircraft. For example, a prop blade has an AOA that's a function of (1) its incidence, (2) its rotational speed and (3) the aircraft's forward speed. Increase the forward speed without changing throttle say by diving), and the blade's AOA goes down. Ditto for rotor blades. However, the rotor disk's AOA (as contrasted with blade elements) does not experience an AAO change from changes in airspeed alone, except for (1) the blowback effect I mentioned earlier, to the extent it exists in that particular rotor) (2) pitching of the airframe that might, or might not, occur, depending on the layout of the frame.

Number 2 can produce either an increase, or a decrease, in AOA, depending again on frame layout. HTL gyros have a habit of DECREASING the rotor's AOA as throttle increases -- producing the opposite of the stable reaction that you mentioned earlier, and that we all (should) want.

[MichaelBurton]

I see no real need to argue this point. As you have pointed out in a wind tunnel it is easy to control AOA and air speed independently as you can eliminate any movement of the wing. Since the wing is not supporting any weight it would appear that AOA is a function of angle of incidence only.

In the real world the Airspeed and AOA are linked. You can not Increase airspeed in any aircraft that uses lift to fly without changing the AOA. As you mentioned an increase in airspeed will change the blow back angle in a gyro. In a helicopter it is even more complexly linked. In the Rotorcraft flying hand book chapter 3 page 6 there is an adequate explanation. In a fixed wing increasing the forward airspeed decreases the AOA with or without any change to the pitch of the air frame. This is not a bad thing. It is one of the factors that aid in power stability.

Here is how AOA changes in your wind tunnel experiment. While creating lift, the airfoil changes the direction of airflow in many ways. With down wash the deflection of air changes the relative wind in the vicinity of the wing to a slightly downward direction. As a result, the true angle of attack is different from the apparent angle of attack. The relative wind is opposite the flight path. But the airfoil alters the relative wind. This is known as “induced relative wind.”

[gyrogreg]

Mike and Doug,

It's probably a mistake to step back in here again, I'm not sure how this old thread got reinvigorated just now again!

I agree with the link between AOA and airspeed. And, that the rotor has additional characteristics that compound, or can compound the situation - such as "blowback". But, IMHO, one big issue with rotorcraft on the AOA/AS issue can be the fact that the rotor is not a fixed wing, or "fixed rotor". The rotor has some freedom of movement relative to the airframe, and the airframe has freedom of movement relative to the rotor.

The issue I have been trying to include lately in the understanding of gyro stability mechanisms is the DYNAMIC actions and interactions. Correct me if I'm wrong, but I believe all of these recent posts are addressing just the STATIC considerations. The principles you are addressing are AFTER the dynamic reactions and responses have steadied out - the STATIC Steady State condition.

I have come to believe that we cannot understand the complex DYNAMIC issues of PIO and buntover unless we look beyond the STATIC steady state analysis. PIO and PPO buntover are DYNAMIC, not static actions. They occur because of the static situation, but the dynamic action is also defined by its dynamic characteristics – do natural short period pitch oscillations exist – inadequate dynamic pitch damping? Do pitching rates accelerate so much, or are not damped adequately, so that buntovers are likely, etc.?

Different than FW airplanes, on a gyro, the rotor and airframe interact with each other. The RTV of the rotor influences and induces a pitch DYNAMIC response of the airframe. And the Airframe influences and induces a pitch DYNAMIC response of the rotor disk - through uncommanded spindle cyclic input when the airframe pitches. There are delays and inertial responses of the airframe and of the rotor that interact DYNAMICALLY in very complex ways. – Like a child swinging their legs on a swing.

In my non Physicist understanding of “DYNAMIC”, the pitch rate and acceleration are DYNAMIC properties that come from more than just the static parameters of thrustlines, AOA, Airspeed, weight, etc. The additional DYNAMIC properties of both the rotor and the airframe are inertias, MOIs, and damping. These are factors that are not considered in a purely STATIC analysis.

IMHO, buntovers and PIO are DYNAMIC responses to STATIC imbalances. How fast can the airframe pitch nose-down (in an incipient buntover), what are the natural oscillation frequencies of the rotor and airframe in pitch? What is the phasing between the rotor and airframe when reacting to disturbances in one or the other? Very complex issues to try to analyze or predict.

One way I have been able to (almost, in my mind), understand the value of a good, dynamically damping HS, is to recognize that this good Dynamically damping HS will force the airframe pitch attitude to track accurately and quickly to the flight path and/or changing flight path determined by the rotor – irregardless of, or overpowering what the other static moments on the airframe are trying to do. This makes the whole gyro essentially a “FIXED-ROTOR” aircraft. This prevents the airframe from disturbing the rotor so much, and the rotor is essentially in charge of AOA and Airspeed.

At the same time, the “Fixed Rotor” is tracking airframe pitch reactions from wind disturbances. The rotor is essentially fixed to the airframe through cyclic action of the spindle. So, the whole system compensates for turbulence automatically – and better than a fixed wing because of the high control power of this cyclic input and the insensitivity of a spinning rotor to turbulence.

In fact, when the airframe is strongly pitch damped, that damping also resists rapid airframe pitch changes from the RTV or prop static moment changes. That HS makes sure that the airframe always pitches into an increasing vertical relative wind – which cyclically adjusts the rotor into a corrective disk AOA. But, perhaps more importantly, that strong dynamically damping HS prevents any swinging of the airframe under the rotor – makes it quickly adjust to and track the flight path dictated by the rotor.

In this “Fixed-Rotor” concept, provided by a strong dynamically damping HS, I get a glimpse of how that HS overpowers or compensates for even statically destabilizing arrangements of static moments. Now, I don’t know how to resolve this into the mathematics that describes all this. But, this allows me to visualize what is happening in terms of FW aerodynamics – which is much simpler to understand.

When I think of this as a “FIXED-ROTOR” system, I can then start understanding the effects of a heavy nose, or the CG behind the lift vector – like in a fixed wing airplane. Forgive me if I interject this now – that is exactly how the Magni flies – like a fixed wing, but without the sensitivity to turbulence, and with the maneuverability that a pilot controlled wing provides to gyros.

Thanks, Greg

[Vance]

According to the Encarta Dictionary: English (North American)

Static: Related to forces or pressures that act without causing movement

Dynamic: Involving or relating to energy and forces that produce motion.

[Mayfield]

You can not Increase airspeed in any aircraft that uses lift to fly without changing the AOA........

.....In a fixed wing increasing the forward airspeed decreases the AOA with or without any change to the pitch of the air frame. This is not a bad thing. It is one of the factors that aid in power stability.

Here is how AOA changes in your wind tunnel experiment. While creating lift, the airfoil changes the direction of airflow in many ways. With down wash the deflection of air changes the relative wind in the vicinity of the wing to a slightly downward direction. As a result, the true angle of attack is different from the apparent angle of attack. The relative wind is opposite the flight path. But the airfoil alters the relative wind. This is known as “induced relative wind.”

Hi Mike,

I need a little help understanding some of your statements. It would appear to me, in an airplane, I can add power, accelerate, and depending on power available, do pretty much what I want to do with AOA. I can decrease AOA and maintain altitude, or maintain a constant AOA and climb.

A quick look at the formula for lift (L= Cl * (air density * velocity squared/2) * wing area) shows us that if velocity increases, lift increases unless one or more of the other variables decreases proportionately.

I'm also not sure I understand "Induced relative wind." I think I know what induced flow (down wash) is and I think I know what the resultant relative wind is, but I don't recognize "induced relative wind."

As far as Doug's example in the wind tunnel: If the AOA is fixed and velocity is increased the result is more lift not a change in the AOA.

R/S

Jim

[gilgsn]

I'm a newbie... Just like Neal, I decided to build a gyro after looking at them for 25 years but being reluctant to do so because of the death rate.

If there is any information here that could save my life, I will damn make sure I read it. So, thanks guys for your explanations. Thanks Chris for that bowl drawing! Visual aids help a lot to understand aerodynamics. I am still trying to understand how the RTV forward of the CG is a problem.. Seems to me like it would create an up pitching moment.. Sure, it gets over my head, and I am sure I am not alone here. However, as I keep reading, I get bits and pieces here and there, and the big picture starts to look better, kind of like a puzzle getting more pieces.

Did I mention drawings help a lot...?

Thanks again!

Gil.

[MichaelBurton]

Hi Jim,

When I teach instrument flight I have the student set the airspeed with AOA and trim to remove all stick pressure. Throttle is then used to climb or descend.
As you know adding power in most fixed wing aircraft causes the nose to rise and the airspeed to drop. We understand that part of the reason is that as airspeed increases lift increases. The other reason is that additional accelerated flow moves over the tail. The Hstab on a fixed wing aircraft produces a downward force so increased flow causes the nose to rise.

However a flying wing has the same tendency. We also know that if the wing is producing lift there is a forward vector of this lift. We know this because this is what causes the gyro rotor to spin. We have all been taught that lift is perpendicular to the lifting vector so we can see that an increase in airspeed changes the AOA even if we don't take into account the induced air flow.

When looking at induced airflow we see that the velocity of air going over the camber of the wing will increase at a greater rate than the air on the face of the wing. This is where induced AOA comes from. The relative low pressure on top of the wing reduced the AOA by sucking additional air over the top wing while the relative high pressure on the bottom of the wing pushes back on the air further decreasing the AOA. This is what I refer to as induced AOA. It is what causes down wash. So we can look at the amount of down wash to determine the amount of induced AOA.

This is also referenced in “Simple Aerodynamics" Part 4 by Stu Moment
copyright 1984, 2004, Sublogic Corporation

[Doug Riley]

The Bible of aerodynamics is Abbott and von Doenhoff's Theory of Wing Sections. It's both an engineer's reference and a university text. Abbott and von Doenhoff were aerodyamicists with NACA, and both of them gravitated to NASA once it was established. Ira Abbott was NASA's Director of Research in the late 50's and forward.

They define angle of attack as the "angle between the plane of the wing and the direction of motion." IOW, the standard meaning of the term EXCLUDES the changes in direction of the relative wind as it comes under the influence of the wing itself.

You can, of course, come up with additional terms that refer to the direction of the local relative wind at points along the wing, AFTER it's been bent by the wing's surfaces. Good old basic AOA, however, is defined by the direction of travel -- or the initial direction of the relative wind, in a wind tunnel.

The lift vector of a wing isn't inclined forward relative to the plane of the wing. If it were, we wouldn't need an engine (we'd have perpetual motion). The lift vector CAN be inclined forward of the line-of-action of gravity -- as it is a glider. This requires that the plane of the wing be inclined leading-edge-down relative to the horizon -- IOW, the airplane must be descending. This forward inclination of the lift vector (compared to gravity) is what opposes the drag of the fuselage in a power-off glide.

A gyro rotor blade is analogous to a glider. The rotor continues to spin only if its rotational axis is inclined aft. The rotor is effectively "descending" relative to the aircraft's flight path. The lift vector of each blade points slightly forward of the spin axis, but not forward of the aircraft's flight path.

The pilot in the cockpit doesn't experience AOA in isolation. He deals with a mixed set of reactions, in which airspeed, AOA, power setting and airframe angle usually bleed over into one another. Still, it's helpful to consider these parameters one at a time when designing aircraft or understanding what a given craft is doing (or will do).

[MichaelBurton]

The Bible of aerodynamics is Abbott and von Doenhoff's Theory of Wing Sections. It's both an engineer's reference and a university text. Abbott and von Doenhoff were aerodyamicists with NACA, and both of them gravitated to NASA once it was established. Ira Abbott was NASA's Director of Research in the late 50's and forward.

They define angle of attack as the "angle between the plane of the wing and the direction of motion." IOW, the standard meaning of the term EXCLUDES the changes in direction of the relative wind as it comes under the influence of the wing itself.

You can, of course, come up with additional terms that refer to the direction of the local relative wind at points along the wing, AFTER it's been bent by the wing's surfaces. Good old basic AOA, however, is defined by the direction of travel -- or the initial direction of the relative wind, in a wind tunnel.

The lift vector of a wing isn't inclined forward relative to the plane of the wing. If it were, we wouldn't need an engine (we'd have perpetual motion). The lift vector CAN be inclined forward of the line-of-action of gravity -- as it is a glider. This requires that the plane of the wing be inclined leading-edge-down relative to the horizon -- IOW, the airplane must be descending. This forward inclination of the lift vector (compared to gravity) is what opposes the drag of the fuselage in a power-off glide.

A gyro rotor blade is analogous to a glider. The rotor continues to spin only if its rotational axis is inclined aft. The rotor is effectively "descending" relative to the aircraft's flight path. The lift vector of each blade points slightly forward of the spin axis, but not forward of the aircraft's flight path.

The pilot in the cockpit doesn't experience AOA in isolation. He deals with a mixed set of reactions, in which airspeed, AOA, power setting and airframe angle usually bleed over into one another. Still, it's helpful to consider these parameters one at a time when designing aircraft or understanding what a given craft is doing (or will do).

Interesting, If we define AOA only according to the plane of movement and not relative wind then the AOA along a rotor would be constant. How are we going to resolve this problem using this definition? Using this definition we also run into problems involving relative motion and every observer would see a different angle of attack.

Having the lift vector forward does not provide for perpetual motion as you will eventually run out of altitude. What it does is offset the drag vector. If it were not slightly forward the aircraft would decelerate in a power off event. It would also be impossible to spin an aircraft or for that mater to fly a gyro. The forward vector of lift is what causes the aircraft and the rotor to spin.

The definition of angle of attack that I have always seen is the angle between the mean cord line and the relative wind.

[MichaelBurton]

A gyro rotor blade is analogous to a glider. The rotor continues to spin only if its rotational axis is inclined aft. The rotor is effectively "descending" relative to the aircraft's flight path. The lift vector of each blade points slightly forward of the spin axis, but not forward of the aircraft's flight path.



I am not quite sure how to respond to this. Perhaps some other informed source would like to take this.

If you concede that there is a forward vector of lift in the rotors plane of rotation I would ask how the laws of physics change when they are applied to the fixed wing aircraft to prevent it from having a forward vector in its plane of motion?

[Resasi]

This thread confirms my supposition that the aerodynamics of gyro flight are complex. I have done my best to grasp the various factors and understand them as fully as I can in my own limited capacity

It has not been easy, certainly for me, and the participation and contributions from the more experienced and educated of our Forum most welcome in helping clarify it.

It seems there are factors in this field that seem to exceed those governing most fixed wing flight, exponentially increasing possible permutations of in-flight behavior.

Most reassuring to see the continuing efforts made by individuals in our sport on behalf of all of us, to understand and explain the theory and methods by which we can increase our safe enjoyment of this of passion of ours.

Thank you to all those who share their knowledge and experience here.

[BEN S]

When I read the posts back and forth between Michael and Doug, I imagine myself as Homer Simpson eating a doughnut and staring blankly at a wall of blinking lights in the nuclear power plant with just the start of some drool coming out of my mouth!
I'm just glad that you don't have to understand WHY they work just HOW to make them work!
Ben S
(Apperently the S doesn't stand for socretes!)

[MichaelBurton]

Doug,

I looked at Abbott and Von Doenhoff's Theory of Wing Sections but was unable to decipher any usable information in regards to our discussion. Perhaps you could point me to a page and reference that we should discuss in relation to this topic.

[WaspAir]

They define angle of attack as the "angle between the plane of the wing and the direction of motion."

The definition of angle of attack that I have always seen is the angle between the mean cord line and the relative wind.

You're saying the same thing. The "plane of the wing" is that containing the chord line (it's a plane if you ignore washout/twist and consider the orientation of only one station along the span at a time). The direction of motion points to where the relative wind appears to come from; it's a vector of the same magnitude along the same line but pointing in the opposite direction. The included angles are equal.

[Doug Riley]

The discussion is fragging into multiple pieces.

The A. & von D. quote I posted is near the bottom of page 4 in the Dover paperback edition.

The lift vector (really the resultant vector of lift and drag) of the (inner portion) of the blade airfoil in an autorotating rotor points forward of the axis of rotation. We've all seen those diagrams. The axis is tipped back so that the axis is not perpendicular to the aircraft's flight path, however. Thus the resultant force can be forward of the rotational axis (pulling the blade around) without also being forward of a line perpendicular to the flight path. (The resultant force on the blade near the tip isn't even forward of the rotational axis; it's aft, tending to slow the blade's rotation. The inner portions do have a forward-of the-axis leaning resultant, however, so these portions "drive" the outer portion.)

The A. and von D. definition of AOA works OK for rotor blades as well as fixed wings. The motion of the blade (which determines the baseline for measuring AOA) is a bit hard to picture, though. It's the vector sum of rotational motion and the aircraft's forward motion. The rotational motion is in the plane of the disk, obviously -- no matter where in its orbit the blade happens to be. When you add in the aircraft's forward motion, the resulting motion of the blade through the air results in positive angle of attack for both blades.

You guys who don't think you need to understand any of this stuff are whistling past the graveyard. These are EXPERIMENTAL aircraft. The analytical work has almost certainly NOT been done for you by your kit "designer," since only a few of them actually know much about aircraft design. It's left to the CFI's to teach you about it, or you to learn it on your own, if you expect to be able to watch your own back.

[BEN S]

There are many levels of understanding.
I understand that the forward motion provided by the propeller makes the rotor spin creating the lift I so desperately seek.
I understand that by looking at the number of kit sold and flying versus the number crashed and burned that some kits MAY be safer than others.And choose accordingly.
I understand that proper training by a certified flight instructor is the ONLY way I could be typing this now.
I understand that although I will do my utmost to comprehend the intricacies of the aerodynamics, I will probably fall short of some others abilities to make mathematical computations regarding the same.
But ultimately I UNDERSTAND that we are ALL "whistling at a graveyard" and if I wanted to live to the ripe old age of 105 eating broccoli through a straw, I wouldn't be flying gyros at all. Or for that matter Diving, De-Fusing bombs riding motorcycles playing with machine guns, jumping out of planes, catching rattlesnakes or any of the other myriad things which make life so worth living!
These are my feelings and not an attack, I hold you and your opinions in the highest regard.
I hope you can understand
Ben S

[Resasi]

Ben you jester you.:)

Ironic for the unfortunate guy who studies hard, gets it all down pat and then has a nut back off 500' up.

But then again that Broccoli bit sounded awful, and those others things such fun.

[Doug Riley]

I eat the broccoli AND fly the gyros. Like drinking, swearing and also going to church -- it covers all your bases.

Just don't rely on homebuilt-kit manufacturers having done their design homework, as they have for certified aircraft. Recall that the chief widowmakers in the "bunt-o-matic" gyro category were relatively high-volume, factory-produced, nicely-finished kit gyros.

Better get back to the topic...

[MichaelBurton]

De we agree then that there is a forward vector of lift? You mentioned it in relation to a rotor can you also see the same relationship for a fixed wing aircraft?

[Doug Riley]

Well, the question is "forward of what?"

The lift (and also the resultant when you combine lift and drag) can't be forward of a line perpendicular to the wing's FLIGHT path. If the resultant were forward, the wing would accelerate itself along the flight path. You wouldn't need gravity or an engine to propel it.

The lift (or lift + drag resultant) can be forward of VERTICAL (= the direction of gravity). That can happen when the wing is gliding downward. The resultant is still points aft of a line perpendicular to the flight path.

Again, the inboard portion of a gyro rotor blade is like a gliding airplane wing. The lift + drag resultant points forward of the axis of rotation, but aft of the blade's individual flight path.

(The same idea applies to a gyro rotor disk in a power-off glide. The disk's front edge is a little lower then its back edge, as you can see in photos. The rotor's thrust is perpendicular to the disk. Therefore the thrust is forward of vertical (i.e of the line of gravity). The gyro is descending, however, so the thrust is NOT forward of a perpendicular to the flight path.)

This stuff is hard to talk about with only words; easier with pictures. I will try to find or scribble some if I get a break.

[MichaelBurton]

In Stu Moment's "Simple Aerodynamics" copyright 1984, 2004 by Sublogic Corporation induced relative wind is explained "While creating lift, your airfoil changes the direction of airflow in many ways. You’re already familiar with down wash. This downward deflection of air changes the relative wind in the vicinity of the wing to a slightly downward direction. As a result, the true angle of attack is different from the apparent angle of attack. In the last lesson, we described this apparent angle of attack by stating that the relative wind is opposite the flight path. But the airfoil alters the relative wind. This is described in the following diagram as “induced relative wind.”


Lift is produced perpendicular to the real, induced relative wind direction. A relative wind in a slightly downward direction will give us a real lift vector with a component toward the rear. This rearward component is known as induced drag. The following figure illustrates the concept of induced drag."
http://classicairshows.com/Education/Aerodynamics/AeroDynamicsImages/AerodynamicsFour2.gif

Here is an additional reference from the pilots reference guide (http://books.google.com/books?id=GB1Cdqf433wC&lpg=PA22&ots=ydfdNSLppN&dq=lift%20vectors&pg=PA23#v=onepage&q=lift%20vectors&f=false)

Here the diagrams for lift clearly show the vectors for various AOA.

[MichaelBurton]

Here is a vector diagram which came from www.engr.umd.edu/~jeffl/autogyros.html.
http://www.unc.edu/~franco/autogyro/vectors.jpg

[Doug Riley]

The Pilot's Reference Guide is rigorous and has it right. The resultant (red line) points straight up (not forward) relative to gravity. See the upper right drawing. In the same drawing, the initial (before getting bent by the wing) relative wind is coming at the wing from below (the wing is in a mush). That is, the flight path is somewhat downhill. The resultant points well aft compared to the (downward) flight path.

OTOH, Moment's book plays around a bit with the standard definitions. That doesn't matter until you try to communicate using his novel terminology. What he calls "apparent AOA" is simply AOA in standard parlance. Lift is roughly proportional to this "apparent AOA."

His "induced relative wind" concept certainly is true for any given point along the wing. The "induced relative wind" will have a different direction, depending on the location on the wing where you measure it. As a number for the wing as whole, I'm not sure what you'd use it for, or even how you'd measure it.

The dotted line in Moment's picture is labelled "vertical reference," which is fine. That's the line that I've referred to as the direction of gravity. He goes "non-standard" when he comes up with a lift + drag vector that's perpendicular to the wing. That would imply that the wing can be propelled through the air, and even make lift, with zero thrust. We wish! In reality, once you add in induced drag, the total of lift and drag (what I've been calling the "resultant") leans aft of a perpendicular to the plane of the wing.

The sketch in #145 looks right. Again, the resultant isn't forward of the BLADE'S flight path.

[MichaelBurton]

Here are some simplistic examples showing lift vectors rearward and forward. from http://www.av8n.com/how/htm/4forces.html

http://www.av8n.com/how/img48/four-slow-descent.png
http://www.av8n.com/how/img48/four-dive.png
http://www.av8n.com/how/img48/four-climb.png

If the wing is not powered and is not decelerating there must be a force that causes balance. this force is the forward vector of lift that is acting perpendicular to the total relative wind. In total relative wind I include flow caused by any forces or wing movement.

[MichaelBurton]

Ok, now lets go back to my statement that caused all the fun in the first place. If airspeed changes AOA changes.

Even in a wind tunnel with the wing held in position and the wind in a constant direction except when acted upon by the wing. The AOA changes with airspeed change due to the induced airflow.

I can imagine also how equal AOA could be realized in two vastly different air speeds and relative winds.

[Doug Riley]

Well, this is one of those discussions that resemble Alice's conversation with Humpty Dumpty. Humpty tells her that he can make words mean just what he wants them to mean. Of course, he's right. The humor is that defining words differently than the other guy makes communication impossible.

AOA is conventionally defined with reference purely to the direction of travel of the wing through the air. Another way of saying the same thing is that it's measured with reference to the oncoming air, far enough upstream so that the stream hasn't yet begun to be deflected by the wing. This is the type of AOA that is directly proportional to lift at a given airspeed.

If you want to use a different definition of AOA (or relative wind), so that you measure the direction of the air AFTER it's come under the influence of the wing, that's OK. The specs conventionally used to describe wing sections (lift coefficient curves and stuff) won't work with this definition, but it may not matter much in the cockpit.

The important fact from the pilot's viewpoint is that an aircraft is usually not perfectly-enough designed so that you can easily alter ONLY its airspeed. Offset thrust, draggy bits, imperfect trim or (in a rotorcraft) rotor blowback are all apt to cause some change in the lifting surface's AOA.

This change of AOA CAN be in either direction, depending on the wholesomeness of the design. A safe design at least doesn't DECREASE its AOA as a result of an increase in power setting or airspeed. Even if AOA doesn't increase with increased airspeed, but just stays constant, the aircraft will climb in reaction to an increase in airspeed.

If the aircraft dives in reaction to increased airspeed, it is statically unstable and quite dangerous.

[MichaelBurton]

I am glad that language is always changing. If it were not then we would have an even harder time communicating. I am not trying to create new science only to explain what has already been discovered accepted and published.

I agree with the accepted definition of AOA. That it is in reference purely to the direction of travel of the wing through the air. We just need to look closer to the wing to get the true direction of the wind.

My vocabulary may be inadequate for the task and I have no problem changing terms to conform to established definitions. If you have better terms for these concepts feel free to enlighten me.

I would agree that an aircraft that reacts so that a further divergence in airspeed caused by a power change would be unstable, I have never said otherwise.

[MichaelBurton]

Here is an additional source I have read that talks about induced air flow and the change that it causes to AOA. Aerodynamics (http://selair.selkirk.bc.ca/Training/aerodynamics/induced.htm)

[Redbaron]

http://www.magnigyro.com/USA/feature_articles/ASTM%20Standard%20Part5.pdf

nuff said! :D

[DaveAndPete]

Personally, I won't be caught above 0 kts IAS and/or 0 ft AGL without the "full, free, and correct" motion of all controls

Wow, I am glad you said that, I was sitting here reading and getting sweaty palms with that fixed stick stuff. Damn, that's too risky for me. I don't care if it is "supposed" to disable instantly, if I can't wave that stick around then the wheels are not getting off the ground.

Dave

[PW_Plack]

...I don't care if it is "supposed" to disable instantly, if I can't wave that stick around then the wheels are not getting off the ground...

Dave, if conducted in the manner Greg recommends, the fixed-stick test is quite safe. Not knowing your gyro's safe envelope is much riskier than the test!