Testing gyros for buntovers potential

WaspAir

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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

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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

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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?
 
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Poor comeback

Poor comeback

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

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anti-venom

anti-venom

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

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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

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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

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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

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Test Flight procedures

Test Flight procedures

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

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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

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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

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Power "chop" test - A

Power "chop" test - A

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

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Power "chop" test - B

Power "chop" test - B

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

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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:
 

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PW_Plack

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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

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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

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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

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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

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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

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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
 
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