horizontal stabilizer...which way is up

[steve hartle]

Acknowledging the fact that an effective hs is neccesary for the safe and stable operation of most, if not all gyros, i pose this question. Given that size matters, that symetric is better than flat, that in propwash is better than not, that some negative incidence is neccesary, would it also be advantageous to use an inverted airfoil design ? I believe Zenith uses such a stabilizer on its STOL 701 model aircraft. Maybe in our quest to save weight, minimize size, and improve effiency someone better versed than I could explain if this design would be appropriate, of significant benefit, and if so what airfoil profile might work best.

[Aussie_Paul]

Acknowledging the fact that an effective hs is neccesary for the safe and stable operation of most, if not all gyros, i pose this question. Given that size matters, that symetric is better than flat, that in propwash is better than not, that some negative incidence is neccesary, would it also be advantageous to use an inverted airfoil design ? I believe Zenith uses such a stabilizer on its STOL 701 model aircraft. Maybe in our quest to save weight, minimize size, and improve effiency someone better versed than I could explain if this design would be appropriate, of significant benefit, and if so what airfoil profile might work best.

An effective small h/stab. A piece of extruded rotor blade with the heavy spar removed works well for this 2.5" HTL machine with a pod. The faster the machine goes as the nose is pushed down the more the h/stab works tp keep it from goi9ng too far.

Aussie Paul. :)

[Passin' Thru]

Acknowledging the fact that an effective hs is neccesary for the safe and stable operation of most, if not all gyros, i pose this question. Given that size matters, that symetric is better than flat, that in propwash is better than not, that some negative incidence is neccesary, would it also be advantageous to use an inverted airfoil design ? I believe Zenith uses such a stabilizer on its STOL 701 model aircraft. Maybe in our quest to save weight, minimize size, and improve effiency someone better versed than I could explain if this design would be appropriate, of significant benefit, and if so what airfoil profile might work best.
.
Steve, that is precisely what I did on my previous dominator. When I first started examining the overall layout, I had to make a few adjustments to accomodate a 68" prop on the Rotax 618-E box. (I really wanted to go 70", but settled for 68.) this neccesitated raising the engine about 4" and an attendant increase in mast height. Next, due to a hip and leg problem I needed to lower the seat, so I reduced the amount of keel drop by about 4" with another increase in mast length. (Gee, I'm really going away from home, aren't I?)
At this point it was only on paper, so I consulted someone who I consider an expert in these matters. After describing what I was trying to accomplish, he suggested the inverted asymmetrical horizontal stab. ( I won't mention his name, he may not want to be party to the crime!:eek: ) The results were very satisfying. It worked great. The machine is completely stable in every test. Hands off and trimed, I could drink a soda and smoke a cigar with my feet on the panel. Just like a Cessna. I really hate I sold it.:sad:
.

[Fl90]

I hate to get off topic, but I have a head full of questions looking at that airfoil.

It looks to have a heavily aft loaded mean line, a laminar attempt with the major thickness location and heavy reflex. I would think the separation at about 3/4 chord would be high at most rotor speeds and the drag on the airfoil would make for an inefficient rotor. I'm just wondering how the airfoil performs as a set of blades, and what disc loading does the manufacturer recommend?

If you would rather PM me, so as to keep this thread on track, that would be fine.

Sorry for the hijack, Phil.

[steve hartle]

Just viewed R&D at BD ,confirmed alot of beliefs I had but as usual far more complicated than I realized so although Iam convinced this would be beneficial I would like to hear some of the more versed opinions . Starting to realize more going on than meets the eye. Speak slowly , use small words.

[Doug Riley]

An asymmetrical airfoil and a symmetrical one with negative incidence are virtually the same thing. The camber contributes the equivalent of 2-4 degrees of incidence, depending on the severity of the camber.

A cambered airfoil is slightly more efficient at generating X pounds of lift than a symmetrical one making the same lift (by using more incidence). The difference is very slight for a wing as small (and slow-moving) as the usual gyro H-stab.

Example: A NACA 0012 airfoil (a typical HS foil) has a drag coefficient of around .007 at a lift coefficient of 0.4. The same airfoil, but with camber, becomes a 2412, and the drag coefficient for the same lift goes down to .006. That's a savings of 14% of the drag. But the drag of a gyro-HS-sized wing at 60 mph at the 0.4 lift level is less than a pound to begin with. Who cares about saving 14% of 1 lb. of drag?

Do the simplest thing.

Chum's rotor-blade HS is too low to catch any propwash and is quite small to boot. You'd need to do a little arithmetic to see if it's powerful enough to prevent PPO and/or drag-over caused by the pod. I'd be surprised if it were.

[steve hartle]

Thank you Mr Riley , clear and to the point as usual.Thought there might be more to be gained . Zenith seemed to make big deal about benefits ( beware the hipe I guess). The point you keep making about hs in the propwash is well taken. FW with T tail above propwash don't experience the same pitch movement under throttle changes as do tails in propwash. Don't know statis of Starbee T tail but seems to me to be the optimum for those searching ( definitely my personal pick). Good job at BD , we all need to do more practical work and less speculating. Wright brothers wouldn't have got off the ground if they had sat back in Ohio and talked about it. For us looking for the Wright stuff the gyrobee is a good inspiration as the Wright Flyer was but it needs to evolve . Just because something is simple, cheap to build and is flyable dosen't mean we should stop there or else we'd all be buzzing around in Wright Flyers. Before the hate mail starts flowing I mean no disrespect and applaud Mr Taggart for inspiring us.

[Aussie_Paul]

Do the simplest thing.

Chum's rotor-blade HS is too low to catch any propwash and is quite small to boot. You'd need to do a little arithmetic to see if it's powerful enough to prevent PPO and/or drag-over caused by the pod. I'd be surprised if it were.

Doug, without the stabs there is a slight tendency to nose down with a rapid application of power, so it is very slightly HTL. With the stab it behaves nicely. I would not say that it could not be improved on BUT it does work well. This was the first single seater built to the Hybrid tested geometry. The fact that Chum is short so he had to keep the seat lower. Otherwise it would have turned out very close to CLT.

Using the upside down rotor blade idea was just an out of interest idea. Once you are close to CLT the h/stab is nowhere near as critical as it is on a RAF. I seem to compare everything to a RAF :sorry: BUT that is where I started from, and use that as my datum I guess. :)

Aussie Paul. :)

[Doug Riley]

Paul, you really need actually to find the CG-thrustline relationship with a double hang or equivalent static test. On gyros, HTL has a nasty habit of sometimes not revealing itself readily in flight tests. The gimbal head and other mechanical factors can mask it -- but that does not mean that it isn't there, waiting to get you.

Body pods are the same. Their effects may only pop out when you shut the engine off and do a fast deadstick dive. In normal flight, they can behave just fine.

Again, some ground analyisis and testing will often reveal the problem more quickly and safely than flying about and trying to find the "coffin corner" of the envelope.

If a gyro fails such a ground test, flight testing should not proceed. The fact that a given craft appears to "fly good" doesn't mean that HTL or other airframe instability can be ignored. Flight tests do NOT trump the math -- not at this basic level.

[Doug Riley]

Steve, fortunately for us, the Gyrobee is a much better gyroplane than the Wright Flyer was an airplane. The Flyer is a canard craft that is NOT set up like a modern Rutan-style canard. The Flyer doesn't use the canard as a lifting wing. As a result, it's pitch-unstable for the same reason that a HTL gyro is: the CG is aft of the lifting surfaces' combined aerodynamic center. This is an unstable setup on any aircraft, whether fling-wing or fixed-wing.

Various 'Bees have tested out at perfect CLT, slightly LTL and an inch or two HTL. All three situations are within the "tolerance" that's easily accommodated by a 5-6 square foot HS in the propwash.

The principal shortcoming of Ralph's old prototype Gyrobee is the use of a Brock HS forward of the rudder and low on the keel. Most 'Bee builders have used a more effective HS than this, though.

[Aussie_Paul]

Hi Doug. Paul, you really need actually to find the CG-thrustline relationship with a double hang or equivalent static test. I agree On gyros, HTL has a nasty habit of sometimes not revealing itself readily in flight tests. The gimbal head and other mechanical factors can mask it -- but that does not mean that it isn't there, waiting to get you. Absolutely

Body pods are the same. Their effects may only pop out when you shut the engine off and do a fast deadstick dive. In normal flight, they can behave just fine. Absolutely

Again, some ground analyisis and testing will often reveal the problem more quickly and safely than flying about and trying to find the "coffin corner" of the envelope. Absolutely

If a gyro fails such a ground test, flight testing should not proceed. The fact that a given craft appears to "fly good" doesn't mean that HTL or other airframe instability can be ignored. Flight tests do NOT trump the math -- not at this basic level.Absolutely

Points taken Doug. Many thanks.

Aussie Paul. :)

[birdy]

If Chum's machine is so it is very slightly HTL, then Mate, i must be sitn on my head. :(
Not only is he sintn low, so is the fuel. And the bulk of a heavy engine is nearly ON the TL.
Add to that the fact that theres a larg pod, with a 'down forcing' windsheild, both way below the TL [ adding more nose down forces] and dragy mains gear even further below the TL.
If this thing flies with even the slightest resemblance of stability, with that pissy little stab to counter all that weight/drap offset, then ill stand r##tn. :(
Yes, it dose have a long mast with heavy rotors, but after wot i saw at Lammeroo with the double hang testing there, Chum's machine could not possably be remotely close to so it is very slightly HTL.

[Aussie_Paul]

If Chum's machine is so it is very slightly HTL, then Mate, i must be sitn on my head. :(
Not only is he sintn low, so is the fuel. And the bulk of a heavy engine is nearly ON the TL.
Add to that the fact that theres a larg pod, with a 'down forcing' windsheild, both way below the TL [ adding more nose down forces] and dragy mains gear even further below the TL.
If this thing flies with even the slightest resemblance of stability, with that pissy little stab to counter all that weight/drap offset, then ill stand r##tn. :(
Yes, it dose have a long mast with heavy rotors, but after wot i saw at Lammeroo with the double hang testing there, Chum's machine could not possably be remotely close to so it is very slightly HTL.

Well once again you are wrong Birdy.

Aussie Paul. :)

[SamL]

The other important factor about sizing HS's is to size them large enough to place the machines centre of presure behind the CofG.
Otherwords allowing you machine to weathercock in pitch correctly.

Sam..............

[Doug Riley]

Certainly, Sam.

Where some machines get into trouble is in regard to the C.P. of the frame at large airframe pitch angles. The C.P. can be behind the CG when the frame meets the air in a level stance, but the C.P. may forward dramatically at a nose-down angle.

This can happen with open-topped pods. When the nose gets low relative to the flight path, the open "bathtub" acts like a scoop, creating tremendous drag way forward and possible pulling the nose over. The rotor may not have enough power to keep the nose up, even at full aft stick, especially if RRPM declines because of the nose-low stance.

Igor Bensen encountered this effect in a famous incident involving the Gyroboat.

It's important when using a pod to make certain the H-stab is large enough to prevent this sort of "drag-over." A simple silhouette (paper cutout) test is a start, but isn't conclusive in this case. A 3-dimensional model tested in a breeze is better.

[birdy]

Another thing to consider wen your lookn for your COD [center of drag] is the actual shape of the structure the air is hitting.
Lookn frunt on to a machine for example, to determan the machines' TL in relation to its COD, it may LOOK like it is centered, but wen you look at indervidual parts 'wind resistance' you could be way out.
One of those AC fuel pods would look similar front on too a similar sized trye, but the trye would be more dragy than the pod, coz of its side profile. The air flow coming off a tyre would be much more turbulant than the pod, so its going to cause more drag.
Streamlined mian gear and engine cowl would make a huge difference to the COD of most gyros.

[Aussie_Paul]

Paul, you really need actually to find the CG-thrustline relationship with a double hang or equivalent static test. On gyros, HTL has a nasty habit of sometimes not revealing itself readily in flight tests. The gimbal head and other mechanical factors can mask it -- but that does not mean that it isn't there, waiting to get you.

Body pods are the same. Their effects may only pop out when you shut the engine off and do a fast deadstick dive. In normal flight, they can behave just fine.

Again, some ground analyisis and testing will often reveal the problem more quickly and safely than flying about and trying to find the "coffin corner" of the envelope.

If a gyro fails such a ground test, flight testing should not proceed. The fact that a given craft appears to "fly good" doesn't mean that HTL or other airframe instability can be ignored. Flight tests do NOT trump the math -- not at this basic level.

Doug, we did the double hang test on Chums machine as I do on every gyro that comes through my hanger.

After a while you start to get a pretty good idea where the CoM will be and how it will fly. Like most things, the more you do the better at it you get.

Aussie Paul. :)

[gyrogreg]

I've got some thoughts on whether the HS should be symetrical or asymetrical with the camber to the bottom:

If you are trying to compensate a really bad nose-down static moment, such as from a very HTL or a very low and draggy cabin, the asymetrical, down-loaded HS might be necessary to do the static job. But, if this is the case, I'd say you ought to correct the offending static pitch moment issue before proceeding.

I'm starting to believe that dynamic stability plays a more important part in flight stability/safety than we realize - good dynamic stability can compensate for some actual statically unstable AOA or G-Load. I say that because it appears to me that the Magni M16 gyro may actually fly in some speed/power conditions with the CG a bit aft of the RTV "Oh No!" It's possible that the Offset Gimbal may compensate some or all of that static instability component (RTV forward of the CG). But, this begs the question why a there seem to be no incidents of PIO or buntovers in a Magni.- and most people fly these in the range of 100 mph in even a lot of turbulence! Static considerations only, it looks like at least a buntover might be possible!

Of course, it can be observed that the HS is so big and powerful as to prevent the rapid forward pitch that could propegate and sustain a buntover divergence. But, this theory relies on the dynamic dampening of the HS. In otherwords, the dynamic characteristics can trump some static issues!

On the other end of the spectrum, we have found that some other gyro configurations that actually test to have reasonable static flight results, have a bad history of PIO and buntovers. Apparently, the offset gimbal and flexible masts, etc. on some obsiously HTL gyros are providing (or faking) reasonable static flight stability results. The difference with these gyros appears that they have little or no dynamic dampening - HS! And when they have good dynamic dampening, with the RTV still obviously forward of the CG - such as an RAF with a very large and long tail and a good airfoil HS - they don't seem to have the buntover or PIO issues - in fact pilots report they are very docile in even rough turbulence! This tells me it is actually the dynamic dampening that is helping to avoid the static issues with the RTV forward of the CG.

So, if dynamic dampening is an important parameter - I think it is very important that the airframe can't pitch faster than the rotor can keep up with the spindle - both buntover and PIO are avoided. (Just this morning, I had a student who had trimmed the M16 very nose heavy - high speed. I was not aware that he had trimmed it that nose heavy because he was holding hard back on the stick at the same time! When I told him to let go of the stick to see if he was in trim at the higher airspeed now - he let go quickly and the nose tucked very smartly - I've jabbed the stick forward like this before too - to see what happens! Nothing happened, 'cept we were suddenly pointing severely nose down and gaining speed rapidly!)

I believe it is the dynamic dampening that is the the important parameter on the Magni. It may be similarly important on a lot of gyro configurations, especially some with lower airframe moment of inertia that can otherwise pitch very rapidly - faster than the rotor can keep up.

Chuck also points out that some stability is gained from HSs because their lift slope is steeper than the rotor's.

Both of these are talking about dynamic dampening with the HS lift curve used in both directions - what the HS does when it is in motion - dampening or providing an opposing moment as a result of its vertical translation when the airframe is pitching up or down.

For these functions, the HS really needs to have good lift characteristics in both directions. When the HS (tail) is lowering, it needs to have good resultant upward lift - increased AOA due to it's downward vertical movement. When the tail is rising, the HS needs to have good downward lift from the increased negative AOA due to it's upward vertical motion!

By the reasoning that we want the HS to do more than compensate the static pitch moment imbalances, we want it to be a good dynamic dampener in both directions (to stop a buntover and and dampen PIO tendencies), the HS might be better to be symetrcial so it can work just as well in both directions! If asymetry is necessary for strong static compensation of the HTL and Low Drag line), I suggest to change those bad static moments, and then use a good symetrical airfoil to provide good dynamic dampening in both directions.

- Just some thoughts! - Greg

[SamL]

Greg, I think we are missing a very important point here.
The point being "What causes the CofG of a gyro to move behind the RTV" ?
It moves during changes in pitch. This can be caused by the aerodynamics of a machine, or by a thrustline offset in relation to its CofG. Incorrect vertical centre of pressure can also acheive the same movement.
In the case of the Magni, even though it is a HTL machine the HS seems to do its job very well, and even in fast level flight it keel fly's level. This means the CofG has not moved behind the RTV, as you have so stated.
If we can understand the causes, we can then move onto solving the problems.

Regards Sam.....;)

[Doug Riley]

Let's be sure that we use words consistently, lest we end up like the construction crew at Tower of Babel, LLC.

Dynamic stability is only an issue if the system is already statically stable. IOW, (1) dynamically stable, (2) dynamically neutral and (3) dynamically unstable systems are three subsets that together make up the larger set of statically stable systems.

A statically unstable system never reaches either dynamic stability or dynamic instability; it just capsizes. There's no potential oscillation; the system just keeps going faster and faster in one direction once disturbed. PPO is a statically unstable event.

PIO is tougher nut to analyze rigidly. It's a man-machine interaction. People have emotions, a range of reaction times, behavioral inconsistencies and "senior moments." As a result, man-machine interactions don't necessarily fall into the cut-and-dried physics of static/dynamic stability.

A skilled pilot can supply something LIKE static stability to certain types of unstable systems. Once this pilot gets the hang of it, it becomes a reflex -- meaning it takes no conscious work at all. That pilot will arrive at the opinion that static stability is no big deal.

We human critters are pre-programmed to able to do this. We can easily stabilize certain statically unstable systems. Standing upright requires continual small corrections through our feet. Ditto riding a bicycle using the steering and body English. Yet, a child can do these things. We're able to supply missing static stability in certain cases, especially if we catch the developing capsize early enough.

Greg may have a point that statically stable but dynamically unstable systems
may be especially difficult to control. We humans aren't particularly well-programmed to compensate for dynamic (oscillatory) instability (caveman Og didn't encounter them), so they throw us. That may be why so many Bensens (with near CLT but ineffective H-stab) PIOed.

[gyrogreg]

Sam & Doug, I enjoy these discussions, mostly because I often have things I don't understand and need to understand for the ASTM standards criteria:

I don't know if the Magni CG ever gets aft of the RTV - or equivalently aft considering the other stability contributors - offset gimbal, lift curves of HS vs. Rotor, etc. The M16 does appear to have strong static stability - stick position fixes airspeed like a rock, response to wind gusts is negligible (strong G-Load negative feedback), etc. But, the Magni does appear to have a 2-4 inch HTL - and the HS is not strongly embedded in the propwash to compensate the HTL that much. In fact, adding power under normal and high speeds results in a bit higher trimmed airspeed. Lowerered power -> slower airspeed. This would be the classic case of an unbalanced HTL! If a HTL is unbalanced, I do believe the RTV can be forward of the CG especially at high power and high airspeeds!

Another thing you can notice in the M16 is that, stick free, adding or reducing power results in the same airspeed - but the stick floats forward and aft to maintain that stick free airspeed. Adding power, stick free, the stick floats aft - indicating the airframe (and CG) is moving aftward - more nose down. Reducing power, stick free, the stick floats forward - indicating the airframe has just swung forward a bit - more nose up. Since all power conditions stick free maintain the same steady state trimmed airspeed, my only conclusion is that the airframe is not maintaining the same level flight attitude under the different power conditions - as you explain, Sam!

In all power/airspeed, stick free or stick fixed though, the aircraft exhibits strong static stability - so what is doing that - especially if the RTV may be forward of the CG - as I suspect it would be under higher power HTL. Could the strong dynamic dampening effect create a suedo static stability?

Doug, certainly there can be no dynamic stability (oscillations damped), or even oscillations (damped or undamped) without static stability. That is what you are refering to as a difficult gyro to fly - the pilot must be the static stabilizer - and if you are not good at that, static divergent buntover. The pilot must also dampen any oscillations - if you get that out of time, there is no dynamic stability (result - PIO). I'm not really suggesting that a statically unstable aircraft is dynamically stable. I'm suggesting that, dynamically stable or not (oscillations dampen or not), the dynamic dampening effect of the HS either presents some suedo inherent static stability, or makes it easier for the pilot to be the static stabilizer.

There are two different dynamic characteristics:

1) Dynamic stability where any oscillations dampen to zero. There can be no oscillations, and dynamic stability is not even a consideration, if there is no statically stable point about which to oscillate.

2) Dynamic dampening: there can be dynamic dampening even if there is no static stability, or there is static stability and the oscillations are divergent - dynamic instability. Dynamic dampening reduces the rate or acceleration of any pitch motions - whether in just one direction, or oscillating up and down. Dynamic dampening is also what causes any pitch oscillations (from a statically stable system) to dampen amplitude and die out!

For oscillation dampening, the dynamic dampening of a HS creates a force that is 90 degrees out of phase with any oscillation peaks - the strongest corrective HS force occurs at the point where the HS is moving vertically the fastest - at the point where the airframe passes through the static steady state point - level keel? This phase condition is really what does the oscillation dampening - the true secret of a HS as a dynamic dampener. The vertical rate of movement of the HS at this point, combines with the relative airflow to produce a high HS AOA at this point - proprotional to the rate of vertical movement of the hS - pitch rate. The highest pitch rate of an oscillation occurs at the point of static neutral (keel leve?) where it does the most good to dampen the oscillation and slow the max pitch rate. If there was no phase lead of this HS lift force, and the highest AOA and greatest lift force happened only at the peak of the oscillations, this would essentially be just the static restorative force, acting mostly like a spring, that would not have any dampening effect. The mechanism by which the HS dampens any oscillations, and how it occurs 90 degrees before the maximum pitch attitude, is the real secret of what a HS does! It is only 1/2 of the stability story to consider the simple static effects of the HS balancing the HTL or other static aerodynamic moments on the airframe. I am suggesting the overall impact of the dynamic damping effect of a HS may be somewhat understated. For this reason, and maybe part of the Magni secret, the HS placed far aft may be understated contributor to full stability safety!

I don't know these answers. I just find it strange that my M16 is so statically stable, when I really have my doubts the CG is truly even forward of the RTV!

- Help me understand, please! I need some understanding of the relative importance of static and dynamic stability in order to suggest some improvements in this area on the gyroplane ASTM standard to better identify, evaluate and assure safe gyroplane characteristics. We are aware that some gyros that seem to meet the static criteria in the standard, are still capable of buntovers and PIO (have no data feedback on dynamic parameters on such gyros!). And, gyroplanes such as the Magni M16 appear to be insulated from PIO and buntovers, and exhibit strong static and dynamic stability, even though they may not meet the static holy grail of the CG equivalently forward of the RTV.

- Greg

[steve hartle]

Gyrogreg, you raised an interesting question I hadn't considered much at the time because I was focusing on nose down pitch primarily but didn't quite hear the answer, what effect good or bad does the asysmetric foil HS cause in nose up pitch or how does it react dymamically. Most of my thought proccess focuses on the static (easier to grasp right now). The answers are out there we just need to ask the right questions.

[gyrogreg]

Gyrogreg, you raised an interesting question I hadn't considered much at the time because I was focusing on nose down pitch primarily but didn't quite hear the answer, what effect good or bad does the asysmetric foil HS cause in nose up pitch or how does it react dymamically. Most of my thought proccess focuses on the static (easier to grasp right now). The answers are out there we just need to ask the right questions.

I would need smarter people than me to tell us if an assymetrical airfoil would have a worse lift curve in the reverse direction. I suspect it would not have as good lift vs AOA in the reverse direction because the purpose of the assymetrcial airfoil is to maximize lift in one direction and not worry about the other direction! But, maybve the difference is minimal!

Doug:? - Greg

[mceagle]

Greg, we can speculate many reasons why a car has a flat tyre - maybe the airpressure inside is less than the air pressure outside, or that the side walls are not as stiff as they could be, or even that if it spun faster centripetal force will hold it up.
Could the real reason be that it has no goddam air in it?

The Magni stability answer might be just as simple. Could it be that the stabilizer volume is just plain and simply powerful enough to overcompensate for thrust line offset, rotor thrust vector placement and vert C of D offset?

Could it also be that nil bunt record of the Magni has something to do with the type of person that can afford, and is prepared to fork out, the $ that are required to buy a Magni?

A fairly simplistic view I know but maybe we can't see the trees for the forrest.

[gyrogreg]

Greg, we can speculate many reasons why a car has a flat tyre - maybe the airpressure inside is less than the air pressure outside, or that the side walls are not as stiff as they could be, or even that if it spun faster centripetal force will hold it up.
Could the real reason be that it has no goddam air in it?

The Magni stability answer might be just as simple. Could it be that the stabilizer volume is just plain and simply powerful enough to overcompensate for thrust line offset, rotor thrust vector placement and vert C of D offset?

Could it also be that nil bunt record of the Magni has something to do with the type of person that can afford, and is prepared to fork out, the $ that are required to buy a Magni?

A fairly simplistic view I know but maybe we can't see the trees for the forrest.

Hi Tim, long time since we've said Hi!

I think you are correct on both counts. I do believe the HS is just so big that it so dampens ANY deviation from airframe attitude that it over-powers even some positive feedback, divergence initiating pitch movements: Such as a reduced load on the rotor that would normally pitch the nose downward is just prevented, dynamically, from doing so because the dynamic dampener, HS, will not let it move much to even start with! I've always said the simple way to look at a Magni is just that the tail is so big, the airframe flies level no matter what static moment distrubances there are! This is why I am suggesting the dynamic dampening actually provides a form of suedo static stability!

Your other point about the person flying it probably has merit too! But, also, Vittorio Magni's safety philosophy could be biasing the accident results too. Their safety philosophy includes not only an aerodynamically stable design, a rugged and reliable and easily maintainable airframe and engine. Their safety philosophy also includes required training by Magni approved instructors/dealers, forever close contact with all Magni owners, and family participation in the process of owning and learning to fly the Magni gyros. Vittorio believes knowledgeble and engaged close family and loved ones will also help the pilots control their urges to fly outside their limitations.

Having said that, I can say for a fact though, that relatively lower time Magni pilots have flown in conditions I would not ever have flown my previous gyros - all with no problems or issues. When I see my lower time pilots flying safely and securely in gusty crosswinds that keep the other people from even going to the airport, and doing this on a regular basis, and taking experienced pilots on rides in those conditions, and hearing the excited confidence of even those previously timid pilots, I have to believe it is more than just the abilities of the pilot! And, every Magni flier you talk with will say the same thing. There is something about the aircraft that is unusually stable in very rough air! And these low time pilots are flying at speeds exceeding 100 mph in very rough winds. We even have an experienced FAA district Manager who flies a Magni at 100+ mph in very rough winds and will regularly tell people that it is no problem!

"See the forest for the trees": Maybe! But, for the ASTM standard purposes, I am searching for objective criteria for the standard - and a test method - that will define safe and stable flight characteristics. So, I need to go further than just saying "powerful enough" horizontal stabilizer! I've become convinced our static stability criteria has some holes in it. In combination with the dynamic criteria, these holes are likely covered. But, dynamic criteria is not safe to test, and the static criteria are not always definitive. I'm looking for some safe-to-test criteria that combines both static and dynamic effects to define a stable and safe gyroplane. I'm becoming convinced, this cannot be done with separate static and dynamic criteria - it is a combination of things - Vittori Magni says it is a "harmony" of all the parts! And, it really does not much concern Vittorio if the Magni does or does not meet all of the static criteria - to him that is not definitive or always necessary!

Here is one test that does seem to be rather telling - and is probably testing the combination of static and dynamic effects! Fly in not-so-smooth air with a truly FIXED stick. Even a gyro that meets the ASTM standard static criteria, if it does not have the adequate combination of static stability and dynamic dampening, will start phugoid (long-period) pitch oscillations that continue to diverge! We don't have a lot of test data - but, there are numbers of reports on what an RAF will do - even an RAF with a stab - start wilder and wilder pitch amplitude oscillations until the pilot decides to release the stick and stabilize out the gyro! I do know in the Magni - with a solidly fixed stick - it simply maintains an airspeed, or returns to that airspeed if disturbed by even the roughest wind gusts and turbulence - no divergent oscillations - only damped oscillations. That is a truly simple test - if it gets so "hairy" after a few seconds of fixed stick flying in less than smooth air, that the pilot is compelled to release the fixed stick and take over the flying, that gyro is probably not adequately statically or dynamically stable to assuredly avoid PIO or buntovers! We really need more feedback on that - and I do intend to reactivate the ASTM task teams to search for some data and answers on all this! And then, how do we put that in standards terminology?

- Greg