Testing gyros for buntovers potential

All_In

Gold Supporter
Joined
Apr 21, 2008
Messages
14,625
Location
San Diego, CA. USA
Aircraft
Piper Archer, Aviomania G1sb
Total Flight Time
Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
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
 

Not Yet

Member
Joined
Nov 3, 2004
Messages
456
Location
Haines City Fl
Total Flight Time
5
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

Member
Joined
Feb 14, 2008
Messages
840
Location
Colorado
Aircraft
Enstrom 480B, CTLS, Ercoupe
Total Flight Time
5000+
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

Gold Supporter
Joined
Jul 2, 2007
Messages
7,356
Location
London/ Kilifi Kenya
Aircraft
Gyrs, RAF 2000/Mgni/Bnsn/Hrnet/Mrlin/Crckt/MT-03/Lyzlle AV18-A/Prdtor. Pax ArrowCopter
Total Flight Time
100+ gyro, 16,000+ other
Nice one Tom.

Very interesting thread.
 

gyrogreg

Senior Member
Joined
Dec 9, 2003
Messages
999
Location
Ste. Genevieve, Missouri, USA
Aircraft
Magni M-16 Gyroplane
Total Flight Time
3000 total, 2000 Gyro
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
 
Last edited:

ckurz7000

Senior Member
Joined
Nov 7, 2006
Messages
3,438
Location
Vienna
Aircraft
ArrowCopter
: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

Senior Member
Joined
Dec 9, 2003
Messages
999
Location
Ste. Genevieve, Missouri, USA
Aircraft
Magni M-16 Gyroplane
Total Flight Time
3000 total, 2000 Gyro
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

Member
Joined
Feb 14, 2008
Messages
840
Location
Colorado
Aircraft
Enstrom 480B, CTLS, Ercoupe
Total Flight Time
5000+
Basics Please

Basics Please

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

Gold Supporter
Joined
Apr 21, 2008
Messages
14,625
Location
San Diego, CA. USA
Aircraft
Piper Archer, Aviomania G1sb
Total Flight Time
Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
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

Senior Member
Joined
Dec 9, 2003
Messages
999
Location
Ste. Genevieve, Missouri, USA
Aircraft
Magni M-16 Gyroplane
Total Flight Time
3000 total, 2000 Gyro
Tom, please refer to Glossary of Gyro Terms 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
 
Last edited:

carlschneider

Member
Joined
Nov 1, 2003
Messages
99
Location
fort madison, Iowa
Aircraft
Carlanater
Total Flight Time
2000+
Ltl

Ltl

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

Gold Supporter
Joined
Apr 16, 2004
Messages
9,673
Location
Florida
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

Member
Joined
Nov 1, 2003
Messages
99
Location
fort madison, Iowa
Aircraft
Carlanater
Total Flight Time
2000+
Ltl

Ltl

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

Gold Supporter
Joined
Apr 16, 2004
Messages
9,673
Location
Florida
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

Senior Member
Joined
Mar 1, 2004
Messages
401
Location
Cave Creek, AZ (Phoenix area)
Aircraft
none presently
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

Member
Joined
Dec 16, 2005
Messages
128
Location
Springfield, MO
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.
 

Attachments

Redbaron

Banned
Joined
Apr 20, 2009
Messages
2,679
Location
earth
Aircraft
bee
good riddance, shouldn't the machine being extremely simple to fly be a summory of its stability! no comment!
 

Doug Riley

Platinum Member
Joined
Jan 11, 2004
Messages
6,323
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.
 
Top