Gyroplane Thrustlines vs. Center of Gravity

[Doug Riley]

Howard, "vertical CG" is a precise concept, not ambiguous at all.

OTOH, the EXPRESSION "vertical CG" is a shorthand term that is nonsensical if you read it literally. An object has only one CG, not a vertical one and then some other kind.

What we mean by "vertical CG," of course, is the Y-axis location of the (one and only) CG. As you observe, the CG will be located somewhere along a plumb line hung from the point of suspension during a hang test. And, yes, the CG will move about as a result of different loading conditions and fuel burn.

The condition we want to avoid is one in which the rotor's thrust holds the nose up against a tendency of engine thrust to push the nose down. This condition makes the gyro statically unstable with respect to rotor angle of attack, power-on. PPO is the extreme result of this particular type of instability.

[fara]

I am not sure why aerodynamically this tendency would "definitely result" in a PPO.
"The condition we want to avoid is one in which the rotor's thrust holds the nose up against a tendency of engine thrust to push the nose down."

How does an airplane with high mounted engines like a Searay etc. not get into a PPO when it gets into negative G's?

Obviously there is more to this than described here?

[Jean Claude]

Originally Posted by fara

How does an airplane with high mounted engines like a Searay etc. not get into a PPO when it gets into negative G's? Obviously there is more to this than described here?

It is only in the absence of aerodynamic control

[gyrogreg]

There is much more to this than just the simple popular STATIC concepts to attempt to determine the capability or likelihood of a buntover (or misnamed PPO). "PPO" is a common but misleading and inadequate description of what theoretically happens anyway if only STATIC concepts are applied. Jean's description is more technically accurate for the STATIC analysis - not a "POWER push over" - simply a loss of rotor drag allowing the Prop thrust acting above the "vertical CG" to "push" the gyro over.

If you are going to limit your concepts to just the incomplete STATIC concept of "vertical CG" and HTL, you are missing much of the STATIC analysis anyway. In a STATIC analysis only, to Doug's point, anything that causes the "CG" (vertical and longitudinal) to be positioned in flight aft of the Rotor Thrustline (Rotor Thrust Vector - RTV), would suggest STATIC instability. But, even this STATIC only analysis ignores other in-flight conditions, other than HTL, that can cause the CG to be aft of the Rotor Thrustline. These other in-flight conditions include low airframe DRAG lines and sloping windscreens that also cause the nose to tilt down and the CG to move aft - less STATICALY stable and possibly even STATICALY unstable - at higher airspeeds. I propose that many gyros, incompletely judged by their CLT or LTL only, are actually flying closer to the STATIC instability point, or are possibly even statically unstable, especially at higher airspeeds, and especially if the prop thrustline is weak such as at low or no power. This STATIC analysis only requires consideration of all the factors that can cause the nose to fly lower in some flight condition – such as all the drag issues as well as prop thrustline. And realize that if you are depending on the prop thrustline to hold the nose up against all the other nose-down factors in flight, what happens when power is off and the prop is no longer even helping to hold the nose up?

In the world of STATIC analysis only, a "BUNTOVER" happens because the total LIFT of the rotor is acting ahead of the "CG" - as Jean's depiction illustrates. Prop thrustline is just one STATIC factor to consider. If you are going to rely on STATIC factors only, you must consider all static factors that, in combination, at various power and airspeed conditions, might set up the STATIC condition that Jean depicts.

However, the DYNAMIC characteristics are an even more essential element of overall protection from buntovers or even "PPO". As Fara suggests, there must be something this STATIC analysis is missing! Those missing element are the DYNAMIC characteristics – that are indeed very complicated and difficult to relate to intuitively. A buntover and even the tendency for PIO are DYNAMIC characteristic that are also importantly related to the strong or weak DYNAMIC parameters of an aircraft design. The most popular gyro configuration element to provide strong DYNAMIC pitch damping are LARGE tails on LONG tails. When pitching rates are properly DYNAMICALLY "damped", the simple static concept of a buntover or "PPO" is wholly inadequate to predict buntover or PIO tendencies. This is because, the rates at which the aircraft can pitch or accelerate in pitch are important factors into whether a buntover is inherently self-propagating or self-damping by the aircraft itself.

AeroDYNAMICS are a difficult subject, to intuit, and are just as difficult to analyze “on paper” or from a simple observation of the gyro’s thrustline configuration. But, they are hugely important in the final flight characteristics of any aircraft. Big and long tails are not the only pitch damping elements or factors in a gyroplane. Other factors include the moment of inertia of the airframe, the weight (inertia) and DYNAMIC damping characteristics of the rotor, and especially how the DYNAMIC characteristics of the rotor and the airframe interact to reverse or improve or aggravate undesirable flight characteristics. (Think of tuning forks that match harmoniously or create disturbing “beat” frequencies!) Especially analyzing two dynamic systems that interact and affect each other requires more than simple STATIC depictions of a PPO. The DYNAMIC characteristics of any aircraft can enhance, negate, or even reverse the flight characteristics that simple STATIC only (prop and rotor thrustlines and CG position) analysis attempts to predict.

The proof of this (inadequacy and misleading STATIC “paper” analysis only), in my humble opinion, is the complete lack of PPO and PIO reported incidents in the highly numerous flight hours flown in such gyroplanes as the Xenon, Magni, Auto Gyro, ELA, etc. Except for arguably the Xenon, these aircraft ARE demonstrably very HTL and actually do fly very STATICALLY stable with even the CG positioned in-flight behind the RTV. What they have in common are large tails on long tail booms. Where are the "smoking holes" if STATIC concepts alone are the sole determinates of static instability and tendencies for PIO and buntovers ("PPOs")?

[shootthrees]

Originally Posted by Jean Claude

It is only in the absence of aerodynamic control

Excellent graphic illustration of this entire sticky started by Chuck Beaty. The CLT thrust design, in my opinion, should be the only option in the Gyro design and build.

I owned and flew a Quicksilver MXL ultralight FW with Rotax 503 hanging down, thereby putting the thrust closer to the center-line, but, it still had a tendency to want to nose down at lower air speed when the throttle was increased. I later owned and flew an Aerolite 103 FW with a top mount 503 and a definite high thrust line and an even greater tendency to nose over with an increase in throttle or thrust (T).

The FW elevator is used to compensate for the PPO and continues to have relative wind (Vo) from the prop, unlike the rotor that must have relative wind (Vo) as illustrated above to continue to provide lift (F) and control.

How can anyone argue against CLT design and continue to promote a Gyro that is not?

[Alan_Cheatham]

The problem is because gyroplanes can fly at little to no airspeed you can have little to no DYNAMIC stability but very high STATIC instability.

.

[Vance]

I don’t know what STATIC means when it is all in capitals.

According to the Encarta Dictionary:

Static: related to forces, weight or pressures that act without causing movement.

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

What is the purpose of redefining these terms?

In my opinion a static analysis is a way to determine the forces that are trying to move something.

I was taught that to achieve dynamic stability; analyze the static forces and find out how they balance without creating an undesirable event.

I was taught that this is done in all of the positions that the object in question might achieve so that you can understand dynamic trends.

I have no formal mechanical engineering training so perhaps I am operating on too low a level to understand what Greg is writing about.

I have had the experience of working with a room full of mechanical engineers to achieve dynamic stability and they were always careful to see that everyone understood the definition of the terms they were using.

I feel that this is an important subject to understand because people pay for their ignorance with their lives.

I am off to fly my statically designed stable gyroplane.

Thank you, Vance

[fara]

Originally Posted by Jean Claude

It is only in the absence of aerodynamic control

I am sorry that's just way too simple an analysis which is ok for a pilot's handbook but that is not all that's going on in any aircraft when you are engineering a system whose interactions with fluid and systems unit systems are dynamic

[fara]

Hi GyroGreg,
I believe even your explanation point regarding large Horizontal stab surfaces further back of correct shape and in flight angle creating some kind of dampening to prevent a PPO etc. is inadequate because the UK CAA study read thoroughly suggests that the Air Command and other gyros they tested with horizontal stab configurations of different sizes did not in fact increase safety regarding PPO but it did help handling and pilot workload (a very well desired characteristic by itself).

Regarding the rotor lift/thrust line being ahead of pr behind the CG in various formats of flight and attitude is an interesting one. The aircraft should always be seeking trim so a balancing dynamic moment can be established using the rotor lift vector and CG trying to keep the aircraft wanting to achieve trim under positive loads. That works. But when the load is gone (zero G), the aerodynamic influence of the whole body (not just the tail section) has to start to correct the situation to get back positive G and thus the proper control. No? This is the reason the UK CAA U of Glascow study suggests that some other gyros even with high thrust lines relative to CG, showed indeed positive pitch stability even near zero G. So its got to be a sum total of factors. Perhaps that's what you suggested

[thomasant]

Originally Posted by fara

I am not sure why aerodynamically this tendency would "definitely result" in a PPO.
"The condition we want to avoid is one in which the rotor's thrust holds the nose up against a tendency of engine thrust to push the nose down."

How does an airplane with high mounted engines like a Searay etc. not get into a PPO when it gets into negative G's?

Obviously there is more to this than described here?

I have only a very basic understanding of physics and so this is how I understand this concept:

I believe that in the case quoted above, even if the initial tendency in an aircraft like the Searay in a negative G condition will be to nose over, the wings will again begin producing lift with the change in angle of attack if there is sufficient altitude to recover provided the wings do not detach. This may be similar to why a stalled airplane would also recover with sufficient altitude, as the wings will again generate lift, thereby restoring equilibrium of the different forces.

In the case of the gyroplane, in a negative G condition, all lift is irrecoverably lost due to the rapid slowing of the rotors, and there is no more equilibrium of forces that were originally in balance during forward flight. Now the turning/rotational moment about the CG due the prop in a HTL machine without a stabilizer will cause the gyroplane to turn downwards about the CG, and the ensuing rotor flap cause destruction of the rotors and tail, etc. I believe that the force generated by a horizontal stabilizer is significant at increased speeds to exert a fairly large downward force on the tail to oppose the force of rotation caused due to the HTL. At low speeds, there will not be much force generated to counter this, unless located in the prop wash. I have not considered the other aspects like the translational movement of the CG, different AUW, etc. in the above situation.

I find it helpful to understand some of these concepts by reviewing the topics on balancing moments of forces about points. As I stated before, my understanding of these concepts may be incorrect or inadequate and I will appreciate it if anyone can point me in the right direction.

[gyrogreg]

Originally Posted by fara

Hi GyroGreg,
I believe even your explanation point regarding large Horizontal stab surfaces further back of correct shape and in flight angle creating some kind of dampening to prevent a PPO etc. is inadequate

Hi Fara. It is often confusing to separate the STATIC forces and moments from the DYNAMIC forces and moments and elements. DYNAMIC forces and moments, and resulting damping of motion only occurs when the system is in motion, not when it is in steady state balance where everyone considers the balance of the static moments and forces on the aircraft. The steady state flight attitude of the HS does create balancing static moments and forces due to its mounting angle, shape, etc. But, the DYNAMIC damping only occurs when the system is moving, such as pitching nose-down. When the gyro is in motion pitching (nose down for instance), the rising HS creates a new relative wind AOA on the HS. That motion creates a new lift or force on the HS, a force in the opposite direction to the movement of the HS, proportional to how fast the HS is rising. In other words, when the HS is rising, that DYNAMIC motion creates a force in the opposite direction against that movement – a damping force that only the motion creates. That’s what DYNAMIC DAMPING does, opposes, slows and eventually stops a motion. That resistance to motion ONLY occurs when the HS is in motion! So, when the nose starts to drop, the HS starts to rise. The relative wind AOA is now angled so as to increase the downward force on the HS and oppose the tail rising and the nose is dropping. Again, this DYNAMIC damping ONLY occurs WHEN the aircraft is pitching, the tail is rising.

(Note that the tail rises faster when it is on a longer arm. Also note that the resulting new dynamic force acts over the leverage of the longer tail arm. Therefore the DYNAMIC damping function is a product of the length of the tail arm SQUARED! But, the static moment from the HS is only the factor of the tail arm, not the tail arm squared! This is the reason for the longer tail on the more stable gyros – or on any aircraft! The longer the tail, the more impact dynamic damping has to improve the handling characteristics and widen the power/speed envelope for buntover resistance.)

Originally Posted by fara

because the UK CAA study read thoroughly suggests that the Air Command and other gyros they tested with horizontal stab configurations of different sizes did not in fact increase safety regarding PPO but it did help handling and pilot workload (a very well desired characteristic by itself).

A HS will always provide some airframe pitch dynamic damping. The larger the HS, the more damping. The better airfoil shape (better coefficient of lift), the better the damping (The AC HS is a flat plate and about half the CoL of an efficient airfoil. Another factor is that a flat plate will stall at a lower AOA than an efficient airfoil – about half the AOA - at some pitching rate it simply stalls before a good airfoil would) The longer the tail, the better the dynamic damping – by a square factor! There is a tipping point where the DYNAMIC damping provided by the HS finally overwhelms the purely static elements to provide actual STATIC stability characteristics in spite of HTL or other statically destabilizing factors. For the AC, the improved dynamic pitch damping will improve handling and increase stability margins and widen the power/airspeed statically stable envelope. But, there is probably some airspeed and power combination for the AC, beyond which, that HS is no longer able to maintain adequate pitch damping to inherently prevent a self-sustaining and propagating PPO. And, if the pitching rate is excessive, the flat plate HS will stall - with a sudden decrease in its static and dynamic functions – at that stalled point, teh flat plate ahs little effect to stop the buntover..

Originally Posted by fara

The aircraft should always be seeking trim so a balancing dynamic moment can be established using the rotor lift vector and CG trying to keep the aircraft wanting to achieve trim under positive loads.

Fara, this is where you may be mixing static and dynamic functions. A dynamic moment is not trying to balance the gyro statically to trim. The trim condition is set solely by the static moments acting on the airframe. The dynamic damping moments only occur and only act to oppose movement.

Originally Posted by fara

That works. But when the load is gone (zero G), the aerodynamic influence of the whole body (not just the tail section) has to start to correct the situation to get back positive G and thus the proper control. No?

Yes, sort of! When the static moments are unbalanced, such as with a decrease in rotor lift (it doesn’t require total rotor lift loss to progress a buntover if there is inadequate dynamic pitch damping), the unbalanced static moments should start it moving back to the static trimmed condition – flight AOA, G-load, airspeed. That is unless the RATE of pitching, the DYNAMIC rate of pitching, is not slowed enough by DYNAMIC damping to prevent the increasing nose-down rate from continuing to increase the unbalanced buntover moments. What an adequately dynamically damping HS does is prevent that G-load from getting so low so fast that the buntover is self-sustaining.

The highly damped airframe from an adequately dynamically damping HS will also respond to a wind vertical wind gust very quickly -think feathers on a long arrow – so as to also provide rotor cyclic input to quickly counter that wind gust. From a purely STATIC analysis, you can visualize the loss of rotor lift and the resultant unbalanced static moments that cause the gyro to buntover. But, the missing element is the DYNAMIC forces that prevent the gyro from reaching that rate of pitching tipping point in the first place. The strongly dynamically damped gyro’s airframe and rotor will quickly restore normal rotor AOA and lift from a wind gust, and it will also create pilot cyclic stick forces that deter the pilot from making overly aggressive commanded inputs.

Originally Posted by fara

This is the reason the UK CAA U of Glascow study suggests that some other gyros even with high thrust lines relative to CG, showed indeed positive pitch stability even near zero G. So its got to be a sum total of factors. Perhaps that's what you suggested

EXACTLY, Fara! Some gyros, and a lot of gyros now, with HTL still demonstrate strong static stability margins. These are the gyros that have strong pitch dynamic damping – such as the Magni, ELA and Auto Gyros. Some of these have very high HTL, but still flight test to be statically stable at all conditions within their published flight power/speed envelopes. The Dynamic Pitch Damping of the large and long and efficient airfoil HS extend the speed/power envelope where they will inherently return to the STATIC trimmed condition when disturbed – this is the definition of static stability, but it is provided and enhanced by the dynamic damping that prevents it from pitching so quickly that it continues to buntover – statically unstable.. It is the sum of all factors, static and dynamic – not just the static moments – that are the whole story. Some say it is a “harmony” of all factors. “Harmony” is a dynamic term. “AeroDYNAMICS” uses the word “DYNAMICS” for a reason. DYNAMICS is not as easy to understand as static moments, but it can’t be disregarded!

Thanks, Greg

[Jean Claude]

All this is true. We must take into account damping, and establish formulas to predict it. For some characteristics, it is quite easy (pitch inertia, lever arm and tail area, HTL, etc.). But the damping produced by the rotor is certainly fundamental and most difficult to find (torsional elasticity, blades balance on the chord, Cm, rotational inertia)

[Doug Riley]

My experience with small gyros (which goes back to the Bensen era of high-RPM, lightweight rotors as well as today's heavier, slower rotors) leads me to prefer designing around a rotor that has only moderate damping. Using such a rotor, rotor damping can effectively be ignored in the design effort.

Instead, I think it is sufficient to view rotor damping as merely a bonus (as "icing on the cake").

Therefore, in my opinion, an airframe should be designed and tested for static pitch stability as if it had zero rotor damping. That is, test the engine-frame unit with a simple mass in place of the rotor. If the sum of static moments about the pitch axis at all airspeeds and power settings is not zero (or some small number that reverses very quickly as the frame reacts), then modify the tail surfaces, body shape or propulsive thrustline location until you reach this goal.

It's possible to create a stable airframe that depends to a large extent on rotor damping. Again in my experience, though, this approach tends to eat into the control lightness that most pilots prefer. In some cases and with some pilots, building in a relatively large amount of rotor damping results in a rotor that lags large control inputs -- enough to produce frequent instances of "hammering" of the teeter stops during maneuvering.

None of this says anything about oscillatory behavior (dynamic stability). Once again in my experience, a gyroplane with adequate horizontal surfaces to produce static stability without the rotor will be dynamically stable in pitch.

(It would probably require wings to produce an equivalent dynamic stability in roll).

[C. Beaty]

Not only should static moments about the pitch axis be considered but also roll axis moments:

The Magni M-24 that splattered in the UK last year succumbed to torque roll, the AAIB postulates.

http://www.aaib.gov.uk/cms_resources...TI%2010-11.pdf

The door wasn’t properly latched and flew open on takeoff. The pilot grabbed and held it shut with one hand while flying with the other.

Setting up for a landing and having only two hands, the pilot turned the stick loose to shut the throttle. Apparently trimmed for cruise power (and torque), the machine flipped over on its side and splattered with propeller torque removed.

There was a similar accident in the US a few years back to a similarly configured RAF-2000. At the top of a zoom climb, the machine rolled over sideways.

Cierva Autogiros usually compensated for propeller torque by differential setting of tailplane incidence. Horizontal tail surfaces were centered in the propeller slipstream.

Pitcarin tried vanes immediately aft of the propeller (catfish whiskers) on the AC-35 as well as a contrarotating propeller but settled on differential tailplane incidence as the most practical solution.

Ron Herron has mentioned setting the main wheels of his LW series on bathroom scales and adjusting differential tailplane incidence for equal readings.

Of course, with tail located outside of the propeller slipstream, no such aerodynamic compensation is possible. Offsetting the rotorhead laterally to make the airframe fly level at cruise power is only cosmetic.

[gyrogreg]

Originally Posted by Doug Riley

Therefore, in my opinion, an airframe should be designed and tested for static pitch stability as if it had zero rotor damping. That is, test the engine-frame unit with a simple mass in place of the rotor. If the sum of static moments about the pitch axis at all airspeeds and power settings is not zero (or some small number that reverses very quickly as the frame reacts), then modify the tail surfaces, body shape or propulsive thrustline location until you reach this goal.

------------

Doug, I love ya Buddy. You've done a lot to improve the safety and understanding in our sport. But I guess we will never agree that there are more things to consider than just STATIC balances, sum of static moments! You are implying the only way to achieve static stability is to have a neutral or positive sum of STATIC moments (that maintain the CG forward of the RTV in flight.) I used to believe that also, until I flight tested machines with high degrees of Pitch DYNAMIC damping that otherwise are certainly not balanced static moments. Yes, I include the HTL Magni gyros. By your descriptions, such machines will always display negative static stability. But, in extensive flight testing of such gyros, not just by me but by professional CAA test pilots, these certainly HTL gyroplanes display not only static AOA, G-Load and Airspeed static stability, but they also demonstrate Dynamic pitch stability. That means they display oscillatory responses to disturbance that are quickly damped to zero oscillation and back to static trimmed condition. I know you know that a system that oscillates around its steady state condition MUST first be statically stable. But, since such machines don’t meet your criteria for static stability per your static-only analysis, there must be something more – as Fara suggested to re-activate this thread.

This does go to support your statement that gyros must be tested for static pitch stability – which in cases of gyros with strong airframe dynamic pitch damping, will confirm that even those HTLs are still statically pitch stable. So, with flight testing, you will get to the same place – in-flight static stability, but not always because you have a balance sum of static moments. If you provide adequate pitch dynamic damping, you will get to static stability, even if you don’t have a balanced sum of static moments. I continue to maintain that the only way you can know for sure if a gyro is statically pitch stable and therefore not prone to buntovers, is to flight test it. The simplest flight test is to simply see if that gyro - at all combinations of power and airspeed – displays an oscillatory reaction to a disturbance. Even if that oscillation were not damped, but continued on ad-infinitum, that gyro would still be statically pitch stable, it is oscillating about its static trimmed condition. [But, a highly dynamically pitch damped gyroplane, will also inherently quickly damp such oscillations (dynamic stability) – also to benefit PIO prevention.)

Originally Posted by Doug Riley

None of this says anything about oscillatory behavior (dynamic stability). Once again in my experience, a gyroplane with adequate horizontal surfaces to produce static stability without the rotor will be dynamically stable in pitch.

Duh! Yes, any statically stable system will oscillate – unless critically dynamically damped. And, conversely, any system that displays oscillations MUST first BE statically stable. Again Doug, I guess we will never agree on this. But the term “DYNAMIC”, as used in “AeroDYNAMIC”, refers to more than just the oscillatory behavior of “dynamic stability” – damped oscillations. “Dynamic” refers to any motion or acceleration, whether oscillatory or not. The elephant in the room, that few, at least few in America, seem to want to consider, is “dynamic damping”, or “pitch dynamic damping”. Pitch dynamic damping not only serves to dampen oscillatory (statically stable) systems, but it can also serve to dampen pitching rates of statically unstable systems. In the case of gyros, adequate airframe dynamic pitch damping can temper or damp the pitching rate to the point where the pitching rate is not adequate to sustain a divergent buntover. In other words, such a gyro inherently can tend to return to its static steady state condition, even though a strictly static analysis of airframe moments suggests it should not.

I’ll mention a little poorly kept secret – again - at the risk of CB finding another way to attack Magni: The M24 has about a 13 inch HTL! But, flight testing by many, including professional CAA test pilots and engineers in Britain determined that that gyroplane is statically and dynamically pitch stable and therefore is not bound by that insane Glasgow prescription of thrustline within 2 inches of the CG! That requirement for a 2” thrustline in the British BCAR Section T standard has been waved for two Magnis and for two Auto Gyros – since all far exceed the 2” criteria, but still demonstrate high static pitch stability margins as the real and critical criteria to avoid buntovers. The reason is the high degree of dynamic damping – from both the rotor inertia, and especially from the highly dynamically pitch damped airframe. (Note this is true for both the heavy rotor Magnis and the light rotor AGs. So, I suggest, not all of the dynamic damping comes from the rotor – but it sure is part of the total pitch damping in the machine – just not always the whole answer without airframe damping.)

Originally Posted by Doug Riley

(It would probably require wings to produce an equivalent dynamic stability in roll).

Finally something we can agree on – gyros are not spiral (roll) stable – stable in roll. They are negatively statically stable in roll – but that has not been a big challenge for pilots to master because the roll rates, at least initially in a spiral, are not excessive and well within the ability of the pilot to balance with little effort. But notice, the lateral MOI of most gyros, especially tandem or single seat, is very low – as compared especially to the longitudinal MOI. But, roll static instability, without pilot roll input, does not create excessive roll rates. That is probably due to the dynamic damping provided by the rotor alone (As you point out, there is no roll stabilizer such as wings). In my experience, the lighter rotor systems are less spirally stable than heavy, highly dynamically damping rotors. If not for the dynamic roll damping of the rotor, by your static only analysis, you might expect, in this low MOI direction, quicker, more highly divergent roll rates that would require constant pilot effort to prevent a “roll-over”. (I’m not talking about “torque over” here, that is a different subject.)

Provided in the interest of improved gyroplane safety in the U.S. – Greg Gremminger