About control and stability

ferranrosello

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I’ve read a lot of threads about gyro’s stability. Some of them are really good, but it is very difficult to explain gyro’s stability and control by moments acting on cg.

I’m a pilot, not an engineer, and I’m more interested in understanding physics of flight than in accurate calculations involving cg’s moments and arms. I think there is an easier and more understandable explanation of gyro’s stability and control. But, it is needed a whole picture vision of gyro’s flight. Please, don’t focus just on cg alone.

Control

We all are used to study gyro’s control by moments acting in the three axis (pitch, roll and yaw). This is no bad, but it is FW culture, and gyro’s control does not have the pitch and roll axis so neatly independent as planes do.

The way in which a rotary wing aircraft is controlled in pitch and roll is by changing rotor’s aerodynamic force direction (by moving the rotor disc). The fuselage, which is hanging from rotor head by a U joint, will follow the rotor’s new direction until it aligns the resultant vector acting on cg (weight plus fuselage drag and power thrust).

In this way it is easy to understand the control mechanics. When you tilt the rotor disc you are tilting the aerodynamic force vector. The fuselage will react aligning the weight plus thrust and drag vector. In simple terms, it will try to maintain the cg aligned with the rotor thrust vector.

Stability

What will happen if a turbulence creates a sudden change in the rotor aerodynamic force?
This is the main question when talking about stability (but not the only one). The good reaction happens when an increase in lift produces a reduction in the rotor disc AOA (this is call statically stable).

A helicopter is not statically stable. After a change in relative speed, the aerodynamic force vector will change and an acceleration will occur. The rotor will move in the space and nothing is going to reset the original trimmed condition (unless you have a wonderful AFCS fitted).

But it doesn’t mean that the helicopter is unstable. But, what happens to an isolated fixed wing?. When the wing’s AOA is increased (because of a wind gust), the wing will suffer a positive pitching moment. So it is not possible to fly a plane without an appropriate horizontal tail surface.

And an autogyro? This is a different story.
Because of the gimbals between pitch and rotor axis, the gyro’s rotor head is positively stable versus AOA. That means that the autogyro’s rotor is statically stable. But it does not mean that the whole aircraft will be stable in all conditions.

The cg position versus rotor thrust is not important for gyros stability. Cg will tend to be aligned with rotor thrust. But cg position is important in order to ensure full control capability. Longitudinal extreme cg locations may reduce control authority, but not stability.

The real problem in a teetering rotor is flying in sustained (several seconds) low g environment. Then the rotor aerodynamic force vector is not there any more. And without this vector the cg has no reason to keep aligned. And then is when the high thrust line and fuselage drag becomes alive.

If this is the case, the only way to stop a sinking nose is an appropriate horizontal surface. Two additional benefits are an improvement in pitch stability and a reduction in the time lag between control inputs and fuselage attitude. Makes flying an autogyro easier and safer.

I hope this lines can help in understanding gyro’s control and stability.

Regards, Ferran.
 

eaglecarob

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I also see that the "horizontal stabilizer" is only a fixed plane set "some distance" behind the engine at "some distance" within the slip stream of the propeller. I've been wondering about an adjustable airfoil there instead of just a flat piece of plywood. Perhaps an all flying tail or even an articulated elevator surface that could be set with a trim wheel. Would this help stability or complicate the control system unwarrantly?
My Benson type has only a plywood slap on the tail along with the "stone shield" under the prop. They really seem too flimsy to have any positive effect. I will be changing it before I fly this machine. And I believe the airfoil shape should be more effective, and therefore more stable.
 

vista

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Two articles that were in the Popular Rotorcraft Association magazine several years ago are memorable because of their theory simplicity.

One compared gyroplane roll and pitch cause and effect to weight shifting in a hang glider.

The other illustrated "center of pressure" (wind vane effect) by simply finding the balance point of a small-scale gyroplane side profile paper cutout (with stacking in the case of dual vertical stabilizers).
 

RockyMeLad

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Over-simplified Stability Argument, Again.

Over-simplified Stability Argument, Again.

Ferran,

You jumped to the same wrong conclusion that I did some time back. As a prior "fixed wing" pilot and not an aviation engineer, I jumped on the same bandwagon. But I am a physicist, so subsequently acquiring an understanding of the aviation specific characteristics was fairly easy for me (I know there are a lot of folks that still can't see the forest for the trees).

Unfortunately it's an over-simplified (and incorrect) assumption that the rotor alone is stable and the airframe just follows along for the ride. The entire flying "body" has to be taken into account. Most of the guys describing "stability" using true engineering methods assume that everyone knows this, but they don't. The engineer types talk about variations from the CoM, CoD, RTL, etc. expecting everyone to "see" the logic of their presentations. The idea that the airframe "hangs" under the rotor sounds reasonable and can be delivered in what appears to be a logical presentation. The flat earth concept was accepted as logical for hundreds of years, even though there were observable facts that didn't fit the accepted theory. There are others on the forum that can delineate the fallacies to the "hanging" argument better than I, but I accept the engineering rational. There are no valid engineering arguments against the "whole flying body" view, only the mantra preached by the those who "need" to perpetrate miss-information for their own purposes.

So yes, you do have to account for ALL the moments/arms that are involved in the "flying body". This is true for ALL aircraft, fixed wings, helicopters, gyros, even PPGs, balloons, and blimps. Thinking that gyros are "different" is simply an excuse for lack of aeronautical engineering knowledge.

Sorry to have stepped on you, but it is important to ensure that other "newbies" don't make the same mistakes. The "stable rotor" idea sounds good, but due to it's inaccuracies can and has killed people.
 

XXavier

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As a newbie, I stay tuned... I'm eager to learn about the right way to think about this matter, that should be already clear by these days... Or perhaps not... After all, there are still conflicting views on the real cause of lift...

Rgds

XXavier
 

XXavier

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Hi all...

Reflecting on the above, I wonder if a flexwing and a gyro are really that different. Aside, of course, of the gyroscopic and aerodynamic peculiarities of a rotor, aren't those aircraft very similar...? In both cases, what I see is a powered nacelle hanging -gimbaled- from a 'lifting entity'...

XXavier
 

Udi

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CG vs. RTV and Stability

CG vs. RTV and Stability

There is one basic concept that is critical for understanding gyroplane stability. An object that is free to rotate in space will rotate unless all of the forces acting about this object are passing right through it's center of gravity, or when these forces balance each other.

A gyro in flight is such an object. When the gyro flies in a trimmed condition - i.e. it is not pitching, rolling, or yawing, all the moments acting about the gyro are balanced. For example - install a flap to create drag on the left side of the cabin and the gyro will want to yaw to the left. The pilot may hold right pedal to deflect the rudder to the right, and hence create an equal and opposite MOMENT to cancel the yaw moment created by the flap.

A moment is defined as force multiplied by arm. But what is the arm? To understand the flap and rudder moments we must understand what these forces are acting about. Since the gyro is free to rotate in space, these forces are acting about the CG - the gyro center of gravity, also known as the center of mass.

So, if the flap sticking to the left is located 1 ft from the CG and the rudder is located 4 ft from the CG, and the moment produced by the rudder is equal to the moment produced by the flap, that means the force produced by the flap is 4 times greater than the opposing force produced by the rudder.

When we discuss gyro pitch stability, we must to take into account all of the moments acting about the gyro CG in the pitch axis. These are mainly the rotor, the engine, and all of the aerodynamic forces (lift and drag) acting on the airframe (including a stab, if there is one). ALL of these forces must be balanced if the gyro is in trim - i.e. in a balanced attitude.

Lets take a simple HTL gyro as an example. This gyro has an engine thrust line that is passing 1 ft above the gyro CG. The gyro has no stab, and let's assume the center of drag is passing right thru the CG and there are no other aerodynamic moments acting on this gyro in pitch. If we assume this gyro has an engine that is producing 500 lbs of thrust, the nose-down pitching moment of the engine acting about the CG is 500 x 1 ft = 500 ft-lb. If there were no other moments available to balance this nose-down pitching moment, the gyro would start spinning forward.

Luckily, there is the rotor thrust vector to balance the nose-down pitching moment of the engine. If the rotor thrust vector (RTV) equals 1000 lbs, it now must pass 6 inches, or half a foot, forward of the CG in order to balance the engine nose down pitching moment. How does the RTV finds itself exactly 6 inches ahead of the CG? Simple - the pilot is pulling back on the stick, tilting the rotor back, moving the RTV forward, until the gyro stops pitching down.

Back to stability - now imagine this gyro flying happily with the RTV 6" ahead of the CG, keeping the engine nose-down pitching moment in check. Suddenly this gyro hits a gust that is causing the rotor to create more lift, and the RTV force increases from 1000 lbs to 1200 lbs. Now the gyro is no longer in equilibrium - the RTV is producing a larger nose-up pitching moment than the engine nose-down pitching moment, so the gyro starts pitching up.

As the gyro is pitching up, the angle of attach of the rotor increases, and the rotor is creating even more lift and the RTV force increases to 1300 lbs, which is causing an even larger nose-up pitching moment. This is an unstable response to angle of attack. This gyro is AOA unstable.

Now think about the opposite scenario. This same gyro is flying happily in trim when it is hit by a strong down-draft. Due to the down-draft the rotor lift is reduced and the RTV force goes down from 1000 lbs to 900 lbs. Suddenly the RTV moment is smaller than the engine moment and the gyro starts to pitch nose-down. As the gyro is pitching nose down, the rotor AOA is reduced and the rotor is producing even less lift, which creates a smaller RTV moment and an even stronger nose-down pitching moment. This is the start of a PPO. Power-Push-Over. The engine is pushing the gyro over, and the gyro AOA instability is assisting, instead of resisting, the progress of the PPO.

So, Ferran, the location of the RTV vs. the CG is indeed important for achieving AOA stability. Having said that, it is possible to improve the stability of the gyro with the aid of a large stab - but remember that stabs are not very effective at low airspeeds. HTL gyros with stabs are mostly susceptible to PPO during low-speed high power climb, when the nose-down pitching moment of the engine is greatest and the effectiveness of the stab is lowest.

A safe gyro must be designed such that when the RTV is reduced during flight, as a result of a down-draft or of pilot maneuvering, the balance of moments acting about the gyro CG will be net nose-up - the correct direction required to restore rotor thrust and control. Any gyro configuration that achieves this goal will be a stable and safe configuration.

Udi
 

C. Beaty

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The physics of spinning yo-yos and tumbling gyros is exactly the same; an unbalanced moment about the CG (more precisely, center of mass).
 

dragonflyerthom

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Chuck B The yo yo is quite the example. Good one



XX


Everything that Udi has said has been stated before. If you will do a search you will find post by Chuck B, Rahgu, Doug Riley, Udi, Chuter, and many others. Udi has explained the entire concept very well. Altho it was using one of my favorite gyros to explain it. The problem that has pleagued the gyro in the last 50 years is most designs leave out one or more of the moments that have to balance to have a stable gyro. The whole gyro concept isn't new but has been identified so there isn't any reason to ignore this any more. It is only in the last few years that the fuselage has been addressed to keep the entire aircraft stable. A lot of the things that have been used as a fix on the gyro doesn't include this entire gyro concept. The stabilator address only the rotor. It tries to do this by trying to control the AOA of the rotor but it misses the RTV of the rotor by not addressing the high thrust of the power plant. As Udi so eloquently explained until the thrust of the engine is closer to the CoM the airframe will continue to be unstable and any disturbance in the line of flight will effect its stability. This divergence in stabality is what can cause either PIO or PPO of the craft.
Now the Stab is used to control the attitude of the fuselage inspite of the rotor thruse vector and the engine thrust. It has to compensate for the unequal moments about the CG. Most of the time it works but when it doesn't then we read or hear about it here on the forum.
Now with all of this knowledge there really isn't any reason for there to be anymore poorly designed gyros. The fix to the poorly designed gyro is available for just a little time and money or modifications can be quite expensive but the point is that it is available.

I hope this helps in your quest for knowledge and flight.

Good post Udi.
 
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Doug Riley

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Ferran has it mostly correct. However, as Udi points out, you cannot accurately describe "trimmed" or "equilibrium" flight without talking about moments... whether or not you actually use the word "moment." Forces have lever arms, and you must account for these lever arms to find equilibrium.

Ferran recognizes that, simply because the rotor is AOA-stable when mounted on a gimbal head (or even without one!), that does not make the whole aircraft stable. An unstable airframe can make the whole aircraft unstable. The frame can overwhelm the rotor. In some such cases, as Ferran also states, the rotor loses thrust. This deprives the pilot of control. With the controls ineffective, all the "training training training" in the world will not help.

Ferran over-simplifies when he states that the rotor thrust will tend to align with the CG (or CM). In fact, in equilbrium (trimmed) flight, the rotor thrust line is usually somewhat ahead or behind the CM. The rotor thrust line will lie where it must in order to cancel the other moments (sorry!) on the frame.

We would like to have the rotor thrust line end up behind the CM. This way, a loss of rotor thrust will lift the nose, increase rotor AOA and tend to restore rotor thrust.

In effect, if a gyro is statically stable, the entire gyro is a giant offset gimbal head. Just as in the gimbal head, the "pitch pivot" of the whole airframe (=the CM) is ahead of the rotor thrust line. A down-loaded HS is analogous to the trim spring.
 

ferranrosello

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Thank you for your answers. I feel the need to arise this question.

I understand Udi' s statements about cg and moments. And when I started flying gyros (3 years ago), I was amazed because of its behaviour.

Accordingly with Udi's explanation of a negative pitching moment in HTL gyro, I was expecting a nose down tendency when applying power. My gyro is an ELA with a ROTAX 914. And when applying power (including full power settings) the tendency is clearly to rise the nose.

In the beginning I thank it was because of the propeller thrust on the tail. But I tried different gyro types, including a single Air Command without a HS. The tendency was to rise the nose when increasing power as well. And I've been thinking about it, and... flying a lot.

I've seen in take off, that if you set more power when the nose wheel is in the air and the main main gear is still rolling on the ground ,the tendency is to low the nose. But in the moment in which the entire aircraft is airborne this tendency is reversed.

Surely this effect can be explained by moments acting on cg, but I think that the real explanation would be much more complicated than Udi's one.

Thinking more about this question, Do you think that the moments acting on cg would be the same than in FW? The fuselage hanging from a U joint does not make a big difference?.

When I have a force acting in a body that is free to rotate, and this force is not acting in the body's cg. of course I have a moment, and the body will rotate.

But do you really think that the gyros fuselage is free to rotate? When a gyro (or a helicopter) is banking, the roll axe is on cg or on the rotor?.

I know that these questions are contrary to the widely accepted concepts in gyro's stability. But I'm seeing that these explanations does not fit with real flying gyro's experiences. Of course the problem may be because of my lack of understanding in this questions, and my explanations could be wrong.

I think that the control mechanics is that fuselage will try to maintain the cg aligned with the rotor thrust vector. Of course this is an over simplification, as Dough has remarked. But if this is a real tendency, the cg’s location is not so important like in FW aircraft, because the RW aircraft has an inherent tendency to put the cg in a good position.

I think that the cg. is relevant versus propeller thrust line (PTL). But in the overall picture you need to consider the arm between PTL and disc rotor axis. This arm is always much higher and its effect on fuselage attitude is much bigger than the PTL moment on cg. I think this is the reason HTL gyros rising the nose under power.

And, in my experience, it is not possible to climb at very low speeds (these speeds in which the HS is not efective). Anyway, in my experience too, this kind of power settings create a nose up pitching moment on the fuselage, not a nose down one.

Thanks a lot for this great forum. I'm sure this is the best place (maybe the only one) to discuss these questions.

Regards, Ferran.
 

Doug Riley

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Ferran, your comment about the early Air Command is interesting. I flew one for several years. I, too, observed that the nose seemed to rise slightly when you added power. I would add a little power when I wanted to fly hands-off, such as when I was taking pictures. It seemed more stable that way. This experience was before I added a horizontal stab. I could never account for it.*

The old Air Command exhibited all the other classic signs of static pitch instability, however. The nose went the wrong way in up- and down-drafts. During hands-off flight, if the gyro was disturbed more than a little, it kept going in the direction of the disturbance and would not return to trimmed flight without a control input. At high airspeeds, the nose rode lower and lower and the machine became very sensitive. You needed to hold back stick to keep it from pitching over.
__________________
* One guess is that adding power at relatively low airspeeds unloaded the rotor somewhat. This would reduce rotor RPM. The gimbal head will add some back pressure to the controls when this happens. You are still in great danger in this situation, however, because a further unloading of the rotor will start a PPO sequence that the gimbal head cannot stop. Many of the RAF PPO accidents have occurred on initial climbout -- when this pseudo-stability would be present.
 

ferranrosello

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I agree, Doug, the old Air Command are not stable. I think the worse thing is power off yaw stability at low speeds. I was surprised by the lack of yaw control authority in power off landings.

I think the nose down tendency versus speed is easily explainable. More speed implies less rotor disc AOA (a flatter rotor disc). This means that the aerodynamic force vector provided by rotor will be less rearwards twisted. And the consequent fuselage attitude will be with a lower nose, in order to get the cg approximately aligned with the aerodynamic force vector.

There will be moments acting on cg and rotor disc hinges because of PTL and fuselage drag too.

An interesting thing is that the power applied to the fuselage balances not only the fuselage drag, but also the rotors one. I think this is the reason because the gyros cg are set in the ground in a nose down attitude. The extra power used to balance the rotor's drag in flight rises the gyro's nose.

The first helicopters had the problem of nose down attitudes at high speeds too. This problem was solved by fitting an horizontal stabiliser.

In the gyro's this problem is not so notable because of the nose up tendency under power.

I think that some problems controlling gyros arises because the most important thing is the rotor disc attitude. But this attitude is no what the pilot sees. He controls the fuselage attitude which can be very different. When flying with some speed the HS aligns the gyro's fuselage into relative wind immediately, making the pilots job much easier. I believe that many times this quality is confused with the gyro being more stable. What really happens it is the time lag between an control input and the fuselage response is significantly reduced.

But at low speeds, the lag time becomes pilot problem again. So, in very stable gyro's like ELA or Magni (the two I better know) take offs and landings are still very difficult tasks for beginners.

I don't believe that the vertical PTL vector component was able to reduce the g load significantly. I think that the PPO does not arise a low g situation. On the contrary, it is a low g environment which triggers a PPO. This is the main limitation of a teetering rotor: very low g environments.

In a very steep climb the speed is rapidly reduced. The lower the speed, the higher the required power. At 0 airspeed you would need infinite power to maintain altitude. The Gyro will damp the climb under that condition, which implies a vertical deceleration and a low g environment. Once it happens with a full power setting the PPO can happen.

All gyro's with teetering rotor heads can suffer this problem, and at very low speeds (when you get the pick of the climb) the HS is not effective.
 

XXavier

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A couple of days ago, and among other things, Ferran stated that...




(...) I was expecting a nose down tendency when applying power. My gyro is an ELA with a ROTAX 914. And when applying power (including full power settings) the tendency is clearly to rise the nose.

In the beginning I thank it was because of the propeller thrust on the tail. But I tried different gyro types, including a single Air Command without a HS. The tendency was to rise the nose when increasing power as well. And I've been thinking about it, and... flying a lot.

I've seen in take off, that if you set more power when the nose wheel is in the air and the main main gear is still rolling on the ground ,the tendency is to low the nose. But in the moment in which the entire aircraft is airborne this tendency is reversed.

(...)



I believe that both situations have a different sum of momenta.

1) Main gear still rolling on the ground: you have to take into account the reaction of the ground on the wheels, and the corresponding momentum.

2)As above but just airborne: the sum of momenta doesn't include the one originated by the reaction of the ground on the main wheels.

Hence, the sum of momenta is different, and the resulting pitching moment may have different signs in cases (1) and (2).

XXavier
 

C. Beaty

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The nose pitching of a gyro upon abrupt application of power is a function of the rotor’s moment of inertia.

The initial response of the rotor to an increase of translational velocity is an increased flapping angle. The resultant nose-up moment from the rotor can be greater than the nose down moment of a high propeller thrust line.

With Shapiro’s inertialess rotor, a HTL machine would of course pitch nose down upon rapid increase of propeller thrust.
 

XXavier

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The nose pitching of a gyro upon abrupt application of power is a function of the rotor’s moment of inertia.

The initial response of the rotor to an increase of translational velocity is an increased flapping angle. The resultant nose-up moment from the rotor can be greater than the nose down moment of a high propeller thrust line.

With Shapiro’s inertialess rotor, a HTL machine would of course pitch nose down upon rapid increase of propeller thrust.

If I understand that well, a sudden thrust 'takes the rotor by surprise', the advancing blade flaps up, the rotor rolls, and the gyroscopic response is a pitch-up of the entire rotor. Got it right?

XXavier
 

C. Beaty

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Cyclic flapping and cyclic feathering are one and the same, depending only upon axis of view.

Viewed from the rotor’s tip plane axis, the rotor doesn’t flap but if it has translational (edgewise) velocity, there is cyclic feathering which equalizes lift between advancing and retreating sides.

The pitch of the rotorblades is fixed relative to the rotorhead so when viewed along the rotorhead axis, there is only cyclic flapping but no cyclic feathering.

With a rapid increase of propeller thrust, the rotor’s inertia prevents an instantaneous increase of rotor RPM Vs. rotor disc angle of attack, so the flapping angle is greater than it would have been with a gradual increase of translational velocity.

Shapiro’s inertialess rotor would have caused the HTL machine to pitch nosedown following a rapid throttle application.
 

XXavier

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Precisely that:

With a rapid increase of propeller thrust, the rotor’s inertia prevents an instantaneous increase of rotor RPM Vs. rotor disc angle of attack, so the flapping angle is greater than it would have been with a gradual increase of translational velocity. [Beaty]


is what I did mean when I wrote that of 'taking the rotor by surprise'...

Any reference to that 'inertialess rotor'? How it's possible that a rotor may have zero m.of i.?

XXavier
 

ferranrosello

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I think that we need some time between applying power and being accelerating to an higher speed. And I think that there is not such a sudden air speed escalation to prevent the rotor regime following air speed. But it may be...

My gyro has a great trimming system. It is very easy to trim it to permit hands off flight in all speeds above minimum power speed (60 mph).

If flying in perfect trim and hands off I increase power smoothly the gyro's exhibits a nose up tendency which slighly reduces trimmed speed. If you reduce power then the gyro's shows a nose down attitude tendency, which leads to a descent profile at a slighly higher trimmed speed.

Believe me, the nose up and down tendency are not because of the flapping variation versus speed. The attitude changes happen prior any speed variation can be felt. And the changes in rotor rpm are nearly unnoticeable.

Ferran
 
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