accidents.. areas to avoid your advice

Doug Riley

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Ferran:

Fortunately, the answer is yes. Cyclic pitch control works normally in zero G and negative G, in the sense that the rotor still tilts in response to stick movements. Without rotor thrust, the change in the angle of the disk is not felt in the frame, but it still occurs.

When the frame rotates nose up, the spindle tips aft relative to the rotor disk, as long as the spindle maintains a fixed angle to the frame (i.e. the stick is fixed). The rotor will experience the usual cyclic pitch change and react accordingly. That is, the advancing blades' pitch will increase, and the retreating blades' pitch will decrease. The disk will respond by flying to a new, positive angle of attack.

Note that the cyclic pitch change itself does not increase the rotor's thrust. A cyclic pitch change does not alter total thrust; it merely alters the distribution of thrust across the disk. The rotor's thrust becomes positive in the advancing sector and negative in the retreating sector, but the net thrust is still zero.

However, as the disk tips aft in response to the momentary dissymmetry of forces, the disk AOA becomes positive. This, in turn, increases the AOA (not the pitch, just the actual AOA) of each blade in each sector of the disk. Therefore, rotor thrust increases, restoring positive G.

If you view the video of the PPO in Japan that Scandtours posted in another thread, you may doubt this analysis. You can see the pilot level the rotor during a zoom climb. The gyro begins to pitch over. You then can see the rotor tip aft, presumably because the pilot pulls the stick aft. The pushover continues, though, even with the rotor disk tipped well aft.

I believe the reason for this is that the gyro was still climbing from its own momentum when the nose dropped in response to forward stick. The nose rapidly rotated so low that the gyro was still climbing by the time the pilot reacted by pulling aft. The climb angle was sufficient that even a full-aft rotor disk only met the air at a zero angle of attack.

If my analysis is correct, the video demonstrates the importance of the H-stab. The stab would keep the nose from dropping while the aircraft was climbing. If the gyro in the video remained pointed into the relative wind, prompt aft stick would probably have saved the pilot's life. If the airframe had had a nose-up bias as I suggested earlier, then the aft stick would not even be necessary; the pilot could just hold the stick still and the frame would supply the aft-cyclic effect.

A Dominator, with its low thrustline, is an example of a gyro that provides automatic cyclic inputs that tend to maintain positive G -- as long as you hold the stick still. I was able to duplicate this behavior in my Gyrobee (despite a slightly high thrustline) by using a large immersed H-stab with -3 degrees of incidence.

I don't know if any of the Magni clones uses either of these design tactics. I understand that the Magni aircraft themselves do not.
 

Steve McGowan

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I have a question (of course a difficult one).

Our general knowledge is that to unload the rotors is very dangerous. It is dangerous for all kind of seesaw rotors, but it is probably most sensitive to gyros, because they have auto rotating rotors with a very special control system.

In spite of these notable limitations we know about it because of lots of tragic accidents that have happened. But all the explanations about this problem are related to PPO’s and PIO…

But with the new stable airframes, with good HS and centered or nearly centered propeller thrust lines, having an unloaded rotor is still a very risky situation suitable to end in a tragedy.

But my question is:

What is the behavior of these stable machines with unloaded rotors?

These are my thoughts on the matter:

I guess that at the beginning they will try to follow a 0g curve flying path… Can anyone imagine what would be the end of this flying path?

I guess that during this 0g curve the pilot can do nothing (no rotor trust, no control). And that the HS will keep the machine flying in the direction of the relative wind. It seems to me a non recoverable situation which has to end with the autogiro pointing directly down.

I guess that during this unloaded flight path the gyro will have to accelerate (because it is a 0g curve). And because of that at any moment, the rotor will start to flap back, and increasing the rotor disc pitch angle, which will reload the rotor.

At this moment the pilot will be pulling the stick against the rear stop (something natural because he wants desperately to raise the nose).

But I guess that during the 0g curve flying path the rotor will have lost a lot of rrpm. Now the rotor is reloaded but without enough centrifugal force to tighten the rotor blades…

I think that this picture could explain the last autogiro’s accidents in Mallorca and in Germany… The new gyros won’t PPO. But what are they going to-do?

Ferran

I'm the last one That needs to comment on accidents,, but as to your comment..

"NEW GYROS ..WON'T PPO". (PILOT Push Over)

Please excuse me for saying it....
But the "TITANIC" was also a "NEW CRUISE SHIP"

It's not what your flying in,,, it's the Blades over head,, and that's ANY Semi-Ridged Rotor System.. Make ,Model or Name Brand anywhere in the world..

And "MURPHY". He ain't any fun...
 
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Doug Riley

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Some new gyros can PPO. Some cannot.

"PPO" is not a catchall term for every kind of "twist outa the sky" crash that is possible. There are several other kinds.

PPO refers to the POWER pushing the aircraft into a forward tumble. This can only (absolutely ONLY) happen when the thrustline is above the CG. Period.

You can drag over -- not a PPO.

You can torque over -- not a PPO.

You can lose RRPM from sustained low G, porpoising or even standing a hot gyro on the prop-- not a PPO.

Yeah, of course, dead is dead. It's still helpful to give these various coffin corners separate, since the fix for each one is different.
 

NoWingsAttached

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Some new gyros can PPO. Some cannot.

"PPO" is not a catchall term for every kind of "twist outa the sky" crash that is possible. There are several other kinds.

PPO refers to the POWER pushing the aircraft into a forward tumble. This can only (absolutely ONLY) happen when the thrustline is above the CG. Period.

You can drag over -- not a PPO.

You can torque over -- not a PPO.

You can lose RRPM from sustained low G, porpoising or even standing a hot gyro on the prop-- not a PPO.

Yeah, of course, dead is dead. It's still helpful to give these various coffin corners separate, since the fix for each one is different.
This may be a first, me disagreeing with you. I maintain ANY gyro can PPO regardless of design. All ya gotta do is round out at the top of a climb, add throttle, or not, unload the rotors and PPO.

There are other maneuvers and instances which I believe can allow a high-powered gyro to overcome rotor lift and PPO, if the rotor speed is not allowed time to catch up with prop thrust, but I can't describe it just now.

But then, this may not apply to what is being thrown about as design, these are pilot error.
 

WaspAir

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I maintain ANY gyro can PPO regardless of design. All ya gotta do is round out at the top of a climb, add throttle, or not, unload the rotors and PPO.
Nope. Aircraft don't automatically pitch down in a low g situation. Some force is needed to cause that pitch change. If the engine thrust is directed to cause no pitching response or an upward pitch, a PPO, cannot, will not, does not happen, period.

Flight path may be on a downward trajectory, but if the nose is not pitched down, that is merely a benign descent and not a PPO.

Unload the rotor on my A&S18A and do anything you like with the throttle and it will not PPO.
 

NoWingsAttached

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I want to add:

1. Flying in wind conditions above pilot's level of experience
2. Flaring too soon
3. Heavy head winds suddenly shifting to 90* cross winds during attempted landing
4. Heavy winds rolling off tree tops can create rolling horizontal "tubes", creating localized micro bursts of quickly alternating up and down drafts, or shears, which may disappear as quickly as they form - like horizontal dirt devils.
5. FW disregard for right of way during landing (one novice pilot froze like a deer when a FW ignored several radio calls and turned onto the RW while the new gyro pilot was landing, he chose to do a vertical drop rather than make the proper choice to go around.)
 

NoWingsAttached

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Nope. Aircraft don't automatically pitch down in a low g situation. Some force is needed to cause that pitch change. If the engine thrust is directed to cause no pitching response or an upward pitch, a PPO, cannot, will not, does not happen, period.

Flight path may be on a downward trajectory, but if the nose is not pitched down, that is merely a benign descent and not a PPO.

Unload the rotor on my A&S18A and do anything you like with the throttle and it will not PPO.
I think you can't uncouple the term PPO from a torque roll, it is all happening at once, so it really doesn't matter which one comes first - the chicken or the egg. We've had a few videos posted on YouTube and linked here with definite torque roll/PPO when rounding out of a steep climb, you haven't seen these?

Maybe a super heavy 18A jump gyro won't torque roll but that does not mean other, much lighter gyros can't torque roll and PPO in conditions where the rotors are unloaded. Heck, if I can torque roll a little 65hp GyroBee at 305# at full throttle and 1/2 g to 20+ degrees w/o out much trouble at all when rolling out a bit fast, what can I do at -1/2G? I've also experience even more torque roll than this with a 110 hp gyro weighing 475#. What am I missing, or am I imagining things??
 

Doug Riley

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Greg, you're describing something other than PPO.

What you describe is a zero/neg G event. Zero or neg G will not, in itself, cause the violent forward flip that characterizes PPO. It takes prop thrust, applied above the CG, to flip the gyro forward hard. It's not a PPO if power isn't pitching the gyro forward.

This is more than just a battle of definitions.

In a PPO-prone aircraft, the aircraft powers ITSELF into zero G. The pilot does not have to create the zero G with an abrupt control movement; the aircraft will do it for him. The flip is automatic and self-amplifying unless you snap the throttle closed. That makes PPO-proneness a particularly vicious flaw in design.

With CLT, the engine's thrust won't amplify a low G. It won't push the nose down.

With LTL, the engine's thrust even will fight the pilot's attempt to create low G. The engine thrust will push the nose UP when rotor thrust is lost. Just the reverse of an engine-induced forward flip.

Low, zero and even negative G by themselves will not automatically flip the gyro (or cause any other kind of catastrophe). To get the flip effect, there must be some built-in instability of the airframe. In a stable gyro, you can take the G's briefly to zero or even negative, and come back to tell the tale. In a PPO-prone one, you can't.

That's not to say that low, zero or neg G condition can't get you into trouble. RRPM can be lost rapidly (some blades lose it faster than others, Bensens being about the worst) when you reduce the rotor disk's angle of attack. But, as long as you don't have HTL or a low center of fuselage drag, there will be no self-amplifying flip.
 

NoWingsAttached

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Which brings up another accident situation (forgive me if it is already listed):

In-flight rotor flap, leading to a rotor strike, due specifically to unloading the rotors
 

NoWingsAttached

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So torque roll and PPO are unrelated in this discussion?
 

Doug Riley

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Torque roll is analogous to PPO, in that the prop-engine unit powers it. Of course, the capsizing of the aircraft is in the roll axis instead of the pitch axis.

PPO and torque roll should be separated, because an aircraft can be prone to one while being incapable of the other. A Brock-Bensen can be made CLT or LTL and will then be PPO-proof. It can still torque roll, however. To fix that, we need to swap the Bensen "shorty" tail for a tall tail, and/or use a large, centered H-stab, possibly with some anti-torque incidence.

I don't know if you could manage to torque over an 18A by putting into zero G at WOT. There's an awful lot of tail surface (including a H-stab the size of Rhode Island) as well as a wide mast fairing.
 

cburg

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Or have contra-rotating props...

Torque roll is analogous to PPO, in that the prop-engine unit powers it. Of course, the capsizing of the aircraft is in the roll axis instead of the pitch axis.

PPO and torque roll should be separated, because an aircraft can be prone to one while being incapable of the other. A Brock-Bensen can be made CLT or LTL and will then be PPO-proof. It can still torque roll, however. To fix that, we need to swap the Bensen "shorty" tail for a tall tail, and/or use a large, centered H-stab, possibly with some anti-torque incidence.

I don't know if you could manage to torque over an 18A by putting into zero G at WOT. There's an awful lot of tail surface (including a H-stab the size of Rhode Island) as well as a wide mast fairing.
 

ferranrosello

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Ferran:

Fortunately, the answer is yes. Cyclic pitch control works normally in zero G and negative G, in the sense that the rotor still tilts in response to stick movements. Without rotor thrust, the change in the angle of the disk is not felt in the frame, but it still occurs.
Doug, thank you very much for your patience.
But I’m not sure that this statement is right. I would like to believe it (because it would mean that flying our gyros is much safer), but I cannot do it.

There two reasons for my lack of faith in your statement: a theory reason and a practical one.

If we have a 0 g situation is because the blade is not producing lift. But also a no lift situation implies that the blade is seeing an effective 0 deg AOA. And without lift I’m not able to understand what kind of force can force the rotor blade to change its AOA.

I think I understand your explanation, but from my point of view: Without rotor thrust, the change in the angle of the disk does not occur… If it occurred you would retain some control authority.

The practical explanation is the case of the mast bumping in a helicopter using a teetering rotor. It is caused because something is tilting the fuselage around its longitudinal axis (the tail rotor thrust). If the airframe was able to effectively change the blade cyclic pitch this would not happened.

Of course the HS is still a compulsory requirement. The thing will get bad much quicker without one. But anyway the 0 g situation is a total loss of control, in which a stable gyro will be in a ballistic flying path. The HS the only thing is going to do is avoid PPO keeping the nose pointing higher than the relative wind.

Besides I have to say that the number of not understood accidents we are suffering in gyros is unacceptable. I feel we need to know what the real cause of it…is so I’m trying to explore these questions.

Of course, Doug, I can be wrong in my understanding, and I know that you understand very well rotors. I have learned a lot reading your posts. So, if you can help me to understand well what is going on in this case …

Ferran
 

bryancobb

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Ferran,

A boomerang reacts to gyroscopic and aerodynamic forces without a fuselage hooked to it. The only weight it is supporting is its own.

I would think a spinning rotor would perform similarly to a boomerang if the fuselage instantly disappeared (zero G). I fantasize that if cyclic inputs could be magically made, you could fly just the rotor all over the sky without an aircraft attached.

Am I wrong?
 

Vance

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Airfoils will be Airfoils.

Airfoils will be Airfoils.

Hello Ferran,

When an airfoil is moving through the air and you increase the angle of attack I would think it would produce lift and want to move particularly if the airfoil on the other side to the teeter has a decreased angle of attack regardless if it is a zero G event.

It seems to me a zero G event just takes the driving force away and the hope would be that the zero G condition would not last long enough to slow the rotor to the point of becoming a non recoverable event.

I have difficulty imagining a sustained zero G event when the gyroplane is upright.

Actually I have difficulty imagining a zero G event in a gyroplane because they seem to fall out of the sky quickly when lift is reduced and it is hard for me to imagine the source of the energy that would create a sustained zero G event.

Would you be more specific about the not understood accidents?

Thank you, Vance
 

Doug Riley

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Ferran, there's a distinction to made here. There can be full cyclic control of the rotor even when the rotor imposes no force on the fuselage.

If the rotor is in zero G, then, yes, each blade, on average, is either stalled or operating at its zero-lift angle of attack (for cambered blades, this AOA isn't zero, but around -2 deg).

Mechanically, our cyclic pitch control system still works, right? That is, when we move the stick, each blade still experiences cyclic changes in its mechanical pitch as it rotates? This much will be true unless the control system breaks. It is even true when the aircraft is parked on the ground.

If each blade, on average, is producing no lift (zero G), but a cyclic pitch change is made, then each blade will acquire a positive AOA in one sector of the rotor disk and a negative one in the opposite sector.

If the blades in one sector have positive AOA, then they will rise, peaking at a point 90 degree later. The fact that the disk rises this way does NOT mean that there is yet any thrust on the fuselage; we are still in zero G for the rotor as a whole. Thus, the disk will precess, but will not pull the fuselage with it.

If our cyclic pitch input was aft stick, then the rotor will precess to a larger aft tilt, relative to the airstream. If the fuselage maintains its own attitude relative to the airstream (i.e. doesn't flip forward), then the aft tilt of the rotor disk will re-establish a positive DISK angle of attack. This, in turn, will re-establish thrust.

Why will tilting the disk aft re-establish rotor thrust? Because tilting the disk aft increases the angle of attack of each blade throughout the blade's travel around the disk. Aerodynamically speaking, tilting a gyro's rotor aft while the gyro is travelling forward is the same as adding collective pitch to a helicopter rotor; it increase the AOA of each blade (not the mechanical pitch, but the actual angle at which the air strikes the blade).

(Mast bumping in helicopters occurs precisely BECAUSE the rotor DOES respond to cyclic control inputs. When the aircraft starts to roll, the pilot typically adds opposite stick. This causes the rotor to reach its flapping stops and therefore to hammer the mast.

(Mast bumping in a helicopter, with its fixed spindle, is more immediately destructive than contacting the flapping stops in a tilting-spindle gyro. Since the gyro's spindle can tilt, the shock of each stop contact will be somewhat less, as long as the spindle assembly hasn't yet contacted its own control stops. Once the spindle assembly contacts its own stops, there's not much difference in result between the tilting spindle and the fixed spindle.)
 

ferranrosello

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Ok, thanks to all.

You are right, in spite of having the rotor in a true 0 g situation the cyclic input still works because it is directly changing the rotor blade AOA. This is the case in the autogiro’s direct control system. Then I think this is correct.

Then, the only way to get a "sustained" pilot commanded 0 g situation is by pushing the stick widely forward flying with some speed.
But what other cases do you think it is possible to reach a 0g situation?
Releasing a fast sideslip centering pedals without moving the stick?
Achieving an attitude with the rotor disc nearly perpendicular to the horizon?

I need to think a little bit more...

Ferran
 

Doug Riley

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The zero G situation that we really MUST cope with, by design, is the strong downdraft. We can avoid the intentional zero G by simply not throwing the stick forward, as you describe. But you can't prevent weather.

It is very educational to fly both a HTL, no-HS gyro, and a stable gyro such as a Dominator, in rough air.

The HTL gyro's nose will DROP when you hit a downdraft. That is, the aircraft, without any pilot input, will exaggerate the effects of the downdraft. The only ways to lessen this effect are to reduce throttle or loosen your grip on the stick ("float" it). "Floating" the stick allows the gimbal head trim spring to pull the head back when G's are reduced.

In contrast, a Dominator handles downdrafts best if you hold the stick firmly. The nose will pitch up to meet downdrafts. The Dom. will maintain altitude and speed better than the HTL craft. (In the HTL craft, if you frequently reduce throttle, sooner or later you'll run out of altitude. Then you'll have to go to full throttle, adding to your potential problems with the downdrafts.)

Again, I don't know if the current Magni and Magni-like Euro-gyros behave more like a Dominator or like a HTL gyro. I would guess they are somewhere in between.
 

ferranrosello

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My experience is flying Elas. And I have flown in very rough air sometimes. The behavior is very closed to what you have described about dominator. When I´ve found such rough conditions I always fly to hold a constant nice attitude closed to Vy, which implies a reduced power setting. It works in all the machines.

I have no experience with gyros without a proper HS in rough air.

One of the questions I´m wondering is how long can a gyro be in a very low g situation before the situation becomes unrecoverable. Flying in rough air with Ela my experience is that I´ve been in unloaded situations for a couple of seconds. But you know that in these situations my time estimation can be very wrong. Besides, I believe that the rough air is never going to produce a sustained 0g situation.

It is obvious that nobody will command a 0g maneuver but… Pilots are human beings, and all of us make mistakes…
Andy’s accident in Mallorca is something that seems not having any sense. The same that the MT accident in Germany. But things do not happen without reason, and medical issues are never a satisfactory explanation. I thing that at any moment something has been made in the controls that has degenerated in an unrecoverable situation…

Because both accidents seem to have happened after a hard sideslip I think that this maneuver has something to do. But a lot of experienced gyro pilots (me, for example) feel comfortable doing sideslip…know. I´m trying to imagine what could be wrong…

Additionally I´m trying to imagine how could develop a sustained 0g maneuver in a gyro. All my imaginations end with a broken machine pointing directly down…

ferran
 

birdy

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PPO refers to the POWER pushing the aircraft into a forward tumble. This can only (absolutely ONLY) happen when the thrustline is above the CG. Period.
I gota hand it to you Doug, your persistant. :)

And there should be alota people that are greatfull for your persistance.
I gave up ages ago.
 
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