Gyroplane Stability and Aerodynamics - Glasgow University

[C. Beaty]

Birdy beat me to it but I’ll repeat, offsetting the rotor to one side has a purely cosmetic effect.

However, when selling gadgets to the general public, aesthetics is everything; the shiniest object always wins. Be it kitchen appliances or flying machines.

 

[raghu]

Jurgen, I think you are getting tied up in terminology. There are a whole range of 3d CFD techniques. Depending on what you like to model they have their pros and cons. For example panel method is a 3D techniques as it models the flow in 3D. It is good for lift and pitching moment but predicts (parasite) drag to be zero for all shapes, as it does not model viscous effects. In other word, if you used it to calculate Xu you would be way off.

As I said in my previous post they obviously did not do a full NS model but that would be way way too complicated and unwieldy to use. Even so, before you can trust the model you would have to do a flight test to validate the model. Aeroelastic forces may dominate and you may have to couple the full NS model with a aeroelastic structural model.Do you pick a non-linear or linear one? What turbulence model do you pick? .. and so on and so forth.

The point is all models simplify reality. The goal of any model builder is to use the simplest model that matches reality in the area of interest. Find attached the model response and flight response for the same longitudinal cyclic movement for a Magni VPM 16. It is pretty darn good. Holger in DLR has got similarly good results (see his paper), though his model may in fact be even simpler.

We can of course spend time perfecting the model but you are going to reach diminishing returns. All we are interested is in trends when you change parameters such as thrust line - CG offset tails etc. These models can do this.

Are there gross errors in the Glasgow work? I do not know. All I can say is when the raw Glasgow results (ignoring the interpretation) are take with the DLR work, Section T CAA flight test and compared with simple closed form analytical solutions I have done, you get similar findings. HS is a big factor in the short term response. The Glasgow work appears contradictory only because they are focused on the long term response.

 

[JAL]

Jordan, propeller torque in FWs is handled very nicely by wings and ailerons. Provably most FW pilots aren’t even aware of propeller torque. But in a gyro, especially one with geared Subaru or the like, blipping the throttle produces a rockabye baby motion. Any yaw resulting from asymmetric dipping of stuff in the spiraling propeller slipstream is handled by rudder/vertical stabilizer.

Why then do aeroplanes have rudder trim? My understanding is to relieve the pilot from applying constant rudder to counteract the yaw created by the propellor slip stream usually used in climb when the power setting is high

Never seen aelion trim on an aeroplane

Cheers

Jordan

 

[Abid]

Certainly offsetting the rotor is also a way of doing it which is similar I guess as having slightly larger wing on one side compared to the other in fixed wings or more like slightly different incidences of each wing to balance in the most used condition (cruise).
True, but only wen you have rotor thrust.
Low G, WOT and round you go.
An immersed TT only works against torque roll wen there is prop wash.
But no prop thrust, no torque roll.

I understand your comments Birdy. I do have a slightly different view. I don't know if you'd agree or not. Basically the preponderance of low G event as mentioned by you is akin to when people do aerobatics in a trike get weightless (in a weight shift control, no weight = no control) and come tumbling down to their demise. But at least we are confident that trikes just don't go weightless or very low G just like that. You really have to do something quite stupid and outside their flight limitations to get there for long enough to have an issue. Transient weather induced low or negative G last only a fraction of a second and that can be dealt with by the structure and dampening of the design

My question would be how often do you think gyroplanes are just going into 0 or low G and remaining there long enough to diminish RRPM to the point where this becomes a problem and how do they get to this extreme low G or negative G events? I know they can't be all weather events or trikes would be coming down like flies all over the world too.
There are flight limitations for each aircraft beyond which there is danger zone that is to be avoided. In gyroplanes unloading the rotor should be an extremely rare event. There should be some standard of how rotors should behave when faced by a low G event and give enough momentum to have a bit of time to recover at a minimum, especially rotors used for auto-rotation as a main means of lift.
Pilots can do some really stupid things and no one can design an aircraft worthy of flight that can idiot proof everything and pilot training should have certain responsibility. However, the videos I have seen of some gyroplane crashes, I don't think the initial problem happened because being in low G. It seems to me low G happened due to bad design and non-dampened characteristics causing pilot to make mistakes and get into negative G and unloading the rotors. Some people here in this very thread earlier re-counted some experiences in some of their own machines of behavior of pitching and so on that are tell tale signs by themselves of those machines having design characteristics that some day can catch you on the wrong foot and make you go where you never intentionally want to go. If then the pilot puts in the wrong input (for example going to WOT instead of pulling the cyclic back or in some other configurations of machines, going to idle really quick and pushing the nose down at the same time), that could possibly exacerbate the situation and cause a big problem. In any other aircraft category those would be warning signs that the design needs work and does not meet a minimum criteria set by the industry or agencies. One can't design for a pilot who intentionally want to keep going to the danger zone but we can design for pilots who can make some minor unintentional mistakes sometimes to allow them to recover and have a forgiving aircraft

 

[kolibri282]

Thank you for your comprehensive answer raghu! I am not trying to find faults in the Glasgow report but rather look for a way to improve my own program to the point where I could take the design data of a gyro and predict stability to within reasonable margins of error. I am sure that I will never be able to come up with a fully fledged 3d CFD solution but a model using Bruno Zillis aeroelastic rotor calculation and adding a Peters He inflow model and prescribed wake model seems to be possible, even if it's a long way to go. The major point for me was that if even that level of modeling would not give any useful results I'd better stop now than to go on wasting time. I take your answer as an indication though that a model as described above should be sufficient for stability calculations at a much better level than is publicly available today, please correct me if I got this wrong.

Again thanks a lot,

Juergen

 

[Abid]

Why then do aeroplanes have rudder trim? My understanding is to relieve the pilot from applying constant rudder to counteract the yaw created by the propellor slip stream usually used in climb when the power setting is high

Never seen aelion trim on an aeroplane

Cheers

Jordan

Generally the propeller torque effect is countered in airplanes by adjusting the thrust line to one side or another or by having a very slight wing incidence difference from one wing to the other to balance the pte or actually literally having one wing slightly longer than the other in span for a particular speed and power setting range (cruise) or in most cases a combination of these.

The propeller yaw effects of which slipstream effect is one, are countered by application of yaw trim or yaw control and yaw aiming devices are rigged for a particular speed and power setting range (cruise) so pilot workload in that regimen is reduced (no rudder). But pilot does have to set the yaw trim or apply rudder when the machine is not in cruise setting. MTO handles the yaw effects by a pretty generous offset angle on the rudder and by bending a rudder trim tab permanently for a particular power/speed setting. But when you are outside of their desired range, if you notice carefully you have to apply rudder in one way or the other to keep coordinated.

 

[Doug Riley]

Fara: Regarding your Post #85. Keep in mind that low or zero G in gyros can be dangerous during a much shorter time interval than it takes to lose significant RRPM. At a certain disk AOA (a little below 0 deg), the rotor instantly loses all thrust. This happens at full RRPM as well as at reduced RRPM. It happens because the reduction in disk AOA reduces the blades' individual mean AOA's (just like applying down collective in a helicopter).

If the gyro is designed so that it does not rely on rotor thrust to stabilize its airframe, then a short-duration low G event will do no harm. The loss of RRPM will not be great enough to matter.

If OTOH the gyro relies on rotor thrust to keep the airframe upright (as does a gyro with uncompensated engine torque and/or high thrustline), then trouble begins the iNSTANT rotor thrust diminishes. The aircraft will begin to roll and/or begin a PPO. The pilot has limited ability to control the craft with the stick, because tilting the rotor does nothing at zero G.

I bring this up because many people don't understand the instant nature of the correspondence between disk AOA and rotor thrust -- a factor that DOES matter with rotor-stabilized airframes and DOESN'T matter with inherently stable ones.

The relatively slower correspondence between disk AOA and RRPM matters to all gyros. IOW, sustained low G will get you into trouble even in a stable gyro.

 

[C. Beaty]

Some people forget that a rotorcraft with central flap hinges is controlled entirely by thrust vector orientation. The rotor thrust vector is displaced about the CG, producing a control moment that initiates a pitch or roll acceleration. No thrust; no control.

 

[Jean Claude]

The Glasgow work appears contradictory only because they are focused on the long term response.

Why they are focused on long term response with the blocked stick?
This is in contradiction with a search for security!

Fara, No thrust, unbalanced roll and unbalanced pitch.

 

[raghu]

If OTOH the gyro relies on rotor thrust to keep the airframe upright (as does a gyro with uncompensated engine torque and/or high thrustline), then trouble begins the iNSTANT rotor thrust diminishes. The aircraft will begin to roll and/or begin a PPO. The pilot has limited ability to control the craft with the stick, because tilting the rotor does nothing at zero G.

o.

Doug,
Consider the Magni or MTO. Depending on the model and loading you get 8 to 13 inch thrust- CG offset for both brands per CAA section-t airworthiness testing. In both cases the prop thrust is balanced by RTV, in other words the RTV is ahead of the CG. The tail is carrying under 10 pounds of load. In other words, per your above quote, you would say these two models are both PPOs waiting to happen.

Yet, they have passed more stringent professionally conducted airworthiness flight tests than any other model.They are more numbers of these two manufacturers flying today than all the others put together. So far not a single case of PPO. Why?

There are two problems with your above analysis:

1. how does the low g happen? If it is not the pilot deliberately flying a parabola then it must be an atmospheric gust. Now, as long as the gyro is AOA stable such a gust will cause the gyro to pitch in a direction that neutralizes the effect of the gust. In the Magni and MTO3 it is the stab that gives the AOA stability despite the RTV being ahead of the CG. PPO is not some special case it is a garden variety of AOA instability. It is a runaway condition a small decrease in AOA resulting in a nose down motion that further reduces the AOA and so on.

2. There is another force in play in a zero G event - gravity- that is ignored in all descriptions of PPO I have seen.

When a gyro is in a zero (or Low) G condition it is falling or accelerating towards the earth. This increases the AOA of the rotor in effect loading the blades. This effect is not small. At 30 MPH airspeed the AOA would increase by about 30 degrees per sec in a zero G event.

Now further if the gyro is AOA stable (due to a stab)it would pitch up into the gust, further increasing the AOA and loading the rotor. Both these effects (falling and AOA stability) work in the same direction and as you can see there is no run away condition and no PPO.

Now if the gyro is AOA unstable the vertical acceleration (falling) would help with loading the rotor while the AOA instability will attempt to unload the rotor further. At low speeds the falling dominates. At some critical speed the AOA instability wins and you get a run away PPO condition.But this is only possible in gyros that are unstable wrt AOA.

Bottom line, AOA stability is the vaccine to inoculate against gust induced PPO. It does not matter where the RTV is wrt CG.

 

[raghu]

Why they are focused on long term response with the blocked stick?
This is in contradiction with a search for security!

Because of section t compliance.

 

[Abid]

Fara: Regarding your Post #85. Keep in mind that low or zero G in gyros can be dangerous during a much shorter time interval than it takes to lose significant RRPM. At a certain disk AOA (a little below 0 deg), the rotor instantly loses all thrust. This happens at full RRPM as well as at reduced RRPM. It happens because the reduction in disk AOA reduces the blades' individual mean AOA's (just like applying down collective in a helicopter).

If the gyro is designed so that it does not rely on rotor thrust to stabilize its airframe, then a short-duration low G event will do no harm. The loss of RRPM will not be great enough to matter.

If OTOH the gyro relies on rotor thrust to keep the airframe upright (as does a gyro with uncompensated engine torque and/or high thrustline), then trouble begins the iNSTANT rotor thrust diminishes. The aircraft will begin to roll and/or begin a PPO. The pilot has limited ability to control the craft with the stick, because tilting the rotor does nothing at zero G.

I bring this up because many people don't understand the instant nature of the correspondence between disk AOA and rotor thrust -- a factor that DOES matter with rotor-stabilized airframes and DOESN'T matter with inherently stable ones.

The relatively slower correspondence between disk AOA and RRPM matters to all gyros. IOW, sustained low G will get you into trouble even in a stable gyro.

Hi Doug:
Ok. I will have to think about this a bit but if this is really the case, honestly I expect a lot of PPO's to be happening in Europe since most of the gyroplanes flying in Europe are HTL with stabilizers, yet there are none to few pure PPO's and very few speculative torque-overs.
Can you tell me how this is different in a light 2-seat helicopter then also because the rotors there keep it up as well. Just asking how your analysis here applies to helis and if it applies at all how do they handle this situation. I understand there is no propeller in tractor or pusher config there but there is still torque compensated by the tail rotor and if somehow the main rotor is able to instantly dump thrust there, I am sure no human pilot is fast enough to match that with compensating effect on tail rotor.

Also, I would like to ask you or anyone else, that from your description I gather that AOA of the disc goes below zero, and thus the rotor thrust disappears instantly. How does the AOA of the rotor disc go below zero? Isn't the fuselage following the rotor disc? or shouldn't it be made that it does? so that the relative path and rotor disc relationship is maintained. This has got to be a desirable design requirement in a gyroplane like it is in trikes or airplanes so things remain stable in abrupt upsets and actions. Going below zero lift angle of attack should be quite difficult while remaining within operating limitations of the aircraft.

 

[brett s]

Teetering rotor helicopters do the same thing if the main rotor is unloaded, they start to roll uncontrollably & if the pilot tries to compensate with lateral cyclic while it's still unloaded very bad things happen - do a bit of research on mast bumping.

 

[raghu]

JC, in addition to my reply to you earlier it is not well appreciated that the only reason the Glasgow team recommended near CLT is that this was the only configuration they found that gave a stable long period mode at low speed (35 mph) compliant with section-T . At higher speeds all configurations CLT, LTL, HTL with or without HS where found to have a stable long period mode and are compliant with section-T. This result (long period instability at low speed) is also confirmed by the work at DLR by Holger Duda.

Given the above (long period benign at high speed) it is not at all likely this long period mode is the cause of the poor handling qualities and safety record. Any one who has flown a RAF without a HS will tell you it is when you get past about 55mph that you start to see the rapid degradation of flying qualities. Most gyros at low speed are fairly benign unless one actively unloads the rotor. Hence it is ironical that this was the driver for the CLT requirement.

Btw the long period mode that Doug R. experience flying HTL gyros is an intermediate mode that is created when the short period mode merges with the rotor speed mode. This mode gets worse with higher speed and is only present in AOA unstable gyros.

 

[birdy]

However, when selling gadgets to the general public, aesthetics is everything; the shiniest object always wins. Be it kitchen appliances or flying machines.
CB beet me to it.

And so did Doug Fara, you don't need to loose rrpm to loose thrust, just like you don't need to loose AS ina trike or FW to loose thrust( lift).BUT, with a gyro, you still have an out if you suddenly find yourself off the seat. You still have rrpm, so you still have cyclic control, so you can quickly reorientate the disc to regain thrust, much faster n a FW can pitch nose up to do same.

 

[birdy]

Ok. I will have to think about this a bit but if this is really the case, honestly I expect a lot of PPO's to be happening in Europe since most of the gyroplanes flying in Europe are HTL with stabilizers, yet there are none to few pure PPO's and very few speculative torque-overs.
Can you tell me how this is different in a light 2-seat helicopter then also because the rotors there keep it up as well. Just asking how your analysis here applies to helis and if it applies at all how do they handle this situation.
I will speculate bout the lack of ppo and torque over incidents in these HTL machines.
It's the general flyn type, combined with the docile/ heavy control.
Coz of their configuration, they are harder to throw around , so less likely to get into pilot induced low g situations.
And they are cruisers, not acrobatic, with no roof, so they are less likely to be flown in low g type air.
Bout the Helies, the thrust of the TR is much less ( Usualy only bout 40% of total power available, at full pedal deflection. Not to mention the fact that at 0 g, you are not near full power or pedal deflection) than that of the gyro prop and is Usualy closer to the COM, so it has less leaverage.

 

[Abid]

Teetering rotor helicopters do the same thing if the main rotor is unloaded, they start to roll uncontrollably & if the pilot tries to compensate with lateral cyclic while it's still unloaded very bad things happen - do a bit of research on mast bumping.

Hi Brett:
Yes I have read a bit about mast bumping and its history in Bell and the way down to SFAR 73. Teetering rotors are definitely a compromise but the bumping is happening with low G situations and flapping. It does not seem like Doug is describing the same thing. He is describing PPO while rotor has even max RRPM. Is that not so?

BTW, searching just now I found this very recent report from ATSB about R44
https://www.atsb.gov.au/media/4533016/ao-2013-203_final.pdf

 

[raghu]

[
I will speculate bout the lack of ppo and torque over incidents in these HTL machines.

How 'bout the simpler explanation mate- they are in fact stable!

 

[Abid]

Ok. I will have to think about this a bit but if this is really the case, honestly I expect a lot of PPO's to be happening in Europe since most of the gyroplanes flying in Europe are HTL with stabilizers, yet there are none to few pure PPO's and very few speculative torque-overs.
Can you tell me how this is different in a light 2-seat helicopter then also because the rotors there keep it up as well. Just asking how your analysis here applies to helis and if it applies at all how do they handle this situation.
I will speculate bout the lack of ppo and torque over incidents in these HTL machines.
It's the general flyn type, combined with the docile/ heavy control.
Coz of their configuration, they are harder to throw around , so less likely to get into pilot induced low g situations.
And they are cruisers, not acrobatic, with no roof, so they are less likely to be flown in low g type air.
Bout the Helies, the thrust of the TR is much less ( Usualy only bout 40% of total power available, at full pedal deflection. Not to mention the fact that at 0 g, you are not near full power or pedal deflection) than that of the gyro prop and is Usualy closer to the COM, so it has less leaverage.

I wouldn't call controls of the MTO "heavy" unless you are made of sugar

Yeah gyroplanes aren't supposed to be acrobatic. There isn't a good out if you misjudge something just like while performing a loop in a trike. Of course you can do it but you lose airspeed and your tumble is inevitable. This idea that gyroplanes should be or could be flown this way, needs to be kept in check.

 

[Abid]

However, when selling gadgets to the general public, aesthetics is everything; the shiniest object always wins. Be it kitchen appliances or flying machines.
CB beet me to it.

And so did Doug Fara, you don't need to loose rrpm to loose thrust, just like you don't need to loose AS ina trike or FW to loose thrust( lift).BUT, with a gyro, you still have an out if you suddenly find yourself off the seat. You still have rrpm, so you still have cyclic control, so you can quickly reorientate the disc to regain thrust, much faster n a FW can pitch nose up to do same.

I see your point. Still I am trying to understand what Doug said about instantly loosing rotor thrust and being doomed even at full RRPM in a HTL machine. I definitely would have expected to see a lot more PPO in Europe in these gyroplanes. I have seen some European pilots fly MTO etc. quite aggressively, as much as I have seen anyone in the US fly a Dom or even a single seat one and perhaps even more. So I don't think that European pilots are all just flying somber. Some are quite aggressive.