gyrogreg
Senior Member
- Joined
- Dec 9, 2003
- Messages
- 1,000
- Location
- Ste. Genevieve, Missouri, USA
- Aircraft
- Magni M-16 Gyroplane
- Total Flight Time
- 3000 total, 2000 Gyro
The thread ”Future of Gyros” prompts me to start this discussion on flight testing of gyros. An ASTM Task Team is working on some refinements to the stability portion of the standard. In this investigation, I believe a major light bulb has gone off – at least for me.
People worry most about getting into PIO or a buntover (PPO in terms of a HTL). One intent of the ASTM standard was to provide a way to easily determine by flight test if any particular gyro would be susceptible to buntover or PIO. A simple flight test, performed and reported on many gyro configurations, could be most informative. As you have all probably read, our ASTM standard focused on the static G-Load stability (actually this is the AOA static stability) of a gyro. The static G-Load stability flight test, a banking constant airspeed turn, was intended to be the test that could say whether a gyro could buntover or not. The presumption was that there was a symmetry from the +G-Load condition of a banking turn to the -G-Load condition of a buntover. A buntover is really an AOA static instability that causes a decreasing G-Load to continue and accelerate in a worsening reducing G-Load – nose down pitching.
It turns out that the banking turn flight test really only works for some severely AOA unstable gyros – such as an old-style Air Command with no HS. For less severe AOA unstable gyros, and especially for gyros with strong offset gimbal trim spring arrangements, this test did not define a buntover susceptible gyro very well. In fact, the revealing fact was when Jerry Tiahrt reported that his RAF 2000 passed the static G-Load test and nearly passed the static Power stability test! So, back to the drawing board we went!!! Apparently, the Static Stability flight tests were not truly definitive – by that I mean we know the RAF 2000 is capable of a buntover, and yet Jerry’s static flight testing did not identify it as such. The major problem with the G-Load test is that the offset gimbal provides the overwhelming aft stick pressure in a banking turn – whether the gyro is AOA stable or not. This test works for a FW airplane, but not for a gimbaled gyro rotorhead!
There may be different technical opinions on this, but what I think we discovered with Jerry’s testing is a new, “back door” way to determine if (or when) a gyro is statically AOA unstable and therefore capable of a buntover! The Static Stability tests are important, but they do not provide a definitive indication of buntover susceptibility, or not. What Jerry noted was that in performing the Fixed Stick static flight testing in his RAF 2000, he found that above a certain airspeed, the gyro responded to moderate turbulence with divergent pitch and airspeed oscillations. This got Jerry’s attention! This is an indication of DYNAMIC INSTABILITY. Jerry reported this was disquieting, and he subsequently lowered his personal Vne for this aircraft. (Jerry, if you are listening, please correct anything I might be relating differently than you reported – I did not go back and research our emails and your postings recently.)
I believe this has pointed us to a new determination of actual static AOA instability – ability to buntover. Please try to follow this: If an aircraft is able to oscillate in pitch, whether convergent, divergent or constant amplitude oscillations, it must be statically AOA stable. The mere fact that it demonstrates Phugoid pitch oscillations (slow rate, constant AOA oscillations), that gyro has static AOA stability. If it was statically AOA unstable, it could not oscillate – it would simply diverge exponentially nose-up or nose-down (buntover)! A phugoid oscillation occurs because there is static AOA stability – it is the small AOA divergences from steady state that provide the negative feedback restoring moment that causes the gyro to oscillate rather than statically diverge.
Fixed Stick flight testing was the key! With a fixed stick, you are evaluating the pure aircraft response and the airframe pitching, whether in the restoring or divergent direction. With a fixed stick, the airframe pitch response to a disturbance couples directly to the rotor (wing) and causes the whole gyro to react. If the airframe pitching response is in the unstable direction, the aircraft is statically AOA unstable. The key is that it is the Fixed Stick condition that really reveals the ability to oscillate in turbulence or if disturbed by a pilot input. This is actually the standard DYNAMIC fixed stick flight test that has always been part of the gyroplane ASTM standard – and that Jerry inadvertently flight tested when his gyro developed diverging oscillations in moderate turbulence!.
What I think we have discovered is that most gyros, such as the RAF 2000, at moderate airspeeds near their Minimum Power Required Speed (MPRS), are statically AOA stable and will exhibit slow rate phugoid pitch oscillations. The existence of oscillations indicate the AOA static stability! However, as in the case of the RAF, some gyros might become statically AOA unstable at higher airspeeds (airframe aerodynamic nose-down moments) and at higher power levels – both of which are moving the “effective” RTV forward of the CG – to the Chuck B. “classic” definition of AOA unstable and able to buntover!
If a gyro approaches static AOA instability (with a worsening power/airspeed condition), the static restoring AOA pitch moment becomes weaker and weaker. As it does so, the phugoid dynamic oscillations become less convergent, neutral, and eventually divergent oscillations. The standard requires that a gyro not be dynamically unstable – in other words, still able to oscillate, but with diminishing oscillations. But, even if the dynamic phugoid oscillations become neutral or divergent, they are still oscillations, and there is still static AOA stability – however weakening! If we require a gyro to be dynamically stable – excited phugoid oscillations eventually diminish to zero - that gyro still has a margin of static AOA stability – it is assuredly statically AOA stable and cannot buntover in that flight (power and airspeed) condition!
So, then how do we test for the ability or not for a gyro to buntover? We conduct the standard Fixed Stick Dynamic Test. The key to this is that the stick must be truly mechanically fixed. It is not good enough to just try to hold the stick steady! An experienced pilot just cannot avoid stabilizing inputs! With the stick fixed, fly at MPRS in afternoon turbulence to observe that the pitch oscillations that are excited do not start getting worse and worse – divergent dynamic response – dynamically unstable and weak static AOA stability. (This test can also be done in calm air with a pilot “step” input on the stick – like professionals do it – but just flying in turbulence will work for informal testing just as well!) Then, increase the Fixed Stick airspeed by about 5 mph – change the fixed position of the stick - and watch for divergent oscillations in moderate turbulence again! Keep increasing airspeed and power to see if there is any airspeed where the pitch oscillations start getting worse and you, the pilot, release the fixed stick to stabilize the oscillations. This is the point, beyond which, that gyro will have meager AOA static stability, or beyond which it will be actually statically AOA unstable and able to buntover! That determines the flight condition (power and airspeed) beyond which that gyro can buntover or PPO. That should identify your Vne condition. Gyros that are capable of bunting over will only do so at statically unstable combinations of power and airspeed – normally at higher airspeed because airframe aerodynamic moments are normally statically destabilizing! This flight testing will determine your “safe operating flight area”. A really good thing to know – especially since buntovers are so insidiously surprising otherwise!
This test should be conducted at higher airspeeds with lower power settings as well – the prop thrust moment or lack of moment can contribute to AOA instability on certain configurations!
This Phugoid dynamic stability test is simple and safe to be conducted by any gyro pilot that is proficient in flying that particular gyro in moderate (afternoon thermal) turbulence conditions. The second post below will describe two ways to safely “fix the stick” for this testing. Jerry Tiahrt helped develop the “chain” or rope method. This is how he identified the buntover susceptible flight conditions for his RAF 2000.
I would appreciate any comments or flight testing feedback on these concepts. I have been evaluating these concepts for some time, and I think it is time to start sharing all this with others to hopefully provide some flight test feedback and validation. I have become convinced that this simple flight test will truly indicate if or when a gyro can buntover (or PPO). But what about PIO? To pass this test, a gyro must have a dynamic damper – a HS! It turns out that a HS, besides providing static stability directly, actually provides dynamic damping that also extends the static AOA stable margins and range. The dynamic damper is also the mechanism that prevents rapid PIO pitch oscillations (the rapid, constant airspeed, varying AOA oscillations) that promote PIO. I don’t have complete theoretical reasoning for this, but it can be expected that any gyro that is shown to be buntover proof from the above testing, will also be highly resistant to PIO due to its strong dynamic damping..
There are many aspects of the above theory that I haven’t discussed here – such as susceptibility to buntover in a push over the top of a zoom! But, the above are the basics. I hope some of you will provide some flight testing – and I hope this flight testing really opens some eyes – as it did for Jerry Tiahrt!
If you do some testing, please share your results.
- Greg Gremminger
People worry most about getting into PIO or a buntover (PPO in terms of a HTL). One intent of the ASTM standard was to provide a way to easily determine by flight test if any particular gyro would be susceptible to buntover or PIO. A simple flight test, performed and reported on many gyro configurations, could be most informative. As you have all probably read, our ASTM standard focused on the static G-Load stability (actually this is the AOA static stability) of a gyro. The static G-Load stability flight test, a banking constant airspeed turn, was intended to be the test that could say whether a gyro could buntover or not. The presumption was that there was a symmetry from the +G-Load condition of a banking turn to the -G-Load condition of a buntover. A buntover is really an AOA static instability that causes a decreasing G-Load to continue and accelerate in a worsening reducing G-Load – nose down pitching.
It turns out that the banking turn flight test really only works for some severely AOA unstable gyros – such as an old-style Air Command with no HS. For less severe AOA unstable gyros, and especially for gyros with strong offset gimbal trim spring arrangements, this test did not define a buntover susceptible gyro very well. In fact, the revealing fact was when Jerry Tiahrt reported that his RAF 2000 passed the static G-Load test and nearly passed the static Power stability test! So, back to the drawing board we went!!! Apparently, the Static Stability flight tests were not truly definitive – by that I mean we know the RAF 2000 is capable of a buntover, and yet Jerry’s static flight testing did not identify it as such. The major problem with the G-Load test is that the offset gimbal provides the overwhelming aft stick pressure in a banking turn – whether the gyro is AOA stable or not. This test works for a FW airplane, but not for a gimbaled gyro rotorhead!
There may be different technical opinions on this, but what I think we discovered with Jerry’s testing is a new, “back door” way to determine if (or when) a gyro is statically AOA unstable and therefore capable of a buntover! The Static Stability tests are important, but they do not provide a definitive indication of buntover susceptibility, or not. What Jerry noted was that in performing the Fixed Stick static flight testing in his RAF 2000, he found that above a certain airspeed, the gyro responded to moderate turbulence with divergent pitch and airspeed oscillations. This got Jerry’s attention! This is an indication of DYNAMIC INSTABILITY. Jerry reported this was disquieting, and he subsequently lowered his personal Vne for this aircraft. (Jerry, if you are listening, please correct anything I might be relating differently than you reported – I did not go back and research our emails and your postings recently.)
I believe this has pointed us to a new determination of actual static AOA instability – ability to buntover. Please try to follow this: If an aircraft is able to oscillate in pitch, whether convergent, divergent or constant amplitude oscillations, it must be statically AOA stable. The mere fact that it demonstrates Phugoid pitch oscillations (slow rate, constant AOA oscillations), that gyro has static AOA stability. If it was statically AOA unstable, it could not oscillate – it would simply diverge exponentially nose-up or nose-down (buntover)! A phugoid oscillation occurs because there is static AOA stability – it is the small AOA divergences from steady state that provide the negative feedback restoring moment that causes the gyro to oscillate rather than statically diverge.
Fixed Stick flight testing was the key! With a fixed stick, you are evaluating the pure aircraft response and the airframe pitching, whether in the restoring or divergent direction. With a fixed stick, the airframe pitch response to a disturbance couples directly to the rotor (wing) and causes the whole gyro to react. If the airframe pitching response is in the unstable direction, the aircraft is statically AOA unstable. The key is that it is the Fixed Stick condition that really reveals the ability to oscillate in turbulence or if disturbed by a pilot input. This is actually the standard DYNAMIC fixed stick flight test that has always been part of the gyroplane ASTM standard – and that Jerry inadvertently flight tested when his gyro developed diverging oscillations in moderate turbulence!.
What I think we have discovered is that most gyros, such as the RAF 2000, at moderate airspeeds near their Minimum Power Required Speed (MPRS), are statically AOA stable and will exhibit slow rate phugoid pitch oscillations. The existence of oscillations indicate the AOA static stability! However, as in the case of the RAF, some gyros might become statically AOA unstable at higher airspeeds (airframe aerodynamic nose-down moments) and at higher power levels – both of which are moving the “effective” RTV forward of the CG – to the Chuck B. “classic” definition of AOA unstable and able to buntover!
If a gyro approaches static AOA instability (with a worsening power/airspeed condition), the static restoring AOA pitch moment becomes weaker and weaker. As it does so, the phugoid dynamic oscillations become less convergent, neutral, and eventually divergent oscillations. The standard requires that a gyro not be dynamically unstable – in other words, still able to oscillate, but with diminishing oscillations. But, even if the dynamic phugoid oscillations become neutral or divergent, they are still oscillations, and there is still static AOA stability – however weakening! If we require a gyro to be dynamically stable – excited phugoid oscillations eventually diminish to zero - that gyro still has a margin of static AOA stability – it is assuredly statically AOA stable and cannot buntover in that flight (power and airspeed) condition!
So, then how do we test for the ability or not for a gyro to buntover? We conduct the standard Fixed Stick Dynamic Test. The key to this is that the stick must be truly mechanically fixed. It is not good enough to just try to hold the stick steady! An experienced pilot just cannot avoid stabilizing inputs! With the stick fixed, fly at MPRS in afternoon turbulence to observe that the pitch oscillations that are excited do not start getting worse and worse – divergent dynamic response – dynamically unstable and weak static AOA stability. (This test can also be done in calm air with a pilot “step” input on the stick – like professionals do it – but just flying in turbulence will work for informal testing just as well!) Then, increase the Fixed Stick airspeed by about 5 mph – change the fixed position of the stick - and watch for divergent oscillations in moderate turbulence again! Keep increasing airspeed and power to see if there is any airspeed where the pitch oscillations start getting worse and you, the pilot, release the fixed stick to stabilize the oscillations. This is the point, beyond which, that gyro will have meager AOA static stability, or beyond which it will be actually statically AOA unstable and able to buntover! That determines the flight condition (power and airspeed) beyond which that gyro can buntover or PPO. That should identify your Vne condition. Gyros that are capable of bunting over will only do so at statically unstable combinations of power and airspeed – normally at higher airspeed because airframe aerodynamic moments are normally statically destabilizing! This flight testing will determine your “safe operating flight area”. A really good thing to know – especially since buntovers are so insidiously surprising otherwise!
This test should be conducted at higher airspeeds with lower power settings as well – the prop thrust moment or lack of moment can contribute to AOA instability on certain configurations!
This Phugoid dynamic stability test is simple and safe to be conducted by any gyro pilot that is proficient in flying that particular gyro in moderate (afternoon thermal) turbulence conditions. The second post below will describe two ways to safely “fix the stick” for this testing. Jerry Tiahrt helped develop the “chain” or rope method. This is how he identified the buntover susceptible flight conditions for his RAF 2000.
I would appreciate any comments or flight testing feedback on these concepts. I have been evaluating these concepts for some time, and I think it is time to start sharing all this with others to hopefully provide some flight test feedback and validation. I have become convinced that this simple flight test will truly indicate if or when a gyro can buntover (or PPO). But what about PIO? To pass this test, a gyro must have a dynamic damper – a HS! It turns out that a HS, besides providing static stability directly, actually provides dynamic damping that also extends the static AOA stable margins and range. The dynamic damper is also the mechanism that prevents rapid PIO pitch oscillations (the rapid, constant airspeed, varying AOA oscillations) that promote PIO. I don’t have complete theoretical reasoning for this, but it can be expected that any gyro that is shown to be buntover proof from the above testing, will also be highly resistant to PIO due to its strong dynamic damping..
There are many aspects of the above theory that I haven’t discussed here – such as susceptibility to buntover in a push over the top of a zoom! But, the above are the basics. I hope some of you will provide some flight testing – and I hope this flight testing really opens some eyes – as it did for Jerry Tiahrt!
If you do some testing, please share your results.
- Greg Gremminger
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