Hi Greg, ------

Of course, knowing the point whence you travel into the land of negative dynamic stability is a very good thing! And never shalt thou trespass beyond it. But the reason for this has nothing to do with static stability, in my understanding. You KNOW you have positive static stability as long as you also have its dynamic sibling. Once you lose the latter doesn't necessarily mean you're any closer to losing the former. The only thing you've lost is one indication that you're still statically stable.

What say you?

-- Chris.

Hi Chris, I also agree these discussions should be kept on this forum. My intent on this thread is to vet this concept, and all thoughts need to be presented to all who might be able to contribute. I need to think about this a bit more, but I do think I agree with all you presented in this last post. True, all we know for sure, if any phugoid oscillation is still present, is that there is still static AOA stability existing. So stopping the test at the simple loss of dynamic stability, when the oscillation is no longer damped, does not insure that static stability will certainly be lost when pushed further. Safety prudence says we should not actually push the testing fully into the dynamic instability realm to find out!

But, I think we are seeing some correlation between accident histories and dynamic instability occurring within that gyro's common operating power/airspeed envelope. We don't have a lot of data, but we do have two reports of the RAF 2000 exhibiting dynamic instability well within the commonly accepted flight envelope - dynamic instability at 80 mph, vs. a published Vne of 125 mph. The SH and Magni are examples at the other extreme - no history of buntovers (or PIO), and no reported dynamic instability within their approved Vne/power operating envelope. Totally anecdotal, I know, but it is precisely this reason that I would like to have more data on more configurations - to validate or invalidate the concept with more correlation between accident histories and occurrences of dynamic instability.

However, I do think there is some reasoning that suggests that a diverging phugoid oscillation might be a valid indicator of weakening static restoring moment. Consider that a tendency for phugoid oscillations to grow in amplitude can be the result of

**two** factors that might be changing with worsening power/airspeed combo:

1) Dynamic damping is getting weaker. The inherent (fixed stick) dynamic damping of a gyro comes mostly (or completely) from the HS. The common thread in all buntovers (and PIOs) is higher airspeeds - where dynamic damping is actually increasing at a square factor rate! It seems improbably that a phugoid oscillation would grow or become dynamically unstable (less damped?) at higher airspeeds when the damping mechanism is becoming stronger! I tend to dismiss decreasing dynamic damping to be the cause of this dynamic instability because damping is increasing at the higher airspeeds where buntovers – static AOA instability - more likely occur!

2) The second factor that can grow the amplitude of phugoid oscillations is a weakening of the restoring moment - static AOA stability margin. Like a weaker spring constant! We do know that the higher airspeed is most likely moving the CG aftward - reducing the static stability margin at higher airspeeds - for HTL, due to higher prop thrust and higher airspeed moments on the airframe; and for LTL due to reducing prop thrust and higher nose-down airspeed moments on the airframe.

The oscillations reported to be growing at this power/airspeed point are growing in amplitude, not in frequency! An increase in frequency, in my thinking, would be the result of an increasing spring constant, an increasing restoring force - increasing static stability. However, if the amplitude of the oscillation is what is growing - as reported so far - IMO this indicates a weakening restoring force - exactly what we expect to happen as a result of the reducing RTV / CG spread occurring under such worsening power/airspeed combination.

I suggest that, because the dynamic damper (HS) should be improving with higher airspeed, the actual cause of growing phugoid oscillation amplitudes would have to be a reducing restoring moment – weakening static AOA stability. And, if it is this weakening static AOA stability margin that is the cause of this dynamic instability onset, we know that is what does happen under the worsening power/airspeed conditions.

I admit I’m not smart enough to devise the equations that express this mathematically – and my intuition may not be technically correct. Maybe a dynamics expert or student can provide some mathematical explanation of what factors are actually indicated by our flight test results. Having been burned before by theory vs. flight test reports, I tend to rely on flight test results correlation with real accident history as the final validation – or at least the validation that would be better understood by most of us in the sport!

I also agree with you that control surface flutter is a poor analogy – control flutter is mostly a resonance with other structural components – a better analogy for PIO where the pilot provides the “resonant” input, than for fixed stick phugoid dynamic instability.

- Thanks, and please continue the conversation - Greg