- Apr 21, 2008
- San Diego, CA. USA
- Piper Archer, Aviomania G1sb
- Total Flight Time
- Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
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!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?
Greg, what you say sounds compelling. But just from a physics point of view (I am a physicist with little to none prior education in aerodynamics) I have to take issue with the quoted statement above: changing the spring constant and changing the restoring force is one and the same and would, as you correctly point out, change the frequency of oscillation -- stronger restoring force/spring constant results in higher frequency. Conversely, lower restoring force/spring constant leads to lower frequency. No matter if we are talking about an "ideal" or "real" system, the oscillation frequency is always primarily affected by the spring constant.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.
This means the amplitude of the oscillations is getting larger and larger (dynamic instability) – rather than “converging” to smaller and smaller amplitudes and eventually disappearing (dynamic stability)OK ... May I have things clarified ... for both deep thinkers and newbies?------
For example ... what do you mean by "Divergent Oscillations"?
Phugoid oscillations are long-period (slower) oscillations – mostly in airspeed. This can be observed in pitch oscillations too – but the Angle of Attack (AOA) of the rotor and aircraft remains essentially the same during phugoid oscillations. Such oscillations are not the same as the dangerous “PIO” oscillations we often talk about. PIO oscillations are actually very short period (fast) oscillations of AOA – with relatively constant airspeed. PIO oscillations are true pitch oscillations as the AOA is going up and down quickly.Yes "Oscillations" (specifically Phugoid Oscillations) mean up and down pitch movements. But when you add the term "Divergent" you may think it means one thing, when actually it can mean something opposite (BTW, my object here is not to criticize but rather to clarify).
“Divergent” oscillations generally means the amplitude of each cycle is getting worse. The oscillation frequency may change – faster or slower – but generally divergence refers to the amplitude of the oscillations.Divergent means the pulling apart or spreading apart of ideas, relationships, movements and in this case "Oscillations". So in a gyros, a divergence of oscillations can mean that they eventually get farther and farther apart until they stop or "smooth out" to reach equilibrium or "stability".
“Convergence” can also refer to both static and dynamic stability. In the case of dynamic oscillations, “convergence” refers to the amplitudes getting smaller and smaller until they (hopefully) disappear. Dynamic stability is achieved with a dynamic “damper”.that creates a force or moment in the opposite direction of a movement - a force or moment that is also proportional to the velocity of that movement. and is created by the movement, not by the actual position (of the HS). The "damping" moment from a HS results from the upward or downward movement of the HS, not the actual AOA of the HS. This is different than the "restoring" force or moment, in timing and source, below.Also ... the word "convergent" can have opposite meanings as it relates to oscillations. Convergent means to get "closer together". This could mean that they get closer together to achieve "stability" (as in a ball rolling into a bowl ... the movements get tighter and tighter until the ball stops in total stability).