On Thu, 1 Jan 2009 10:22:18 -0800 (PST), kd6veb
wrote:
[snip]

PS I have researched flutter without finding any really definitive
papers on the subject. It is widely said that if flutter occurs at say
200mph at sea level it will occur at the same speed at any altitude. I
find this difficult to believe. I always try to apply limit reasoning
to these kinds of problems. Say there was virtually no air would the
wing flutter in free space at 200mph. Of course not. So this reasoning
suggests to me that as the air density diminishes flutter speeds
increase. Now intuition sometimes let you down and there may be an
explanation why my take here is incorrect. Again any comments?

As a first, repeat FIRST, approximation, flutter depends on true
airspeed because it's a resonance effect.

When a wing is oscillating in torsion, the leading edge generates a
train of positive and negative pressure pulses that propagate back
along the chord to the trailing edge. If a positive pulse on the upper
surface reaches the trailing edge just as that edge is on the "up"
side of an oscillation, it will oppose the twist and tend to damp out
the oscillation; if it arrives when the TE is "down", it will
reinforce the oscillation. The relative timing depends on two things:
(1) the natural vibration frequency of the wing, and (2) the time it
takes for a pressure pulse to travel from LE to TE. The latter depends
directly on the true airspeed.

But there are a lot of other factors. For instance, the taper of the
wing means the pulse travel time differs at different spanwise
positions. The aeroelastic properties of the wing can put one part of
it on an "up" cycle when other parts are "down". The indicated
airspeed affects the amount of force the pressure pulses can
exert...and so forth.

So it's hard to say what speed really counts. Bottom line: If you fly
faster than the factory test pilot flew the machine, you're an
experimental test pilot...;-)

rj