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Post by Br. Marius on Apr 22, 2015 16:17:14 GMT
Justify the following:
(a) Pitch divergence does not depend on elastic axis position.
(b) A great effort is made to place cg and elastic axis at the quarter-chord position.
(c) After the blade was built, flight-testing showed flap-lag flutter. Any quick fix you can suggest to remedy this problem.
(d) Pitch-flap flutter does not depend on thrust level.
(e) If you reduce the rotational speed, can it be a concern for aeroelastic instability? Flap-lag or pitch-flap or both?
(f) During the wind tunnel testing of a rotor, the large amplitude vibration signal was observed from a strain gage mounted near the root of the blade. How would you identify whether it is flutter condition or just a regular forced vibration? If it happens to be a flutter condition, what type of flutter is it?
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Post by matthorr on Apr 26, 2015 21:31:32 GMT
Here's what I have. Thoughts?
a) The static instability condition for pitch divergence is dependent on the chord-wise distance of the center of gravity behind the aerodynamic center. If the c.g. is ahead of the a.c., there is no possibility of pitch divergence, regardless of the location of the elastic axis. b) Pitch divergence can be prevented by keeping the c.g. on the a.c., and pitch-flap flutter can be prevented by keeping the c.g. on the elastic axis. Keeping all three coincident prevents both instabilities. c) Flap-lag flutter can be quickly remedied by the addition of more lag damping through a mechanical or elastomeric damper near the blade root or by introducing pitch-lag coupling (negative \(k_{p_\zeta}\) or \(\alpha_4\)). d) Pitch-flap flutter is caused by the coupling of the pitch and flap modes, but does not require aerodynamic forcing. It can occur even at zero thrust. e) If rotational speed \(\Omega\) is reduced, the values of the rotating torsion, lag and flap frequencies \(\nu_\theta\), \(\nu_\zeta\) and \(\nu_\beta\) are all reduced. For flap-lag, the flutter boundary is determined by both \(\nu_\zeta\) and \(\nu_\beta\), but with hinge offset, the effect of reducing \(\Omega\) is not the same on both, but in any case could bring the rotor inside the flutter boundary. Pitch-flap flutter can become unstable with reduced \(\Omega\) if the distance between the c.g. and a.c. are large enough, and reducing \(\Omega\) can cause pitch divergence. Therefore, reducing rotational speed can be a concern for both flap-lag and pitch-flap instabilities. f) The I would test to see if the vibration changes linearly with thrust level (indicative of an aerodynamically forced vibration) or exhibits non-linear behavior, indicative of flap-lag flutter. Since the vibration is measurable and not destructive, I would rule out pitch-flap flutter.
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Post by Br. Marius on Apr 27, 2015 18:34:21 GMT
With (e), wouldn't a decrease in rotational speed increase the torsion, lag, and flap frequencies? For a contribution of \(\frac{1}{\Omega^2}\) to each frequency, as \(\Omega\) decreases, the value of that fraction should increase (since the natural non-rotating frequency is constant), and hence the torsion, lag, and flap frequency should increase.
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Post by matthorr on Apr 27, 2015 18:42:53 GMT
Yes, you're right. For flap-lag, you could move up into an ellipse, so could be bad. For pitch-flap, divergence, increasing \(\nu_\theta\) is good, but you could move up into flutter instability. Though, in most cases, I think reducing \(\Omega\) would probably be good.
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Post by Br. Marius on Apr 28, 2015 0:38:28 GMT
Agreed. Sounds good!
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