Okay, lets assume we have an airplane with some sort of superstrong wing, but otherwise conventional configuration. We accelerate it gradually. Eventually, it will reach flutter speed, at which the wingtips will flap up and down with ever increasing amplitude in a standing wave. But we don't stop there - we keep accelerating. 1.5x flutter speed. 2x flutter speed. 5x flutter speed. 10. What happens to the flutter? Does it disappear, then reappear again with more peaks and troughs of the "standing wave", at every multiple of the initial flutter speed? Or does it continue to get worse and worse?
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asked Jul 6, 2020 at 17:52
Abdullah is not an AmalekiteAbdullah is not an Amalekite
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1 Answer
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Flutter happens when elastic and aerodynamic frequencies converge. Normally, the elastic one is (almost) fixed and the aerodynamic one increases with flight speed. When speed increases further and the frequencies move apart again, flutter will die down. However, with further increasing speed more elastic modes will be encountered with ever smaller gaps between them.
There are indeed cases of airplanes which have a flutter mode below minimum speed but are safe to operate within their certified speed limits. The linked article doesn't mention this, but if you fly that thing slowly enough, the tail surfaces will wildly swing caused by a torsional elastic mode of the rear fuselage being in frequency neighborhood with oscillating separations on the tail surfaces. Pretty scary but harmless at that low speed.
answered Jul 6, 2020 at 18:31
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1$\begingroup$ The Breezy I flew back in the 70s would do that because the skinny truss wan't very torsionally stiff, and you avoided looking back because the oscillations of the J-3 Cub tail assembly it had, at certain power settings and speeds, would scare the bejeebies out of you. It seemed to be mostly the natural frequency of the truss matching the beat frequency of the pusher prop's pressure waves, only about 5 feet ahead. $\endgroup$– John KCommented Jul 6, 2020 at 21:15
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$\begingroup$ ever smaller gaps means smaller speed increments, right? $\endgroup$ Commented Jul 7, 2020 at 6:42
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1$\begingroup$ I've witnessed that in a Libelle that flew overhead at my field abt 500 ft at Vne, dumping water at the end of the day, and there was a BRRRRR sound that lasted for about 1 sec, sort of like a drum roll sound, maybe 20 hZ. The pilot landed safely, with no apparent damage to the glider, somewhat alarmed to say the least. The Libelle's almond shaped cross section, with the narrow axis the lateral one, seems to be particularly prone to that sort of thing. $\endgroup$– John KCommented Jul 7, 2020 at 14:16
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1$\begingroup$ @JohnK: Scary indeed. The Libelle is rather lightweight and delicate, so even when this lasted only a second I would be concerned about the control linkages and their attachment points. After the flutter movie with the SB-9 was finished, the plane was inspected and the aileron linkages were found to be almost worn out. The next flight would had most likely resulted in a torn lug in the aileron linkage. $\endgroup$ Commented Jul 7, 2020 at 20:14