Sunday 26 May 2013 06:42 UTC
The place for technical discussions about GA and flying.
Technical discussions about GA only, please.
I've seen flutter in some types at relatively low speeds. It's a hellishly complex business, but yes flutter onset is a function of TAS, and *usually* happens at higher airspeeds.
Regardless however, if you see something you think may be flutter - change conditions immediately. Fly slower, faster, more g, less g - anything to change the flight conditions. Then stay away from the speed/g/altitude combination that the flutter occurred at until you've landed, looked hard at it, and thought hard about it.
A few types have a reduced Vne at altitude because of a TAS triggered flutter onset, but not all that many. Whether the Vans is one of those, I've not idea.
Been there, lived!
Van's have stored a good article that recalls the particular RV-4 incident at http://vansaircraft.com/pdf/hp_limts.pdf.
It alludes to why he doesn't like turbo-charged engines in RVs as they may encourage 170 knots IAS at 20,000' and the possible consequences of that.
I have the same problem not that we need to go close to VNE anymore. Its almost vertical and gravity is a large part of it with a fixed pitch prop in the Musketeer i cruise at 110mph and VNE is 175 mph.
Worse aircraft i have had at VNE was an Aircoupe.
And sometimes something asty does happen; dive testing of light aircraft is definitely still a bit scary. You have to make a positive attempt to induce flutter while flying at Vd, which basically means making an abrupt (but not too large) control movement and watching to see if anything falls off. I had the pleasure of working with a test pilot who had to do this on a Rockwell Commander 112 in 1971. While in a dive at 208kt, he applied the required pulse to the elevator. The horizontal stabiliser, most of the fin and the rudder broke off 0.3 seconds later. The pilot got out - or rather was thrown out as the rest of the aircraft broke apart under the sudden high loads. The flight test engineer (who had done the flutter calculations that said the test should be safe) wasn't so lucky.
For certified aircraft the manufacturer has to prove by data analysis that the aircraft will not experience flutter at speeds up to 1.2 Vd. That leaves a very comfortable margin beyond any speed that will be experienced in normal flight.
Damn, I was going to buy an MCR.
A fine aircraft with fat margins.
Seriously, what worries me is that the flight testing is done by the manufacturer and at the end of the test certain numbers are written on a piece of paper. There was probably only 1 person present on the flight. You get the idea?
I've seen flutter several times, either as a TP or FTE, and can fully appreciate how this happened, plus I've worked with several of the leading expert analysists of the subject. The worst aeroplane I've seen it on was the thankfully defunct E-series Shadow, the only example of which being exported eventually to a country with no airworthiness oversight and I sincerely hope that it's now firmly scrapped and rotting in the corner of a field somewhere. (That was elevator trim tab flutter which coupled with a natural short period pitch oscillatory mode, and was really rather disturbing.)
It's left me with a healthy disrespect for flutter analysis. It's a black art at the best of times, and even now with the amount of computing power potentially available, any calculation of flutter modes and onset speeds, is frankly an inspired guess. I'd always have this analysis, but equally I'd never trust it and always want at-least one, preferably two, clear survival strategies available if doing that sort of testing.
In 1971 of-course the computing power wasn't there and the level of analysis wasn't there. I know nothing about the incident, but I'm inclined to believe that that FTE, if he was properly trained and supported, was just unlucky rather than stupid in any way.
Flight testing is a dangerous game - always was, always will be. Even in 1971 however, it was mature enough as a field to recognise the risks and the likelihood of errors in the flutter predictions, and plan for those.
I'm finding the contributions here educational, thanks!
What is it that physically casues the onset of flutter, is it simply airflow over the surfaces at specific angles and speeds resulting in a ferquency of airflow which, when it matches the necessary frequency, transfers the movement to the surfaces?'
Here's a video of the DG-300 sailplane flutter test.
Unlike most power planes, sailplanes, especially in competitions are frequently flown at Vne, even in rough air
It's co-incidence of Von-Karman vortex shedding frequency from a piece of structure, and the resonant frequency of that structure relative to it's larger supporting structure - along with neutral to negative overall systems damping.
The most visually impressive example of flutter in history was the Tacoma Narrows bridge....
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