Quote:
Originally Posted by Aviator168
I guest you did this at altitude. You were very brave.  |
I intend to do a lot of aerobatics, cloud chasing, zoomies, whifferdills, etc. If you fly these aircraft exuberantly long enough, someday you'll screw up a end up on the wrong side of VNE, maybe without a parachute and with a passenger. In the very unlikely event that my aircraft had a flutter problem, I would rather find it when I was all by myself, mentally prepared for the event, wearing parachute and helmet, ready to bail out if needed, at a safe altitude, etc.
The service history tells us that Van's designs have quite a bit of margin, as there are over 6000 of these things flying, and there have been no accidents that were clearly due to flutter. There was an
RV-4 accident in Australia where all tail surfaces separated from the aircraft while the aircraft was in a steep, power-on spiral dive. The investigators speculated that the tail surfaces separated because of the aerodynamic and inertial forces from the high speed, tight, power-on spiral dive. I think the fact that the elevator balance weights had separated from the tail surfaces suggests that elevator flutter may have occurred, at an unknown airspeed, possibly well in excess of VNE, but this is far from conclusive. Rob Ray did report a high frequency vibration and stick shaking in his hand in his RV-4 (see right side of page 3 of this
RVator article). Note that later RVs have thicker tail surface skins than RV-4s, which would increase the torsional rigidity, reducing the odds of flutter at any given speed. I know of no other event that even hints at possible flutter in RVs.
I am not suggesting that every RV builder should do a flutter flight test. There are many good reasons why most builders should feel no need to do this test.
Quote:
Originally Posted by logansc
Kevin: Have you reported on your flutter check methods, techniques, safety measures, and results? If you did, I missed them. I have plagiarized heavily from your test plan!
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Lee - The first step is preparation, both physical and mental. Physical - you need parachute and helmet, and you need to sort out the details of bailing out of the aircraft. I replaced the two bolts that secure the canopy bow to the rollers with removable quick release pins, so that I would only need to pull the canopy aft about two inches, then push up to allow the airstream to pull it off the aircraft. Before the flutter test points, I pulled the quick release pins, to make one less thing I needed to do before bailing out.
Mental - you should figure out every detail of the bail out procedure, and practice the steps in your aircraft until you can do them quickly.
The test itself is fairly simple. Chose an area with open land underneath, so you aren't risking people on the ground. You want smooth air and a good horizon. Altitude around 5,000 ft AGL, or higher. Review the bailout procedure, then accelerate in a dive to the first target speed, trimming the aircraft. Release the stick, and give it a good sharp rap laterally. If there is any vibration or noise, immediately pull back on the stick and pull the power back to slow down. Land ASAP and inspect the aircraft. If no vibration is felt, give a stick rap in the pitch direction. If no vibration, then do a rudder pedal kick.
Warning: be careful on all these stick raps and rudder kicks to not make much of a control deflection. A large control deflection at high speed could cause structural failure. It is better to start with small raps and kicks, and do another bigger one if needed, than to start too big. The airspeed should be built up in small increments, with stick raps and rudder kicks at each increment, rather than simply going to the end point and doing it once.
After the flight, do a detailed inspection of the structure, paying particular attention to control surfaces.
Results - I saw no hint of flutter at 220 KIAS at 5000 ft in my RV-8.
Airspeed range
Linear Mode
