Flutter?

LAST EDITED ON Aug-23-03 AT 06:51 PM (CDT)
 
Watched one of those strange episodes of flutter put one into the turf today. The plane was a scratch built Chief with a Fox 35 flying a good 4-2-4 in a 5-8 MPH wind. The flutter started around 3 1/2 minutes into the flight and one of the flyers said it only fluttered on the upwind side of the circle. When it went by the elevator and flaps were shakin' to beat the band. The plane had quite a few flights on it and I don't recall it exibiting anything like this before. Four or five laps after the flutter started something let go and the plane went straight in from about 5 feet like you had crammed it on full down. Lots of pieces but amazingly the prop didn't break. Is flutter one of those freak things that happen or is there some things that can be done to avoid it?

Flutter is usually the result of slop in the control system. Flexible control surfaces and engine vibration can also be a cause. It is possible to statically balance control surfaces to eliminate the posibility but with our airplanes it usually isn't worth the effort.

Have you checked your hinges? It is possible one or more of the hinges let go, and then the problem escalated.

Maybe the owner will bring all of the pieces back to the field and we can do a decent post crash inspection. If it was a faulty hinge it would probably be hard to determine at this point. Wish I'd had the video camera handy today. It was a strange thing watching the controls vibrate so wildly.

Tendency to flutter can be reduced by having the surfaces mass forward. This applies to wing, tail, and control surfaces. Torsional stiffness is important, as is the stiffness of the control system, as already pointed out. Be sure your pushrod can't travel up/down in the bellcrank, L/R in the flap horn and elevator horn. Some folks leave off the bearings at the flap horns (tubes, springs or straps glued to the TE), allowing some fore-aft at the middle as control is applied. This is tough on the hinges, and could lead to failure. An autopsy would be interesting. Please report on the results if one can be arranged. This is a pretty rare thing in stunters. Steve

>Tendency to flutter can be reduced by having the surfaces
>mass forward. This applies to wing, tail, and control
>surfaces. Torsional stiffness is important, as is the
>stiffness of the control system, as already pointed out. Be
>sure your pushrod can't travel up/down in the bellcrank, L/R
>in the flap horn and elevator horn. Some folks leave off the
>bearings at the flap horns (tubes, springs or straps glued
>to the TE), allowing some fore-aft at the middle as control
>is applied. This is tough on the hinges, and could lead to
>failure. An autopsy would be interesting. Please report on
>the results if one can be arranged. This is a pretty rare
>thing in stunters. Steve

I have seen flutter problems a few times on stunt planes, but it always turned out to be the result of something being broken. For instance, I had a flutter in the stab of an airplane, and I fought it all sorts of ways. But it turned out that the stab had broken in the middle and the only thing holding it together was the fuselage sides.

This case seems pretty clearly related to something being broken, causing it ti flutter, and then later broke all the way. I think Leonard's guess was pretty likely - one hinge broke or pulled loose, the surface fluttered, leading to the others breaking or coming loose, and on the way out, gave it full down.

I would agree that it was probably a broken hinge. But watch balancing control surfaces. That can cause flutter, too. Don't ask me how I know!
Bare

In the 60's, I had a Chief that fluttered. I solved it by moving the pushrods on the bellcrank, the flap and the elevator to the holes further out from the hinge. In the process, I probably put them on holes that were less worn out, i.e. less play. It seemed to work.

I am not real sure what happened. This was on the second flight and started after some outside loops. It only fluttered going into the wind. I had crashed this Chief before and the wings were broken off. I believe I did a good job putting it back together but maybe the wing load caused something to let go. All the controls were still connected and none loose. I used fabric hinges and all are still together except for the left wing flap. Actually not much is left of the left wing so maybe the hinges giving way was to be expected. I will take it to the field this weekend and get the "Expert Tulsa Gluedobber Crash Investigation Team" to do their thing. I had just put a wind screen on the cockpit and it was looking good. You know, I wonder why it went straight down instead of straight up? It was dramatic as it was but would have been much more after a wing over. By the way, no one was injured in the accident.

Bob Beasley

Bob,
Very sorry to hear about the Chief. It was looking good again after the rebuild and the new graphics. What kind of handle were you using?
Probably didn't have anything to do with the flutter, but ask Mitch how fast a plane will nose in if everything isn't tight on your handle.

Peter Garrison wrote a (typically) fine article for AIR&SPACE on the issue of flutter. He titled it 'The Hammer' to attempt to convey what flutter can do to an airframe.

There is a copy of the piece online at the A&S website:

http://www.AirAndSpaceMagazine.com/ASM/Mag/Index/2001/FM/Hamm.html

And if you follow the links you will find our 'flutter lab', a collection of quicktime movies showing some spectacular examples of flutter in action.

http://www.AirAndSpaceMagazine.com/asm/mag/supp/FM01/hammer.html

Cy