what is it?

Usually it is when the plane goes fast enough that the controls slam back and forth uncontrollably. Makes a funny noise and if you don't slow down, can lead to spontanious dissassembly of the plane. Cause is usually sloppy controll linkages.

Causes can also be due to morons flying a light 3D plane like its some kind of high speed racing plane. And a piss poor quality kit.

Take a piece of paper and hold it out your car window while you're driving. That's flutter. It is speed dependent and may be a moron flying a plane in a way not intended and may be a poor quality kit, but more often than not it's a sloppy control setup or flexible controls. Actually, the misusing the plane thing is pretty common too.

I'm one of those morons who likes to put my planes in terminal velocity power dives to see what happens. I've lost two elevators because of that. My controls are much better now and I still do the power dives, but I don't lose the planes any more.

SJM55! How are ya? The only thing that hasn't been mentioned is if your linkage isn't set high enough up on the control horn. It might be safe to say that if you are 3/4 of an inch up on the control horn, the linkage geometry will be able to hold the flap steady. If you are controlling your flap from one extreme end [like with some aileron setups] and the wood is too flimsy that can contribute to the problem. Not having sealed gaps can contribute. The cause can come from just one, or several contributing factors. The buzzing sound from extreme flutter is unmistakable. immediately throttle down and land. You can disintegrate the tiny plastic teeth on the servo gears in the blink of an eye. Unless you are disciplined enough to fly below flutter speed always, it's a good idea to try the simple fixes to eliminate the problem.

Unfortunately... the geometry of the control linkages is only one factor in flutter control, and the distance from hinge line to clevice on the control horn needs to be larger as the aircraft becomes larger. (and some other factors can change what will work and what won't)

Do a search on Flutter, and you will find a LOT of threads discussing the ways to prevent it.

The most common thing to break first with flutter is the hinge, or the servo gears. I have seen aileron flutter rip a wing off of one aircraft.

There's control surface flutter (which, technically isn't actually called "flutter", but we all call it flutter anyway)

And there's flutter of the entire structure, the whole wing or stab fluttering or flexing due to the aerodynamic loads on it.

Lots of causes for both, and lots of things to do to try to stop it. And yes, it's nasty. I stripped an aileron servo while testing out a new combat design last season. I went from flying fine to having 0 aileron control in an instant, and it was a fairly shallow dive that did it.

In general, flutter is when an outside force stimulates a structure at its natural frequency. Any and all surfaces will flutter if excited at the proper frequency. When that happens, a small input can produce a large output, enough to tear the structure apart. Keeping contol rods ridged, stiff with no slop or spring back prevents the initial initialization. Building stiff and ridgid (but light) surfaces also helps prevent flutter from ever starting (this raises the natural frequency above the exciting frequency). The biggest cause for flutter initiation in models is sloppy hinges, soft control systems or flimsy structure. Hinge gap is seldom a cause of flutter although it does cause surface deflection to be less effective.

What is it called?

Oh, you would have to ask that. I don't remember. I just call it flutter like 99% of all modelers. However, if you ask over on the aerodynamics forum, where guys can get really picky about exact terms for these things, you might get an answer.

More to the point, when you go searching for info on flutter in aerodynamic books and websites, they always are talking about the whole wing or stab fluttering around, they don't mention the control surface moving while the wing stays still.

But, really, who cares .

I thought about mentioning the wing and stab flutter also, but it is a lot less common. The control surface fluttering is the main problem I have seen in RC. Thats the only name I have ever heard it called, other than harmonic vibration, but I don't recall that being in RC.

There is a video of the full size F 117 stealth at an airshow. When they showed it in slow motion the whole wing flapped about three times, like a bird, and disintegrated in about two frames.