Guys, "Flutter" is cause by exceeding the airframes capabilities(flying faster than the airframe can handle). Gap has very little to do with it. Slop in the control surface can cause it.

Many planes with airelons are very capable of creating Flutter. If the airelon goes all the way out to the edge of the wing. Especially if the airelon is very "Soft" "(ie twists easily) then in flight it flops like laundry on the clothes line. Look at a flag in a high wind. That is what the surface is doing.

In twenty plus years I have had quite a few different airplanes "flutter" during high speed flight. Most are overpowered.

It is very easy to "stop" the flutter. Slow the airplane down. Quit exceeding power requirements. build a fixed area on the outboard area of the wing and shorten the airelon one to two inches. Thus the airflow around the tip is not amplifying the flutter and the airelon end is not in the tip vortex.

I had a 60 size pattern plane not overpowered and the airelons fluttered during high speed flight. Upon inspection I found the servo arm (gear lash) had "slop" thus causing the the airelon to flutter, putting a small input stopped it. I replaced the servo and no more flutter.

In most cases there is a "Mechanical" problem that is causing the flutter. A proper detailed inspection can find the cause.