The biggest cause of flutter is the ability of the surface to move in the airstream. While a large gap can contribute to flutter it is rarely the cause. As you noted, many plane have large gap and never suffer flutter. The gap is more likely to affect the lift and effectiveness of the surface. For the flutter and type of movement can cause the flutter. Any slop in your control surfaces can lead to it. Control linkages that are too small for the application can lead to it. Weak or damaged servos can lead to it. Worn our servo arms and control arms can lead to it. Wallowed out holes in control arms can lead to it. There are several others that can lead to it as well, but these are the biggest causes that will usually be culprits in flutter. Check your control surface to see if there is any movement with the radio turned on. Any free movement in the control surface is going to put the plane at risk for flutter.

All of the above it good to prevent it, but sometimes a plane can be in perfect shape and suffer from flutter (although it's sometimes rare for this to happen). Flutter can actually start as a harmonic resonance in the airframe that moves to flutter. This can and does happen usually in a matter of seconds and the plane will literally come apart in the air. Sometimes with control surfaces flutter you can cut the throttle and get down immediately before the airframe is destroyed. If you do get your plane back on the ground never ever go back up again until you determine what was causing the flutter. Going back up again without fixing the problem is just inviting flutter to destroy your plane.

Ken</p>