seal the gap isn't for flutter,it's to stop the uneven air flow of the controls making the plane more precise in the controls,read a study once as if there is even 1/32 " of gap then sealing helps,I seal all the controls.FLEX the control futher than you'll need then seal with tape or iron on covering,one side only,usually the bottom,on the rudder,pick a side.You can seal both sides but I've never needed to,If the gap isn't to large I use clear silicon