RE: Questions regarding Flutter
The plane looks well made. I would say check your linkages to the ailerons, you should have zero slop. Two things I would look into, only because you said you had one before and it got flutter:
First make sure that the servo tray that the aileron servo screws into is not flexing, also make sure it''s glued in well if it breaks loose you could get flutter and destroy the plane so quickly you just thought it was the flutter that caused the problem, when really it was a broken out aileron servo causing/allowing the aileron to flutter.
Second I''d really crank down on the servo mounting screws to take out as much of the servo mounting grommet flex as possible. Knowing I had flutter last time on the ailerons, I might even remove the grommets entirely, put some Goop on the mount and screw the servos in.