Frmo what I have seen on this model, the aileron horn plates in the ailerons are inadequate, as are the supplied horns...

If you replace the horns for more suitable items then there is not sufficient meat to locate them to in the aileron... you really need to peel back a square of the covering, inset a block and ply plate, and then re cover, fix superior horns with a large footprint and you should be better off....

Take a close look at where your horn mounts... hold the servo (or with digis just switch on !) and ush the surface back and forth - look closely at the point of contact of the horn to the surface, and that general area - see whats moving !

Ive seen flutter occur on many models with large surfaces where insufficient attention has been given to the proper spread of load from control rod , thru horn, to surface...

Its not your fault - it is in my view a design flaw and one that ZN will hopefully fix - its an easy job. At the same time they need ot move the aileron servo bay over a little to provide scope for large arms and proper geometry of the control rod..

Nice plane through isnt it !