Hello jaav
I also build and fly SPADS. I suggest you use a separate servo for each aileron, because they are very large. Move the servos out on the wing so the rod is near the center of the ailerons. Move the servos closer to the aileron to shorten the rods so they are stiffer. Put about a 10 inch long stiffener rod, such as a bamboo skewer or a carbon hollow rod, into the flute of the ailerons, centered under the aileron bracket to which the servo rods connect. You cn get more help in the SPAD forum, below. You can see my latest SPAD there, http://www.rcuniverse.com/forum/m_8007330/tm.htm. Mine has no aileron flutter, but it did have elevator flutter. In my case, I reduced the elevator area a little.