RE: Flaperons on an Avistar
cczarnik is referring to a conventional outboard aileron installation -- and he's correct in that regard. However, the Avistar & virtually all trainers have strip ailerons, so tip stalling with flapperons isn't a problem -- but the flapperons add little to a trainer (unless it is very nose heavy).
Taking the dihedral out of a wing is much less work than building a new wing.
Cut the wing with a band saw (or whatever you have available)
make a new straight-across wing joiner
Cut rectangular openings in the inboard ribs adjacent to the main spar for the new joiner
epoxy the new joiner into place
make wedges to fill the gap at the inboard ribs & glue into place
reinforce the joint with fiberglas or 1/32 balsa sheet (grain crosswise)
re-cover the center section
go flying