The book "Basics of R/C Model Aircraft Design" by Andy Lennon, ISBN 0-911295-40-2, is the definitive sourcebook for information on STOL and high-lift devices. This will gvie you more information on flaps than you'd care to know.

Flaps aren't necessary on small models, especially trainers, but they are useable. Several years ago, on my Sig LT-40 trainer, I modified the full-span ailerons to include flaps. I cut the ailerons and turned the inner 1/3 into simple flaps, operating the "new" flaps from the original single center aileron servo. The outer part of the ailerons were still ailerons and I added two separate aileron servos in the wing. Not a necessity, but I did get experience with setting up and using flaps for later planes.

--Bill