If you look at most flap design most are designed to add lift at 7-15 degrees. Then at the higher levels of 20-35 degrees you end up with more drag than lift. Stall speed are only a result of the true angle of attack relitive to the models movement thru the air. Fowler flaps create more wing area and greatly let you slow the plane down for landing. I test all of my 1/2 scale planes in a sim that you can buy of the web called X-plane. It will let you build any aircraft from models to full scale and test your design. It has helped me with everything form cg, stall speeds and wing designs.