Want to add flaps to a strip aileron . I've looked all over and can'f find a formula for how much of the aileron you can use for effective flaps and still retain good aileron control. Can anyone help?

10% of the wing area. You lose some authority at low speeds (if you're a high alpha/3-D flier) by removing them from the slip stream. And if you put all your aileron on at the wingtips you increase the likelyhood of stalling a wing and snap rolling at low speeds, also.

Have you considered flaperons that do both?

I have a Contender that has 12" of apron flap along the wing root - one solid piece below the fuselage and all - on a 60" +/- overall wingspan. Don't know of any formula.

Thanks for the info Charlie. The plane is a Bridi "Krafty 60" from BlueJay Models. It has a 60 or so inch span. It's an advanced trainer. I'm using an OS .46 AX. My radio has a flap knob, but no mixing capacity. I'll be modifying the kit to use in-wing aileron servos and will use the center wing servo for the flaps... like I did on a Senior Telemaster. The Sen. Tele. plans, however, showed where you could cut the ailerons for flaps. I'm not a hotdogger. I would just like to slow the Krafty down for landings and give it a little more lift for shorter takeoff runs. I liked the flaps on the Telemaster, but don't have enouth serious knowledge of aerodynamics to know how much flap area is enought, but not too much.