Don't really know if they're "needed".
I put flaps on mine between the nacelles and the fuselage. same chord as the ailerons.
They are effective especially when used with a small amount of flaperon outboard.
I had to mix in a couple clicks of down elevator to neutralize a pitch-up tendancy.

If I were to do it again, I would increase the flap span to include a couple inches outboard of the engine nacelles. I would also increase the flap chord.

Mine are driven by a pair of very short torque rods connected to a common servo in the wing center. Ailerons are driven by dedicated wing mounted servos.

Cheers,
-E