I find on my RV3 with half, (maybe 25 degrees), flaps the ailerons get sluggish or have diminished response. Does anyone know the aerodynamic reason for this?

Because the model is flying so much slower.

I have done stalls and spins in a 152 Cesna. As you approach the stall speed the ailerons have less and less effect. In a full stall they have none. You keep the plane flying as long as you can using the rudder, of course it gets to the point it will not fly anymore,then comes the thrill. Not very thrilling if you are to close to the mother earth! If you want to do a spin you kick the rudder full over and hold full up elevator ,you will spin untill you release them and add power. Bottom line, as you get closer to the stall speed the ailirons start to lose their effect. Models are most usually over powered,and if you had one that was powered on the same level as the full scale you would soon see what I mean. Of course a plane like the Pitts ,etc. , that are made to be aerobatic fly more like a lot of our models do! Even these will stall and crash.

Thanks, but this happens at cruise speeds?? Also the plane does not pitch up or down noticeably.