This "flap" as you refer to it is more commonly known as a trim tab. It is used to affect the aerodynamic qualities of the craft to sometimes compensate for the torque created by the motor, or cause the plane to turn or fly in a desired direction. It's just a lot easier to do this than to bend, warp, or move normal control surfaces to get the plane to fly to your satisfaction. This usually takes some experimentation as to placement of the tab and it's size. I have seen as many as 3 used at the same time creating varied results. Here again "experiment" is the key to success. Hope this helps. Good flying.
thanks a lot for the explanation, i've add a weight for the torque and balance the plane, for new plane i use this solution
In the future if you post other photos of your plane please can post a detail of this "flap" ?
thanks still, sorry form my englis, and my basic question
I think what you're looking at on the Me-109 is a drag flap. It looks like it is straight down or perpendicular off the bottom surface of the wing. It looks like it's working too as the model is in a nice left hand bank in that shot. I like using these drag flaps because you can adjust the turn by increasing or decreasing the length of it. I've seen some guys glue a length of 1/16" square under the left wing. It does the same thing and you can just cut a length off as adjustment is needed.