When I am ready to land I throttle back to about 1/3 throttle, then drop the retracts and then drop to full flaps. I do all those things before getting into the landing approach. On my TF P-51 the only thing the plane does when I drop to flaps is the attitude of the plane is a little more nose down while flying level (maybe 2 or 3 degrees) and the plane slows down a little quicker when throttle is lowered. With the flaps full down on landing the plane seems to stick better on touchdown. To answer patrnflyr's question, I leave the flaps full down after touchdown and taxi back to pit area. Here is a picture of the amount of flaps my P-51C has. Note the top surface of the flaps is about the same angle as the joint between the scoop and fuselage.