What type of flap helped a World War II fighter turn best?

The F4U Corsair's wing received NACA-type slotted flaps, and at the same time the ailerons' span was increased to improve roll rate, with a corresponding reduction in flap span. Split flaps, used on aircraft such as the Supermarine Spitfire, generated considerable drag and little lift. With the flaps fully deployed to 50°, the Corsair could fly slowly enough to land on a carrier, despite its streamlined shape and high-speed capability.

The F4U-4 incorporated a dedicated 20° maneuver setting, usable at 200 knots indicated airspeed or below. In a descending speed spiral, that setting markedly reduced stall speed. That let the pilot briefly tighten the turn while the opposing aircraft remained above its own flap-extension speed limit. If the Corsair reached 200 knots first, its pilot could deploy the maneuver flaps and momentarily out-turn an opponent still constrained by speed.

Data from the National Advisory Committee for Aeronautics showed that flaps only improve turning performance once the aircraft is approaching its stall-limited speed.