Dive brakes
From FlightSim
Dive brakes are devices controlled by the pilot which extend out from the bottom of the wing or from the side of the fuselage to add drag, to aid in control of speed and descent.
Gliders and sailplanes often employ both spoilers and dive brakes to aid in glide path control. Few other civilian aircraft use dive brakes, but they've been common on some military aircraft since WWII or before. Dive bombers, such as the German Stuka, used dive brakes to enable them to dive very steeply without gaining excessive speed, thus improving their bombing accuracy.
With the advent of high speed propeller driven fighter aircraft in WWII (P-38, P-51, etc.), pilots started running into problems of controllability and even in-flight breakup when chasing enemy fighters in a high speed dive, as they approached a high percentage of the speed of sound. Eventually, dive brakes were found to be the solution, allowing the pilot to avoid getting into the sped regime that generated dangerous shock waves that wrested control from the pilot and, often, damaged or destroyed the aircraft.
See also the dive brake page or the air brake (aircraft) page on Wikipedia.
