The F4U Corsair is an iconic WWII fighter that had a lot of success in the Pacific and later in Korea. It's most striking feature by far is it's strange wing shape which few other planes, especially comparable fighters, used.
So why did the Corsair have a wing like that?
Short answer: it helped to fly faster
Long answer below.
When Vought designed the F4U Corsair in the early 1940s it had a requirement for speed. Japanese A6M Zeros were dominating the Pacific early in the war with extreme maneuverability. But flight testing from captured zeros revealed they had a relatively low top speed and no hydraulic boosted control surfaces. This meant their maneuverability was crippled at high speeds and robbed them of their advantage. Late war fighters with much higher speeds could engage and disengage in battles with zeros completely at their leisure. The F4U was no different.
The primary source of the Corsair's extreme speed was it's massive engine - the Pratt and Whitney R-2800 Double Wasp.
It was an 18 cylinder, 46 liter, supercharged radial engine that could produce over 2000 horsepower. A powerful engine like that needed a large propeller to soak up all that horsepower and turn it into thrust. So the Corsair was fitted with a 3 or 4 bladed constant speed propeller that was a bit over 13 feet (4m) in diameter.
The problem with a large propeller is that it needs a lot of ground clearance. The plane should be able to have landing gear tall enough to allow the fuselage to be level without the propeller blade tips striking the ground. Tall landing gear isnt usually a problem because they have plenty of room to retract inwards into the fuselage or outwards into the wings. The Corsair couldn't do either because it was designed to operate from an aircraft carrier.
The wings were required to fold for storage on and below deck. Because of that the gear didn't have room to retract inward and couldn't be retracted outward into a wing folding mechanism. So Vought designed the gear to retract rearward into the trailing edge of the wing.
The drawback now is that the landing gear struts had to be shorter than the wing chord length to fit. So Vought decided to use a special wing design to add more length to the landing gear.
It's known as an inverted gull wing. This is referring to the shape of a seagull wing and its notable kinked shape, only upside-down.
The initial downward angle of the wings helped the landing gear dip down far enough to provide the extra length required. And the upward dihedral angle provided aerodynamic stability and wingtip clearance. The inverted gull wing also gave one more advantage that helped the F4U get a bit of extra speed - it reduced interference drag.
Interference drag is caused by airflow through the angles of structures as they are attached to the aircraft. Its especially strong where the wings attach to the fuselage.
The F4U had a low wing which attached to the lower section of the fuselage. A straight wing or one with an upward dihedral angle would have created a narrow connection and increased interference drag. The F4U had a relatively round fuselage and by using an inverted gull wing, the wings could be attached at nearly a 90° angle thanks to the intial downward angle.
This minimized the interference drag and made the Corsair more slick than other comparable fighters.
All of this and more made the F4U the fastest US fighter ever when it entered service at over 400mph in level flight. Later improvements pushed it to over 450mph. This gave it an enormous edge over Japanese aircraft that resulted in an 11:1 kill ratio.
