Flaps are a “high lift / high drag” device. Not only do they improve the lifting ability of the wing at slower speeds by changing the camber, or curvature of the wing, but when extended fully they also create more drag. This means an aircraft can descend (or lose altitude) faster, without gaining airspeed in the process. Flaps come in 4 main varieties: plain, split, slotted and fowler

The plain flap is the simplest of the four varieties. It works by lowering the aft portion of the wing, increasing its camber, which in turn causes the wing to produce more lift. Plain flaps are typically
used only when the aircraft is required to be as simple to construct as possible.

The split flap works by lowering just the bottom section of the wing. Rather than providing additional lift, the split flap is primarily used to allow an aircraft to descend quickly without gaining forward momentum, or airspeed. As such, it is typically found on aircraft that have to operate in smaller areas, such as those used for crop dusting, or in the Alaskan bush. It was invented by Orville Wright in 1920 and became popular in the 1930’s but, due to the large quanitity of drag it produces, has been rarely used since then.

The slotted and fowler flaps are both designed to physically increase the overall surface area of the wing, literally making the wing bigger. In addition, the slotted flap, as the name implies,
creates one or more slots within the wing. These slots provide additional energy to the air on the upper surface of the wing, ensuring that as the airspeed decreases, the air still has sufficient momentum to reach the rear of the wing. In technical terms, it is referred to as preventing the separation of the boundary layer.