Flap (aeronautics)

In aeronautics, a flap is a high-lift device used to manage an aircraft's flight performance. These components are usually mounted on the trailing edges of a fixed-wing aircraft. Their primary purpose is to reduce the stalling speed of the wing. The stalling speed is the minimum velocity required for a wing to generate enough lift to maintain flight. By using flaps, pilots can decrease both take-off and landing distances.

Flaps function by altering the physical properties of the wing. When extended, they increase the wing's camber, which is the curvature of the airfoil. This change increases the maximum lift coefficient, or the upper limit of lift a wing can produce. Some flap designs also increase the total surface area of the wing. According to the airplane lift equation, increasing the wing area (S) or the lift coefficient allows the aircraft to generate the same amount of lift at a lower airspeed (V). This mechanism is essential for Short Take-Off and Landing (STOL) operations.

Most aircraft use partial-span flaps, which extend from near the wing root to the inboard end of the ailerons. Extending these flaps changes the spanwise lift distribution across the wing. This causes the inboard half of the wing to provide a larger proportion of the lift. Meanwhile, the outboard half of the wing provides less lift. This reduction in outboard lift also reduces the angle of attack on that part of the wing. This is beneficial because it increases the margin above the stall for the outer wing. This helps maintain aileron effectiveness and reduces the risk of an asymmetric stall or spinning.

While flaps increase lift, they also increase drag. This happens because the altered lift distribution increases lift-induced drag. Some flaps also increase parasitic drag by increasing the wing's surface area. During an approach to landing, this extra drag can be useful. It allows the aircraft to descend at a steeper angle. However, flaps are retracted when they are no longer needed to avoid unnecessary resistance.

There are several distinct types of flap designs. Plain flaps are the simplest, where the rear portion of the airfoil rotates downward on a hinge. Split flaps, invented by Orville Wright and James M. H. Jacobs in 1920, only move the lower surface of the wing. These can act like spoilers by adding significant drag. Slotted flaps are more advanced because they create a gap between the wing and the flap. This gap allows high-pressure air from below to flow over the flap, keeping the airflow attached. Fowler flaps are a type of split flap that slides backward before hinging downward. This movement increases both the camber and the wing area.

The history of flap development shows a steady progression in aviation technology. In 1913 and 1914, the Royal Aircraft Factory and the National Physical Laboratory tested flaps in the United Kingdom. Although these tests were successful, the flaps were not yet installed on actual aircraft. In 1916, the Fairey Aviation Company improved upon these ideas. They created the Fairey Hamble Baby, the first aircraft to fly with flaps. These were full-span plain flaps that also acted as ailerons, a design known as flaperons. By 1917, Breguet had incorporated automatic flaps into the lower wing of their reconnaissance bombers.

Flaps are used in many different flight scenarios. During takeoff, pilots may use partial flap settings to reduce the ground roll. For example, the Cessna 172S Pilot Operating Handbook recommends 10 degrees of flaps for short runways or soft ground. During landing, flaps are often fully extended to allow for a slower approach speed. In gliders, flaps are used to optimize the wing for different speeds. Pilots may use them to fly more slowly while thermalling in rising air. Even fighter aircraft, such as the Nakajima Ki-43, have used special flaps to improve maneuverability during combat.