Flaps for Landings and takeoffs
Flaps are hinged surfaces on the trailing edge of the wings of a fixed-wing aircraft. As flaps are extended, the stalling speed of the aircraft is reduced. Flaps are also used on the leading edge of the wings of some high-speed jet aircraft, where they may be called slats or Krueger Flaps.
Flaps increase the camber of the wing airfoil, thus raising the lift coefficient. This increase in lift coefficient allows the aircraft to generate a given amount of lift with a slower speed. Therefore, extending the flaps will reduce the stalling speed of an aircraft.
Some trailing edge flap systems increase the planform area of the wing in addition to changing the camber. In turn, the larger lifting surface will allow the aircraft to generate a given amount of lift with a slower speed, thus further reducing stalling speed. Although this effect is very similar to increasing the lift coefficient, raising the planform area of the wing does not itself raise the lift coefficient. The Fowler flap is an example of a flap system that increases the planform area of the wing in addition to increasing the camber.
The general lift equation for an aircraft in flight demonstrates these relationships:
where:
L is the lift,
ρ is the air density,
V is the velocity of the aircraft (airspeed),
S is the planform area of the wing and
CL is the aircraft lift coefficient
Here, it can be seen that increasing the area (S) and lift coefficient (CL) will allow a similar amount of lift to be generated at a slower airspeed (V).
Extending the flaps also increases the drag coefficient of the aircraft. Therefore, for any given weight and airspeed, flaps increase the drag force. Flaps increase the drag coefficient of an aircraft because of higher induced drag caused by the distorted spanwise lift distribution on the wing with flaps extended. Some flaps increase the planform area of the wing and, for any given speed, this also increases the parasitic drag component of total drag.
Depending on the aircraft type, flaps may be partially extended for takeoff. With light aircraft, use of flaps for takeoff may be optional and will depend on the method of takeoff (e.g., short field, soft field, normal, etc.) When flaps are partially extended for takeoff it is to give the aircraft a slower stalling speed but with little increase in drag. A slower stalling speed allows the aircraft to take off in a shorter distance. Flaps are usually fully extended for landing to give the aircraft a slower stalling speed so the approach to landing can be flown more slowly, allowing the aircraft to land in a shorter distance. The higher drag associated with fully extended flaps allows a steeper approach to the landing site. This is the benefit of the higher drag coefficient of fully extended flaps.
Flaps used in air combat
Flaps could be used for the higher drag coefficient to slow the aircraft and to reduce the stalling speed. Forcing an overshoot in a rolling sissors.