Flaps

flaps

Leading-edge slats

Leading-edge slats increase the wing’s chamber area and mean aerodynamic chord (MAC), thereby increasing its coefficient of lift (CL) maximum, which reduces the aircraft’s stall speed.

Krueger flaps

Krueger flaps are leading-edge wing flaps used to increase the wing chamber and therefore increase the coefficient of lift maximum.

Fowler flaps

Fowler flaps are trailing-edge wing flaps (usually triple slotted) used to increase the wing area and chamber, which increases the coefficient of lift maximum for low flap settings, e.g., 1 to 25°. High flap settings increase drag predominately more than lift and therefore are used to lose speed and/or height, most commonly during an approach to land.

What is the primary use of flaps on a jet aircraft?

The primary use of flaps, especially on a jet aircraft, is to increase lift by extending the geometric chord line of the wing, which increase its chamber and area.

What are the effects of extending flaps in flight?

Lowering the flaps in flight generally will cause a change in the pitching moment. The direction and degree of the change in pitch depend on the relative original position of the center of pressure and the center of gravity. The factors that contribute to this are:

1. The increase in lift created by the increased wing area and chamber will lead to a pitch-up moment if the center of pressure remains in front of the center of gravity.

2. If the associated rearward movement of the center of pressure is behind the center of gravity, then this will produce a nose-down pitch.

3. The flaps will cause an increase in the downwash, and this will reduce the angle of attack of the tailplane, giving a nose-up moment.

4. The increase in drag caused by the flaps will cause a nose-up or nosedown moment depending on whether the flaps are above or below the lateral axis. The overall change and direction in the pitching moment will depend on which of these effects is predominant. Normally, the increased lift created by extending the wing chord line when the flaps are extended is dominant and will cause a nose-up pitching tendency because the center of pressure normally remains in front of the center of gravity.

What are the effects of raising flaps in flight?

The raising of flaps in flight, if not compensated for by increasing speed and changing attitude, will result in a loss of lift.

How do flaps affect takeoff ground run?

Flaps set within the takeoff range: A higher flap setting, within the takeoff range, will reduce the takeoff ground run for a given aircraft weight. The use of flaps increases the maximum coefficient of lift of the wing due to the increased chord line for a low drag penalty, which reduces the stall speed (Vg) and consequently the rotation (VR) and takeoff safety (V2) speeds.

This provides good acceleration until it has sufficient kinetic energy to reduce the takeoff ground run. Typically, various flap settings from the first to the penultimate flap setting are available for takeoff (i.e., takeoff range). The higher the flap setting within this range, the less is the takeoff run required because the drag is not significantly increased because the angle of attack is low. However, the drag increment is higher when the aircraft is in flight and out-of-ground effect because of the aircraft’s angle of attack is much higher.

Note: Initial and second-segment climb performance thus will be reduced with a high takeoff flap setting.

Flaps set outside the takeoff range: A high flap setting outside the takeoff range will result in a large drag penalty that will reduce the aircraft’s acceleration, and therefore, the takeoff run will be greatly increased before VR is attained. No or a very low flap setting outside the takeoff range on takeoff will result in a low coefficient of lift produced by the wing for a given speed, and thus a higher unstick (Vfi) speed is required to create the required lift for flight. Therefore, an increased takeoff run to attain the higher VR is required.

How does the use of flaps affect the aircraft’s takeoff performance?

The effect of flaps on the takeoff performance, i.e., TOR/TOD, and climb performance varies between different aircraft types, especially between swept-wing (jet) and straight-wing (turboprop) aircraft and further with the degree of flap deployed on individual aircraft.

Swept-wing (jet) aircraft: Swept-wing aircraft require a low flap setting, i.e., takeoff flap, to improve the CL during the takeoff. This has two positive effects.

First, a low flap setting reduces the aircraft’s takeoff run required (TORR) because the higher CL lowers the stalling speed (Vs), which in turn reduces the V2 and VR speeds and results in the aircraft reaching its liftoff speed from a shorter ground takeoff run (TORR).

Second, a low flap setting reduces the aircraft’s takeoff distance required (TODR) because the increased CL benefits outweigh the increased airborne drag and thereby improve the climb performance of the aircraft, resulting in the aircraft reaching the screen height over a shorter distance. Maximum takeoff flaps may be used to reduce the ground takeoff run required when the field length is limiting or the runway surface is poor. However, airborne climb performance may be compromised due to an increase in drag, which reduces the lift-drag ratio and results in a reduced rate of climb (climb gradient) performance.

The use of flap settings outside the takeoff range would increase aerodynamic drag during the ground run, causing a slower acceleration that results in an increased TORR and then once airborne would significantly degrade the climb gradient performance because of the poor lift-drag ratio that results in an unacceptable increase in TODR.

Straight-wing (turboprop) aircraft: Straight-wing (turboprop) aircraft usually require a low flap setting to improve the CL during takeoff. This has the effect of reducing the aircraft’s takeoff run required (TORR) because the higher CL lowers stalling speed (Vs), which in turn reduces VR speed and results in the aircraft reaching its liftoff speed (Vfi) from a shorter ground run (TORR). However, the takeoff distance required (TODR) to the screen height may not be reduced significantly, if at all, because, as well as increasing lift, flap deployment also increases drag, thus reducing the lift-drag ratio, which results in a lower rate of climb.

For this reason, only a small takeoff flap setting is used for the takeoff to maintain an adequate airborne climb performance. However, large takeoff flap settings may be used to reduce the ground takeoff run as much as possible as long as the climb performance to the screen height is not compromised when the field length (runway) is limiting or the runway surface is poor. Conversely, no flaps may be used when takeoff distance is limiting.

For large flap settings above the takeoff range, e.g., >20 degrees, the increased aerodynamic drag during the takeoff ground run causes a slower acceleration that results in an unacceptable increase in the takeoff run required and then once airborne significantly reduces the lift-drag ratio, which degrades the climb gradient performance to an unacceptable level. The principles of flap deployment on the takeoff performance for both swept- and straight-wing aircraft is similar, but the effects are much more acute for swept-wing aircraft. Therefore, the use of takeoff flap deployment is much more rigid on swept-wing aircraft, whereas straight-wing (turboprop) aircraft have more flexibility and variation among different aircraft types.

 

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