Leading-Edge Devices: Airplane Slots and Slats

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Flaps usually describe high-lift devices located on the wing’s trailing edge, but similar effects can be achieved at the wings leading edge. The following mechanisms are but a few among many fitting a broad category of leading-edge devices, which generally serve to accomplish the same goal as trailing-edge flaps.

Airplane Slots
Slots are a cleverly designed gap in the wing or flap and are used primarily to enable the wing to fly at higher angles of attack. Many of the more complicated fowler flaps employ double, and occasionally triple, rows of slots as required to meet specific runway performance requirements. Flaps of this nature are referred to as slotted flaps. Slots can do much to enhance the effectiveness of a wing flap, and also work well at the forward edge of the wing.

A typical leading-edge slot is comprised of an open gap aft of the leading edge, running spanwise, parallel to the spar.

Figure illustrates the difference between the airflow over a slotted airfoil at high angle of attack, as compared to that of a wing without slots. As you can see, air is able to flow through the slot, then along the upper surface of the wing in a manner preventing an undesirable stall. When the wing is at lower angles of attack, the slot has less effect on the flow of air. Slots are often placed in front of the ailerons, allowing better aileron control at high angles of attack.

Airplane Slats
Slats work by extending the leading edge downward, and forward, much like flaps work on the trailing edge of the wing. Figure depicts a leading-edge slat. Slats normally work on slides and rollers, allowing the leading edge of the wing to move down and out from the wing.

Slats are frequently designed to create a slot between the slat and the wing when the slat is extended. This provides additional ability for the wing to produce lift at higher angles of attack. Leading-edge slats can be extended mechanically with motors or hydraulics, or by natural aerodynamic loads. In the case of the latter, the force of air pushing against the slat holds it in a streamlined position at low angles of attack/high speeds. But when the plane slows sufficiently and angle of attack increases, the slat begins developing lift and flies itself into the extended position. When combined with fowler flaps, leading-edge slats can dramatically increase the camber of the airfoil.

Leading-edge flaps
A simple increase in camber can be achieved by simply drooping the leading edge of the wing. The effects are similar to trailing-edge flaps, and when combined with everything else, can dramatically increase an airplane’s ability to fly at slow speeds.

Figure depicts a leading-edge flap in both retracted and extended positions. Notice that the extended position greatly increases the airfoil camber. This increase in camber also increases both lift and drag. Interestingly, an airliner with both leading- and trailing edge devices fully extended requires an enormous amount of thrust to maintain level flight, and substantial amounts just to make a landing approach. Some airports restrict full-flap landings for large jets whenever possible for noise reasons.

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