thumb|upright=1.14|A leading-edge slot on a [[STOL aircraft]]
A leading-edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of attack and thus reduce the stall speed.
Purpose and development
thumb|right|Full-span leading-edge slot in the wing of the PZL-104M Wilga 2000
At an angle of attack above about 15° many airfoils enter the stall. Modification of such an airfoil with a fixed leading-edge slot can increase the stalling angle to between 22° and 25°.
Slots were first developed by Handley Page in 1919 and the first aircraft to fly with them was the experimental H.P.17, a modified Airco DH.9A. Their invention is credited jointly to Sir Frederick Handley Page and Gustav Lachmann. The first aircraft fitted with controllable slots was the Handley Page H.P.20. Licensing the design became one of Handley Page's major sources of income in the 1920s.
Similar, but retractable, leading-edge devices are called slats. When the slat opens, it creates a slot between the slat and the remainder of the wing; retracted, the drag is reduced.
A fixed leading-edge slot can increase the maximum lift coefficient of an airfoil section by 40%. In conjunction with a slat, the increase in maximum lift coefficient can be 50% or even 60%.
Unlike trailing edge flaps, leading-edge slots do not increase the lift coefficient at zero angle of attack since they do not alter the camber.
Operation
thumb|Partial-span leading-edge slot in the wing of a Stinson 108-3
thumb|[[Fieseler Storch with full-span slots]]
thumb|A [[Zenith STOL CH 701 showing its full-span slot.]]
A leading-edge slot is a fixed (non-closing) gap behind the wing's leading edge. Air from below the wing can accelerate through the slot towards the low pressure region above the wing, and exit from the slot moving parallel to the upper wing surface. This high-speed flow then mixes with the boundary layer attached to the upper surface and delays boundary layer separation from the upper surface.
Slots naturally exact a penalty on the aircraft in which they are used. This is because they contribute to drag compared to an unslotted wing. The extra drag at low speed is acceptable because of the beneficial reduction in stall speed and improvement in handling characteristics, but at higher speeds the extra drag contributed by slots is a significant disadvantage because it reduces cruising speed and increases fuel consumption per unit distance flown.
One way to reduce the cruise drag of slots is to make them able to be closed. This arrangement is known as leading-edge slats. Aerodynamically, slats work in the same way as fixed slots but slats can be retracted at higher speeds when they are not needed. Slats, in turn, are heavier and more complex than slots.
In aircraft other than specialist STOL aircraft, full-span slots have serious drawbacks because, to take advantage of the high angle of attack at the stall, they usually necessitate long undercarriage legs that either cause high drag or are longer than can be accommodated easily inside the airframe.
Partial-span slots are usually found only on the outboard portion of the wing where they ensure airflow over that portion of the wing will remain unstalled at higher angles of attack than the inboard portions of the wing. This ensures the wing root stalls first and contributes to docile stall behaviour and maintaining aileron control throughout the stall.