Leading-edge slot

Lore & Background

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, and licensing the design became one of Handley Page's major sources of income in the 1920s.

A leading-edge slot is a fixed gap behind the wing's leading edge. Air from below the wing accelerates through the slot toward the low-pressure region above, exiting parallel to the upper surface. This high-speed flow mixes with the boundary layer, delaying separation and allowing higher angles of attack before stall. Unlike trailing edge flaps, leading-edge slots do not increase the lift coefficient at zero angle of attack because they do not alter the camber.

Slots naturally exact a drag penalty compared to an unslotted wing. At low speeds the extra drag is acceptable due to the reduction in stall speed and improved handling, but at higher speeds it reduces cruising speed and increases fuel consumption. One way to reduce cruise drag is to make the slot closable, resulting in leading-edge slats, which are heavier and more complex. At low angles of attack the airflow through the slot is insignificant, but at high angles it becomes significant, re-energising the boundary layer.

Reader's Guide

Leading-edge slots are significant as one of the earliest forms of boundary layer control. By re-energising the boundary layer through a fixed gap, they allow wings to operate at higher angles of attack before stalling, increasing the maximum lift coefficient by 40% for a fixed slot and up to 60% when combined with a slat. This makes them valuable for short take-off and landing (STOL) aircraft, where full-span slots are used on types like the Fieseler Storch, Dornier Do 27, PZL-104M Wilga 2000, and Zenair CH 701 STOL. However, full-span slots often require long undercarriage legs, which cause high drag or are difficult to accommodate. Partial-span slots, found on the outboard portion of the wing, ensure the wing root stalls first, maintaining aileron control and docile stall behaviour, similar to washout. Examples include the Stinson 108, Bristol Beaufort, Lockheed Hudson, and Dornier Do 28D-2 Skyservant. The legacy of the leading-edge slot includes its role in enabling safer low-speed flight and its influence on later retractable slat designs.

Did You Know?

• A fixed leading-edge slot can increase the maximum lift coefficient of an airfoil section by 40%.
• The first aircraft to fly with leading-edge slots was the Handley Page H.P.17, a modified Airco DH.9A.
• Slots were first developed in 1919 and are credited jointly to Sir Frederick Handley Page and Gustav Lachmann.
• Partial-span slots ensure the wing root stalls first, contributing to docile stall behaviour and maintaining aileron control.