Leading-edge extension

Lore & Background

The leading-edge extension (LEX) is a general term that can refer to several specific devices. One common type is the leading-edge root extension (LERX), a small, roughly triangular fillet running forward from the wing root to a point along the fuselage. On modern fighter aircraft, LERXes induce controlled airflow over the wing at high angles of attack, delaying the stall and consequent loss of lift. In cruising flight, the effect is minimal, but at high angles of attack—as in a dogfight or during takeoff and landing—the LERX generates a high-speed vortex that attaches to the top of the wing, maintaining airflow attachment well past the normal stall point.

Another type of leading-edge extension is the dogtooth, a small, sharp zig-zag break in the leading edge of a wing. It is usually used on a swept wing to generate a vortex flow field that prevents separated flow from progressing outboard at high angles of attack. The dogtooth can also be used on straight wings in a drooped leading edge arrangement. Many high-performance aircraft use the dogtooth design, including the F-15 Eagle (stabilizer), F-4 Phantom II, F/A-18 Super Hornet, CF-105 Arrow, F-8 Crusader, and the Ilyushin Il-62. Where the dogtooth is added as an afterthought, as on the Hawker Hunter and some Quest Kodiak variants, it is created by adding an extension to the outer section of the leading edge.

A further development is the leading-edge vortex controller (LEVCON), a continuation of LERX technology with actuation that allows leading edge vortices to be modified without adjusting the aircraft's attitude. This system has been incorporated in the Russian Sukhoi Su-57 and Indian HAL LCA Navy. When combined with thrust vectoring control, aircraft controllability at extreme angles of attack is further increased, assisting in supermaneuverability stunts such as Pugachev's Cobra.

Reader's Guide

The leading-edge extension, in its various forms, is a significant aerodynamic device for improving aircraft performance at high angles of attack and low airspeeds. The article notes that the primary reason for adding any such extension is to improve airflow, handling, and delay the stall. The LERX, first used on the Northrop F-5 'Freedom Fighter' in 1959, has since become commonplace on many combat aircraft, with especially large examples on the F/A-18 Hornet, Sukhoi Su-27, and CAC/PAC JF-17 Thunder. The Su-27's LERX helps enable advanced maneuvers like Pugachev's Cobra, the Cobra Turn, and the Kulbit. The dogtooth extension serves a similar purpose, inducing a vortex over the wing to control boundary layer spanwise extension, increasing lift and improving stall resistance. Its use on notable aircraft such as the F-15 Eagle, F-4 Phantom II, and F/A-18 Super Hornet underscores its legacy in high-performance design. The LEVCON system represents an evolution, adding actuation to LERX principles for improved performance, including increased departure-resistance in the event of thrust vectoring control failure at a post-stall attitude, trimming the aircraft, and optimizing lift-to-drag ratio during cruise. The article also notes that a leading-edge cuff is a fixed device that introduces a sharp discontinuity like a dogtooth, often with a drooped leading edge for low-speed characteristics.

Did You Know?

• The leading-edge root extension (LERX) was first used on the Northrop F-5 'Freedom Fighter' which flew in 1959.
• A dogtooth extension can be created as an afterthought by adding an extension to the outer section of the leading edge, as on the Hawker Hunter and some Quest Kodiak variants.
• The Sukhoi Su-27's LERX helps make advanced maneuvers possible, such as Pugachev's Cobra, the Cobra Turn, and the Kulbit.
• The LEVCON system on the Sukhoi Su-57 can be used for trimming the aircraft and optimizing the lift-to-drag ratio during cruise.