High-lift device
A component that increases wing lift for takeoff and landing.
A high-lift device is a part of an aircraft wing, either fixed or movable, designed to boost the lift the wing produces. Movable types include flaps and slats, while fixed examples are leading-edge slots, leading-edge root extensions, and boundary layer control systems. These devices solve a design compromise: a wing built for fast, efficient cruising—where jets spend most of their time—isn't ideal for slow takeoffs and landings. High-lift devices add extra lift when needed, lowering the speeds and distances required for safe operation. For instance, the Boeing 747-400's high-lift systems expand the wing area by 21% and increase lift by 90%.
The most common device is the flap, a movable section of the wing that lowers to increase camber. Flaps are typically on the trailing edge, though leading-edge flaps exist. Simple hinged flaps became widespread in the 1930s with the rise of fast monoplanes. Split flaps hinge the lower surface down while the upper surface stays fixed or moves separately. Traveling flaps extend backward to lengthen the wing chord and increase area, emerging just before World War II after development in the 1920s and 30s. Slotted flaps consist of multiple small airfoils that separate, hinge, and slide past each other when deployed; large modern airliners use triple-slotted flaps for the massive lift needed at takeoff.
Another common device is the slat, a small airfoil mounted just ahead of the wing's leading edge. It redirects airflow over the upper surface at high angles of attack, allowing the wing to produce more lift. The gap between slat and wing is called a slot. Slats can be fixed, with a permanent slot, or retractable. They may run the full span or only part of the wing, often outboard near the ailerons to keep them effective if the rest of the wing stalls. The first slats were developed by Gustav Lachmann in 1918 and independently by Handley-Page, who patented one in 1919. By the 1930s, automatic slats appeared, opening or closing based on flight conditions—typically closed by airflow pressure and opened by springs at slower speeds. Modern slats, like modern flaps, are often deployed hydraulically or with servos.
Powered high-lift systems use engine airflow to shape airflow over the wing.
- Boeing 747-400 wing area increase
- 21%
- Boeing 747-400 lift increase
- 90%
- First slats developed
- 1918 by Gustav Lachmann and simultaneously by Handley-Page (patent 1919)
- Simple hinged flaps common use
- 1930s
- Travelling flaps appeared
- just before World War II
Lore & Background
The most common high-lift device is the flap, a movable portion of the wing that can be lowered to produce extra lift by re-shaping the wing section to give it more camber. Flaps are usually located on the trailing edge, though leading edge flaps are used occasionally. Simple hinged flaps came into common use in the 1930s with the arrival of the modern fast monoplane. Split flaps have the lower surface hinge downwards while the upper surface remains fixed or moves independently. Travelling flaps extend backwards to increase wing chord and area, appearing just before World War II. Slotted flaps comprise several separate small airfoils that separate, hinge, and slide past each other; large modern airliners use triple-slotted flaps for massive lift during takeoff.
Another common device is the slat, a small aerofoil attached just in front of the wing leading edge that redirects airflow to allow operation at higher angles of attack. A slot is the gap between the slat and the wing. Slats may be fixed or retractable. The first slats were developed by Gustav Lachmann in 1918 and simultaneously by Handley-Page, who received a patent in 1919. By the 1930s automatic slats had been developed, typically operated by airflow pressure and small springs. Modern systems are deployed hydraulically or with servos.
Powered high-lift systems include blown flaps, which take bleed air from the jet engine's compressor or exhaust and blow it over the rear upper surface to re-energise the boundary layer. The Blackburn Buccaneer had a boundary layer control system using compressor air blown onto wings and tailplane. Another approach places a flap into the engine exhaust path, as on the C-17 Globemaster III. Leading-edge root extensions (LERX) are small triangular fillets that generate a vortex at high angles of attack to increase lift, common on modern fighters and some civil types. A Co-Flow Jet (CFJ) wing uses injection and suction slots to augment lift, increase stall margin, and reduce drag.
Reader's Guide
High-lift devices are significant because they resolve a fundamental design trade-off: a wing optimized for cruise efficiency is smaller and produces less lift at low speeds, but high-lift devices compensate by adding lift during takeoff and landing. This allows modern passenger jet wings to be designed for speed and efficiency during cruise, where the aircraft spends most of its flight time, while still achieving safe low-speed performance. The article notes that on the Boeing 747-400, high-lift devices increase wing area by 21% and lift by 90%. The variety of devices—from simple hinged flaps to triple-slotted flaps, slats, slots, boundary layer control systems, LERX, and Co-Flow Jet wings—demonstrates the breadth of engineering solutions. The historical development, from the first slats in 1918 to automatic slats in the 1930s and travelling flaps before World War II, shows a steady progression toward more complex and effective systems. Modern systems are often hydraulically or servo-actuated, and powered systems like blown flaps use engine bleed air to further enhance lift. The legacy of these devices is their essential role in enabling safe, efficient flight across a wide range of speeds and conditions.
More in Aircraft Components 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced