Dispositifs Hypersustentateurs

Overview

High-Lift Devices transform a wing optimised for efficient cruise into one capable of flying slowly enough for safe takeoff and landing. Without them, modern jet airliners — which cruise at speeds exceeding 900 km/h — would need runway lengths of several kilometres to accelerate to the required V-speeds or decelerate after touchdown. Leading-edge slats and trailing-edge flaps are deployed in discrete steps (configurations) to progressively increase maximum lift coefficient while keeping the stall margin acceptable.

How It Works

Trailing-edge flaps increase wing camber and (on Fowler-type flaps) wing area by moving aft and down, adding up to 30–40% more lift coefficient at a given angle of attack. Simultaneously, leading-edge slats extend forward and down, opening a slot that energises the boundary layer over the main wing element, delaying separation to higher angles of attack and thus raising the stall angle. The combined effect can more than double the maximum lift coefficient compared to the clean configuration, allowing approach speeds 40–60 knots slower than cruise. Both slats and flaps are actuated by hydraulically or electrically driven jackscrews and torque-tube distribution shafts that ensure symmetric deployment; asymmetric deployment (one side only) would create an immediate roll that must be countered.

Key Components

• Trailing-Edge Flaps: Single-slotted, double-slotted, or Fowler flaps depending on aircraft generation. Extend on tracks driven by central screwjacks. See Flaps.
• Leading-Edge Slats: Full-span or partial-span devices that slide forward on curved tracks. Krueger flaps used on some inboard sections (747, 737 inboard leading edge).
• Hydraulic / Electric Drive System: Distributes power to all panels symmetrically via torque tubes; Wing Tip Brakes (WTB) lock surfaces in position to prevent asymmetry if drive continuity is lost.
• Flap Position Sensors: Report actual surface angle to the cockpit and to the Stall Protection System, which adjusts stall warning thresholds for each configuration.
• Flap/Slat Control Lever: Cockpit lever with detented positions (0, 1, 2, 3, Full on most jets) that select the target configuration; the control system drives surfaces to the commanded position.

Aircraft Applications

• Boeing 737-800: Triple-slotted trailing-edge flaps with leading-edge slats and Krueger flaps; optimised for short-to-medium-haul operations from shorter regional runways.
• Airbus A320: Single-slotted Fowler flaps with full-span slats; FBW monitors surface position and reconfigures stall protection automatically with each flap setting.
• Boeing 787: Simple single-slotted flaps enabled by advanced aerofoil design and optimised slat profiles; electrically-powered drive system (no hydraulic flap motor).
• Airbus A380: Eight-panel trailing-edge flap system spanning the very wide wing planform; inner and outer slat groups independently scheduled for optimised high-lift performance across the large wing.

Advantages and Limitations

High-lift devices make modern airports practical — enabling heavily loaded jets to use runways of 2–3 km rather than 5+ km. Each additional flap setting increases drag significantly, which is beneficial during approach (helping to maintain the glidepath without excessive speed) but problematic if flaps are not retracted promptly after takeoff. Extended flap operation at high speed is prohibited by placard speeds (V_FE). Flap system failures — particularly asymmetric deployment — are among the highest-priority items in emergency checklists, requiring immediate action and a modified approach procedure.