High-lift devices are fitted to both the leading and trailing edges of the wing. On the leading edge they take two principal forms: slats and Krueger flaps. A slat is a movable panel that, when extended, leaves a slot between itself and the fixed wing. This slot accelerates airflow, permitting safer flight at high angles of attack. Krueger flaps, used on many jet airliners, increase camber by hinging panels forward from the lower surface. By enlarging effective camber and area, these devices augment lift during take-off and landing, allowing an aircraft to become airborne sooner and to touchdown at lower speed.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What are leading edge flaps?
Leading edge flaps are devices on the leading edge which increase camber and the depth of the wing. Airplane designers try to increase the wing area and change the airfoil shape by putting moving parts on the wings' leading edges. Pivoting the leading edge downward increases the effective camber of the airfoil.
A leading edge droop flap is a device on the leading edge of aircraft wings that comprises the rounded front part of a wing in movable form. This movable form is designed to boost airflow at high angles of attack.
What is the difference between leading edge flaps and trailing edge flaps?
Leading edge flaps increase camber at the front of the wing. Trailing edge flaps are devices positioned at the back of the airplane's wings. Leading edge flaps increase camber which increases lift. The main purpose of trailing edges is to generate lift and reduce drag. When deployed, the split flap trailing edge lowers away from the trailing edge of the wing, enlarging wing area and curvature. Trailing edges can be lowered and tabs are exclusive to trailing edges. Trailing edge flaps are operated with hydraulic power which automatically shut off when protection logic activates.
Leading edge devices modify the forward part of the airfoil, delay separation, and add little extra drag. Airflow over the top of the wing remains the same until either device is extended. Together, leading edge flaps and trailing edge flaps let airplane designers increase the wing area and create the high lift needed for safe low-speed flight without sacrificing cruise efficiency.
What are the types of leading edge flaps?
Leading edge devices are split into three main types: Krueger flaps, slats, and slots. Within the Krueger group, the simple Krueger flap and the folding bull-nose Krueger flap are generally used as two-position devices biased toward an optimum landing configuration. A third take-off position is possible but demands a more intricate mechanism. The hinged leading-edge droop nose is a further variant that increases wing camber by drooping downward. Slots and slats energise the airflow over the top of the wing, delaying stall and augmenting lift without the marked drag rise associated with split flaps.
What are slats?
Slats are leading edge devices located on the wing's leading edge. They create a slot that increases the stalling angle of attack. This allows the wing to continue flying at slower speeds. Slats do not directly generate lift but maintain smooth airflow over the wing during slow flight. When not needed, they are retracted to minimize drag.
What are leading edge flaps used for?
Leading edge flaps are lowered to provide greater lift at slow speeds. They rotate downward, increasing wing camber so the wing produces the same amount of lift at a lower airspeed. During landing they are fully extended, shortening runway required and enabling landing at slower speeds. For takeoff they increase lift, shortening the runway required. Throughout low-speed operations - approach, landing, and takeoff - leading edge flaps provide greater control, boosting maneuverability while delaying airflow separation.
Leading edge flaps handle airflow over the wing and slow stall. By postponing the stall spot, they let the aircraft operate safely at lower velocity and shorten required airstrips. Their main function is to extend usable performance capability during departure and arrival stages.
When are leading edge flaps used?
Leading edge flaps are mainly used in the low speed situations of take off, approach, initial climb and landing. Deploying the devices increases camber, area, and lift coefficient. The gap that opens when a leading edge flap is fully extended lets air pass from the bottom to the upper surface. This energises the boundary layer so that, beyond 20 degrees when flow normally separates on the upper surface, the slot promotes good low speed handling and the wing continues to produce lift.
How do leading edge flaps extend?
Leading edge flaps extend using a motor. The motor moves a linkage that extends the flaps on both sides of the plane. Slats move along metal tracks built into the wings. Extending flaps increases lift and drag.
What does the leading edge flaps transit indicate? Illuminated amber indicates any LE device in transit. Any LE device in transit is highlighted by the LE Devices TRANSIT lights on the panel. LE FLAPS EXT light illuminated green indicates a related LE device fully extended. LE FLAPS FULL EXT indicator lights up when the related LE device is fully extended. Extinguished light indicates the related LE device is retracted.
What planes have leading edge flaps?
A320 family planes, A320/321 and A380 carry slats or other forms of leading edge high lift devices. Boeing 707 and Boeing 747 used Krueger flaps on the wing leading edge. The simple Krueger flap is used on the inboard wing of the Boeing 707 while the Boeing 727 uses a mix of inboard Krueger flaps and outboard slats. Airbus A380 has droop flaps between the fuselage and each inboard engine. These droop flaps are called droop noses by Airbus. Short takeoff and landing airplanes are more likely to have leading edge slots.
An Airco DH.9A was modified as a monoplane with a large wing fitted with full-length leading edge slats. Messerschmitt aircraft employed automatic spring-loaded leading edge slats as general rule. A VC leading edge was tested on NASA's Advanced Flight Technology Integration (AFT I) 111 experimental airplane. VC Krueger changes the main Krueger panel from a rigid to a flexible panel, refining the airfoil shape of the Krueger. The tail of the Cessna Cardinal is the most famous example of slots on a standard GA airplane.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
