How Wing Flaps and Slats Let Planes Fly Slowly

Flaps and slats are movable parts on an aeroplane's wings that let it produce enough lift at low speed. Flaps sit on the rear edge of the wing and slats on the front. Pilots extend them for takeoff and landing, when the aircraft is slow, and tuck them away in cruise, when speed does the job instead.

The basic problem

NASA's educational pages explain that lift depends on the shape of the wing's cross-section (the airfoil), the wing's area and the aircraft's speed. A wing designed to cruise efficiently at high speed does not make much lift when the airspeed is low. Takeoff and landing happen at low speed, so the wing needs help. Flaps and slats provide it.

Three things a flap does

• Adds area. On many designs the flap slides backward along tracks, and slats slide forward, so the total wing area grows.
• Adds camber. Camber is the curve of the wing. Tipping the front edge of a slat and the rear edge of a flap downward increases the effective camber, which NASA says raises lift.
• Adds drag. A big flap angled into the airflow slows the aircraft, which is useful when landing.

Wikipedia's article on flaps adds that extending them raises the maximum lift coefficient, so a given weight can be supported at a lower speed. That lowers the stall speed, the slowest speed at which the wing still flies. It allows a slower and steeper approach. Flaps also tend to lower the nose, which improves the pilot's view of the runway.

Types of flap

• Plain flap: the back part of the wing hinges down. It is simple and cheap but less effective than other types.
• Split flap: only the lower surface of the trailing edge hinges down. It makes a lot of drag at full deflection.
• Slotted flap: a gap between wing and flap lets higher-pressure air flow over the flap's upper surface, which helps keep the airflow attached and raises the maximum lift beyond a split flap. Some designs use several slots.
• Fowler flap: the flap first slides backward, lengthening the wing, then tilts down. It adds both area and camber and always creates a slot effect.

Wikipedia credits Harlan D. Fowler with the Fowler flap in 1924. It first flew on the Martin 146 prototype in 1935 and entered production on the 1937 Lockheed Super Electra. The article says it is still widely used.

Slats and leading-edge devices

Slats are mounted at the front of the wing. When extended they form a slot that guides air over the upper surface. This lets the wing be tilted to a higher angle against the airflow before it stalls, improving low-speed control and shortening takeoff and landing distances. Some fast fighter aircraft with thin wings instead use a leading-edge flap, in which the whole front edge rotates downward.

How they are used

NASA notes that a moderate flap setting at takeoff gives plenty of lift with fairly low drag. For landing, flaps and slats are fully extended, giving high lift at low speed and high drag to help slow down. After touchdown, spoilers on the top of the wing are raised to reduce lift, which keeps the aircraft on the ground, and to add more drag.

A short history

• 1913 to 1914: plain flaps were tested in the United Kingdom but not fitted to aircraft.
• 1916: the Fairey Hamble Baby is credited as the first aeroplane to fly with flaps.
• 1917: Breguet fitted automatic flaps to the Breguet 14.
• 1920s and 1930s: split and slotted flaps were developed, and split flaps became common in the 1930s.
• 1935 to 1937: the Fowler flap entered service.

Wikipedia flags the credit for inventing the split flap as unverified, so this article does not name an inventor for it.

Next time you sit by an airliner's wing, you may see sections sliding backward and down as the plane slows to land. That is the wing being reshaped, in a few seconds, for a different job than the one it does at cruising speed.

Sources

• Flaps and Slats, NASA Glenn Research Center
• Flap (aeronautics), Wikipedia)