High-lift devices on a fixed-wing aircraft are divided into two families: those that extend from the trailing edge and those that extend from the leading edge. Flaps are hinged surfaces mounted on the trailing edges of the wings. They can be plain, split, or slotted, with slotted flaps offering the best balance of additional lift and moderate drag. Because they increase camber and effective wing area, flaps are deployed during takeoff and landing so that the airplane can become airborne or touch down at lower speeds.
Slats are located at the front of the wing. These extendable panels move forward and pivot downward, increasing both wing area and effective camber. Like flaps, slats are used during low-speed operations - takeoff, initial climb, approach, and landing - to let the airplane fly more slowly without stalling.
Together, flaps and slats reshape the wing: the trailing-edge flaps add lift with some drag, while the leading-edge slats postpone stall and permit even lower speeds.
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Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between aircraft flaps and slats?
Flaps and slats are both movable surfaces, yet they sit at opposite ends of the wing. Slats are the parts on the leading edge; they slide forward to boost lift at low speeds and high angles of attack, opening leading edge slots that let higher-energy air delay stall. Flaps are the parts on the trailing edge. By pivoting, the trailing edge of the flap increases effective camber which produces more lift at the same angle of attack. Krueger flaps are hinged and extend forward from the lower surface near the leading edge, while Zap and Junkers varieties reshape the camber of the wing.
Because both devices let airplane designers try to change airfoil shape for the same purpose - more lift at low speed - and because they are used at the same time, pilots refer to them together. The extra lift flaps create also adds drag. Yet drag benefits during approach, making the pairing even more useful.
Where are flaps and slats located on an aircraft?
Flaps sit on the trailing edge of the wing and slats are built into the leading edge of the wing. On many jet airliners, Krueger flaps also occupy the leading edge, extending from the lower surface so that a high-lift device is present both front and back of the same airfoil.
When are flaps used compared to slats on an aircraft?
Pilots use flaps during takeoff and landing, with flaps set downward at a moderate setting for takeoff and fully deployed during landing. Slats come out when selecting first detent, providing additional lift. Takeoff requires high lift and low drag, while landing requires high lift and high drag.
Which creates more lift: flaps or slats?
Slats create more lift. A wing creates more lift when slat is deployed because the slat re-energizes the boundary layer and postpones separation, allowing the wing to work at a higher angle of attack. Flaps moving downward create more lift as well, but their primary benefit is a simultaneous increase in camber and wing area. The same flap extension that raises lift also produces a noticeable rise in drag and a steeper lift-curve slope. Slats create more lift without the heavy drag penalty that accompanies large flap deflections. Flaps create more lift, yet that lift arrives with a sharp increase in induced and parasitic drag, so take-off and approach speeds must account for the extra drag. Because slats create more lift at lower angles of attack, the airplane can fly more slowly before the wing stalls, whereas flaps create more lift but push the stall angle downward. Hence, pilots respect new stall speeds and pitch limits when both devices are used together. A wing creates more lift when a slat is deployed and retains a gentler stall, giving the flight crew a wider margin between approach speed and natural stall, while flaps create more lift yet steepen the drag curve and demand higher throttle settings to hold the glide-path.
How are flaps controlled compared to slats?
Flaps are moved by the flap lever. Flaps are mechanically actuated and controlled by the pilot in the cockpit. Moving the lever provides a signal to the flap Power Drive Unit (PDU). The flap Power Drive Unit moves the flaps to the selected position.
Slats are moved by using the same control. Slats are operated with hydraulic power. A signal is sent to the Slat PDU as the flaps move toward their selected position.
How do flaps and slats differ in their design?
Flaps and slats differ in design because flaps pivot down when you extend them, while slats slide forward along tracks. Plain flaps pivot down; a slotted flap is a modification of a plain flap, and the Fowler is a type of flap that extends on rails.
What are the advantages and disadvantages of flaps?
Flaps help increase the camber of an airplane wing so the surface area of an airplane wing becomes larger. For this reason, flaps increase lift. When the wing produces more lift, flaps lower stall speed and permit lower landing speed. Flaps reduce landing distance because they decrease take-off distance as well. Flaps reduce the length of the landing roll, so aircraft can use shorter runways.
Disadvantages of flaps are that they increase drag. As a result, flaps reduce weight on wheels and make brakes less effective. Flaps affect pitch behavior: lift primarily on the rear portion of the wing produces a nose-down force, so flap extension has a definite effect on the airplane's pitch behavior. They also bring crosswind problems, since flaps increase the tendency to weathervane into the wind.
What advantage do slats give an airplane?
Slats enable fresh air to flow over the wing, allowing the wing to reach a higher CL max and fly at slower speeds. By increasing the stall angle of attack, slats provide stall protection at low speeds, boost stall margin, and allow shorter takeoff and landing distances. This capability lets aircraft operate effectively in a wide range of conditions, contributes to safety and efficiency of flight operations, and allows a slower approach for landing while maintaining a safe stall margin.
