A Brief Explanation of Aircraft High-Lift Systems

In addition to their specially shaped wings and control surfaces, most modern airplanes also feature a number of flaps and slats called the “high-lift system” which can provide additional lifting capacity at slow speeds. This system takes the form of trailing edge flaps, leading edge slats, or a combination of both. Overall, it lends additional lifting power to the aircraft so that a pilot can effectively raise their vessel even while flying at relatively slow speeds. Just like the other control surfaces on board, these flaps and/or slats work by adjusting the wing’s shape, manipulating the airflow around them to push the aircraft in just the direction the pilot wants it to go. This blog will provide a brief overview of the high-lift systems in use on today’s aircraft.

Why Do We Use High-Lift Systems?

Aircraft typically operate across a wide range of speeds, but the wing is designed to be most efficient while at cruising speeds. In general, this makes the wings less suited to low-speed flight, meaning taking off and landing would be much more difficult and dangerous at these reduced speeds without making any modifications. Therefore, the high-lift system presents a safe and effective way of adjusting the wing design to also operate well at low speeds.

How Do Flaps and Slats Work?

Flaps and slats work by increasing the camber of a wing through the mechanical actuation of leading-edge devices (slats) and trailing edge devices (flaps). In this case, camber is a measure of the curvature of the plane’s wing on its surface and underside. By changing the wing’s camber, a pilot can adjust the speed and direction in which the wind passes over and under the wing. Generally, a highly cambered wing will produce more lift at a given angle of attack, so a deployed flap or slat can temporarily increase the curvature of the wing to increase its lifting capabilities. This in turn reduces the stall speed of the aircraft, making it safer to take off and land. Moreover, larger flap angles also reduce the angle of attack needed to maintain a given lift value so that the pilot may fly with the nose lower down during an approach, giving them better visibility at a critical part of flight.

Types of Trailing Edge Flaps

There are many different types of trailing edge flaps in use today, including plain, split, slotted, and fowler flaps. Plain flaps are the simplest of the types, consisting of a moveable edge of the wing that can deflect downward to increase camber, thereby producing lift. Conversely, on split flaps, only the lower surface of the trailing edge is deflected downward. This leaves an upper section that remains in its same position throughout the flight. Slotted flaps are designed in such a way so as to create a slot between the trailing edge of the wing and the flap. This slot allows higher pressure air from the lower surface to move through the slot, as well as re-energizes the airflow over the upper flap surface, reducing the tendency for the air to separate. Finally, there are fowler flaps which both extend backwards and downward to increase the camber and the total wing area. For this reason, fowler flaps are the most effective flap systems, though they are also more complicated to design and implement.

Leading Edge Slats

Leading edge slats only come in one variety, but they rely on the same principles as trailing edge flaps. By dropping downward, the slats are able to increase the camber of the airfoil, and a slot or gap between the slat and the parent wing allows the passage of air from the lower surface to the upper surface. More often used on larger jet airliners, slats are not as frequently employed, but they can still provide an abundance of assistance where high lift is needed.


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