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What Is a Flap Actuator?

By Paul Scott
Updated May 23, 2024
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A flap actuator is mechanical device used to adjust the flaps on an aircraft's wings. The flaps are adjustable control surfaces on the rear edge of the wing that are extended or retracted to adjust the profile and surface area of the wing to facilitate efficient flight at low airspeeds. The flat actuator is typically a lead screw type of mechanism driven by an electric or hydraulic motor. The actuator is mounted in a fixed position within the wing and features a moving nut on the lead screw with an integral bracket attached to the flap assembly. When the actuator is activated, the lead screw rotates, moving the nut up or down its length, extending or retracting the flap as it does so.

Fixed-wing aircraft are able to get into, and stay in, the air due to the lift generated by their wings. The amount of lift that the wing generates is a product of its design and the speed at which it moves through the air. Planes with a modest range of airspeeds are fairly easy to design wings for, a single profile and surface area generally being sufficient for all stages of flight. As soon as the aircraft's performance increases and a greater range of air speeds are encountered during normal flight, the picture becomes a little more complicated. In most cases, the wing will then be designed to efficiently carry the aircraft at the higher end of its flight speed envelope.

Unfortunately, aircraft take off and land at speeds considerably lower than their cruise speeds. As lift is a product of both wing design and airspeed, this creates a situation where the wing's basic design no longer generates sufficient lift at the lower speeds. To counter this deficiency, sets of adjustable control surfaces known as flaps are included along the rear, or trailing, edge of the wing. Theses devices can be extended or retracted to effectively change the wing's profile and its surface area to facilitate lift generation at low speeds. The flaps are adjusted by a device known as a flap actuator.

Typically consisting of a linear, lead screw mechanism driven by an electric or, in the case of larger aircraft, a hydraulic motor, the flap actuator is located within the wing's interior. The lead screw is fitted with a nut that travels up and down its length when the screw rotates. A mounting bracket is attached to the nut that serves to connect it to the flap mechanism. When the flap actuator is activated by the crew, the lead screw rotates moving the lead screw and the flaps in the process. The aircraft's flap controls are calibrated to extend the flaps in standard, predictable increments to accommodate varying airspeed conditions.

Not all aircraft employ a lead screw flap actuator. In some cases, particularly in the case of older or smaller aircraft, the actuators may be little more than a set of hinged rods controlled by a lever in the cockpit fitted with a notched locking mechanism for choosing the various flap settings. In some aircraft, steel cables running from the cockpit control lever to a set of hinged rods on the wing's trailing edge are used to actuate the flaps.

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