More on Aircraft Secondary Flight Surfaces
An aircraft flight control surface is an aerodynamic device that allows the pilot to control and adjust the flight attitude of an aircraft. The most commonly known flight control surfaces are the ailerons, elevator, and rudder, but these are not the only ones. However, there are also many lesser known flight surfaces known as secondary or auxiliary flight surfaces. These include the flaps, slats, spoilers & speed brakes, and tabs. This blog will cover each secondary flight surface and its function.
Flaps are found on nearly all types of aircraft. They are usually inboard on the wing’s trailing edge, but leading edge flaps are also common. Flaps are used to increase the camber of the wings to provide more lift and greater control at low speeds. This allows aircraft to land at slower speeds and shortens the amount of runway required for takeoff or landing. Flaps can typically extend up to 45-50 degrees, but the angle is controlled from the cockpit. Flaps are usually made from the same material as the other control surfaces of an aircraft. Depending on the type of aircraft, they can be constructed from aluminum or composite materials.
There are three types of flaps: plain, split, and fowler. Plain flaps are at the rear of the wing and, when in the retracted position, form the trailing edge of the aircraft wing. Plain flaps are hinged, allowing them to increase wing camber and provide greater lift. Split flaps are commonly a flat metal plate hinged at several places along the wing’s leading edge. When deployed, the split flap lowers away from the trailing edge, increasing camber and lift. Fowler flaps, in addition to lowering the trailing edge, also slide aft, increasing the total area of the wing. This increased surface area creates more lift.
Aviation slats are another leading-edge device that extends wing camber. These are operated independently of the flaps and have their own switch in the cockpit control panel. Not only do slats extend out of the leading edge to increase camber and lift, when fully deployed, slats leave a slot between the trailing and leading edges of the wings. This allows the angle of attack to increase while maintaining laminar airflow, allowing the pilot to maintain control of the aircraft even when flying at slower speeds.
Spoilers & Speed Brakes
A spoiler is a device used to disrupt laminar airflow and reduce lift. These are found mainly on heavy and high-performance aircraft, where they are stowed flush to the upper surface of the wing. They are made from similar materials as the other control surfaces on the aircraft, often flat plates with honeycomb cores. At low speeds, spoilers are designed to operate in conjunction with the ailerons to assist lateral movement and stability of the aircraft.
Spoilers can also be fully deployed on both wings to act as speed brakes. The reduced lift and increased drag can quickly reduce the speed of an aircraft in flight or upon landing. The speed brake control in the cockpit is capable of fully deploying all spoiler and speed brake surfaces. As they are important in landing, these surfaces are commonly rigged to deploy automatically when engine thrust reversers are activated during landing.
At high flight speeds, the force of air can make it very difficult for control surfaces to move or remain in their deflected position. To help with this, tabs are used. There are many different types of tabs, each offering different effects. The most common types of tabs are trim, balance, servo, anti-balance, and spring tabs. Trim tabs deflect opposite of the primary control surface, thereby balancing the air and allowing the control surface to maintain its desired position. Balance tabs are connected to the control service via linkage, and aid the pilot in overcoming the force needed to move the control surface.
Servo tabs are directly linked to flight control input devices, and are used to aerodynamically position control surfaces that require too much force to manually move. The anti-balance tab, or anti-servo tab, increases the force needed by the pilot to change the flight control position, therefore making the flight controls less sensitive. Finally, the spring tab is a tab used only in high-speed flight to enable the motion of control surfaces.
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