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Primary flight controls (aileron, elevator, and rudder), assisted by the trim devices, reduce control loading and provide positive feed to the pilot. The trim tab is either servo or anti- servo. [Figure 2-13]  Anti-servo tab movement is opposite to the control surface providing a positive feedback (or feel) to the pilot. Servo tabs move in the same direction as the control surface and allow the pilot to remove (or lighten) control load, reducing fatigue during flight and providing aerodynamic trim. 

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Over the years, the shape of the empennage has taken different forms. Early gliders were most often built with the horizontal stabilizer mounted at the bottom of the vertical stabilizer. This type of tail arrangement is called the conventional tail. Other gliders were designed with a T-tail, and still others were designed with V-tail. T-tail gliders have the horizontal stabilizer mounted on the top of the vertical stabilizer, forming a T. V-tails have two tail surfaces mounted to form a V. V-tails combine elevator and rudder movements. This combination of elevator and rudder are referred as ruddervators. 

## Towhook Devices 

An approved towhook is a vital part of glider equipment. The towhook is designed for quick release when the pilot exerts a pulling force on the release handle. As a safety feature (on most of the bellyhooks (CG hook)), if back pressure occurs from either getting out of position during the tow or overrunning the towrope, the release automatically opens. Part of the glider pilot’s preflight is ensuring that the towhook releases properly with applied forward and back pressure. 

The glider may have a towhook located on or under the nose and/or under the center of gravity (CG), near the main landing gear. The forward towhook is used for aerotow. The CG hook is used for ground launch. A glider with only a CG hook may be approved for aerotow in accordance with the Glider Flight Manual/Pilot’s Operating Handbook. [Figure 2-14] 

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## Powerplant 

## Self-Launching Gliders 

Self-launching gliders are equipped with engines powerful enough to enable them to launch without external assistance. The engines may also be used to sustain flight if the soaring conditions deteriorate. Self-launching gliders differ widely in terms of engine location and type of propeller. There are two types of self-launching gliders: touring motor gliders and high-performance self-launching gliders. 

Touring motor gliders are equipped with a fixed, nose-mounted engine and a full feathering propeller. [Figure 2-15] Touring motor gliders resemble an airplane to the untrained eye. They do have some basic airplane characteristics but are not certified as an airplane. On other types of self-launching gliders, the engine and propeller are located aft of the cockpit. High-performance self-launching gliders generally are seen with engines and propellers mounted behind the cockpit that completely retract into the fuselage for minimal drag in the soaring mode. [Figure 2-16] Propellers may fold or may simple align with the engine and retract completely. This configuration preserves the smooth low drag nose configurations important for good soaring efficiencies. When the engine and propeller are not in use, they are retracted into the fuselage, reducing drag and increasing soaring performance. These types of self-launch engines are usually coupled to a folding propeller, so the entire powerplant can be retracted and the bay doors are closed and sealed. 

