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. 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] 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. 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. 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.  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. Towhook Devices Figure 2-13.  Additional empennage components. Figure 2-14.  Towhook locations. Powerplant Self-Launching Gliders  Aerotow Ground launch CG CG Center of gravity CG towhook Nose-mounted towhook Stabilator Anti-servo tab 2-7