becoming more common to find them being manufactured with tricycle and conventional (two fixed main wheels – i.e. a "tail-dragger") landing gear configurations. Since the additional drag of the stopped propeller and landing gear reduces their gliding performance, TMGs are seldom used in competition. Retractable propeller The retractable propeller is usually mounted on a mast that rotates up and forward out of the fuselage, aft of the cockpit and wing carry-through structure. The fuselage has engine bay doors that open and close automatically, similar to landing gear doors. The engine may be near the top or bottom of the mast, and newer designs have the engine fixed in the fuselage to reduce noise and drag. Unlike TMGs, most gliders with retractable propellers are also fitted with a tow-hook for aero-towing or ground launch. They have a single-axle retractable main wheel on the fuselage like most unpowered gliders, so they do require assistance during ground operations. The two-stroke engines commonly used are not efficient at reduced power for level cruising flight, and instead must use a "saw-tooth" flight profile where the glider climbs at full power, then glides with the propeller retracted. Sustainer Sustainer motor gliders must be launched like an unpowered glider, but can climb slowly to extend a flight once the engine is deployed and started. They generally do not have an alternator or starter motor, so the engine is started by "wind-milling" the propeller in flight. The propeller may be a rigid 2-blade design, or may have more than two blades that fold at the hub when the engine is retracted. The propeller hub is usually attached directly to the crankshaft, but there is at least one example of a sustainer with a belt reduction drive, the DG-1000T. The smaller sustainer engines are usually not equipped with a throttle, but instead have a cable to open decompression valves in each cylinder to allow the engine to turn freely for starting. Sustainer engines are typically two-stroke two-cylinder air-cooled engines in the range of 18–30 hp (14–22 kW). They are lighter in weight, and simpler to operate than self-launching powerplants. Self-launching Self-launching retractable propeller motor gliders have sufficient thrust and initial climb rate to take off without assistance, or they may be launched as with a conventional glider. The engines also have a starter motor and a large battery to allow the engine to be started on the ground, and