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A motor glider is a fixed-wing aircraft that can be flown with or without engine power. The FAI Gliding Commission Sporting Code definition is: a fixed-wing aerodyne equipped with a means of propulsion (MoP), capable of sustained soaring flight without thrust from the means of propulsion.
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.
Another solution is the single-blade propeller that offers the advantage of a smaller opening in the fuselage to retract the engine.
Internal combustion engines can benefit from mounting in the fuselage, rather than on the propeller mast. This allows them to be connected to a larger muffler for reduced noise when operating, something which is mostly relevant to European operation. It also allows the belt tension to be relieved when the engine is retracted to extend the life of the belt and bearings. The drawback of this arrangement is that engines fixed low in fuselages are more difficult to pre-flight and service, and highly stressed power transmission belts should not be bent or twisted.
Self-launching engines are equipped with a throttle that allows the engine power to be adjusted for ground operations. Self-launching engines are typically in the range of 50–60 hp (38–45 kW). The higher engine output power requires liquid cooling with a separate radiator mounted on the propeller mast. Engines commonly used are two-stroke piston engines, or Wankel rotary engines.
They are fitted with front-mounted engines, similar to a small airplane. The large of TMGs provide a moderate gliding performance, not as good as that of unpowered gliders. However TMGs are more efficient than conventional light aircraft.
Most TMGs are designed with engines of 80 to and typically cruise (under power) at 85–. Most have fuel tanks capable of holding between 50 and 100 liters (13 to 26 US gallons) of fuel, giving a range under power of up to 450 nautical miles (approximately 830 kilometers). Modern TMGs like the Phoenix Air Phoenix are capable of higher speeds and longer range under power.
Some TMGs are equipped with folding wings to allow them to fit in standard small airplane . Tow hooks are unnecessary, since aircraft with self-launch ability do not require access to winch or tow plane for launching like a conventional glider.
Some TMGs, like the Europa Aircraft or the Phoenix, can also be supplied with interchangeable wings or wingtips so that they can be flown as a standard touring aircraft as well as a TMG..
The landing gear configuration on TMGs usually incorporates two fixed main wheels, allowing it to be taxiing on the ground without a wing walker. While some TMGs have only one main wheel, with auxiliary trolley wheels on the wings for taxiing, it is 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.
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.
On the AMS Carat, the propeller folds forward, pointing straight ahead like a spear.
Advantages of electric power over gasoline include:
Electric sustainer systems (with power to sustain flight but not self-launch) are optional for the Schempp-Hirth Ventus-3, HPH Shark and ESAG LAK-17B. These three all use the FES as used for self-launching lighter gliders, such as the Alisport Silent 2.
Other self-launching types include the experimental LET L-13TJ Blaník, a version of the Alisport Silent Club and a version of the TeST TST-14 Bonus.
In soaring competitions, starting the engine is usually scored the same as an out-landing in an unpowered glider. To detect the use of the engine, GNSS Flight Recorders used in motor gliders must have a noise sensor that allows recording the sound level along with position and altitude. In many competitions, the rules require that the pilot start the engine at the beginning of the flight, before starting the task, to ensure an engine start later in the flight will be detected.
Gliders without an engine are lighter and, as they do not need a safety margin for an engine-start, they can safely thermal at lower altitudes in weaker conditions. So, pilots in unpowered gliders may complete competition flights when some powered competitors cannot. Conversely, motor glider pilots can start the engine to extend a flight if conditions will no longer support soaring, while unpowered gliders will have to land out, away from the home airfield, requiring retrieval by road using the glider's trailer.
The presence of an engine can increase the safety of gliding, as a powerplant increases the ability of the pilot to avoid storms and off-airport landings. An opposing view is that motor gliders are against the spirit of the sport, and, more importantly, that they sometimes give pilots a false sense of security.
Touring motor gliders are seldom used in competition, but they can be useful in training for cross-country flights. After take-off, the engine is switched off, and the trainee flies the aircraft as a glider. Landings in unfamiliar fields can be practiced while the motor idles. If the trainee chooses an inappropriate field, or misjudges the approach, the instructor can apply power and climb away safely.
In South Africa, Touring Motor Glider (TMG) is an independent National Pilots License category under Recreation Aviation, Part 62, subpart 17.
In the United States, a private glider pilot certificate allows the pilot to fly unpowered gliders, self-launching motor gliders (including touring motor gliders and gliders with retractable engines or propellers), and sustainer motor gliders. An instructor must provide instruction and sign the logbook of the pilot to authorize the launch method, which may be by airplane towing, ground launch (winches, bungee, auto tow), or, in the case of a suitable motor glider, by self-launching. In the US, motor gliders are classified as gliders, and may be operated by a glider pilot without the medical certificate required to operate an airplane.
In the United States, a powered glider may be certified for up to two occupants, up to 850 kg maximum weight, and with a maximum ratio of weight to wing span squared of 3 kg/m2.. Similar requirements exist in European JAA/EASA regulations, at a maximum weight of 750 kg.
In Canada, a glider pilot license allows the pilot to fly unpowered gliders. For self-launching motor gliders (including motor gliders and gliders with retractable engines or propellers), and sustainer motor gliders an aeroplane permit or licence is required. An instructor must provide instruction and sign the logbook of the pilot to authorize the pilot to carry passengers.
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