Motor glider

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DG-808B 18m in self-launch.

See Also: glider

A Motor Glider is a fixed-wing aircraft that can be flown with or without engine power. More specifically, the FAI Gliding Commission Sporting Code definition is:

MOTOR GLIDER: A fixed wing aerodyne equipped with a means of propulsion (MoP), capable of sustained soaring flight without thrust from the means of propulsion.^[1]

[edit] Types of motor gliders

Scheibe SF25C - an older touring motorglider with tube and fabric fuselage construction

Grob G109B touring motor glider, with more modern Fibre-reinforced plastic construction

Most motor gliders are equipped with a propeller, which may be fixed, feathering, or retractable.

[edit] Fixed or feathering propeller

[edit] Touring motor gliders

Motor gliders with fixed or full feathering propellers are generally classified as Touring Motor Gliders (TMGs). TMG's can be take off and cruise like an airplane or soar with power off like a glider. They are fitted with front mounted engines, similar to a small airplane. The large wingspans of TMGs provide a moderate gliding performance, though worse than that of unpowered gliders. However TMGs are more efficient than conventional light aircraft.

Most TMGs are designed with engines of 80 to 100 hp and typically cruise (under power) at 85 - 100 kts. Most have fuel tanks capable of holding 50 and up to 100 liters (13 to 26 US gallons) of fuel, giving TMG's great range under power of up to 450 US nautical miles (aprox. 830 kilometers).

Some TMG's are equipped with folding wings to allow them to fit in standard small airplane T-hangars. Touring motor gliders must self-launch, because they are not equipped with tow hooks for launching like a conventional glider. The self-launch requirement however, is most desirable to those pilots who wish to fly a glider, but have no access to tow planes or ground launching equipment.

The landing gear configuration on TMGs usually incorporates two fixed main wheels, allowing it to be taxied 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 TMGs 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.

[edit] Retractable propeller

A Schleicher ASH 26e self-launching motor glider, with the engine mast extended. A Stemme S10 is in the background with the nose cone extended

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 TMG's, most gliders with retractable propellers are also fitted with a tow-hooks 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 engine retracted.

[edit] 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.^[2]

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.^[3]

[edit] Self-launching

Powerplant from a Schleicher ASH 26e self-launching motor glider, removed from the glider and mounted on a test stand for maintenance at the Alexander Schleicher GmbH & Co in Poppenhausen, Germany. Counter-clockwise from top left: propeller hub, mast with belt guide, radiator, Wankel engine, muffler shroud.

Self launching retractable propeller motor gliders have sufficient thrust and initial climb rate to take off safely 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 an alternator to recharge the battery. A two blade propeller is typically coupled to the engine via a belt reduction drive, and the propeller alignment must be checked by the pilot using a mirror, before it is retracted into the fuselage.

These larger engines 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. It also allows the belt tension to be relieved when the engine is retracted to extend the life of the belt and bearings.

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.^[4]

[edit] Other Types

[edit] Cross-over

On the Stemme S10, the propeller folds into the nose cone, and is connected to the rear mounted engine with a drive shaft. It also has two retractable main wheels, allowing it to be taxiied without assistance, and to soar with low drag. These features make it a cross-over between the touring and retractable propeller motor gliders. It does not have a tow-hook, so it must self launch. The S10-VT variant has a two-position variable pitch propeller and a turbocharger on the engine, which allows the aircraft to cruise at altitudes up to 30,000 feet (9000 m).^[5]

[edit] Electric

Although most motor gliders have gasoline fueled internal combustion engines, recently electric powered self-launchers such as the Antares 20E and the Silent 2 Targa LE (Lithium Electric) have been developed.^[6]

[edit] Jet

There was also one example of a factory produced self-launching motor glider fitted with a jet engine, the Caproni Vizzola A21J Calif. The jet engine was mounted inside the fuselage behind the wing, with fixed intake and exhaust ducts coupled to the outside air stream for engine operation. The Jonkers JS-1 Revelation is a current design available with a sustainer jet engine. The highly modified Alisport Silent-J is a self-launching aerobatic motor glider on the U.S. airshow circuit,^[7] powered by twin AMT-USA AT-450 jet engines (200 N (45 Lbf) of thrust each) originally developed for radio-controlled aircraft.^[8]

[edit] Use of engines

A self-launching Eiri-Avion PIK-20E in flight with engine running (NASA photo)

The ability to use an engine allows glider pilots additional operational flexibility not possible in an unpowered glider. However, the engine cannot always be relied upon to start in flight, so the pilot must allow for this possibility. The generally accepted practice is to get in position for landing at a suitable airport, or off-airport out-landing field, before extending the propeller and attempting an engine start. This allows for a safe landing in the event that the engine cannot be started in time..^[9]

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.^[10] Conversely, motor glider pilots can start the engine if conditions will no longer support soaring flight, while unpowered gliders will have to land out, away from the home airfield, requiring retrieval by road using the glider's trailer.

The most important point in favor of powered gliders (retractable engine high-performance types) is that it helps pilots to avoid outlandings. Outlandings, while they are not necessarily dangerous, can be an expensive and time-consuming nuisance for competitive pilots who need to be back home at a set time. Another consideration is that a retrieve crew is needed on stand-by. However the sense of achievement in completing a difficult cross-country is lessened if an engine has been available.

Some people argue that an engine makes the glider safer, because the pilot can avoid storms, and off-airport landings^{[citation needed]}. However, the additional complexity of engine operation is also a factor in accidents, so this point is not necessarily supported by an analysis of accident data^{[citation needed]}. 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^{[citation needed]}. Even with a motor glider, it is still important never to be out of gliding range of a landable area.

Touring motor gliders are seldom used in competition, but they can be useful in training for cross-country flights.^[11] 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, after pointing out the error.

[edit] Licenses or certificates

Recently, pilot licensing terms have changed in Europe. Powered gliders are now categorized into gliders with retractable propellers/engines, which can be flown with an ordinary glider pilot license (GPL), and touring motor gliders (TMG), which require a specific license extension to the standard GPL. In the United Kingdom, where gliding is regulated by the British Gliding Association, pilots of self-sustaining gliders, like those of pure gliders, do not have to be licensed with the United Kingdom Civil Aviation Authority.

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 propellors) 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, etc.) or, in the case of a suitable motor glider, by self-launching. In the US, motor gliders are classified as gliders, and so may be operated by a glider pilot without the medical certificate required to operate an airplane.

[edit] References

1. ^ FAI Sporting Code Section 3 - Gliding » Chapter 1: General Rules and Definitions. Retrieved on 2006-11-23.
2. ^ DG Flugzeugbau GmbH, DG-1000T Information. Retrieved on 2006-11-24.
3. ^ Schempp-Hirth GmbH, Discus 2T Sustainer Engine Information. Retrieved on 2006-11-24.
4. ^ Alexander Schleicher GmbH & Co., ASH 26 E Information. Retrieved on 2006-11-24.
5. ^ Stemme AG, S10-VT Information. Retrieved on 2006-11-24.
6. ^ Antares 20E Lange Flugzeugbau GmbH, Antares 20E Information. Retrieved on 2006-11-24.
7. ^ Silent Wings Airshows, jet sailplane. Retrieved on 2006-12-13.
8. ^ AMT-USA jet engine product information. Retrieved on 2006-12-13.
9. ^ Information about self-sustaining gliders. Retrieved on 2006-09-04.
10. ^ Guide to Self-launching Sailplane Operation. Retrieved on 2006-09-04.
11. ^ Motor gliding training syllabus for instructors. Retrieved on 2006-09-04.