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Motor glider

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DG-808B 18m self-launching

Amotor glideris afixed-wing aircraftthat can be flown with or without engine power. TheFAI Gliding CommissionSporting Code definition is: a fixed-wingaerodyneequipped with a means of propulsion (MoP), capable of sustainedsoaringflight without thrust from the means of propulsion.[1]

History[edit]

Carden-Baynes Auxiliary engine

In 1935, an occasional or auxiliary motor that could be retracted was suggested bySir John Carden.[2] This was incorporated into theCarden-Baynes Auxiliarythat first flew on 8 August of the same year. A later version of theBudig gliderwas powered.

Categories[edit]

Most motor gliders are equipped with apropeller,which may be fixed, feathered (e.g.AMS-Flight Carat), or retractable. However jet engine-powered motorgliders are now available from some manufacturers, some of which are intended for use only as "sustainer" engines, i.e. for sustaining gliding flight rather than as self-launching aircraft.

Sustainer Motor Gliders[edit]

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 analternatororstarter 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.[3]

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 typicallytwo-stroketwo-cylinderair-cooled enginesin the range of 18–30 hp (14–22 kW). They are lighter in weight, and simpler to operate than self-launching powerplants.[4]

Self-launching Motor Gliders[edit]

Powerplant from aSchleicher ASH 26E self-launching motor glider, mounted on a test stand for maintenance atAlexander SchleicherinPoppenhausen,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 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 beltreduction drive.In older designs, the propeller alignment must be checked by the pilot using a mirror, before it is retracted into the fuselage; however in current production gliders, propeller alignment is fully automatic.

Another solution is thesingle-blade propellerthat 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 aretwo-stroke piston engines,orWankel rotary engines.[5]

Touring Motor Gliders[edit]

Motor with fixed or full feathering propellers are generally classified as Touring Motor Gliders (TMGs). TMGs can take off and cruise like anairplaneor soar with power off, like aglider.

They are fitted with front-mounted engines, similar to a small airplane. The largewingspansof TMGs provide a moderateglidingperformance, not as good as that of unpowered gliders. However TMGs are more efficient than conventionallight aircraft.

Most TMGs are designed with engines of 80 to 100 hp (75 kW) and typically cruise (under power) at 85–100 knots (190 km/h). 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 thePhoenix Air Phoenixare capable of higher speeds and longer range under power.

Some TMGs are equipped with folding wings to allow them to fit in standard small airplaneT-hangars.Tow hooks are unnecessary, since aircraft with self-launch ability do not require access to winch or tow plane forlaunchinglike a conventional glider.

Some TMGs, like theEuropaor 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.[6]

Grob G 109B touring motor glider, withfibre-reinforced plasticconstruction.
Motor gliderSF25 MotorfalkeoverYstad2020.

Thelanding gearconfiguration on TMGs usually incorporates two fixed main wheels, allowing it to betaxiedon 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 additionaldragof the stopped propeller and landing gear reduces their gliding performance, TMGs are seldom used in competition.

Motor Configurations[edit]

Retractable propeller[edit]

Schleicher ASH 26eself-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 thefuselage,aft of thecockpitand 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.

Cross-over[edit]

On theStemme 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 taxied 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-positionvariable-pitch propellerand aturbochargeron the engine, which allows the aircraft to cruise at altitudes up to 30,000 feet (9,000 m).[7]

On theAMS Carat,the propeller folds forward, pointing straight ahead like a spear.

Electric[edit]

Although most motor gliders havegasoline-fueledinternal combustion engines,a number of electric-powered self-launchers have been developed including theLange Antares 20Eand23E,Schempp-Hirth Arcus E,Schleicher AS 34Me,Pipistrel Taurus Electro G2,Silent 2 Targa LE(Lithium Electric), and the closely relatedAir Energy AE1 Silent,Yuneec Apis 2,Yuneec EViva,Alpaero Exel,Aériane Swift,ElectraviaandAlatus AL12.[8][9][10][11]Some types use a pylon behind the cockpit to extend a propeller. 11 different types, from 7 manufacturers, such as theAlisport Silent 2 Electro,use thefront electric sustainer(FES) system with a folding propeller in the nose.[12]

Advantages of electric power over gasoline include:

  • Faster and more reliable transition from gliding to powered flight, as the time and risk required to start and warm up an internal combustion engine is eliminated.[13]For example, the Antares 20E transitions from gliding to powered flight in under 12 seconds.[citation needed]With a FES system maximum power can be available in 1–2 seconds.[12]
  • For types that use a pylon, lower drag and sink rate are achieved with the pylon extended and the motor not running, as there is no radiator as required to cool a gasoline engine. For FES systems, the drag from the folded propeller in the nose is negligible if the engine does not start. For safe operation, the "Engine out and not running" performance is used to plan in-air restarts in the event of a problem (even for electrics). The reduced sink-rate of electric-powered motor-gliders "engine out and not running" allows a lower altitude for a safe in-air restart.[13]
  • For types that use a pylon, belt reduction drive and associated reliability issues are eliminated. As an electric motor can turn at a lower RPM than typical gasoline engines, it can be mounted at the top of the mast without a reduction system, eliminating belt-drive issues.[citation needed]
  • Greatly reduced noise, allowing take-off from locations where other powered aircraft are not permitted.[13]

Disadvantages of current electric powered motor-gliders as compared to gasoline include:

  • Reduced range or climb altitude, as the energy stored per unit weight for available batteries is less than gasoline.[citation needed]
  • Gliders with pylon-mounted electric motors are slightly heavier than those with gasoline engines due to the batteries. An FES system weighs about the same as a two-stroke engine.[13]
  • Increased cost. While electric motors are comparably inexpensive, low-weight batteries suitable for motor-gliders are a little more expensive than gasoline engines.[citation needed][when?]

Electric sustainer systems (with power to sustain flight but not self-launch) are optional for theSchempp-Hirth Ventus-3,HPH SharkandESAG LAK-17B.These three all use the FES as used for self-launching lighter gliders, such as the Alisport Silent 2.[12]

Jet[edit]

Bob Carlton's jet-powered glider.

The first production self-launching motor glider fitted with ajet enginewas theCaproni Vizzola 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. Since then a new generation of jets have been offered for theSchempp-Hirth Ventus 2,Jonker JS-1 Revelationand theHpH 304,all featuring sustainer jet engines.

Other self-launching types include the experimentalLET L-13TJ Blaník,[14]a version of theAlisport Silent Cluband a version of theTeST TST-14 Bonus.

Use of engines in self-launching sailplanes[edit]

A self-launchingEiri-Avion PIK-20E in flight with engine running.

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.[15]

Insoaring 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 Recordersused 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.

Schempp-Hirth Nimbus 4M with engine running.

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.[16]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.[17]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.

Licenses or certificates[edit]

Fournier RF 4D in England (2009).

In Europe, powered gliders are categorized into gliders with retractable propellers/engines, which can be flown with an ordinaryglider pilot license(GPL), and touring motor gliders (TMG), which require a license extension to the standard GPL. In the United Kingdom, where gliding is regulated by theBritish Gliding Association,pilots of self-sustaining gliders, like those of pure gliders, do not have to be licensed with theUnited Kingdom Civil Aviation Authority.

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 certificated 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.[18]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.

See also[edit]

References[edit]

  1. ^"3. Gliding, chapter 1: General Rules and Definitions".FAI Sporting Code.Archived fromthe originalon 7 October 2007.Retrieved23 November2006.
  2. ^"Flying on 250 cc",Flight,1935.
  3. ^"DG-1000T Information".DG Flugzeugbau. Archived fromthe originalon 5 December 2006.Retrieved24 November2006.
  4. ^"Discus 2T Sustainer Engine Information".Schempp-Hirth. Archived fromthe originalon 16 October 2006.Retrieved24 November2006.
  5. ^"Alexander Schleicher GmbH & Co., ASH 26 E Information".Archived fromthe originalon 8 October 2006.Retrieved24 November2006.
  6. ^Europa Aircraft(homepage).
  7. ^"Stemme AG, S10-VT Information".Archived fromthe originalon 17 September 2012.Retrieved24 November2006.
  8. ^"Lange Flugzeugbau GmbH, Antares 20E Information".Archived fromthe originalon 15 October 2006.Retrieved24 November2006.
  9. ^Arcus-E,Schempp-Hirth Flugzeugbau, archived fromthe originalon 6 July 2015.
  10. ^World Directory of Leisure Aviation,2011.
  11. ^"Going Vintage Electrically".Café foundation.
  12. ^abc"LZ Design web-site".Retrieved14 November2018.
  13. ^abcd"LZ Design web-site".Retrieved14 November2018.
  14. ^"Turbojet engine TJ 100, PBS Velká Bíteš, a.s."Retrieved4 December2009.
  15. ^"Information about self-sustaining gliders".Retrieved4 September2006.
  16. ^"Guide to Self-launching Sailplane Operation".Archived fromthe originalon 12 March 2008.Retrieved4 September2006.
  17. ^"Motor gliding training syllabus for instructors".Retrieved4 September2006.
  18. ^Advisory circular(PDF),Regulatory & guidance library, FAA[permanent dead link].
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