Aircraft engine starting

Many variations ofaircraft engine startinghave been used since theWright brothersmade their first powered flight in 1903. The methods used have been designed for weight saving, simplicity of operation and reliability. Early piston engines were started by hand. Geared hand starting, electrical and cartridge-operated systems for larger engines were developed between the First and Second World Wars.

Gas turbineaircraft engines such asturbojets,turboshaftsandturbofansoften use air/pneumatic starting, with the use ofbleed airfrom built-inauxiliary power units(APUs) or external air compressors now seen as a common starting method. Often only one engine needs be started using the APU (or remote compressor). After the first engine is started using APU bleed air,cross-bleed airfrom the running engine can be used to start the remaining engine(s).

Piston engines

Hand starting/propeller swinging

Hand starting of aircraft piston engines by swinging the propeller is the oldest and simplest method, the absence of any onboard starting system giving an appreciable weight saving. Positioning of the propeller relative to thecrankshaftis arranged such that the engine pistons pass throughtop dead centreduring the swinging stroke.

As the ignition system is normally arranged toproduce sparks before top dead centrethere is a risk of the engine kicking back during hand starting. To avoid this problem one of the two magnetos used in a typical aero engine ignition system is fitted with an 'impulse coupling', this spring-loaded device delays the spark until top dead centre and also increases the rotational speed of the magneto to produce a stronger spark. When the engine fires, the impulse coupling no longer operates and the second magneto is switched on.^[1] As aero engines grew bigger incapacity(during theinterwarperiod), single-person propeller swinging became physically difficult, ground crew personnel would join hands and pull together as a team or use a canvas sock fitted over one propeller blade, the sock having a length of rope attached to the propeller tip end.^[2]^[3]Note that this is different from the manual "turning over" of radial piston engine, which is done to release oil that has become trapped in the lower cylinders prior to starting, to avoid engine damage. The two appear similar, but while hand starting involves a sharp, strong "yank" on the prop to start the engine, turning over is simply done by turning the prop through a certain set amount.

Accidents have occurred during lone pilot hand starting, high throttle settings, brakes not applied or wheel chocks not being used, all resulting in aircraft moving off without the pilot at the controls.^[4]"Turning the engine" with the ignition and switches accidentally left "on" can also cause injury, as the engine can start unexpectedly when a spark plug fires. If the switch is not in start position, the spark will occur before the piston hits top dead center, which can force the propeller to violently kick back.

Hucks starter

The Hucks starter (invented byBentfield Hucksduring WWI) is a mechanical replacement for the ground crew. Based on a vehicle chassis the device uses aclutchdriven shaft to turn the propeller, disengaging as the engine starts. A Hucks starter is used regularly at theShuttleworth Collectionfor starting period aircraft.^[3]

Pull cord

Self-sustaining motor gliders(often known as 'turbos') are fitted with small two-stroke engines with no starting system, for ground testing a cord is wrapped around the propeller boss and pulled rapidly in conjunction with operatingdecompressorvalves. These engines are started in flight by operating the decompressor and increasingairspeedto windmill the propeller. Early variants of theSlingsby Falkemotor glider use a cockpit mounted pull start system.^[5]

Electric starter

Aircraft began to be equipped with electrical systems around 1930, powered by a battery and small wind-drivengenerator.The systems were initially not powerful enough to drive starter motors. Introduction of engine-driven generators solved the problem.^[6]

Introduction of electric starter motors for aero engines increased convenience at the expense of extra weight and complexity. They were a necessity for flying boats with high mounted, inaccessible engines. Powered by an onboard battery, ground electrical supply or both, the starter is operated by a key or switch in the cockpit. The key system usually facilitates switching of the magnetos.^[6]^[7]

In cold ambient conditions the friction caused byviscousengine oil causes a high load on the starting system. Another problem is the reluctance of the fuel to vaporise and combust at low temperatures. Oil dilution systems were developed (mixing fuel with the engine oil),^[8]and engine pre-heaters were used (including lighting fires under the engine). TheKi-Gasspriming pump system was used to assist starting of British engines.^[9]

Aircraft fitted withvariable-pitch propellersorconstant speed propellersare started in fine pitch to reduce air loads andcurrentin the starter motor circuit.^{[citation needed]}

Many light aircraft are fitted with a 'starter engaged' warning light in the cockpit, a mandatory airworthiness requirement to guard against the risk of the starter motor failing to disengage from the engine.^[10]

Coffman starter

The Coffman starter was an explosive cartridge operated device, the burning gases either operating directly in thecylindersto rotate the engine or operating through a geared drive. First introduced on theJunkers Jumo 205diesel engine in 1936 the Coffman starter was not widely used bycivil operatorsdue to the expense of the cartridges.^[11]

Pneumatic starter

In 1920Roy Feddendesigned a piston engine gas starting system, used on theBristol Jupiterengine in 1922.^[3]A system used in earlyRolls-Royce Kestrelengines ducted high-pressure air from a ground unit through acamshaftdriven distributor to the cylinders vianon-return valves,the system had disadvantages only overcome by conversion to electric starting.^[12]

In-flight starting

When a piston engine needs to be started in flight the electric starter motor can be used. This is a normal procedure formotor glidersthat have been soaring with the engine turned off. During aerobatics with earlier aircraft types it was not uncommon for the engine to cut during manoeuvres due tocarburettordesign. With no electric starter installed, engines can be restarted by diving the aircraft to increase airspeed and the rotation speed of the 'windmilling' propeller.^[13]

Inertia starter

An aero engine inertia starter uses a pre-rotatedflywheelto transferkinetic energyto the crankshaft, normally through reduction gears and aclutchto prevent over-torque conditions. Three variations have been used, hand driven, electrically driven and a combination of both. When the flywheel is fully energised either a manual cable is pulled or asolenoidis used to engage the starter.^[14]

Gas turbine engines

Starting of a gas turbine engine requires rotation of thecompressorto a speed that provides sufficient pressurised air to thecombustion chambers.The starting system has to overcome inertia of the compressor and friction loads, the system remains in operation after combustion starts and is disengaged once the engine has reached self-idling speed.^[15]^[16]

Electric starter

Two types of electrical starter motor can be used, direct-cranking (to disengage as internal combustion engines) and starter-generator system (permanently engaged).^[17]

Hydraulic starter

Small gas turbine engines, particularlyturboshaftengines used in helicopters andcruise missile turbojetscan be started by a gearedhydraulic motorusing oil pressure from a ground supply.^[18]

Air-start

With air-start systems, gas turbine engine compressor spools are rotated by the action of a large volume of compressed air acting directly on the compressor blades or driving the engine through a small, gearedturbinemotor. These motors can weigh up to 75% less than an equivalent electrical system.^[15]

The compressed air can be supplied from an on-boardauxiliary power unit(APU), a portablegas generatorused by ground crew or by cross feedingbleed airfrom a running engine in the case of multi-engined aircraft.^[19]

TheTurbomeca Paloustegas generator was used to start theSpeyengines of theBlackburn Buccaneer.Thede Havilland Sea Vixenwas equipped with its own Palouste in a removable underwing container to facilitate starting when away from base.^[20]Other military aircraft types using ground supplied compressed air for starting include theLockheed F-104 Starfighterand variants of theF-4 Phantomusing theGeneral Electric J79 turbojetengine.

Combustion starters

AVPIN starter

Versions of theRolls-Royce Avonturbojet engine used a geared turbine starter motor that burnedisopropyl nitrateas the fuel. In military service thismonofuelhad theNATOdesignation of S-746 AVPIN. For starting a measured amount of fuel was introduced to the starter combustion chamber then ignited electrically, the hot gases spinning the turbine at high revolutions with the exhaust exiting overboard.^[21]

Cartridge starter

Similar in operating principle to the piston engine Coffman starter, an explosive cartridge drives a small turbine engine which is connected by gears to the compressor shaft.^[22]

Fuel/air turbine starter (APU)

Developed for short-haul airliners, most civil and military aircraft requiring self-contained starting systems these units are known by various names including Auxiliary Power Unit (APU), Jet Fuel Starter (JFS), Air Start Unit (ASU) or Gas Turbine Compressor (GTC).^[21] Comprising a small gas turbine which is electrically started, these devices provide compressedbleed airfor engine starting and often also provide electrical and hydraulic power for ground operations without the need to run the main engines.^[23] ASUs are used today within the civil and military Ground Support to serve Aircraft on main engine start (MES) and pneumatic bleed-air-support for the Environmental Control System (ECS) cooling and heating

Internal combustion engine starter

Riedel two-stroke starter motor of theJunkers Jumo 004.Note the pull-cord handle

An interesting feature of all three German jet engine designs that saw production of any kind before May 1945: the GermanBMW 003,Junkers Jumo 004andHeinkel HeS 011axial-flowturbojetengine designs was the starter system, which consisted of aRiedel10 hp (7.5 kW)flat twin two-strokeair-cooled engine hidden in the intake, and essentially functioned as a pioneering example of anauxiliary power unit(APU) for starting a jet engine — for the Jumo 004, a hole in the extreme nose of the intake diverter contained a D-shaped manualpull-cordhandle which started the piston engine, which in turn rotated the compressor. Two small petrol/oil mixtanks were fitted in the annular intake.^[24]

TheLockheed SR-71 Blackbirdused twoBuick Nailheadsas starter motors, which were mounted on a AG-330 Start Kart trolley, later with big-block V8Chevrolet 454engines.

In-flight restart

Gas turbine engines can be shut down in flight, intentionally by the crew to save fuel or during aflight testor unintentionally due tofuel starvationorflameoutafter acompressor stall.

Sufficientairspeedis used to 'windmill' the compressor then fuel and ignition are switched on, an on-board auxiliary power unit may be used at highaltitudeswhere the air density is lower.^[16]

During zoom climb operations of theLockheed NF-104Athe jet engine was shut down on climbing through 85,000 ft (26,000 m) and was started using thewindmill methodon descent through denser air.^[25]

Pulse jet starting

Pulse jet engines are uncommon aircraft powerplants. However, theArgus As 014used to power theV-1 flying bombandFieseler Fi 103R Reichenbergwas a notable exception. In this pulse jet three air nozzles in the front section were connected to an external high-pressure air source,butanefrom an external supply was used for starting, ignition was accomplished by aspark pluglocated behind the shutter system, electricity to the plug being supplied from a portable starting unit.^[26]

Once the engine started and the temperature rose to the minimum operating level, the external air hose and connectors were removed, and the resonant design of the tailpipe kept the pulse jet firing. Each cycle or pulse of the engine began with the shutters open; fuel was injected behind them and ignited, and the resulting expansion of gases forced the shutters closed. As the pressure in the engine dropped following combustion, the shutters reopened and the cycle was repeated, roughly 40 to 45 times per second. The electrical ignition system was used only to start the engine; heating of the tailpipe skin maintained combustion.^[26]

References

Notes

^Thom 1988, p. 166.
^Lumsden 2003, p. 40.
^^a ^b ^cGunston 2006, p. 86.
^Thom 1988, p. 202.
^Hardy 1982, p. 174.
^^a ^bGunston 2006, p. 87.
^Thom 1988, p. 167.
^Gunston 2006, p. 89.
^Gunston 2006, p. 85.
^Thom 1988, p. 165.
^Gunston 2006, pp. 87–88.
^Rubbra 1990, p. 40.
^Williams 1975, p. 59.
^FAA 1976, p. 263.
^^a ^bFAA 1976, p. 270.
^^a ^bStewart 1986, p. 33.
^FAA 1976, pp. 271–272.
^Gunston 1997, p. 82.
^FAA 1976, p. 277.
^Flightglobal archive -Flight,March 1965Retrieved: 15 August 2012
^^a ^bGunston 1997, p. 81.
^FAA 1976, p. 281.
^FAA 1976, p. 283.
^Gunston 1997, p. 141.
^Bowman 2000, p. 173.
^^a ^bJane's 1998, p. 284.

Bibliography

Bowman, Martin W.Lockheed F-104 Starfighter.Ramsbury, Marlborough, Wiltshire, UK: Crowood Press Ltd., 2000.ISBN 1-86126-314-7.
Federal Aviation Administration,Airframe & Powerplant Mechanics Powerplant HandbookU.S Department of Transportation, Jeppesen Sanderson, 1976.ISBN 0-89100-079-8
Gunston, Bill.Development of Piston Aero Engines.Cambridge, England. Patrick Stephens Limited, 2006.ISBN 0-7509-4478-1
Gunston, Bill.The Development of Jet and Turbine Aero Engines.Cambridge, England. Patrick Stephens Limited, 1997.ISBN 1-85260-586-3
Hardy, Michael.Gliders & Sailplanes of the World.London: Ian Allan, 1982.ISBN 0-7110-1152-4.
Jane's Fighting Aircraft of World War II.London. Studio Editions Ltd, 1998.ISBN 0-517-67964-7
Lumsden, Alec.British Piston Engines and their Aircraft.Marlborough, Wiltshire: Airlife Publishing, 2003.ISBN 1-85310-294-6.
Rubbra, A.A.Rolls-Royce Piston Aero Engines - a designer remembers: Historical Series no 16:Rolls-Royce Heritage Trust, 1990.ISBN 1-87292-200-7
Stewart, Stanley.Flying the Big Jets.Shrewsbury, England. Airlife Publishing Ltd, 1986.ISBN 0 906393 69 8
Thom, Trevor.The Air Pilot's Manual 4-The Aeroplane-Technical.Shrewsbury, Shropshire, England. Airlife Publishing Ltd, 1988.ISBN 1-85310-017-X
Williams, Neil.Aerobatics,Shrewsbury, England: Airlife Publishing Ltd., 1975ISBN 0 9504543 03

[1] Thom 1988, p. 166.

[2] Lumsden 2003, p. 40.

[Gunston86-3] Gunston 2006, p. 86.

[4] Thom 1988, p. 202.

[5] Hardy 1982, p. 174.

[Gunston87-6] Gunston 2006, p. 87.

[7] Thom 1988, p. 167.

[8] Gunston 2006, p. 89.

[9] Gunston 2006, p. 85.

[10] Thom 1988, p. 165.

[11] Gunston 2006, pp. 87–88.

[12] Rubbra 1990, p. 40.

[13] Williams 1975, p. 59.

[14] FAA 1976, p. 263.

[FAA270-15] FAA 1976, p. 270.

[Stewart33-16] Stewart 1986, p. 33.

[17] FAA 1976, pp. 271–272.

[18] Gunston 1997, p. 82.

[19] FAA 1976, p. 277.

[20] Flightglobal archive -Flight,March 1965Retrieved: 15 August 2012

[Gunston81-21] Gunston 1997, p. 81.

[22] FAA 1976, p. 281.

[23] FAA 1976, p. 283.

[24] Gunston 1997, p. 141.

[25] Bowman 2000, p. 173.

[Jane's-26] Jane's 1998, p. 284.

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Piston engines

Hand starting/propeller swinging

Hucks starter

Pull cord

Electric starter

Coffman starter

Pneumatic starter

In-flight starting

Inertia starter

Gas turbine engines

Electric starter

Hydraulic starter

Air-start

Combustion starters

AVPIN starter

Cartridge starter

Fuel/air turbine starter (APU)

Internal combustion engine starter

In-flight restart

Pulse jet starting

See also

References