Aircraft Apu

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  • Topic: Gas turbine, Auxiliary power unit, Jet engine
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  • Published : September 19, 2011
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THE AIRCRAFT APU (Auxiliary Power Unit)

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The Auxiliary Power Unit is a system designed to provide a secondary source of energy to a particular plant or craft. In aviation, this system has evolved to become as essential system in today’s commercial transport aircraft. This report provides an analysis of the history, functionality and future development of the aircraft Auxiliary Power Unit. It covers the technological evolution of the APU with reference to specific makes and models, and their direct impact on the aviation industry. The report also details the main components of Auxiliary Power Units, and highlights their impact on the aircraft’s flight controls. The report concludes with a summary of the impact of APU’s on global aviation, and their development into the future.

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The development of an aircraft Auxiliary Power Unit was first undertaken by British aircraft designer and manufacturer Noel Pemberton Billing in 1916 (Andrews & Morgan, 1987). His P.B.31.E prototype “Nighthawk”, was a quadruplane designed to hunt German airships at night during WW1. For that purpose, it was equipped with a nose-mounted searchlight powered by a 5 horsepower, 2stroke gasoline piston engine, courtesy of the All British Engine Company (ABC) (Andrews & Morgan, 1987). Although the Nighthawk never entered service, it has achieved historical significance as the first ever example of an APU equipped aircraft. Fig 1: Early ABC APU (ABC and other aero auxiliary engines , 2002)

During WW2, the All British Engine Company once again took up manufacturing of APU systems for military aircraft. ABC APU’s were installed in a variety of aircraft, primarily providing secondary electrical power to various electronic systems (Andrews & Morgan, 1987). The ABC MkII APU was a powerful air-cooled OHC flat-twin engine Capable of providing 300 watts at 24 volts (Green, 1962). Designed for the British S.25 Short Sunderland flying boats, the MkII was also connected to 2,500 gallon bilge pumps to drain the aircrafts bilge and floats, as well as an onboard fuel transfer pump to allow for self-fuelling. Later variants of the Short Sunderland Bomber also utilised the MkII to power air compressors, capable of charging engine air start bottles to 400psi in under 10 minutes (Green, 1962). Fig 2: ABC MkII APU (ABC and other aero auxiliary engines , 2002)

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The first turbine APU was adapted by British aircraft manufacturer Avro in 1956 (Darling, 2007). In a joint venture with the British Air Ministry, the Avro Vulcan B.1 (the RAF’s strategic jet bomber) was fitted out with a Rover 1S Series gas turbine, re-named the Rover Airborne Auxiliary Power Pack (AAPP). Mounted inside the wing, behind the starboard main undercarriage bay, the AAPP ran a single Rotax 208 volt, 3 phase alternator through a reduction gearbox, providing ample power to the aircraft including its four turbojet starter motors (Smith, 1955). This enabled the aircraft to be operated independently of ground systems, as well as providing for the rapid ignition of all four main engines, allowing the aircraft to be ready for taxi in less than eight seconds (Darling, 2007). Rover also developed the first turbine Auxiliary Power Units for use in commercial aviation (Tapper, 1988). In 1959, British aircraft manufacturer Armstrong Whithworth delivered the first of seven AW 650 Argosy turboprop aircraft to American cargo airlines Riddle Airlines. Each aircraft was equipped with the Rover 1S-60 gas turbine APU, the same APU currently in use by the Arvo Vulcan at that time (Tapper, 1988). One Rover 1S-60 was even modified into a turboprop engine (the Rover TP.90) and fitted to a converted de Havilland Mk 22A Chipmunk (Jackson, 1987).

Fig 3: 1959 Rover...
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