Editing Auxiliary power unit (section)

====As mechanical "startup" APUs for jet engines====
The first German [[jet engine]]s built during the [[Second World War]] used a mechanical APU starting system designed by the German engineer [[Norbert Riedel]]. It consisted of a {{convert|10|hp|kW}} [[Two-stroke engine|two-stroke]] [[flat engine]], which for the [[Junkers Jumo 004]] design was hidden in the engine nose cone, essentially functioning as a pioneering example of an auxiliary power unit for starting a jet engine. A hole in the extreme nose of the cone contained a manual pull-handle which started the piston engine, which in turn rotated the compressor. Two spark plug access ports existed in the Jumo 004's nose cone to service the Riedel unit's cylinders in situ, for maintenance purposes. Two small "premix" tanks for the Riedel's petrol/[[Two-stroke oil|oil]] fuel were fitted in the annular intake. The engine was considered an extreme short stroke (bore / stroke: 70&nbsp;mm / 35&nbsp;mm = 2:1) design so it could fit within the in the nose cone of jet engines like the Jumo 004. For reduction it had an integrated [[planetary gear]]. It was produced by [[Victoria (motorcycle)|Victoria]] in [[Nuremberg]] and served as a mechanical APU-style starter for all three German jet engine designs to have made it to at least the prototype stage before May 1945&nbsp;– the [[Junkers Jumo 004]], the [[BMW 003]] (which uniquely appears to use an electric starter for the Riedel APU),<ref>{{cite web |url=http://legendsintheirowntime.com/LiTOT/Content/1946/Av_4603_DA_BMW003.html |title=Design Analysis of BMW 003 Turbojet - "Starting the Engine" |last1=Schulte |first1=Rudolph C. |date=1946 |website=legendsintheirowntime.com |publisher=United States Army Air Force - Turbojet and Gas Turbine Developments, HQ, AAF |access-date=September 3, 2016 |quote=Starting procedure is as follows: Starting engine is primed by closing electric primer switch, then ignition of turbojet and ignition '''and electric starting motor''' of [[Norbert Riedel|Riedel engine]] are turned on (this engine can also be started manually by pulling a cable). After the Riedel unit has reached a speed of about 300 rpm, it automatically engages the compressor shaft of the turbojet. At about 800 rpm of the starting engine, starting fuel pump is turned on, and at 1,200 rpm the main (J-2) fuel is turned on. The starter engine is kept engaged until the turbojet attains 2,000 rpm, at which the starter engine and starting fuel are turned off, the turbojet rapidly accelerating to rated speed of 9,500 rpm on the J-2 fuel |archive-date=September 29, 2018 |archive-url=https://web.archive.org/web/20180929074301/http://legendsintheirowntime.com/LiTOT/Content/1946/Av_4603_DA_BMW003.html |url-status=dead }}</ref> and the prototypes (19 built) of the more advanced [[Heinkel HeS 011]] engine, which mounted it just above the intake passage in the Heinkel-crafted sheetmetal of the engine nacelle nose.<ref>Gunston 1997, p. 141.</ref>

The [[Boeing 727]] in 1963 was the first jetliner to feature a [[gas turbine]] APU, allowing it to operate at smaller airports, independent from ground facilities. The APU can be identified on many modern airliners by an exhaust pipe at the aircraft's tail.<ref>{{cite news|last1=Vanhoenacker|first1=Mark|title=What Is That Hole in the Tail of an Airplane?|url=http://www.slate.com/blogs/the_eye/2015/02/05/what_s_that_thing_unveils_the_mystery_of_that_hole_on_the_tail_of_the_airplane.html|access-date=20 October 2016|work=[[Slate (magazine)|Slate]]|date=5 February 2015}}</ref>