|Role||Observation and liaison aircraft|
|First flight||28 January 1917|
The Junkers J.I (manufacturer's designation J 4) was a German "J-class" armored sesquiplane of World War I, developed for low-level ground attack, observation and Army cooperation. It is especially noteworthy as being the first all-metal aircraft to enter mass production; the aircraft's metal construction and heavy armour was an effective shield against small arms fire over the battlefield.
In an extremely advanced design, a single-unit steel "bathtub" that ran from just behind the propeller to the rear crew position acted not only as armour, but also as both the main fuselage structure and engine mounting in one unit, engine access being provided by twin vertically-hinged, aft-swinging triple-piece armor-steel panels, one on either side of the nose. The armour was 5 millimetres (0.20 in) thick and weighed 470 kilograms (1,040 lb). It protected the crew, the engine, the fuel tanks, and radio equipment (when fitted). The flight control surfaces were connected to the aircraft's controls by push-rods and bellcranks – not with the usual steel cable control connections of the era – as push-rods were less likely to be severed by ground fire.
There was a significant size difference between the upper and lower wings – the upper wing had a total area of 35.89 m2 (386.3 sq ft), over double the total area of the lower wing – 13.68 m2 (147.3 sq ft). This is a form of biplane known as a Sesquiplane.
The aircraft had two fuel tanks with a total capacity of around 120 litres (32 US gal). The main tank (divided into two parts for redundancy) was supplemented by a smaller, 30-litre (7.9 US gal) "gravity tank". This was intended to supply fuel to the engine by gravity feed in the event of an engine fuel pump failure; it contained enough fuel for thirty minutes on full power. There was a manual fuel pump for use when the gravity tank became exhausted.
The aircraft could be disassembled into its main components: wings, fuselage, undercarriage, and tail, to make it easier to transport by rail or road. A ground crew of six to eight could reassemble the aircraft and have it ready for flight within four to six hours. The wings were covered with 0.19-millimetre-thick (0.0075 in) aluminum skin which could be easily dented, so great care had to be taken when handling the aircraft on the ground.
The J.I was well liked by its crews, although its ponderous handling earned it the nickname "furniture van". The aircraft first entered front service in August 1917. They were used on the Western Front during the German Spring Offensive of 1918.
The aircraft could be fitted with two downward-firing machine guns for ground attack, but they were found to be of limited use because of the difficulty of aiming them. The J-Is were mainly used for army co-operation and low-level reconnaissance. They were also used for dropping ammunition and rations on isolated or cut-off outposts that could not be easily supplied by other means.
The production at Junkers works was quite slow, because of poor organization. Only 227 J.Is were manufactured before production ceased in January 1919. At least one was lost to ground fire, shot down by a French anti-aircraft machine gun that was firing armour-piercing rounds. Although this was apparently an isolated event as some sources state none were lost in combat. A few were lost in landing accidents and other mishaps.
Only one relatively complete aircraft survived, bearing German military serial number J.I 586/17. It is preserved at the Canada Aviation and Space Museum in Ottawa. This aircraft, construction number 252, was manufactured in 1918 and was a war trophy that was sent to Canada in 1919. It was in the possession of the Canadian War Museum before being transferred to the Canada Aviation and Space Museum in 1969.
Additionally, one Junkers J.I fuselage exists, at the Museo Storico dell Aeronautica Militare Italiana in Vigna di Valle. This aircraft was previously exhibited at the Leonardo Da Vinci Museum at Milano and was restored at the Technical Museum of Berlin between 2005 and 2010. 
Some flyable Junkers J.I replicas are under construction in Hungary.
- Crew: Two, pilot and observer
- Length: 9.1 m (29 ft 10 in)
- Wingspan: 16.00 m (52 ft 6 in)
- Height: 3.4 m (11 ft 2 in)
- Wing area: 49.4 m2 (531 ft2)
- Empty weight: 1,766 kg (3,893 lb)
- Gross weight: 2,140 kg (4,718 lb)
- Powerplant: 1 × Benz Bz.IV, 149 kW (200 hp)
- Maximum speed: 155 km/h (97 mph)
- Range: 310 km (193 miles)
- Service ceiling: 4,000 m (13,123 ft)
- 1 × trainable, rearward-firing 7.92 mm (.312 in) Parabellum MG14 machine gun; five, 200-round drums of ammunition.
- Aircraft of comparable role, configuration and era
- Grosz 1993, p. 3
- Taylor 1989, p. 538.
- Flight 18 March 1920, p. 315.
- Gray, Peter; Thetford, Owen (1987). German Aircraft of the First World War. Putnam. p. 156. ASIN B007O00RQG.
- Flight February 1920, p. 229.
- Flight 18 March 1920, p. 317.
- Grosz 1993, p. 4.
- Grosz 1993, pp. 6–7.
- Grey 1919, pp. 320a–321a.
- "Junkers JI." Canada Aviation and Space Museum. Retrieved: 6 September 2011.
- "Ministero della Difesa" (in German). Retrieved 2017-05-21.
- Grosz 1993, pp. 7–8.
- Grey, C. G. Jane's All the World's Aircraft 1919. London: Putnam, 1919.
- Grosz, P.M. Junkers J.I, Windsock Datafile 39. Hertfordshire, UK: Albatros Productions Ltd., 1993. ISBN 0-948414-49-9.
- "Report on the Junker Armoured Two-Seater Biplane, Type J.1." Flight, 26 February 1920
- "Report on the Junker Armoured Two-Seater Biplane, Type J.1." Flight, 4 March 1920
- "Report on the Junker Armoured Two-Seater Biplane, Type J.1." Flight, 11 March 1920
- "Report on the Junker Armoured Two-Seater Biplane, Type J.1." Flight, 18 March 1920.
- Taylor, Michael J. H. Jane's Encyclopedia of Aviation. London: Studio Editions. 1989, p. 538.
- World Aircraft Information Files. London: Bright Star Publishing, p. File 898, Sheet 01.
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