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A parasite aircraft is a component of a composite aircraft which is carried aloft and air launched by a carrier aircraft or mother ship to help or protect the carrier. The carrier craft may or may not be able to later recover the parasite during flight.
The first parasite aircraft flew in 1916, when the British launched a Bristol Scout from a Felixstowe Porte Baby flying boat. The idea eventually developed into jet bombers carrying fully capable parasite fighters. With the advent of long-range fighters equipped with air-to-air missiles, this role was seen as less and less necessary.
Parasite aircraft have long been used in experimental situations, particularly when the experimental aircraft is to be operated at high altitudes. Having another aircraft lift the experimental aircraft to its start altitude saves the fuel that would otherwise be needed to raise it, allowing the experimental aircraft to be lighter, faster, or otherwise better-suited for the experimental work.
Until the middle of the 20th century there was military interest in parasite fighters - fighter aircraft intended to be carried into a combat zone by a larger aircraft, such as a bomber. If the bomber were threatened, the parasite would be released to defend it. Parasite fighters have never been highly successful and have seldom been used in combat. A major disadvantage of a parasite aircraft was that it reduced the payload capacity of the carrier aircraft. Projects for this type were designed to overcome the great disparity in range between bombers and their escort fighters. Development of aerial refueling has made parasite fighters obsolete.
The first parasite fighters were launched and recovered from trapezes mounted externally to military airships. In 1915 Neville Usborne and another British officer worked on a plan to lift a BE.2C fighter under a SS-class non-rigid airship. This would allow the fighter to reach the height of a raiding Zeppelin rapidly while also conserving fuel. In the first experimental flight on 21 February 1916, the envelope lost pressure and the plane was prematurely separated from it at 4,000 feet. Both the officers were killed and there was no further experimentation with small airships.
In May 1916 a Bristol Scout flown by Flt. Lt. M. J. Day was mounted above the top wing of a Porte Baby flying boat flown by Sqn.Ldr John Cyril Porte, and was successfully released at a height of 1000 ft (300 m). Although successful the scheme, intended to provide long-range defence against Zeppelins, was not pursued. 
The Germans also experimented with the idea, suspending an Albatros D.III fighter aeroplane below a Zeppelin and releasing it altitude: the intention was to use the aeroplane to defend airships against the British seaplane patrols encountered over the North Sea. Although the single trial, made on 25 January 1918, was successful the experiments were not continued.
On 12 December 1918, in a test to determine the feasibility of carrying fighter aircraft on dirigibles, the airship C-1 lifted a US Army Curtiss JN-4 aircraft to 2,500 feet over Fort Tilden, New York, and at that height released it for a free flight back to base. The airship was piloted by Lieutenant George Crompton, Dirigible Officer at NAS Rockaway, and the airplane by Lieutenant A. W. Redfield, USA, commander of the 52nd Aero Squadron based at Mineola (Long Island, NY).
The British Imperial Airship Scheme of 1924 envisaged a commercial airship that could also carry five fighter aircraft if put into military use, but this requirement was abandoned. In 1925 first the DH.53 light aeroplane and then Gloster Grebes had been launched from the airship R.33.
In 1930, the USS Los Angeles was used to test the trapeze system developed by the US Navy to launch and recover fixed wing aircraft from rigid airships. The tests were a success, and the U.S. Navy airships USS Akron and USS Macon, were designed to carry parasite aircraft inside a hangar bay within the hull of the airship. The airships could carry up to five single-seat Curtiss F9C Sparrowhawks for scouting or two-seat Fleet N2Y-1s for training. In 1934, two two-seat Waco UBF XJW-1 biplanes equipped with skyhooks were delivered to the USS Macon.
The temporary system was removed from the Los Angeles, which never carried any aircraft on operational flights. In 1930, the Los Angeles also tested the launching of a glider over Lakehurst, New Jersey.
Although operations of these parasite aircraft were quite successful, the ultimate loss of both airships, the USS Akron in 1933 and the USS Macon in 1935, put an end to the program.
The first bombers to carry parasite fighters did so as part of the Zveno experiments carried out in the Soviet Union by Vladimir Vakhmistrov from 1931. Up to five fighters of various types were carried by Tupolev TB-1 and Tupolev TB-3 bombers.
In August 1941, these combinations would fly the only combat missions ever undertaken by parasite fighters. TB-3s carrying Polikarpov I-16SPB dive bombers attacked the Cernavodă bridge and Constantsa docks, in Romania. After that, this squadron, based in the Crimea, carried out a tactical attack on a bridge over the river Dneiper at Zaporozhye, which had been captured by advancing German troops.
Later in World War II, the Luftwaffe experimented with the Messerschmitt Me 328 as a parasite fighter, but problems with its pulsejet engines could not be overcome. Other late-war rocket-powered projects such as the Arado E.381 and Sombold So 344 never left the experimental stage. By contrast, the Empire of Japan were able to get the Yokosuka MXY7 Ohka kamikaze rocket plane type into active service, typically using the Mitsubishi G4M (Betty) bomber class to carry them within range. However, their effectiveness proved minimal in part because Allied air naval defense took advantage of the weight of the parasitical aircraft payload slowing the carrying bombers, making them vulnerable to interception before the rocket plane can separate and launch.
During the early years of the Cold War, the United States Air Force experimented with a variety of parasite fighters to protect its Convair B-36 bombers, including the dedicated XF-85 Goblin, and methods of either carrying a Republic F-84 Thunderjet in the bomber's bomb bay (the FICON project), or attached to the bomber's wingtips (Project Tom-Tom). These projects were all soon abandoned, partly because aerial refueling appeared as a much safer solution to extend the range of fighters.
Examples that have flown include:
- A Bristol Scout was flown from a Porte Baby to become the first parasite aircraft (1916)
- An Albatros D.III was flown from L 35 (LZ 80) to become the first parasite fighter flying from an airship (January 26, 1918)
- A Sopwith Camel was flown from airship HMA 23 (1918)
- A Sperry Messenger biplane was launched and recovered by non-rigid airships Tc-3 and Tc-7 (1923)
- Several DH 53 Hummingbird monoplanes were launched and recovered by airship R33 (1924), followed by two Gloster Grebe fighters (1925).
- A glider and a biplane were recovered by the USS Los Angeles. These were followed by the F9C Sparrowhawk escort fighter which flew operationally from USS Akron and Macon (1935).
- Ohka Rocket plane on the Mitsubishi G4M.
- The Polikarpov I-16 modified into a dive bomber carrying two 250 kg bombs (variant TsKB-29), flown from a Tupolev TB-3 as the Zveno-SPB ("composite dive bomber"), was the first parasite aircraft to see combat (1941)
- The Messerschmitt Me 328 escort fighter was intended to fly from the Dornier Do 217/, but this was unsuccessful due to engine problems.
- The XF-85 Goblin was an attempt to equip B-36 bombers with their own escort fighters (1948)
- The F-84 was a more successful attempt to provide the B-36 with a parasite escort fighter in the FICON project(1952)
F9C Sparrowhawk inside Akron's hangar.
F9C Sparrowhawk successfully hooks on to Akron trapeze, May 1932.
A Republic F-84E on FICON trapeze
- Mistel—German WWII project in which a piloted fighter aimed, then released, a pilotless ("bunker-buster") bomber with an explosive warhead in its nose
- H. J. C Harper "Composite History" Flight 1 November 1937
- "The Felixstowe Flying Boats". Flight. 2 December 1955.
- Robinson, Douglas H. (1971). The Zeppelin in Combat (3rd ed.). Henley-on Thames: Foulis. pp. 246–7. ISBN 0 85429 130 X.
- Imperial Airship Service, The Airship Heritage Trust. Accessed 10 June 2009.
- "Plane Hitched To Dirigible by Hook in Flight." Popular Mechanics, August 1930.
- "Big Changes Give Giants Of The Air Far Wider Range." Popular Science, September 1930; rare photos in article.
- "Dirigible Launches Glider." Popular Mechanics, April 1930.
- Lesnitchenko, Vladimir Combat Composites: Soviet Use of 'Mother-Ships' to Carry Fighters, 1931-1941 Air Enthusiast No.84 November/December 1999 pp. 4-21
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