The diminutive XH-20 "Little Henry" was developed by a team led by Marvin Marks in close co-operation between the Helicopter and Propulsion Division of McDonnell and the AAF/USAF Rotor Wing Branch, Propeller Laboratory and Rotary Wing Unit, Aircraft Projects Section, Wright-Patterson AFB. At the time of its inception, the XH-20 was unique in being powered by two McDonnell-developed 18.42cm ramjet units weighing only 4.5kg each, mounted at the tips of the two-blade rotor and fed from tanks beside the pilot. The fuel, originally propane but later gasoline (motor car petrol), was boosted through a feed line to a delivery valve on the rotor head. From there centrifugal force took over to convey the fuel through the blades to the tip-mounted ramjets.
Having undertaken privately-funded studies of ramjets as powerplants for helicopters, McDonnell submitted an unsolicited proposal to the Air Materiel Command at Wright Field in the spring of 1946 and in July was awarded a contract for the development and testing of two experimental machines. It was then hoped that the novel powerplant would make it feasible to build small, lightweight helicopters by reducing or eliminating complex and heavy engine parts, gear systems, and transmissions. Furthermore, it was claimed that this powerplant installation would result in increased efficiency and would completely eliminate the need for a conventional anti-torque rotor. Accordingly, the two XH-20s were ordered as flying test-beds and were not intended as production prototypes. Their airframe was kept as simple as possible and consisted of welded tubes carried on three vertical legs ending with free-castoring wheels. Conventional helicopter controls were fitted, and the pitch lever incorporated a motorcycle-type throttle to regulate the fuel flow.
Fitted as a single-seater, the first of two prototypes (46-689 and 46-690) made both its first tethered flight on 5 May, 1947, and its first free flight on 29 August in St Louis with Charles R. Wood Jr. at the controls. With the project team then benefitting from the experience of Friedrich von Dobhloff, the Austrian engineer who had pioneered the application of jet principles to helicopters during the war, tests continued for four years. During that time, one of the two XH-20s was modified as a two-seater and, like the single-seater, proved generally satisfactory. However, the ramjet units had excessive fuel consumption, which drastically limited range and endurance when compared to conventionally-powered helicopters, and rate of descent in autorotation was excessive. Accordingly, the Air Force terminated the programme in 1951. The first XH-20 was preserved and is now part of the collection of the Air Force Museum at Wright-Patterson AFB, Ohio.
Rene J. Francillon "McDonnell Douglas Aircraft since 1920: Volume II", 1997
This single-seater built in 1947 was powered by blade-tip ramjets delivering 30hp each. The high fuel consumption prevented it from having a satisfactory range and the production programme for the American armed forces was never undertaken. Only one was built.
G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984
