HUGHES   XH-17
^"FLYING CRANE^"

 Por un requerimiento enviado a las constructoras del Comando de Material, el 31 de enero de 1946, se especificaba el diseño y construcción de un helicóptero experimental voluminoso. Esta aeronave debería ser capaz de elevar una carga externa en un container de 2,44 m x 2,44 m x 6,10 metros de largo y hasta un peso de 4536 Kg., a una velocidad de 105 Km/h a una altitud en vuelo estacionario de 915 metros, teniendo un radio de 160 Km y una duración de hasta 30 minutos de vuelo. Se lo utilizaría para transportar equipamiento, provisiones y personal, a lugares donde el aterrizaje de los helicópteros fuese impracticable. Por eso, este helicóptero grúa debería ser fácilmente adaptable al transporte tanto de personas como de carga.

Aconsejados por el ingeniero austriaco Friedrich von Dobhloff, quien fue uno de los pioneros en el principio de la aplicación del  jet en helicópteros y que había trabajado ya en McDonnell en el XH-20 y otros helicópteros, el Departamento de Helicópteros de la Fuerza Aérea consideró que se necesitaría 1.- un método de construcción de un rotor diferente al utilizado hasta el momento, 2.- rotores movidos por turbinas y por chorros de aire y 3.- servo controles, alerones en las palas u otros mecanismos que redujeran las altas fuerzas que se esperaba tendría un rotor de 22,86 a 24,38 metros. Finalmente, para minimizar los costos, recomendó que el fuselaje y otros componentes no críticos, fuesen construidos en la forma mas simple posible.

Conforme a lo solicitado, los oferentes enviarían las propuestas para un programa de dos fases, la primera consistiría de un estudio de diseño y la segunda, la cual se emprendería solo después que la Fuerza Aérea aprobara los estudios, de la fabricación del aparato. Después de la  invitación, Kellett Aircraft Corporation de Upper Darby, Pennsylvania, fue notificada el 2 de mayo de 1946, que había ganado la competencia.

El estudio de un año reveló que existían problemas técnicos que no se habían anticipado, al necesitar un rotor de un diámetro mas grande y por lo tanto encontrándose dificultades en la construcción y control. La elección del motor también resultó ardua y Kellet se vio forzado a recomendar una instalación de una biturbina en lugar del monomotor que quería la AAF. Esto y otras recomendaciones técnicas fueron aceptadas por el Comando de Material al finalizar la fase de estudio el 27 de agosto de 1947. Kellet consiguió el contrato para construir una plataforma de prueba para el XR-17 (el cual se denominaría XH-17 en junio de 1948, cuando se creara la nueva USAF)

Mientras se construía, Kellett cayo en dificultades financieras y en 1948 vendió los derechos y la plataforma parcialmente construida a Hughes por 250.000 dólares. Contenta con el trabajo realizado, la USAF aprobó la transferencia. También, los miembros claves del equipo de Kellet fueron transferidos a Hughes en California, donde se reanudó el trabajo rápidamente hasta completar la plataforma a finales de 1949.

Para reducir costos, Hughes incorporó componentes de otras aeronaves, tales como los asientos de la cabina del Waco CG-15, el tren de aterrizaje delantero del North American B-25, el tren de aterrizaje trasero del Douglas C-54, y las celdas de combustible del Boeing B-29. Potenciado por dos General Electric 7E-TG-180-XR-17A generadores de gases con un sangrado de aire en la etapa intermedia del compresor, que alimentaba el aire hacia la cabeza del rotor y luego por medios de conductos especiales hacia cuatro quemadores en las punteras de cada pala, donde se le adicionaba combustible para efectuar el quemado. Esta instalación dual de motores proveía el equivalente a 3480 HP, cuando el rotor giraba a 88 RPM. El rotor, de 39,62 metros de diámetro, descansaba sobre láminas de torsión.

Las pruebas comenzaron el 22 de diciembre de 1949, encontrándose algunos problemas. Sin embargo, se realizaron progresos hasta junio de 1950, donde un engranaje del cíclico falló, dañando duramente a la plataforma. Ocho meses mas tarde, la Hughes y la Fuerza Aérea se sentían lo suficientemente confiados como para pasar a la siguiente fase y modificar la plataforma de prueba en una plataforma de vuelo standard. Por lo tanto, en lugar de solamente reparar la plataforma de prueba, Hughes también le incorporó los cambios necesarios requeridos para que el XH-17 pudiera volar. Estos cambios incluían un ajuste al sistema hidráulico, y la instalación de un rotor de cola. Debido a la ausencia de torque, el diámetro del rotor de cola podía ser pequeño en comparación al del rotor principal, colocándosele uno de un Sikorsky H-19 sobre una extensión tubular del fuselaje.

Las pruebas en tierra se reanudaron después de dos años, durante los cuales se le incorporaron estas modificaciones, sin prisa. Finalmente, después de recibir el número de serie de la Fuerza Aérea (50-1842), el XH-17 voló por primera vez el 23 de octubre de 1952 en Culver City por el piloto Gale Moore. Sin embargo, este vuelo duró solo unos minutos debido a que tuvo que ser suspendido debido a que se debía ejercer mucha fuerza sobre el control direccional. En particular, la alta vibración sobre las palas del rotor principal fue difícil de corregir y el XH-17 permanecía mas en tierra que volando hasta que se le incorporaban alguna modificaciones. El final del programa de pruebas finalizó cuando las palas del rotor alcanzaron su vida límite en diciembre de 1955.

Teniendo en cuenta todo esto, el XH-17 había confirmado el concepto y su desarrollo fue satisfactorio. Se demostraba también que el concepto de una grúa volante era factible aunque el consumo de combustible fue muy grande y no se lograban los 160 Km. de alcance que solicitaba la especificación de 1946 aunque sí elevaba la carga de 4665 Kg. . Los pilotos alababan la alta respuesta del colectivo y el cíclico y la poca vibración que se percibía en la cabina, pero se quejaban sobre la lentitud de su control direccional.

 By letter dated 31 January, 1946, Materiel Command requested qualified manufacturers to submit proposals for the development of a large experimental helicopter. This rotary-wing craft was to be capable of carrying externally an 2.44m by 2.44m by 6.10m cargo package weighing up to 4536kg at a top speed of 105km/h, to hover at an altitude of 915m, and to have a tactical radius of 160km and an endurance of 30 minutes. It was intended for use in transfer of ordinance, equipment, supplies, and personnel, with loads to be lowered into or lifted out of areas in which even helicopter landings were impossible. Moreover, this sky crane helicopter had to be easily dismantled for transport on standard trailer trucks.

Advised by Friedrich von Dobhloff, the Austrian engineer who during the war had pioneered the application of jet principle to helicopters at the Wiener Neustadter Flugzeugwerke and who also worked with McDonnell on the XH-20 and other helicopters, the AAF Rotary Wing Branch recognized that achieving such capabilities would require major and risk-fraught technological advances and, accordingly, it encouraged bidders to consider using (1) a method of rotor construction other than conventional steel tube spar and wood; (2) gas-turbines and jet-driven rotors; and (3) servo controls, blade ailerons, or other devices to reduce the high control forces inherent in the use of a main rotor which was expected to have a diameter of 22.86 to 24.38m. Finally, to minimize costs, the Rotary Wing Branch recommended that the fuselage and other non-critical components of this flying test-rig be constructed in the simplest form possible.

As requested, the bidders submitted proposals for a two-phase programme with the first phase consisting of a design study and the second phase, which would be undertaken only after the AAF approved the design study, calling for the fabrication of a flying test-rig. After proposals evaluation, Kellett Aircraft Corporation of Upper Darby, Pennsylvania, was notified on 2 May, 1946, that it had won the design competition and soon after a contract for the first phase design study was negotiated.

The one-year design study phase revealed that technical problems would be even more difficult than anticipated, notably as the result of the need to use a rotor of much greater diameter, and hence more difficult to manufacture and control. Powerplant selection also proved arduous as, although concurring with AAF personnel at Wright Field that the use of gas-turbine and rotor-tip combustion was likely to provide the best solution, Kellett was eventually forced to recommend a twin-turbine installation instead of the single turbine favoured by the AAF. This and other technical recommendations were endorsed by the Air Materiel Command at the end of the design study phase and on 27 August, 1947, Kellett was awarded Contract AC15011 to build a ground test-rig for the XR-17 (which became the XH-17 in June 1948 when the newly created USAF adopted a revised designation system). Provision was incorporated in the contract for the modification of the ground rig into a flying test-stand at the Air Force option.

While the XH-17 ground test-rig was under construction, Kellett ran into financial difficulties and in 1948 sold the rights and partially completed rig to Hughes for $250,000. Pleased that work on the largest helicopter then under development would be continued, the USAF approved the transfer of the partially completed rig from Upper Darby to Culver City. At that time, key members of the Kellett design team were hired by Hughes and moved to California where work was resumed quickly and the test-rig completed in late 1949.

To save costs, as had been recommended in the request for proposals and provided in the Kellett design, Hughes incorporated components from other aircraft in the XH-17 test-rig. The two-seat cockpit came from a Waco CG-15, the single-wheel front undercarriage from a North American B-25, the twin-wheel main undercarriage from a Douglas C-54, and the 2407-litre fuel cell from a Boeing B-29 bomb bay tank. Power was provided by two General Electric 7E-TG-180-XR-17A gas generators (modified J35 turbojets) with bleed air from the intermediate compressor stage being fed into the rotor hub and then through ducts to four tip burners at each blade tip, where fuel was added and burned. This dual powerplant installation was predicted to provide the equivalent of 3480hp when the rotor turned at its normal rate of 88 rpm. The 39.62-m diameter two-blade rotor relied on tension-torsion straps for blade retention.

Ground running began on 22 December, 1949, and, as was expected from such a drastic advance in rotary wing size and complexity, soon brought to light a number of teething problems. Nevertheless, satisfactory progress was made until June 1950 when a cyclic gear failed, badly damaging the rig. Eight months earlier, however, Hughes and the Air Force had felt sufficiently confident in the project to proceed to the next phase and the manufacturer was funded under Contract AF8907 awarded in October 1949 to modify the ground rig into a flying test-stand. Hence, instead of merely repairing the rig after the June 1950 accident, Hughes also incorporated changes required to prepare the XH-17 for flight. Notably, these pre-flight modifications included the fitting of a dual hydraulic system, each featuring three variable displacement hydraulic pumps, and the installation of a tail rotor. Due to the absence of driving torque resulting from the use of a pressure-jet cycle system to drive the main rotor, the diameter of the tail rotor could be reduced in comparison with that which would have been required for a conventionally-powered helicopter with the overall size of the XH-17, and for yaw control Hughes was able to use the tail rotor of a Sikorsky H-19 mounted on a tubular fuselage extension.

Ground testing resumed after a two-year hiatus during which these modifications were incorporated at a leisurely pace while design of the servo control was being refined. Finally, having received the Air Force serial 50-1842, the XH-17 was first flown by Gale Moore at Culver City on 23 October, 1952. That flight, however, had to be cut short after the XH-17 had been airborne for barely a minute as directional control forces were excessive. While correction of this deficiency could be made quickly, difficulties uncovered later in the trials required more time. In particular, high vibratory stresses in the main rotor blades were difficult to correct and the XH-17 was repeatedly grounded while modifications were incorporated. The off and on test programme ended when the rotor blades reached their design life in December 1955.

All in all, the XH-17 had validated the design concept and its development into a satisfactory sky crane would have been feasible had it not been for the high fuel consumption rate which could not be reduced to any significant extent and which precluded ever achieving the 160-km tactical radius requirement set back in 1946. On the positive side, this large sky crane helicopter possessed several remarkable assets, including its demonstrated ability to airlift a standard Air Force remote communication trailer and loads of up to 4665kg. It was indeed ahead of its time in many respects apart from its obvious size. Pilots notably praised its exceptional responsiveness to collective and cyclic pitch control movements and the negligible level of vibration felt in the cockpit. Conversely, they complained about the sluggishness of its directional control.

Rene J. Francillon "McDonnell Douglas Aircraft since 1920: Volume II", 1997

Characteristics
Características

Information
Información

Characteristics
Características

Information
Información

1952

2 G.E
7E-TG-180

2 / ?

3480 HP

28563 Lb

Ft

43500 Lb

Ft

70 Kts

13100 Ft

80 Kts

40 NM