JET for ROTARY WINGS Power Plant Laboratory Wright Field. Ohio The Army Air Forces has always encouraged the development of rotary wing aircraft by direct investigation and by development contracts. This interest is based upon the wide range of miscellaneous applications for this type of aircraft in military operations. So far this type of airplane has not yet proved its suitability for the prime functions of bombing and combat operations, and for this reason the initial emphasis of jet engine applications have been with the conventional aircraft. It is well to note, however, some of the important characteristics of the jet engine that make it peculiarly suited for rotary wing aircraft. Jet engines, in the broad sense, include all four types: 1. Turbo-jets, with a mechanical air compressor, combustion chamber and turbine to drive the compressor. 2. Pulse-jets, the German buzz bomb engine, having intermittent burning with compression from ram and partial confinement. 3. Ram-jets, with continuous burning and with compression only from the ram pressure of forward velocity. 4. Rockets, that carry all the chemicals for combustion in a tank and have, in a sense, precompression by chemical compounding of oxygen in the fuel. Each of these jet engines have different characteristics and different suitabilities for various applications, but when compared with a conventional or reciprocating engine, they all have a very definite group of distinguishing characteristics. First, the jet engine itself, in addition to being more simple, has a materially lower weight in lb. per hp; second, the engine power output per unit area at existing high speeds is materially higher; third, if the jet discharge is located at the rotor tip, the problem of power transmission through heavy shafts and gearing is non-existent, as is likewise the problem of torque reaction from the mechanical drive to the rotor that now requires a power drive to a propeller on the tail. Conversely, it should be realized that at the rotor tip speeds now used, these engines in terms of pounds of fuel per horsepower hour will have a higher fuel consumption that may limit their application to short range operations. This would mean a large pay load for short range and a smaller pay load for longer ranges. Centrifugal stresses of hot structure is also a very serious problem. It is possible that the advantages listed above may be exploited to offset the higher fuel consumption by providing a more efficient aircraft. The cleanness of the P-80 airplane, and its transcontinental performance, is a tribute to this type of design and the cumulative benefits that may be derived from simplification, and from the elimination of such parts as radiators, oil coolers, intercoolers, superchargers, cowl flaps, etc. The mutually beneficial combination provides not only advantages of the jet engines for rotary wing aircraft, but also inherent advantages of the rotary wing aircraft for jet engines. Jet engines are essentially high speed propulsion devices, and they are neither efficient nor relatively effective at low speeds. The rotor tip presents the opportunity to provide the high speed needed for the jet engine during take-off, landing, and other low speed operations. It is, for example, the only type of aircraft that could use exclusively as its main power plant the ram-jet, because this engine produces no material thrust at low speeds. It is the only type of aircraft where rockets could be used to materially increase the load carrying capacity (beyond the rocket thrust) at given take-off or landing speeds. Variations and combinations of these different jet engines with many ingenious locations of compressors, air ducts, pre-burning and after burning may be expected to preclude the formation of any fixed idea of what a rotary wing aircraft and its power plant should-look like. It is subject to change without notice. One possibly favorable combination of the jet engine with the rotary wing would be based upon a very substantial modification of the existing concept of a helicopter. In the present configuration with the axis of the rotor perpendicular to the flight path, a jet in the rotor tip (unless operated intermittently with synchronization) would be pushing the airplane backward half the time and forward half the time. The only usable thrust would therefore reside in the rotation imparted to the rotor. The extreme change in air velocity during the forward and rearward swing of the rotor, in addition to its aerodynamic limitation, might present a serious problem in jet engine operation with some rypes of installation. If the axis of rotation were shifted to a position parallel to flight, similar to that of a propeller, but not necessarily located at the front of the aircraft, the engine efficiency would be materially increased during forward flight because the discharge jet would provide a continuous forward thrust to the aircraft in addition to its rotational problem for aerodynamists. Consideration of its operation suggests the contradictory term "a rotary ornithopter." The arrangement does, however, eliminate the extreme variations in Mach number during each rotation and it was under active consideration by the Germans.