This paper deals with the parameters that must be considered in the selection and design of propulsion systems for military VTOL aircraft. Some of these parameters, for instance lightweight, are applicable to engines for all types of aircraft. For the VTOL aircraft, special emphasis must be placed on many of these parameters since aircraft takeoff gross weight determines engine size. Other significant considerations in the selection of the propulsion system include: (a) the ratio of subsonic cruise thrust to maximum thrust; and, (b) exhaust downwash characteristics. Consideration (a) is important because, in the case where no auxiliary lift engines or devices are used, subsonic cruise thrust is about 25 to 30 percent maximum, and at this low power setting, specific fuel consumption is increasing rapidly. Exhaust downwash characteristics are significant because of the variety of landing and takeoff sites likely to be encountered (i.e., shipboard or unprepared fields).

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