Design philosophy and operational requirements of subsonic vtol aircraft

Abstract

The growth-factor concept is used to investigate the influence of operational requirements (hovering altitude, temperature, and time) on gross and empty weight of three VTOL configurations (mechanically driven compound helicopter, tilt wing, and jet lift). Conventional STOL is also considered. With additional criteria of the relative productivity and fuel index, all these concepts are evaluated vs a combat mission (payload 1500 Ib, 3 hr on station at 225 knots, 6 min of hovering at 6000 ft, 95°F; ground run for STOL is 300 ft). For this mission, tilt wing appears to be the best over-all configuration. Relaxation of the on-station speed requirements (150 knots) and more emphasis on hovering and low-speed operation greatly increases attractiveness of the compound helicopter. By contrast, shifting of speed requirements to higher values makes the jet lift quite competitive. To make the STOL competitive with the VTOL's, 300-ft ground-run requirements would need relaxing. A similar analysis of four configurations for the Intra-Theater Transport mission (10-ton payload, over 500-naut mile radius) shows that for the defined mission requirements, the shaft-driven propeller or rotor systems have the necessary capability, high productivity, and excellent fuel indexes required to provide a low-cost intra-theater transportation service.