The Operational Potential of Manned, Ducted-Fan VTOL Vehicles 2005-01-3184

Compared to helicopters, the safety and operational advantages of ducted fan VTOL vehicles that do not rely on large, exposed rotors overhead seem promising. The alternative to the overhead rotor that naturally comes to mind is a smaller, enclosed rotor or multitude of rotors, sometimes referred to as ‘fans’, or, in combination with their enclosure, ‘ducted fans’. During the 50's and 60's, numerous experimental VTOL vehicles were developed, that relied on ducted fans as their main source of lifting power. The experience with the early vehicles was marginal, both in terms of the propulsive efficiencies attained, as well as stability and control problems caused among others by the ducted fan's inherent sensitivity to wind gusts. Today, with the advent of higher efficiency engines and fly-by-wire flight control systems, ducted fan vehicles are again being considered as a potential method for providing a safer, more compact alternative to the traditional helicopter. However, in the current competitive environment, any proposed design, also needs to posses (at least in the civil market segment) some economic advantage compared to helicopters in use today. With the relatively high inherent fuel burn rate of any ducted fan vehicle compared to a helicopter designed to carry an equivalent load, the task of finding an economic justification for such ducted fan vehicles becomes very challenging. The present paper discusses these issues, as well as design parameters that offer potential for reduced operational cost and increased profitability of future commercially operated ducted fan VTOL vehicles. The paper also describes briefly one candidate vehicle that has the potential to meet these requirements--Urban Aeronautics' X-Hawk--a tandem lifting fan, twin turbine powered, VTOL utility vehicle, featuring among other things a fly-by-wire, closed-loop vane control system (VCS), enabling it to operate in rough weather conditions and obstacle rich environments.