Helicopters and Propeller-Type VTOL Aircraft in the Light of Technologies 650193
Pure and compound helicopters (including those with stoppable or stowable rotors) as well as propeller-type VTOLs are examined regarding their possibilities and limitations resulting from aerodynamics and structures. Trends in effective lift-to-drag ratio with flying speed and agility are discussed, along with other performance items. Aeroelastic, dynamic and structural aspects of rotary wing aircraft are considered and specific limitations due to those phenomena indicated. An interplay between design concepts and gross and empty weights is illustrated by the example of operations reflecting a combat type mission. The totality of these considerations indicates that a 200-knot pure helicopter is feasible with the possibility of not exceeding cyclic stress and vibration levels of contemporary 150-knot helicopters. However, the effective lift-to-drag ratio at speeds of about 200 knots would be quite low. Compounding improves lift-to-effective drag ratio at such speeds slightly and moves the various aeroelastic barriers, up to speeds of 250 knots. However, this leads to higher gross and empty weights. Simple stopping of rotors brings little improvement in the speed and range capabilities at the expense of still higher weights. Folding is more promising as far as the performance is concerned, and gross and empty weight penalties are no higher than in stopping. Retraction of the rotor improves aerodynamic cleanness to the levels of propeller type VTOL (tilt wing) aircraft, but weight penalties are the highest. Because of its superior performance, high level of agility and absence of fundamental aeroelastic limitations, the tilt wing type is seen to be outstanding for the combat type missions considered.