The Scissors Rotor

A four-bladed scissors rotor was evaluated in model and full-scale tests to investigate the feasibility of such a concept. Flight speeds over 150 knots were reached and high-speed maneuvers over two g’s. Hub mobility was instrumental in greatly reducing oscillatory blade loads. With respect to these loads, as well as with respect to acoustics and vibrations, the scissors rotor resembles more a two-bladed rotor than a four-blader. The single seesaw hinge combined with a hub spring gave essentially a constant hub moment with flapping, which contributed noticeably to stability and to control power. Some degradation of hovering power was noted in comparison with the performance of a symmetric four-bladed spacing, and at high speed, a slightly higher power required was measured due to increased hub drag. In high-gross-weight hover a blade motion instability was encountered in the reactionless mode. This instability could be explained and eliminated. The scissors rotor offers considerable weight savings over symmetrical inplane stiff multibladers, but will be heavier than soft-in-plane and two-bladed rotors. While the mainstay of Bell’s rotors has been the simple two-bladed rotor, the company has made significant efforts to explore other concepts. Bell flew its first rigid rotor (designed by W. Cresap) in 1958,1 and went on to test versions with three and four blades and with hubs incorporating various flapwise stiffnesses. The company also tested gimbaled multibladed rotors with and without hub springs. 2,3 One result of these efforts is the rotor for the NASA-Army-Bell XV-15 tilt rotor aircraft now under construction --a three-bladed gimbaled rotor with elastomeric hub springs.4 All Bell production and test rotors flown thus far (except the articulated rotors of the initial version of the XV-3 tilt rotor aircraft 5) have a common characteristic: the rotating natural frequency of the blades in the plane of rotation is above the rotational speed of the rotor. As Feingold and Coleman have shown6,7 this essentially precludes mechanical instabilities (ground and air resonance) regardless of the type of pylon isolation, the landing gear configuration, or the surface condition of the takeoff and landing site. Although this positive elimination of resonance is important, there are more factors to consider. The development program for multibladed rotors at Bell therefore includes also soft-in-plane and articulated concepts. A logical step in this multibladed rotor program is the scissors rotor. It attempts to combine attractive features of two-bladed and four-bladed stiff-in-plane rotors while mitigating their short-comings. This paper discusses results of experiments with a scissors rotor, after first treating the philosophy behind the concept.