Dynamic Longitudinal Stability Measurements on a Single-rotor Helicopter (Hoverfly Mk. I)

Abstract

Flight measurements have been made of the phugoid motion of the Hoverfly Mk. I helicopter, following an arbitrary longitudinal displacement of the control, the latter being returned to its initial position and held fixed. The tests were done throughout the speed range for power-on conditions and in autorotation for various centre of gravity positions and for forward and backward initial displacement of the stick. In power-on flight there is a large variation in the dynamic stick-fixed stability with speed. From zero airspeed up to 35 m.p.h. and at airspeeds above 50 m.p.h, the phugoid motion is divergent, but for the speed range 35 to 50 m.p.h. the helicopter is stable. In autorotation, there is little change in the dynamic stability with speed. Below about 30 m.p.h. the phugoid amplitude tends to increase slowly, and above this speed the amplitude tends to decrease slowly. There is no variation in the character of the longitudinal phugoid motion with change in centre of gravity position. Neither was any difference detected in the character of the oscillations produced by initial backward movement of the stick, compared with those produced from initial forward displacement. The theoretical estimation of the power-on stability agrees with the flight tests at low airspeeds, but it shows little variation in stability with speed. In autorotation, the theoretical work agrees very well with the flight tests throughout the speed range. The discrepancy in the power-on tests is felt to be due to a large variation of the fuselage pitching moment with speed, particularly due to the effects of the induced flow from the rotor. (Scan courtesy of Juergen Humt.)