The longitudinal stability and control of the tandem-rotor helicopter. Parts 1 & 2.

Abstract

Part 1. A simple method of calculating downwash interference is presented and comparison of theoretical and flight test trim curves indicates that the method is reasonably accurate. Since the stability of the tandem-rotor helicopter depends largely on small differences between the thrusts of the front and rear rotors it is necessary to calculate the rotor thrust derivatives far more accurately than for the single-rotor helicopter. More accurate expressions than those given in Ref. 8 have therefore been calculated. The downwash interference causes a reversal of stick position with speed for part of the speed range with an associated divergence in the dynamic stability. This may be eliminated by choosing a suitable value of swash-plate dihedral angle. If, in addition, a suitable differential delta-three hinge angle is applied the tandem rotor helicopter appears to be stable over the whole speed range except at hovering and very low speeds. If the swash-plate dihedral is too small the normal acceleration curve, following a step input of control, flattens out and then increases again. Thus the tandem-rotor helicopter may satisfy the N.A.C.A. manoeuvrability criterion yet possess unsatisfactory response characteristics. It is suggested that the N.A.C.A. criterion is Unnecessary if stability of the short and long period modes is ensured. Again, a proper choice of swash-plate dihedral and differential delta-three hinge enables satisfactory control response characteristics to be obtained. Part 2. The lateral stability and control of the tandem-rotor helicopter with a basic configuration similar to that of the Bristol 173 has been investigated. A method of calculating the derivatives is given. Values calculated for the HUP-1 helicopter show fairly good agreement with those obtained from flight measurements. The stability investigation shows that a tall fin may provide an effective dihedral several times larger than that of the rotors and lead to an unstable Dutch-roll oscillation which becomes progressively worse with increase of speed. The corresponding spiral mode is stable. If the rotors provide the only contribution, the Dutch-roll oscillation is stable but the spiral mode may become unstable. Simple approximations are given for the estimation of the damping and period of the stability modes and show quite close approximation to the more exact calculations.