The main rotor, the fuselage and the horizontal stabilizer are assumed to be the only contributors to the longitudinal stability of the helicopter. Before going further it is worthwhile revising two important stability definitions:

(1) Trim. An aircraft is in trim when all the forces and moments acting on it are in balance. The aircraft is in a state of equilibrium and would continue in that condition unless acted upon by a gust, or affected by pilot action.

(2) Static stability. A body is statically stable if there is an initial tendency for it to return to its trim condition after an angular displacement or after a change in the transitional velocity. In helicopter parlance static stability, in forward flight, describes the response of the helicopter following a change in translation velocity (speed stability). Manoeuvre stability is the response to angular changes (AOA stability).

4.7.1 Hovering flight

Evidently a helicopter possesses neutral static stability with respect to angular displace­ments when the definition of static stability, given above, and the rotor response, detailed earlier, are considered. If a helicopter suffers an angular disturbance while hovering no direct aerodynamic moment arises which will restore it to its original attitude. The resultant rotor thrust always passes through the centre of gravity irrespective of the angular position of the helicopter (assuming no download on the horizontal stabilizer and fuselage). The response is equivalent to the neutral stability in roll displayed by a conventional fixed wing aircraft. In both cases it is the subsequent generation of a translational velocity that may give rise to a stabilizing response. In the fixed wing case the roll disturbance causes a lateral velocity to develop and the dihedral of the wings combined with this velocity produces a moment which will tend to return the aircraft to the trim condition. Similarly for a helicopter the angular displacement will result in a translational velocity due to the unbalanced horizontal component of the thrust vector. The positive speed stability of the rotor will then lead to the development of a moment tending to return the helicopter to the trim position. If a hovering helicopter is subjected to a disturbance in translational velocity then it is only the flap-back effect from the rotor that will tend to return the helicopter to its original attitude. At very low speeds contributions from the horizontal stabilizer and fuselage may be ignored.

4.7.2 Cross-coupling : collective to pitch attitude

Before describing the static and manoeuvre stability in forward flight it is important to understand the effect of changes in collective pitch, at constant airspeed and load factor, on the pitch attitude of the helicopter. The initial response of the helicopter will be dictated by the change in the magnitude, and direction, of the thrust vector and hub moment from the main rotor. As the pilot raises the collective lever the lift produced by all blades is increased thereby increasing the thrust and the coning angle. In forward flight the increased collective pitch will have a greater effect on the advancing side and consequently the amount of nose-down longitudinal flapping will be reduced. The net result of these changes is to reduce the size of the nose-down pitching moment generated by the main rotor. The helicopter will, therefore, pitch nose up as the collective lever is raised. Subsequent control activity and cyclic stick displacement from the level flight position will result from the effect of the relative airflow on the horizontal stabilizer and fuselage.

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