NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS LANGLEY FIELD VA LANGLEY AERONAUTICAL LABORATORY
A preliminary analysis has been made of the conditions of stability of free oscillations of a hinged rotor in hovering flight. The case analyzed in a rotor with hinges allowing freedom in flapping and lagging and having a completely reversible cyclic pitch-control system, so that a twisting moment on a blade moves the control stick without hindrance from spring or friction constraint. The principal results of this study are presented in the form of a stability chart in which the quantities specifying average coning angle, pitch setting, moments of inertia, chordwise and spanwise mass distribution are combined into generalized parameters in such a way that the stability condition can be plotted on a single chart. The results show that the stability is extremely sensitive to chordwise center of mass, and that forward movement of the center of mass increases the stability. It is also shown that the stability depends upon coning angle and pitch setting. For all examples in which the coning angle is determined by a balance between lift forces and inertia forces, the condition for neutral stability corresponds to the same point of the chart. Several lines of attack for further theoretical work are suggested, which would extend the generality and strengthen the validity of the analysis.