Rotor‐Fuselage Dynamics of Helicopter Air and Ground Resonance

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Abstract
An analytical investigation of the air‐ and ground‐resonance characteristics of simplified hingeless‐rotor helicopter configurations was undertaken. The objectives were to identify and interpret the unique characteristics of coupled rotor‐body modes that determine air‐resonance stability characteristics, as contrasted to ground‐resonance phenomena. The study examined the influence an air‐resonance stability of unusual rotor‐body modal characteristics such as nonoscillatory body modes. The results indicate that air‐resonance instability does not require distinct coalescences of lead‐lag and rotor‐body modal frequencies, and unusual rotor‐body frequency characteristics do not significantly alter the occurrence of air‐resonance instability or the effectiveness of aeroelastic couplings. The character of air‐resonance and flap‐lag instabilities was found to change significantly at high rotor speeds for high‐Lock number and low‐inertia configurations. It was found that the rotor‐body modes that participate in both air and ground resonance can be related to one another.

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