The Effect of Cyclic Feathering Motions on Dynamic Rotor Loads

The dynamic loads of a helicopter rotor in forward flight are influenced significantly by the geometric pitch angles between the structural axes of the hub and blade sections and the plane of rotation. The analytical study presented includes elastic coupling between in-plane and out-of-plane deflections as a function of geometric pitch between the plane of rotation and the principal axes of inertia of each blade. In addition to a mean collective-pitch angle, the pitch of each blade is increased and decreased at a one-per-rev frequency to evaluate the dynamic coupling effects of cyclic feathering motions. The difference in pitch between opposed blades gives periodical coupling terms that vary at frequencies of one- and two-per-rev. Thus, an external aerodynamic force at n-per-rev gives forced responses at n, n+1, and n plus-minus 2 per rev. The numerical evaluation is based on a transient analysis using lumped masses and elastic substructure techniques. A comparison of cases with and without cyclic feathering motion shows the effect on computed dynamic rotor loads. The magnitude of the effect depends on the radial location of the pitch change bearings.