Experiments on a model helicopter rotor operating in the vortex ringstate.

Unsteady aerodynamic characteristics of a model helicopter rotor operating in the vortex ring state have been experimentally investigated. Time histories of thrust, torque, and rpm of the rotor were measured by use of sensing devices that were installed on the shaft of the rotor. The results obtained show that in the vortex ring state, the thrust fluctuates violently, whereas the torque fluctuates very little; in fact, it is hardly observable. The induced power obtained from thrust and torque data has been compared with that obtained from the momentum theory. Existence of periodicity in the thrust fluctuation also has been observed in some regions of the vortex ring state.