Inertial Dynamics of a Propeller Driven Rotor: Vacuum Chamber Experiment and Validations with Predictions

A propeller driven rotor uses small electric motors and propellers attached to the rotor blade to spin the main rotor. Recent propeller driven rotor hover test campaigns suffered propeller failures at relatively low main rotor rotational speeds. The dynamics of spinning a fast propeller at the end of a spinning main rotor blade were the suspected cause of the propeller blade failure. An experiment using the 10 ft diameter vacuum chamber was designed to isolate and measure the propeller flapping motion of an articulated propeller blade from inertial loads. A Coriolis coupling exists between the propeller and the main rotor, resulting in large 20° sinusoidal propeller flapping motions. The vacuum chamber experiment also demonstrated that for the propeller/rotor speed ranges tested, increasing the propeller or the main rotor speed resulted in larger propeller flapping motion. An analytical model was developed to study the coupled propeller flapping motion due to the main rotor rotation. The predicted solutions for the analytical model capture the vacuum chamber experimental results well. Key insights were obtained by analytically solving the propeller flapping equation of motion assuming periodic coefficients (β0, β1c, β1s). This analytical solution method showed that the β1s term was associated with the Coriolis coupling between the main rotor and the propeller, whereas the β0, β1c terms result from the location of the propeller forward of the rotor 1/4 chord location.