Performance Advantages and Resonance Analysis of a Variable Speed Rotor Using Geometrically Exact Beam Formulations

The efficiency and operating envelope of rotorcraft are constrained by the speed of the rotor. Most helicopters operate with a constant rotor speed. Varying the speed of the rotor based on the operating condition could significantly improve the rotor's performance. In this study, a hingeless rotor model with elastic blades is built-in DYMORE to study various aspects of variable speed rotor technology. The rotor blades are modeled as one-dimensional beams using state-of-the-art beam theory known as the geometrically exact beam theory. An unsteady aerodynamics model with dynamic stall and finite-state dynamic inflow is used to obtain the aerodynamic loads acting on the rotor. The power savings that can be achieved at various advance ratios by varying the speed of the rotor is evaluated. Maximum power savings of 41% was achieved at a nominal advance ratio of 0.2. However, changing the rotor speed leads to vibration issues when a rotor blade passes through a resonance point. A methodology to identify the important resonance points for a given flight condition and rotor speed transition is also provided. The forces acting on the rotor blade during resonance crossings at different advance ratios are evaluated. It is found that the amplitude increase during resonance crossing is strongly dependent on the amplitude of the cyclic pitch angles during resonance.