No abstract

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.

The efficiency of the rotorcraft could be improved significantly by varying the speed of the rotor based on the flight condition. Due to this reason, variable-speed rotor (VSR) technology is becoming popular, especially among urban air mobility vehicles. However, one of the biggest challenges of a VSR is the vibration of the rotor blade when it passes through a resonance point. A detailed study on the characterization of these resonances was performed by the authors (“Performance Advantages and Resonance Analysis of a Variable Speed Rotor Using Geometrically Exact Beam Formulations,” Journal of American Helicopter Society, Vol. 67, May 2022, Paper 042006). In this study, methods of reducing loads during resonance crossings are explored. The rotor model is built in Dymore, which is a multibody dynamics simulation tool. The geometrically exact beam theory is used to model the rotor blades as one-dimensional beams. Peters et al.’s (“Finite State Induced Flow Models Part I: Two-Dimensional Thin Airfoil,” Journal of Aircraft, Vol. 32, No. 2, March–April 1995, pp. 313–322) unsteady aerodynamics model with finite-state inflow and dynamic stall is used to obtain the aerodynamic loads acting on the rotor. Load reduction studies were carried out by varying the transition time, structural damping, and lag stiffness of the blade. The longer the rotor took to traverse a resonance region, the greater were the resonance loads. However, there were torque limitations on how quickly a rotor could pass through resonance. Increasing the structural damping was a very effective way of reducing the resonance loads. Increasing the lag stiffness of the rotor blade deteriorated the torque response due to 4/rev crossings. Combination studies were performed by combining the ideal transition times with 7% lag damping.

The efficiency and operating envelope of a rotorcraft is 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 (VSR) technology. The rotor blades are modeled as one-dimensional beams using state of the art beam theory known as the geometrically exact beam theory (GEBT). 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 41:47% was achieved at mN = 0:2. However, changing the rotor speed leads to vibration issues when a rotor 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 is evaluated.