Cycloidal Rotor Systems

  • Robert Worthing

Student thesis: Doctoral ThesisPhD


Historically the development of the cycloidal rotor into a viable propulsion concept has been hampered compared to more conventional approaches due to limitations in computational modeling capability and availability of low mass high stiffness materials. However, advancements in these areas have improved the viability of the cycloidal rotor over recent years, which has renewed interest in the cycloidal rotor in the quest to develop more efficient propulsion technologies. The cycloidal rotor is a novel thrust generation technology suitable for crewed and uncrewed flight. Thrust generation is created via blades rotating parallel to the rotor’s global rotation axis, pitching cyclically. Precise thrust vectoring is achieved by controlling the blade cyclic pitch phase angle, enabling the cycloidal rotor to operate with a wide operational envelope in pure thrusting and forward flight applications.

To date, all cycloidal rotor research has concentrated on mean rotor performance, which is essential in the preliminary design stage, but its applicability is limited during detailed design. Standard rotorcraft are often subject to high levels of vibration emanating from a number of sources, with the rotor being a significant contributor. Rotorcraft rotor blades operate in a highly unsteady aerodynamic environment, subject to fluctuating aerodynamic and inertial loading, generating large vibratory blade and hub loads, leading to problems such as component fatigue and passenger discomfort. In comparison to standard rotorcraft, the vibratory response of the cycloidal rotor is little researched and understood.

The current thesis presents the development of reduced-order computational aerodynamic models and computational fluid dynamic models (CFD) to characterize the vibratory response of a cycloidal rotor in hover with increasing blade cyclic pitch amplitude and varying rotor speed. Experiments with a new four-blade cycloidal rotor test rig were undertaken to investigate the efficacy of the reduced-order computational and CFD models in predicting the mean and vibratory hub loads.

A method of rotor force sensor dynamic calibration was developed to take account of the test rig dynamic response and characterize the cycloidal rotor vibratory response fully for the first time. The reduced-order computational model and 2D CFD analysis showed good agreement with experimental data in calculating mean rotor performance. It was found that the rotor vibratory loads were dominated by the rotor 4/Rev vibratory response, which saw increased modulation with increasing blade cyclic pitch amplitude. The correlation of computational model vibratory loads with experimental data improved with model fidelity, with both models showing broad agreement with the experimental data. The computational models provided insight into the physical mechanisms behind rotor vibration, with blade wake interactions identified as having a strong influence on the overall rotor vibratory response.

With high levels of cycloidal rotor vibration identified from initial tests, rotor vibration reduction was identified as a critical factor in cycloidal rotor optimization and further development in the future. One such vibratory response reduction methodology is higher harmonic control (HHC). The thesis describes the first known investigation into the use of HHC for mitigating cycloidal rotor vibration, providing a robust demonstration of the efficacy of HHC both computationally and experimentally, identifying that the inclusion of HHC successfully reduced the cycloidal rotor vibratory response in all cases. Furthermore, it is shown that faithfully modeling the blade vortex shedding and stall behavior is key to accurate simulation of the rotor and rotor vibration control.
Date of Award28 Jun 2023
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAnna Young (Supervisor) & Patrick Keogh (Supervisor)

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