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Cyclo-Rotor Acoustics
Cyclo-Rotor Acoustics
The cyclorotor is a type of propeller capable of thrust vectoring. Unlike conventional propellers, where the blades are attached radially to a central shaft, the cyclorotor consists of a series of parallel blades fixed at one or both ends. These blades undergo cyclic pitching as they rotate, producing a net thrust in a specified direction. Despite its unique design, the acoustics of cyclorotors have not been extensively studied from a theoretical standpoint.
The acoustic pressure is generated by forces acting on the normal components of the blade surfaces. Consequently, the radiation pattern of a cyclorotor exhibits significantly different directivity compared to that of a conventional propeller.
In the reference frame of the blade, the direction of the lift force reverses with every half propeller rotation. In contrast, the drag component remains in a consistent direction, although its magnitude fluctuates due to changes in the induced drag component resulting from variations in the blade's angle of attack.
After clearly defining the time-dependent properties of the blade force, the radiated sound pressure produced by the system can be calculated.
Our findings indicate that, from an acoustics perspective, conventional propellers offer a significant advantage over cyclorotors, as the force acting on propeller blades remains constant throughout the rotation.
The acoustic properties of cyclorotors are more comparable to those of installed propellers. At low RPMs, cyclorotors tend to produce higher noise levels than installed propellers. However, their noise scales less rapidly with rotational frequency, rendering them quieter at higher RPMs.
The isolated propeller, on the other hand, is significantly quieter across all RPMs, leading us to recommend its use over a cyclorotor in most scenarios where acoustics is a critical consideration. In order to optimise the noise characteristics of a Cyclotor, one must make sure that the rate of pitch change of the rotor blade is as small as possible.
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