The Boeing GoFly Prize sets a minimum standard for noise. The Zero features low noise, multi-bladed propellers. We started with custom 5-blade propellers manufactured for ZEVA. We have designed and fabricated thrust-pod test stands for both the scaled model as well as the full-size motors and propellers. The driving forces causing us to develop our own propellers are the low noise and small diameter requirements. Rigid, multi-bladed propellers will produce less noise by minimizing flexure and defusing the noise output across a wider spectrum. Careful attention will be paid to the design of the blade tips as they are the predominant contributors to overall propeller noise. Propellers on ZERO will also have odd numbers of blades which helps lower peak harmonics.
The eight motors of the ZEVA ZERO will contribute a portion of the overall noise. The noise is primarily due to the cogging of the motor and its frequency spectrum is determined by the numbers of slots and poles of the motor. This noise can be reduced by increasing the number of poles/slots and skewing the slots so that they transition across them is not as abrupt. We will also look into the design of the motor controller to provide a sinusoidal drive current in order to reduce noise.
The interaction between the wash of the propellers and the elements of the body also contribute to the noise of the craft. This noise can be harmonically rich and in order to minimize this noise, we will provide suitable spacing between the propellers and struts holding the motor nacelles and also the main body of the craft. The motor struts are also curved to limit abrupt edges.
Noise is an area which will require some experimentation to achieve the targets for the craft. The above noise sources are considered to the leading ones that will require investigation and will drive design choices.