How do the Zipline Silent Propellers Work?

In a recent video, YouTuber Mark Rober showcased some innovative Zipline drones that are designed to deliver medical supplies in Rwanda. While the drones themselves are impressive, there is one aspect of their design that has caught the attention of many: the Zipline Silent Propellers. Specifically, the propellers are shaped in a way that reduces noise, but how exactly does this work?

Zipline’s Silent Propellers Explained

To understand the noise reduction properties of these propellers, we must first delve into the source of sound in a spinning propeller. As the blades pass a stationary point, they create a pressure disturbance that moves away from the blade, generating a sound called the blade passage frequency. This frequency is determined by the rotational speed and the number of blades on the propeller.

Traditional propeller designs space the blades evenly, resulting in a loud frequency at the blade passage frequency and its harmonics. However, by spacing the blades unevenly (e.g., 30 degrees apart), the sound is spread out over a wider range of frequencies, including lower ones. This means that the same acoustic energy is distributed over a broader spectrum, with the most crucial frequency being half the blade passage frequency.

The human ear is less sensitive to low-frequency sounds, perceiving them as quieter than they actually are. Uneven blade spacing takes advantage of this fact, creating a lower frequency that we perceive as less noisy. This principle has been applied to cooling fans and helicopter rotors for some time.

The zipline drone propellers go a step further by placing the blades on different levels. This eliminates the overlap between the blades, which would otherwise cause broadband noise that counteracts the noise reduction benefits of uneven blade spacing. By raising one blade slightly higher than the other, the broadband noise is minimized.

A practical way to achieve this design is by stacking two double-blade propellers on top of each other. The result is a propeller with noticeably lower tones, perceived as quieter and less annoying than a traditional design. Zipline has adapted this concept by using only two blades with a counterbalance on the opposite side, further reducing noise.

An alternative propeller design developed by MIT, the toroidal propeller, also effectively reduces noise. By reshaping the propeller to minimize vortex formation, overall sound levels are lowered. This design, however, sacrifices some efficiency, requiring a faster rotation to achieve the same wind speed as evenly spaced blades.

Interestingly, the noise reduction achieved with uneven blade spacing depends on the listener’s position. It is more effective when in front of or in the same plane as the blades, making it particularly useful for reducing flyover noise in planes. However, it might not be as effective for drones, where the listener is typically below the blades. It remains to be seen how zipline drones would perform in such scenarios.

The innovative propeller designs employed by zipline drones are just one example of how creative engineering can help address real-world challenges. By reducing noise pollution, these drones can contribute to improving the quality of life in communities where they operate. As we continue to explore new technologies, it is essential to remain open to unconventional solutions that may lead to more sustainable and efficient designs.

Link to paper mentioned in the video: Reduction of Tonal Propeller Noise by Means of Uneven Blade Spacing