TOROIDAL PROPELLERS

Abstract

This paper discusses the need for less noise in UAVs, how these toroidals can benefit the drone industry, how these propellers came to be and how exactly they reduce the noise. Also, in this paper an experimental validation study for the above-mentioned propellers is performed using a 3D printed toroidal Propeller and a market-available standard propeller of the same size. The noise and thrust are compared for the propellers at varying throttle ranges.

Introduction

The Drone industry is on the rise. Researchers and Engineers have been working passionately to make these technologies more efficient and sophisticated so that they can become part of our daily lives like any other vehicles but there are lots of roadblocks to be cleared before we reach that point. One of the major problems that has come to light is noise produced by the drones which at times can become annoying for people[1]. A survey by mckinsey.com showed us that noise is one of the major concerns for people in various countries[2].

Recently, MIT University cambridge came out with an interesting research in which they tested a type of propeller called the toroidal propeller. It was claimed that these particular propellers produced less higher pitch noise than conventional propellers without losing efficiency[3][4]. This research can prove to be a game changer for the drone industry.

The concept of toroidal propellers has been around for several decades, and it has been studied and developed by various researchers and companies over the years.

The initial Idea started with just rotating the Airfoil in Toroid-like shape called the Ring wings. It was found that they reduced noise and also increased efficiency. Later, building on this concept many researchers and marine companies developed various propellers that are still being used today. Some of those companies being Voith, Wartsila, SCHOTTLE and many more. In 2017, Sharrow Marine LLC patented a Propeller design for Marine applications[5], which they claim increased efficiency and reduced noise significantly[6]. Ducted or Ring wing designs for propellers in the aerospace industry have been used for a long time but this particular design of propellers was first patented in the 1980s but they were not significantly discussed. Now that MIT has come up with solid research backed by real experiments, it makes things very interesting and opens up various prospects for more research and utilization of this propeller design concept.

Noise in propellers is created due to vibrations at the Tip of the blade due to formation of vortices.How do toroidal propellers reduce noise? In toroidal propellers the reduction in noise is by reducing the tip vortices that are generated by the propeller blades. Tip vortices are swirling masses of air that form at the tips of the propeller blades as they rotate, and can contribute to noise and vibration. The toroidal shape of the propeller can help to minimize the formation of tip vortices by confining the airflow and preventing it from spilling over the edges. Noise and vibrations also depend on factors like pitch and Number of blades. Defining pitch in toroidal propellers is quite complex due to the curved shape of the blade and non-linear relation between blade angle and performance.

The toroidal propellers reduce the noise in the high frequency spectrum without actually losing the efficiency.

Methodology

A 3 Blade propeller of 10 inch size is designed in the CAD software and 3D printed using PTEG Material. A standard 10 inch propeller nylon is used as a comparison for the experiment. Both the propellers are mounted on the thrust stand separately. Each propeller is tested for thrust and noise levels over a series of tests. The data is compiled and processed. The voice recordings are analyzed using a Spectrography tool and sound pressure vs frequency graph is plotted in matlab.

Results

Both the propellers are tested up to 60% thrust and the noise levels at that particular thrust are analyzed and plotted over the graph as shown below. It can be seen that both the propellers have comparable sound pressures and frequency ranges. This can be due to the quality and weight of the propellers. The Standard propeller only weighs 9 grams whereas the 3d Printed Propeller weighs 25 grams. Also, the 3D printed propellers are not as smooth at the edges as the normal one which can lead to vortices and efficiency loss as well as increase in noise vibrations.

However, the interesting thing is that both the propellers produce almost comparable thrust and noise values input values.

Conclusion

This Experiment was performed to understand and analyze the use of toroidal propellers in future UAVs. Although the toroidal propeller was made using 3D printed material with much higher weight and bad surface quality, there was not much difference between a normal and toroidal propeller when compared for noise and frequency levels, also, it was interesting to know that both propellers produced comparable thrust results at similar inputs.

It was concluded that if the toroidal propeller is manufactured using more sophisticated methods where we can have comparable weight and much more refined surface, it can outperform the standard propeller easily in efficiency and noise performance Although, more tests and research is needed to confirm this claim.

The toroidal propellers show us great prospects for future drone use cases and can be game changing for various drone applications.

References

1. Wing Drone Delivery Noise: Australians complain, “It’s gone too far!” - eVTOL Insights

2. Up in the air: How do consumers view advanced air mobility?

3. Toroidal Propeller | MIT Lincoln Laboratory

4. Toroidal Propeller - US Patent (MIT)

5. Sharrow Engineering LLC- Patent

6. Sharrow Propeller™ vs Standard on a Regal 33 SAV with twin Yamaha 300s