טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMaor Yonatan
SubjectNoise Abatement Techniques for Small UAV Propellers
DepartmentDepartment of Aerospace Engineering
Supervisor Dr. Oksana Stalnov


Abstract

The use of small drones and UAVs has seen a large surge in recent years. Drones have been applied in new industries, including personal, commercial, military and more. A major concern for this industry that is still to be adequately addressed is the noise emission during flight, which can have implications in both a military setting and in a commercial environment, where regulation concerning noise levels is increasing. 

Due to design considerations, small scale drones tend to use electric motors, which are quieter than other means of propulsion. This makes the propeller the most prominent source of noise, and the prediction and abatement of this emission a concern. Propeller noise is dominated by two components. Tonal noise appears at frequencies related the angular velocity and the number of blades, while the broadband component appears across the entire spectrum.

Considering that the diameter of a small-scale UAV is usually in the order of half a meter, there is very little information on noise characteristics of relevant rotors or propellers. These vehicles generally employ small rotors, where the Reynolds number is on the order of 105 or less, treading the region of fully laminar to transitional boundary layers.

In this experimental study propeller aerodynamic and aeroacoustic performance was measured using a set of microphones and a load sensor. Measurements were conducted in an anechoic chamber, in hover conditions, with no incoming flow. In addition to loads and noise emission, angular velocity was also measured using a magnetic encoder. Several APC propellers were treated to reduce their noise signature using thin strips of roughness grit of several types. An additional set of propellers that were 3D printed were treated using trailing-edge serrations was also tested.

Initially untreated propellers of varying diameters were used to establish baseline measurements, as well as to observe the behavior of an untreated propeller. Following this, treated propellers were tested. Grit treated propellers showed noise reduction of several decibels when compared to the baseline of the broadband component, while the tonal component showed no significant change. Propellers treated with trailing-edge serrations showed a reduction in noise emission mainly at higher RPMs in the broadband component. Aerodynamic loads were not significantly affected by this treatment.

A second part of this study included a comparison between Amiet's model for trailing-edge broadband noise and the experimental results. The model was coded using a span-wise strip division of the blade, thus allowing to consider varying conditions along the span. The model's behavior is very similar to that obtained in the experimental setting, both as a function of frequency and directivity.