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.
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.
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.
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.
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.
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.