Optimised propeller design cuts noise pollution from electric aircraft

Electric aviation typically has to battle with the dilemma that the more energy-efficient an electric aircraft is, the noisier it gets.

Electrification is seen as a way to reduce emissions from aviation in the future, but due to the challenges posed by longer ranges, interest is chiefly focused on electric propeller planes covering shorter distances.

Propellers connected to electric motors are considered the most efficient propulsion system for regional and domestic flights but often create a lot of noise pollution.

This noise not only disturbs air passengers but also those on the ground, as electric aircraft will need to fly at relatively low altitudes, with noise disturbance reaching residential areas and animal life.

“We can see that the more blades a propeller has, the lower the noise emissions. But with fewer blades, propulsion becomes more efficient and the electric aircraft can fly for longer. In that sense, there is a trade-off between energy efficiency and noise. This is something of an obstacle for electric aircrafts that are both quiet and efficient,” said researcher Hua-Dong Yao.

The team has succeeded in isolating and exploring the noise that occurs at the tip of the propeller blades – the ‘tip vortices’ – a known but less well-explored source of noise.

In isolating this noise, the researchers were able to fully understand its role in relation to other noise sources generated by propeller blades. By adjusting a range of propeller parameters, such as pitch angle, chord length and number of blades, the team found a way to optimise the propeller design and even out the trade-off effect between efficiency and noise.

The method can now be used in the design process of quieter propellers for future electric aircraft, the researchers said.

“Modern aircraft propellers usually have two to four blades, but we’ve found that by using six blades designed using our optimisation framework, you can develop a propeller that’s both relatively efficient and quiet. The propeller achieves a noise reduction of up to 5-8 dBA with only a 3.5 per cent thrust penalty, compared to a propeller with three blades”, said Hua-Dong Yao.

“That’s comparable to the noise reduction of someone going from speaking in a normal conversation voice to the sound you would perceive in a quiet room”.

A-weighted decibel (dBA or dB(A)) is an expression of the relative loudness of sounds as perceived by the human ear. A-weighting gives more value to frequencies in the mid-range of human hearing and less value to frequencies at the edges as compared to a flat audio decibel measurement. A-weighting is the standard for determining hearing damage and noise pollution.