Propeller research advance paves way for quiet electric aviation
Electrification plays an important role in fossil-free aviation but the more energy-efficient an electric aircraft is, the noisier it gets, so researchers at Chalmers University of Technology in Sweden claim to have now optimised propeller design to pave the way for quiet electric aviation.
In recent years, electrification has had an important role in reducing emissions but interest has been mostly focussed on electric propeller planes covering shorter distances, considered to be the most efficient propulsion system for regional and domestic flights.
While the noise from the propeller blades may disturb air passengers, future 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 an electric aircraft can fly for longer,” explained Hua-Dong Yao (pictured), associate professor and researcher in fluid dynamics and marine technology at Chalmers University of Technology.
“In that sense, there is a trade-off between energy efficiency and noise, and this is something of an obstacle for electric aircrafts that are both quiet and efficient.”
Now, Hua-Dong Yao and his research colleagues have succeeded in isolating and exploring the noise that occurs at the tip of the propeller blades, or ‘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, described in a study published in the journal Aerospace, can now be used in the design process of quieter propellers for future electric aircraft.
Hua-Dong Yao continued: “Modern aircraft propellers usually have two to four blades, but we have found that by using six blades designed using our optimisation framework, we can develop a propeller that is 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. That is 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) 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.
The study, ‘Blade-Tip Vortex Noise Mitigation Traded-Off against Aerodynamic Design for Propellers of Future Electric Aircraft’, was published in the journal Aerospace. The authors are Hua-Dong Yao, Zhongjie Huang, Lars Davidson, Jiqiang Niu and Zheng-Wei Chen.
