Electric planes have a noise problem, and more propellor blades may be the answer

Electric planes are an important part of developing a fossil-free aviation industry, but unlike electric cars they have a major problem: They make an absolute racket, and are generally noisier than their combustion engine counterparts.

According to researchers at the Chalmers University of Technology in Sweden, the more energy efficient an aircraft gets, the noisier it gets.

This is because, as the researchers explain, that the propeller noise becomes “relatively dominant in the absence of the primary perceived noise from turbomachinery, combustion chamber, and high-speed jet existing in conventional gas turbo engines.”

And, electric planes will likely need to fly at relatively low altitudes, thus increasing the risk of noise pollution over residential areas and critical environmental locations.

The dilemma, then, is solving the trade-off between an electric aircraft that is both quiet and energy efficient, and the answer might be to have more blades on the propeller.

“We can see that the more blades a propeller has, the lower the noise emissions,” said Hua-Dong Yao, associate professor and researcher in fluid dynamics and marine technology at Chalmers University of Technology.

“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.”

In an effort to address this trade-off problem, Hua-Dong Yao and his research colleagues have succeeded in isolating and exploring the noise that occurs at the tip of the propeller blades, known as the tip vortices.

By isolating the noise the researchers were able to better understand its role in relation to other noise sources generated by propeller blades.

More importantly, however, the researchers were able to adjust a range of propeller parameters – such as pitch angle, chord length, and the number of blades – to optimise the propeller design and even out the trade-off effect between efficiency and noise.

The researchers believe that their findings, published in the journal Aerospace, can now be used to make quieter and efficient propellers for future aircraft.

“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,” said Hua-Dong Yao.

“The propeller achieves a noise reduction of up to 5-8 [(A-weighted decibel (dBA)] with only a 3.5% thrust penalty, compared to a propeller with three blades. 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.”