Study points to ideas for reducing noise at the coming air taxi vertiports

UK researchers measure rotor noise at various distances from different kinds of surfaces

Air taxis will need to be incredibly quiet if they are to revolutionize travel and commuting and potentially replace cars, but mechanical engineers in the United Kingdom realized a few years ago that there were very few, if any, scientific studies describing the role of distance from the ground or hard landing surfaces in determining the noise profile of an aircraft.

The researchers at the University of Bristol put an electric rotor with a blade radius of 12.7 centimeters in a soundproofed, or anechoic, chamber and operated the rotor in tests over a flat panel that was shifted up and down to simulate flight at various altitudes.

They found a significant increase in noise when the rotors were closer to the surface, and they quantified the increase. While that knowledge may be helpful in anticipating aircraft noise and designing aircraft or vertiports for noise considerations, the researchers aren’t stopping there. They’re now using their findings to explore how grated landing surfaces or foam treatments might reduce such noise, lead author Liam Hanson told me.

“We found an overall increase in the order of a couple of decibels, up to 4 dB, as a kind of reflective effect, depending on the proximity to the ground and angle,” said Hanson, a researcher in the university’s Department of Mechanical Engineering. “A small factor like this could play into the overall noise of the vehicle.”

For context, a whisper is about 30 dB, and normal conversation is about 60.

The findings were published in Hanson’s paper, “Experimental investigation of propeller noise in ground effect,” online by the Journal of Sound and Vibration on April 28.

“The propeller noise within ground effect has been shown to be highly directional and sensitive to the relative distance between the propeller and the ground plane,” the paper states. “This should be taken into consideration in the design process of new aircraft configurations to shield the highly directional noise from propellers operating” near the ground.

Hanson said it’s well known in aviation that rotorcraft taking off or landing vertically derive more aerodynamic lift when closer to the ground as pressure waves are compressed. “Some manufacturers might be thinking they will get some benefit from being close to the ground, but if you factor in the acoustics, that might be something you have to consider because it could be much louder,” he said.

If an aircraft is landing on a roof, the angle of the soundwaves as they reach people nearby could also alter the noise level, he said.

Hanson is working with a colleague on a follow-up paper that will explore how sound is impacted by porous surfaces. Their research has been funded by Embraer.

“We’re thinking along the lines of a grid or grate, or perforated flooring, which would allow airflow through it, and we will study what benefit that might have in mitigating noise,” he said.