German researchers study noise reduction for UAM systems
Noise is one of the biggest hurdles facing the future of urban air mobility (UAM).
Researchers at the Institute of Propulsion Technology in Germany (DLR) are currently investigating aerodynamic noise sources and their acoustic effects to enable low-noise designs and facilitate the integration of these aircraft into urban airspace.
The research is part of the ongoing project Virtual Acoustic Twin of Distributed Propulsion (VIRLWINT), which focuses on understanding and reducing tonal noise—particularly in fan stages that use fewer stator blades than rotor blades.
In conventional engine design, stator blades typically outnumber rotors helping them to dampen tonal noise.
However, reversing that ratio can simplify manufacturing and reduce broadband noise, making it an appealing option for low-speed fans like those expected to power eVTOLs and other urban aircraft.
Tonal noise reduction
DLR’s team has been studying two key mechanisms that help suppress the tonal sound generated in these less traditional fan configurations.
One involves aligning the angle of acoustic wave propagation with the angle of the stator’s leading edge, while the other—known as the “inverse cut-off” effect—relies on blade combinations and rotor tip speeds to block certain sound frequencies from propagating.
Both effects have been validated in test campaigns using specifically designed fan stages, featuring 31 rotor blades and 10 stator blades.
These were tested at DLR’s CRAFT facility, a dedicated acoustic noise rig capable of simulating representative flight conditions and measuring noise with high spatial accuracy.
The test setup includes more than 160 flush-mounted microphones along the fan duct and a rotating microphone array to map acoustic modes across thousands of positions.
Impact on UAM designs
Now, the measurements are being used to inform the design of a concept UAM aircraft powered by 26 small ducted fans.
The researchers are assessing three different fan stages—one baseline and two noise-optimised variants—for their acoustic performance without compromising on thrust or efficiency.
Each fan measures 46 cm in diameter, and data from the tests feeds into details simulations of aircraft flyovers.
According to DLR, the directional characteristics of the fan noise vary significantly between designs.
The baseline configuration emits tonal noise mostly behind the aircraft, meaning ground observers hear the tones after it passes.
In contrast, the low-broadband variant shifts that sound to the front, making it audible before the aircraft arrives.
And, a third configuration—the low-tone fan—combines both noise reduction methods to achieve a softer, more broadband-dominated sound that reduces the prominence of tonal frequencies altogether.
Furthermore, the next step, according to DLR, is to determine how these different noise signatures are perceived by the public.
They have already planned listening studies at the Institute of Aerospace Medicine in Cologne, where participants will evaluate simulated flyovers based on real test data.
The results will help gauge which noise profiles are most acceptable in urban settings, where sound annoyance could make or break the adoption of future air mobility systems.
In conclusion, with public acceptance hinging not just on safety and convenience but also on how disruptive these aircraft sound from the ground, the latest research by DLR could help shape the propulsion technologies that ultimately get the green light to fly over cities.