As the world shifts towards greener air travel, electric and hybrid aircraft are becoming a key part of the future.
But there’s one big issue: their engines are incredibly loud and annoyingly high-pitched.
Engineers from the University of Bristol and the University of Salford have discovered why these aircraft engines make such unpleasant noise—and what can be done to make them quieter.
The main culprit behind this annoying noise is a technology called Boundary Layer Ingesting (BLI) engines.
These engines are designed to pull in air that flows around the aircraft’s body, or “boundary layer,” and use it to help power the plane.
This design is great for efficiency but terrible for noise. When the boundary layer’s turbulent airflow enters the engine’s fans and ducts, it creates two distinct noise patterns that scientists call “haystacking.”
Haystacking refers to the way turbulent air disrupts sound waves inside the engine, spreading the noise across a range of frequencies.
This makes the engine noise not only louder but also more irritating to our ears. The research team broke this down further, explaining that there are actually two types of haystacking: “duct haystacking” and “fan haystacking,” each happening at different points during a flight.
At low thrust, such as when the aircraft is cruising, the engine’s fan doesn’t pull in much air. This means that only the tips of the blades are exposed to low-energy, turbulent air from the boundary layer.
In this phase, most of the noise comes from the engine’s duct, which vibrates as air flows through it. This is what the researchers call “duct haystacking.” The sound that comes out is spread across many frequencies, creating that annoying, whirring noise.
When the aircraft is taking off, the engine’s fan pulls in air with much more force. This sucks in faster and more turbulent air, which disrupts the smooth airflow around the blades.
As the blades slice through this unsteady air, they produce even louder noise across more of the blade’s surface. This is known as “fan haystacking,” and it’s especially irritating to people on the ground.
Dr. Feroz Ahmed, who led the research at the University of Bristol, says that understanding these noise patterns is crucial for building quieter aircraft.
“These two hidden sound signatures—haystacking—make future embedded aircraft engines feel perceptually irritating, not just loud,” he explained. By linking how air flows into engines with how people perceive noise, engineers can finally work towards designs that actually sound quieter—not just measure quieter.
The researchers used a high-tech wind tunnel to study the noise patterns in detail. This allowed them to see exactly how turbulent air creates these irritating sounds.
Now, they are developing new ways to control the noise at its source, both in the fan and the duct.
Their work could lead to quieter engines for not only large electric aircraft but also new electric air taxis that could soon be flying over cities.
The team hopes that with these new insights, manufacturers can create electric aircraft that people will actually want to hear in the sky—not cringe away from. The next step for the researchers is to expand their work to other types of electric engines, with the goal of making quiet aviation a reality.
