Anyone who has been in close contact with a drone, big or small, knows how unpleasant the noise these flying things make is to the human ear – the feeling of being charged by a tiger mosquito the size of a cow, a drill for sensitive eardrums, the discretion of a lawnmower trying to reach the clouds. It’s the essence of propellers, designed to push the air behind them and thus provide propulsion to the device they carry: they make noise. Sometimes a lot. Always too much.
As New Atlas explains, the frequencies reached in the air are similar to those of a baby’s cries – so it’s not surprising to find them irritating. And it’s not just the case in the air: marine propellers are also very loud. Which can be a bit boring – ask a submariner, for example.
As the site also points out, the general shape of the propeller, whether designed for an aircraft or for a ship, has only changed marginally in recent centuries and millennia, with limited gains in efficiency.
A team from the Structural and Thermal-Fluids Engineering Group of the Lincoln laboratory, which depends on MIT, looked into the question: what if, after all this time, we change their shape a little? “When people were coming up with crazy ideas in the early 1900s or during World War II, a few designs were, basically, ring wings. So I wondered what one of those ring wings would look like if you turned it into a propeller.”says Dr. Thomas Sebastian, one of the members of the aforementioned laboratory.
“Our initial concept was to use a toroidal shapethis ring-shaped wing, to, we hoped, turn it into a silent propeller»he continues, indicating that one of his trainees, an expert in 3D printing, has created and tested many iterations of the thing.
From air to water
Their instinct did not deceive them: as you can see in the video below, not only is the invented propeller much quieter, but it does not “play” the same frequencies. Result: A drone, at full power, sounds more like a fan than an army of furious drones.
The toroidal propeller, one of the Lab’s @RD100Awards winners, has a unique, closed-form propeller design that makes it a significantly quieter alternative to common multirotor propellers on commercial uncrewed aerial vehicles. https://t.co/hgda3NgYIz pic.twitter.com/5XkIxNVPHd
— Lincoln Laboratory (@MITLL) January 6, 2023
In addition, the team of course tested the pure propulsion performance of its propeller. By making less noise, has it lost efficiency? It seems not, it’s even the opposite: at equal power, this contraption that they baptized B160 provides more thrust than a conventional propeller. Bonus all the more appreciable as its less pronounced noise may allow drones to fly closer to humans, its closed ring shape makes the toroidal propeller less dangerous in the event of contact with the flesh.
As New Atlas explains, the team is still in its infancy. It is impossible to say up to what scale the toroidal helix can be applied. Will it be for taxis and flying cars, military vertical take-off and landing aircraft, or only for the smallest drones? In addition, a toroidal propeller remains, to date at least, more complex to manufacture than a normal propeller.
What MIT will do with its propeller therefore remains a mystery. But others, in a different field, have already successfully applied the concept of the toroidal shape. New Atlas thus tells the story of Sharrow Marinea company that has adopted the idea and applied it to the nautical field.
The results are reportedly spectacular, both in terms of noise and thrust. Unlike conventional propellers, the tips of Sharrow Marine’s toroidal propellers do not create any vortices. Much of the noise previously generated is thus eliminated and the assembly propels more water, thus also gaining in efficiency. At equal power, the boat goes faster and consumes less (it is a question of 20% fuel savings), and has the advantage of being able to accelerate without cutting off the discussion of its occupants.
The downside, there must be one, is the price: Sharrow Marine, which adapts them to demand and to different engines, sells its toroidal propellers for around 4,600 euros, when a lambda propeller costs about ten times less. Silence is golden, they say, for fish too.
