A team of researchers from the Massachusetts Institute of Technology empirically selected the shape of a toroidal screw, which creates the lowest sound volume in the frequency range of 1-5 kHz. This is the most sensitive area for the human ear – babies cry at this frequency and nasty multicopter motors screech. The latter became an incentive for research – if people want to see a lot of useful drones around in the future, they should be quiet.
The toroidal propeller has blades in the form of a closed circle, curved in two planes. This design has a lot of variations, the optimal bending parameters have yet to be found. The main thing is that such a blade has no beginning and end, and therefore it moves uniformly in the air. Because of this, there is no tension at the ends of the blades, the interaction of their parts with the air flow is the same everywhere, and therefore they are much less noisy.
Prototypes of toroidal propellers are several times quieter than conventional propellers – and in addition, they provide more thrust. Even better, toroidal blades behave as part of boat propellers, where, in addition to noise reduction, the movement of water flows through the propeller is stabilized. The boat rises a little above the water, friction is reduced – this is how fuel savings of an incredible 20% are achieved.
The disadvantage is obvious: it is not easy to make such a complex structure and it will be very expensive. For example, Sharrow Marine manufactures toroidal propellers for motorboats for $4,999 each, while a conventional propeller costs no more than $500. And here the potential of 3D printing can be fully revealed, which will make it possible to reduce the cost of production of toroidal blades for any industry.
