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The basic configuration of traditional propellers has not fundamentally changed since the first powered flight\u00a0by the Wright brothers in 1903<\/a>.<\/p>\n However, as engineers learn more about aerodynamics and attempt new experiments, propellers are evolving to more complex shapes. These feature multiple blades, high sweep angles, blade tip devices and other features to optimise performance in different conditions.<\/p>\n A recent advancement in propeller technology are \u201ctoroidal\u201d propellers. These devices are ring-shaped, with the blades looping around each other. A few recent\u00a0articles<\/a>\u00a0and\u00a0videos<\/a>\u00a0have been hyping these \u2013 but how \u201crevolutionary\u201d are they, really?<\/p>\n In 2017, researchers at\u00a0MIT filed a patent<\/a>\u00a0for toroidal propellers. Their patent claims the invention is more efficient than traditional propellers and is less noisy.<\/p>\n Coincidentally, already in 2012, US engineering company\u00a0Sharrow Marine<\/a>\u00a0also developed a toroidal propeller for boats; they have demonstrated it to be more efficient and quieter than traditional marine propellers.<\/p>\n The reason toroidal propellers may be quieter is because of their complex shape \u2013 it minimises the strength of the vortex (a spiralling movement of air, water, or another fluid) that naturally happens over propeller blade tips.<\/p>\n This happens because there\u2019s a high-pressure region under the blade, and low pressure above it. As high-pressure air from under the blade moves towards the low-pressure region above it, it travels in a spiral \u2013 a vortex.<\/p>\n This phenomenon is not unique to propeller blades, which are essentially rotating wings. The wing of a plane also experiences this phenomenon. Engineers have done much research on\u00a0wingtip devices<\/a>\u00a0that can minimise this.<\/p>\n The use of closed-loop structures \u2013 like in a toroidal propeller \u2013 is one way of reducing tip vortices.<\/p>\n Even though the basic propeller shape has remained the same since its invention, many propeller blade designs have been put forward. To test these, engineers need to perform design trade-off studies. Some of these approaches have been tested to try and make\u00a0helicopter blades<\/a>\u00a0and\u00a0drones<\/a>\u00a0more efficient and less noisy.<\/p>\n It\u2019s important to understand propeller geometry must be optimised for a specific \u201coperational envelope\u201d. This means the properties of the fluid or air it operates in, rotation speed, forward speed, and other details. Outside that envelope, the propeller will perform poorly.<\/p>\n So far, nobody has achieved the magical propeller geometry that will achieve low noise and high efficiency for all operating conditions and scales. Toroidal propellers are no exception \u2013 from the sparse results available so far, their advantages are not yet fully quantified.<\/p>\n Comparing a well-designed toroidal propeller to a poorly designed traditional propeller will show a significant improvement, but is not a fair comparison.<\/p>\n Well-designed toroidal propellers may have advantages in specific operating conditions, such as dense fluids or a specific range of speeds. However, the question remains as to how a toroidal propeller compares to a\u00a0well-designed traditional propeller<\/a>\u00a0for the same conditions.<\/p>\n This is a challenge, since improvements are always relative to a benchmark \u2013 which may not be the most efficient design to start with.<\/p>\n Another aspect of a fair comparison that doesn\u2019t seem to have been published for toroidal propellers is comparing different propellers at the same\u00a0thrust force<\/a>. Only then you can see the true advantages regarding noise reduction and energy required to spin the propeller.<\/p>\n An\u00a0MIT announcement<\/a>\u00a0earlier this year about toroidal propellers winning one of MIT Lincoln Laboratory\u2019s 2022 R&D 100 Awards generated significant excitement. There\u2019s been\u00a0widespread experimentation<\/a>\u00a0with 3D-printed toroidal propellers, but not all of these have delivered positive results.<\/p>\n This may be due to un-optimised geometry and poor scientific rigour by the general public conducting some of these experiments. This gives the scientific community a research opportunity \u2013 to truly assess and optimise toroidal\u00a0and<\/em>\u00a0traditional propellers to enhance their performance.<\/p>\n Previous optimisation studies have been conducted, some of which even use\u00a0machine-learning<\/a>\u00a0techniques to identify suitable geometries. Engineers are also trying to make propellers that sound less annoying, by considering how\u00a0humans perceive sound<\/a>.<\/p>\nRefining the shape<\/h4>\n
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No magic propeller<\/strong><\/h4>\nHome experiments are not representative<\/strong><\/h4>\n