A noise-killing game changer in air and water

Propellers are generally designed to take in a fluid, such as air or water, and push it out using rotational motion. They, in a sense, evolved from the Archimedes screw likely used in ancient Egypt thousands of years before it was described by Archimedes in 234 BC.

However, for devices designed to rotate, there have been few revolutionary design changes for a very long time. The stanchion-driven aircraft still uses his bladed stanchion twisted aerofoil, a design similar to the bamboo copters that Chinese children enjoyed 2,400 years before him. A screw propeller, a variant of which dates back to his 1700s.

So I’m fascinated by the fact that several groups claim to have demonstrated great advantages in both air and water using markedly different geometries. , but could take a big leap, especially since it’s much more efficient in ocean space.

A potential game changer in the air

One of the key issues with multicopter props is the annoying noise, often described as “grunting” because it falls in the same frequency range as a baby’s cry. Humans tend to be most sensitive to sounds around 100 Hz to 5 kHz. This makes evolutionary sense. It is a place to listen to the vowels that are the key to verbal communication. But whether multicopters can live up to their potential and fill the skies with fast, cheap and clean air transportation services is a key question. Residents and lawmakers don’t want to add annoying noise to city life.

A team working on a quiet ion thruster at MIT’s Lincoln Labs wondered if multi-rotor prop noise could be mitigated with differently shaped propellers.

“Propellers, as we know them, are pretty noisy,” says Dr. Thomas Sebastian, a senior staff member in the Structural and Thermal Fluids Group at Lincoln Labs. “And you can look at the wings to see how it works. During the early 1900s and World War II when people came up with all sorts of crazy ideas for airplanes, the basic There were a few designs that were typically these ring wings, so I wondered what would happen if we turned the ring wings into propellers.”

“We came up with this initial concept of using a toroidal shape, this annular wing shape, to make a quieter propeller,” continues Sebastian. “I had an intern of mine execute a totally amazing idea. He took the concept and used a 3D printer to create many iterations.”

In a few trials, the team actually found a design that not only reduced the overall noise level at a given thrust level, but specifically the noise in the 1-5 kHz range.

In fact, they are more like breezes than propellers and produce an unobtrusive sound. Anecdotally, according to the team, drones running these props make annoying levels of sound at about twice the distance of regular drones. Hear it in the video below.

“The key thing we thought about to make the propeller quieter was the fact that we were distributing the vortices generated by the propeller throughout its shape, not just at the tip,” says Sebastian. “This effectively causes it to dissipate faster in the atmosphere. That vortex doesn’t propagate far, so it’s less likely to be heard.”

Propeller noise can be partially addressed by placing a ring of acoustic treatment around the path of the propeller. This also works as a propeller guard from a safety point of view. However, they increase parasitic mass and reduce battery life. Also, the wind makes it difficult to keep the drone stable in outdoor conditions.

The team analyzed these odd-looking toroidal props to see if they had a thrust efficiency penalty. Apparently not: The team’s top-performing B160 design not only was quieter at a given thrust level than the best standard propeller we tested, it produced more thrust at a given power level. The development behind them and these toroids are in a very early stage and there is still a lot of optimization going on.

Additionally, the looped shape not only increases structural stability, but also reduces the chances of props banging, clipping, or catching on anything. You still wouldn’t want them to hit you in the face, but there are probably marginal safety improvements out there.

On the downside, these are fairly complex shapes, making them much more difficult to manufacture than standard props using cheap and easy injection molding. the kind you need. But even with propellers doubling or tripling in price, these are low-cost parts for drones, and the overall impact might not be as harsh on your waist pocket.

At this stage, it is unclear whether such designs will be relevant on a larger scale, replacing conventional propellers in fixed-wing aircraft, or indeed replacing electric VTOL air taxis. The latter already seem much quieter than helicopters, but when it comes to flooding city airspace with fast, cheap and environmentally friendly air transport, every decibel of noise counts when it comes to public and regulatory resistance. The question is what frequencies these large propellers occupy in the audio spectrum, and whether toroidal propellers shift sound in a more human-friendly direction.

The team has patented the design, and it’s not clear if there are any plans to commercialize it, but MIT appears willing to license it to interested manufacturers.

ADDITIONAL ADVANTAGES IN THE WATER

Drones and aviation are one and the same, but aerodynamics and hydrodynamics go hand in hand, and it turns out that there are already products closer to production in the ocean space that take a very similar approach.

Sharrow Marine has frankly gotten great results from boat propellers that use toroidal loops instead of standard blades. After years of development, the company is now testing their propellers against hundreds of standard propellers and the difference is incredible.Sharow props do not generate tip vortices. . It is a major source of energy loss and a surprisingly large component of the overall outboard engine noise.

Toroidal propellers greatly reduce the amount of liquid that “slides down” rather than being forced out the sides of the propeller, sucking in more water per turn and propelling the boat further forward. Periodically he doubles the speed the boat can achieve at low and medium RPM, radically increasing the effective rev range of the motor. It also reduces fuel consumption by approximately 20%. This is a huge win given the enormous energy requirements of propeller-driven boats and the size of the industry.

Shallow says it has the interesting effect of greatly reducing the tendency of the boat to pitch backward as it accelerates. Instead, the entire boat floats above the water and remains much more level. On top of all this, the noise impact is quite severe, as you can see in the video below.

In fact, the company claims it’s more or less a propeller that you can stick on your outboard, blast quietly at 30 miles per hour (48 km/h), and have a conversation on board without raising your voice. said. Amazing stuff.

Sharrow already sells toroidal propellers that are CNC machined to fit a wide range of popular outboards from most major manufacturers. The downside here is the price. A normal propeller can cost him close to US$500, but will cost you US$4,999 per pop, regardless of model. But again, this is a fairly small component of the overall cost of many boats, and given the voracious appetite for fuel, the expenditure not only pays for itself in the short term, it also costs less to the passengers. Feel much more comfortable. , bystanders, and for underwater marine life.

In the age of the energy transition, these things should also be of great interest to anyone building an electrified boat. Watch Sharrow’s fairly brief presentation in the video below.

Source: MIT, Sharrow Marine