December 6, 2023

Whisper Aero says its electric aircraft propulsion system is “20% more efficient and 100 times quieter than anything else on the market”. They’re designed for drones, airplanes, and possibly eVTOLs, as well as a funky-looking concept aircraft.

eVTOL and delivery drones promise to bring large numbers of aircraft closer to urban life than ever before. Most people find today’s drones too noisy to want them whizzing overhead every day, and Whisper has raised about $40 million and won some military grants as the company builds the quietest electricity on earth propulsion system.

So for a company focused on silence, there’s been some noise around Whisper in recent years. Now, it has finally unveiled the technology it hopes to revolutionize electric aviation: an ultra-quiet, efficient electric ducted jet.

Inside the Whisper propulsion unit, there’s a “propulsion disc,” which is essentially a fan with an unusually large number of strong, stiff blades connected at the outer circumference by a grommet for extra strength. It is a small diameter fan in order to maintain low tip speeds while spinning at flight-related RPMs.

The fan itself uses many blades, with a grommet connecting the blade tips
The fan itself uses many blades, with a grommet connecting the blade tips

Whisper Air

The large number of lobes pushes the “lobe pass frequency” up to 16,000 Hz, well beyond the range of human hearing, Whisper said.Please note that according to the Aerospace Americathe frequency wasn’t pushed high enough to scare the dogs: “We had five dogs physically close to our fan test and they didn’t respond to them at all,” says co-founder and CEO Mark Moore, who also co-founded Uber Elevate, and worked as an engineer at NASA.

With lots of blades, you can also spin “slower than any propeller or turbofan,” Moore continued. “We’re spinning so slowly that the centrifugal force on this rim isn’t so strong that it would tear.” The shielded edge connecting the fan tip also eliminates the gap between the blade tip and the duct casing, further lowering the blade. Tip eddy noise.

Energy lost to noise is energy not converted into thrust, and Whisper says it has been able to demonstrate rotor efficiencies as high as 92 percent, “even at a 6-inch (15cm) fan diameter.”

In acoustic tests, Whisper’s 6-inch fans were completely inaudible to 30 decibels of background noise at a distance of 200 feet (61 meters). They then compared it to two ducted fans and two open propellers, which Whisper describes as the quietest commercial products on the market, both producing the same 7.8 lbf (34.7 Newtons) of thrust.

At a distance of 100 feet (30.5 meters), the Whisper Fan recorded a staggering 34.1 dBA noise level. The Schubeler ducted fans measured 44.9 and 52.1 dBA, respectively, and the Aeronaut CAMcarbon open propellers measured 49.4 and 58.7 dBA, respectively. You can hear the difference below.

Whisper Aero Thruster Comparison Test (Web)

According to Whisper, this means that the company’s propellers are “100-500 times quieter than ducted fans and 100-1,000 times quieter than open propellers.”

Now I’m not a rocket surgeon, but I’ve always thought that the decibel scale, A-weighted or not, is a logarithmic measure of change in sound pressure level, where 10 decibels represents a tenfold increase in intensity, not a hundredfold. Perceptually, a 10 dB difference tends to “feel” more like doubling the noise level. So I’m not sure where the 100x number comes from.

But either way, it does seem quieter. The Tennessee-based company unveiled the propulsion system yesterday at the American Society of Aeronautics and Astronautics’ Aviaton Forum 2023, showing off a “Whisper Jet” model designed to demonstrate the new fan’s potential.

The Whisper Aero team shows off the Whisper Jet concept model
The Whisper Aero team shows off the Whisper Jet concept model

Whisper Air

The Whisper Jet concept is an odd-looking duck that uses a Blohm and Voss outboard horizontal tail (OHT) configuration, similar to Virgin Galactic’s VSS Unity. It has a row of propulsion units on the inner wing surface. In theory, it can carry a pilot and nine passengers, flying up to 200 miles on a charge at speeds of up to 288 mph (463 km/h) using conventional runway takeoff and landing (CTOL) (322 km). With a hybrid system, range can be extended to more than 500 miles (800 kilometers).

It’s just a concept; Whisper is more interested in selling propulsion systems than entering the aircraft manufacturing business. But the company says the propulsion system will be suitable for short-take-off and landing (STOL) aircraft, drones of all sizes, and may eventually be used in eVTOL air taxis. It is working on a range of fan diameters, including 4-inch, 10-inch and 24-inch options, designed specifically for AFWERX’s high-speed VTOL program, which will compete with some other fascinating propulsion technologies.

Low noise and high efficiency are of course two important factors for an electric aircraft. It will be interesting to see how Whisper’s device performs in the real world, and what kind of customers pick up the technology and put it to work.

source: Whisper Air