Aircraft

Whisper Aero claims radical advance in quiet, efficient electric jets

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The Whisper Jet concept would produce less than 50 decibels of noise on the ground when flying at a low 500ft altitude
Whisper Aero
The Whisper Jet concept would produce less than 50 decibels of noise on the ground when flying at a low 500ft altitude
Whisper Aero
The fan itself uses a lot of blades, with a shrouding ring joining the blade tips
Whisper Aero
The Whisper Aero team presents a mockup of the Whisper Jet concept
Whisper Aero
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Whisper Aero says its electric aircraft propulsion systems are "20% more efficient and 100x quieter than anything else on the market." They're designed for drones, planes and possibly eVTOLs, and there's also a funky-looking concept plane.

eVTOLs and delivery drones promise to bring large numbers of aircraft closer than ever before to urban life. Most people find today's drones annoyingly loud and wouldn't want them whirring overhead on a daily basis, and Whisper has raised some US$40 million and bagged some military grants on the basis that this company is building the quietest electric propulsion systems on the planet.

So for a company focused on silence, there's been a fair bit of noise around Whisper for a couple of years. Now, it's finally unveiled the technology it hopes will revolutionize electric aviation: an ultra-quiet, highly efficient electric ducted jet.

Inside the Whisper propulsion unit, there's a "propulsion disc" that's essentially a fan, one with an unusually large number of strong, stiff blades, joined at the outer circumference by a shrouding ring for additional strength. It's a small diameter fan, in order to keep the blade tip speed low while it's spinning at flight-relevant RPMs.

The fan itself uses a lot of blades, with a shrouding ring joining the blade tips
Whisper Aero

The large number of blades, Whisper says, push the "blade passage frequency" over 16,000 Hz, and thus out of the range of human hearing. Mind you, according to an interview with Aerospace America, the frequency doesn't push high enough to freak dogs out: "We’ve had five dogs in physical close proximity to our fan tests, and they don’t react at all to them,” said 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 "way, way, slower than any propeller or turbofan," Moore continues. "We spin so slowly that the centrifugal forces on this rim aren't that significant that it would tear apart." That shrouding rim joining the fan tips also eliminates the gap between the blade tips and the duct housing, further reducing blade tip vortex noise.

Energy lost to noise is energy that's not converted to thrust, and Whisper says it's been able to demonstrate rotor efficiencies up to 92%, "even at 6-inch (15-cm) fan diameters."

In acoustic testing, Whisper's 6-inch fan tested as completely inaudible from 200 feet (61 m) away over a 30-decibel background noise floor. They then compared it against two ducted fans and two open propellers, described by Whisper as the quietest commercially available products on the market, all making the same 7.8 lbf (34.7 N) of thrust.

At 100 ft (30.5 m) distance, the whisper fan recorded a remarkable 34.1 dBA noise level. The Schubeler ducted fans measured 44.9 and 52.1 dBA, and the open propellers, from Aeronaut CAMcarbon, measured 49.4 and 58.7 dBA. You can hear the difference below.

According to Whisper, this shows that the company's propulsor is "100-500 times quieter than the ducted fans, and 100-1,000 times quieter than the open propellers."

Now I'm not a rocket surgeon, but I always thought the decibel scale, whether or not it's A-weighted, was a logarithmic measure of sound pressure level changes in which 10 decibels represents a tenfold increase in intensity, not a hundredfold. Perceptually, a 10 dB difference tends to "feel" more like a doubling of the noise level. So I can't square where that 100x figure comes from.

But either way, it does seem a ton quieter. The Tennessee-based company launched the propulsion system at the American Institute of Aeronautics and Astronautics' 2023 Aviaton Forum yesterday, presented with a mockup of a "Whisper Jet" designed to showcase the potential of the new fans.

The Whisper Aero team presents a mockup of the Whisper Jet concept
Whisper Aero

The Whisper Jet concept is an odd-looking duck, using a Blohm and Voss Outboard Horizontal Tail (OHT) configuration something like Virgin Galactic's VSS Unity. It's got banks of propulsion units along its inner wing surfaces. Theoretically, it could carry a pilot and nine passengers, up to 200 miles (322 km) on a battery charge, at speeds up to 288 mph (463 km/h), using conventional runway takeoff and landing (CTOL). The range could be extended to 500-odd miles (800 km) with a hybrid system.

It's just a concept; Whisper is more interested in selling the propulsion system than going into the business of making planes. But the company says the propulsion system will work well for short takeoff and landing (STOL) aircraft, drones of all sizes, and potentially, eventually, eVTOL air taxis. It's working on a range of fan diameters, including 4-in, 10-in, and a 24-in option specifically designed for use in the AFWERX High Speed VTOL program, where it'll compete with some other fascinating propulsion technologies.

Low noise and high efficiency are certainly two important factors for electric aircraft. It'll be interesting to see how Whisper's units perform in the real world, and what kinds of clients pick this technology up and put it to work.

Source: Whisper Aero

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14 comments
John_H
Thanks for the article; there's some surprising info there. I'm a non-practicing rocket scientist, working with dB in electronics instead. The difference between about 30 and 50 dB is a 100x change. 10dB is 10x, another 10dB makes it 10*10x = 100x. The sound approximately doubles with 3dB of change. Fun stuff.

How many hair driers does it take to generate lift? I'd love to experience the sound and thrust of one of these. Thanks again.
Nobody
Since the storage of electric power is limited, a small electric turbine seems too inefficient to be practical. Open propellers may be noisier but far more efficient at slow flying speeds. The faster you go, the more power it takes.
Bruce McCaskey
How are the fans made?

What rpm are they running ?

How do they quantify efficiency ? Pounds of thrust per amp hour ?

How much air is entrained in the duct in other words how large is the intake beyond the outlet diameter.?
Bruce McCaskey
What airfoils/ twist/ taper are they using in the fan and the stator?

How are they compensating for the change in Reynolds number from root to tip ?

jayedwin98020
To the Author:

"Now I'm not a rocket surgeon..."? Never heard of a "rocket surgeon", rocket scientist, okay.

When you're using an expression, you might want to first determine if it's really an actually a expression, or not.

Just saying.
jerryd
Another scam. Small turbines have to turn fast to get any thrust at all. These style ducts cause a lot of airdrag on the walls of very fast air, the efficiency is 10% of a large prop.
Fact is with load, battery, it won't even lift off physics says.
And noise both behind and ahead will be bad, again selective data by taking readings from the side.
Jinpa
The headline labels this vehicle as a jet. It is not a jet as conventionally understood. Jets as we know them are combustion-engine aircraft, with the combustion occurring in axial tubes aft of the rotor/stator sets which compress the intake air, developing thrust as a reaction to the compression/combustion/exhaust sequence. Power to make that compression occur comes from the turbine wheel that is in the exhaust area of the jet engine, which on a horizontal shaft, the front end of which is locked to the rotor blades up front. Those rotor blades push air backward against the stator blades (which are fixed to the engine housing), compressing the incoming air stream. There are several sets of rotors and stators, to build up the compression, which is then ducted into the combustion chambers. This vehicle appears to use only electric-powered turbine units to generate thrust, although the writer didn't say that, with no apparent rotor/stator sets, so no compression effect. More detail would be highly appropriate.
vince
The range could be extended to much greater distances when the following batteries are a reality--they are all in the works and just need a little more experimental work and commercialization--maybe 3 to 5 years.

1) 600 miles with an Aluminum air battery

2) 1,000 miles with a Lithium Sulfur battery

3) 1,400 miles with a Lithium Silicon battery

The future for our grandkids is great if we just pursue intently the research necessary to get over the hump and make these realities. Of course, eliminating the opposition pushed by Republicans would be of great help too.
David F
Whisper Aero's use of a fan instead of prop is a welcome improvement, one that should be trialled by other eVTOL companies.
riczero-b
Very interesting idea but isn't the OHT design ill-matched to the narrow wing? This will have to bear large twisting forces on a structure full of complex machinery.