Mark Moore had a long career at NASA, followed by his leadership of Uber Elevate, the car-sharing service’s attempt to emulate their terrestrial cab competition in a more skyward realm. Moore left all that behind to form his own company, Whisper Aero, which recently unveiled its Whisper Jet electric ducted fan propulsor.
“Whisper” is more than just a trade name here. Moore and his team have created an electric ducted fan propulsion system that seems to be far quieter than anything else flying. Whisper Aero plans to supply these systems to airframe manufacturers, but not to produce any aircraft itself.
What’s All the Noise About?
Moore works from the standpoint that reducing noise is paramount in gaining community acceptance of AAM (Advanced Air Mobility)* machines. If Mark is correct in his assessments, which seem borne out by recent tests, his ultra-quiet propulsors should be a big part of future urban and regional flight.
One dictum of propeller design is that slow rotational speed aids in reducing noise. The other end of this equation is the tip speed of the individual propeller blades. Once a large propeller’s tips reach a high Mach number, the noise climbs to a point where it becomes pretty unbearable.
If designers make the propeller smaller, it can rotate at high speeds and the tips will still not reach those high Mach numbers. Enclosing the multiple blades in a duct allows high rotational speed, but low noise. A rim, somewhat like a bicycle wheel, connects a high number of blade tips to help keep things together, and that rim fits in a slot in the duct. Whether that will further reduce tip noise remains to be seen, but tests so far reinforce the Whisper in Whisper Jet.
We end up with the counter-intuitive picture of a small fan rotating quickly, but with low tip speeds. Moore explains the outcome in an AIAA interview: ““Our blades are just like spokes on a bicycle wheel, with a rim so that they are very stiff. They have to be very rigid.
“The entire disc of Whisper’s ducted fan, including the thin blades and outer rim, could be made in one piece through pressure injection of thermoplastics, or meltable plastic polymers, Moore said.
“’We spin substantially slower — way, way slower — than any propeller or turbofan. We’re not going to say the precise number, but we spin so slowly that the centrifugal forces on this rim aren’t that significant that it would tear apart,’ he said.”
Moore further explained low tip speed and unspecified number of blades results in sound in the ultrasonic range, not perceptible to human ears but not annoying to animals. You can see the video of the Whisper Jet EDF in noise comparison with other electric drive systems here.
A single Whisper Jet ducted fan has been tested on a 25-kilogram (55-pound) drone, which flying at 200 feet overhead is “not audible to the human ear.”
Now in its ninth generation of design and testing, the propulsors are being prepared for possible sale to the Department of Defense, which has provided funds under six contracts valued at $2.2 million. Subsequent work will involve “partnering with an aircraft manufacturer to develop the Whisper Jet.”
Mounted along the leading edge of a wing, an array of Whisper Jets would blow air over the upper surface of the wing, theoretically augmenting lift and slowing stall speeds. David Ullman is working on a similar device and has been able to find a corporate backer in CubCrafters. Although the speed difference between even an augmented Cub-type aircraft and the designs shown on Whisper Aero’s site is considerable, the benefits are similar.
The world may become quieter overall, with Whisper Jet’s quiet ducted fan used in leaf blowers or vacuums, or any normally noisy device required to move air.
*Advanced Air Mobility (AAM) is an “air transportation system that transports individuals and property between points… using aircraft, such as remotely piloted, autonomous, or vertical take-off and landing aircraft, including those powered by electric or hybrid driven propulsion, in both controlled and uncontrolled airspace,” according to the Advanced Aviation Infrastructure Modernization Act. (Wikipedia)