For two years, Whisper Aero has been quietly developing what it says will be the quietest and most efficient propulsion system on the market. Upon closing a $32 million Series A funding round last week, the company finally broke its silence to reveal some details about its ultra-low-noise thrusters.

We now know that Whisper’s proprietary propulsion system is some kind of next-generation electric ducted fan that can be scaled for applications ranging from aircraft to leaf blowers. However, Whisper isn’t ready to divulge exactly what makes its ducted fan quieter and more efficient than anything else on the market.

Ducted fans have been used in aerospace applications for decades, with the most common ducted fans being the turbofan engines that power many large airplanes. Some eVTOL aircraft developers, like Lilium and Volocopter, are also working ducted fans into their designs. According to Whisper, its proprietary electric ducted fan design is not only significantly quieter than other ducted fans but also 20 percent more efficient.

Whisper Aero CEO Mark Moore told FutureFlight that the company “developed a way to achieve nearly the absolute optimum in terms of noise and efficiency” by mimicking a theoretical construct called an actuator disk.

“Instead of having a few blades that cause pressure pulses as they rotate…this imaginary construct of an actuator disk has the lowest possible change in pressure because the entire disk is creating that [pressure] uniformly,” Moore explained. “Not only is that the quietest way you could generate thrust, but it's also 100 percent efficient—it's as efficient as you can get.”

While the concept of an actuator disk is purely theoretical and imaginary, Whisper managed to recreate a real-world version of one in its propulsion system. Doing so “requires some very unique structures, and that's the part that we don't want to give away, exactly how we've been able to recreate what an actuator disk does,” Moore said.

He added that the “magical outcome” of this actuator disk-like design is that Whisper’s ducted fan has tip speeds that are slower than what any electric propulsion company is using or developing today, while simultaneously creating the least amount of noise that’s audible to the human ear. Other low-noise propulsion systems for electric aircraft, such as the eVTOL air taxis being developed by Joby and Archer, have to sacrifice performance to achieve the low tip speeds necessary to reduce noise. But Whisper has found a way to optimize performance while practically eliminating noise—or at least limiting the noise that’s detectable to the human ear.

When it comes to reducing noise, “it's all about isolating where all the tones are, being able to drastically reduce the amplitude of those tones, and then pushing the tones into frequencies that are pleasant to the human ear,” Ian Villa, Whisper Aero’s COO and chief product officer, told FutureFlight. Whisper managed to push some of the tonal noise into ultrasonic frequencies, which are inaudible to the human ear.

Whisper Aero's electric ducted fan is pictured inside the anechoic chamber.

Whisper has been testing its electric ducted fans in an anechoic chamber. (Photo: Whisper Aero)

To test the noise produced by its ducted fans, Whisper has taken detailed acoustic measurements in an anechoic chamber and measured the sounds produced by demonstrator drones in flight. The company has also done some subjective testing with humans as well as animals, including dogs and birds. “We have five dogs here on our innovation campus who also get to listen, and we see if they react to different tones and different propulsors to make sure that not only are we human friendly, but we're also dog friendly, pet friendly,” Moore said.

In flight testing, Whisper was able to validate that a 55-pound demonstrator drone equipped with the company’s electric ducted fans was completely inaudible when flying 200 feet overhead. The 55-pound drone is one of five drones the company has flight tested while working through nine iterations of its electric ducted fan design.

After spending two years focused on research and development, Whisper is ready to begin producing and selling its first propulsors and generating revenue streams. For now, the company is focused on defense applications, and it already has six contracts with the U.S. Department of Defense, which is looking at using the company’s ultraquiet propulsion system for intelligence, surveillance, and reconnaissance purposes.

As for commercial applications, “I think there's definitely a path for a first civil aviation product that uses our propulsors in the 2028 to 2030 timeline,” Moore said. “That is certainly what our goal is.” Moore added that Whisper is having discussions with potential partners that are interested in using the electric ducted fans in their aircraft designs, and it’s working with the FAA to ensure that the new propulsors can be certified for the civilian market.

Thanks to the success of Whisper’s latest funding round, the company is looking to double the size of its team, which has fewer than 40 employees. “We want to bring in folks who can help us manufacture or set up our first manufacturing lines,” Villa said. Whisper is also using the new funds to open two facilities in Tennessee: a 4,000-sq-ft production facility near its headquarters in Crossville, and an 8,000-sq-ft office in Nashville that will be dedicated to engineering, analysis, and rapid prototyping.

Whisper’s latest $32 million Series A funding round was led by Menlo Ventures, EVE Atlas, Capricorn’s Technology Impact Fund, and Connor Capital. Some funding was also provided by Kindred Ventures, Abstract Ventures, Moving Capital, AeroX Ventures, Cosmos Ventures, Linse Capital, and LaunchTN, a public-private partnership supporting entrepreneurs in Tennessee.

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