Whisper Aero has lifted the curtains on a new aircraft concept known as the Whisper Jet, which utilizes the company’s next-generation, ultraquiet propulsion system. For more than two years, the secretive Tennessee-based start-up has been developing what it claims will be the quietest and most efficient propulsion system on the market.
Just a few weeks ago, Whisper revealed that its propulsion system uses a special type of electric ducted fan, but the company was not yet ready to divulge exactly what makes its ducted fans quieter and more efficient than any other. Now Whisper is finally sharing some more of the recipe for its secret sauce.
This week Whisper co-founders Mark Moore and Ian Villa are presenting their work at the American Institute of Aeronautics and Astronautics (AIAA) Aviation Forum in San Diego, California. The company just published four white papers detailing how the propulsion system works and how it can be integrated into a variety of airframes.
Whisper’s Electric Ducted Fan: The Secret Sauce
Ducted fans are not a new concept for aircraft propulsion systems. In fact, they’re already used in turbofan engines in many large airplanes, and some eVTOL aircraft developers like Lilium and Volocopter are incorporating electric ducted fans into their designs. Whisper says it has redesigned the ducted fan concept “to achieve nearly the absolute optimum in terms of noise and efficiency,” Moore told FutureFlight.
To do this, Whisper engineered a ducted fan with a very high blade count and low tip speed. When rotating at a high enough speed, this puts the blade pass frequency in the ultrasonic range. Blade pass frequency, which is calculated by multiplying the number of fan blades by the rotational frequency of the rotor, is responsible for most of the noise created by fans and propellers. By pushing those tones into the ultrasonic range, the sound produced by the spinning of the blades is barely audible to the human ear, and much of it is dampened by the atmosphere.
Generally speaking, fans that spin at higher rotational speeds (measured in revolutions per minute, or rpm) tend to have higher blade tip speeds and higher tip speeds create more aerodynamic noise over a broader range of frequencies. Whisper has managed to achieve a low tip combined with a high rpm by reducing the length of the blades. In contrast, eVTOL aircraft developers attempting to reduce the noise produced by their propellers have generally done so by reducing the tip speeds, which hinders the performance of their electric motors, Moore said.
To increase the number of blades in its ducted fan, Whisper also had to make them thinner, which inevitably makes them less rigid. To reinforce those thinner blades, Whisper’s ducted fan blades are tensioned to an outer shroud similar to spokes on a bicycle wheel, according to Whisper. Adding this shroud also gets rid of the gap between the blade tips and the duct. In conventional ducted fans, that small gap contributes to noise production and reduces overall efficiency.
While Whisper’s ducted fan design can power airplanes, the potential applications for this technology span far beyond aviation. According to Whisper, this design can be scaled for a variety of other applications, including delivery drones, leaf blowers, bathroom fans, and blow dryers. Just about any type of device that moves air can be made quieter and more efficient using Whisper’s basic engineering principles.
Introducing the Whisper Jet
The Whisper Jet concept, which Whisper revealed during the AIAA forum this week, is the first example of an aircraft design that utilizes the company’s proprietary electric ducted fans. It features an outboard horizontal tail configuration and has 22 ducted fans built into the leading edge of its 50-foot (15 meters) fixed wing. The design looks similar to the ducted fan array on the Lilium Jet eVTOL aircraft, but Lilium’s ducted fans are located on the trailing edge of the wing, rather than the leading edge.
“This alternative leading-edge integration shows promise, specifically for enabling lift augmentation with a distributed array of small, yet efficient, electric ducted fans,” Whisper engineers wrote in one of the white papers published this week. The authors added that this feature “has shown promise for providing similar high lift performance to [NASA’s] X-57 configuration but is able to do so through a clean sheet of air without the choppy lift distribution provided by leading edge-propeller integrations.”
Whisper says the Whisper Jet could carry nine passengers plus one pilot on regional flights of up to 500 miles (800 kilometers) using a hybrid-electric powertrain. Alternatively, it could use a fully electric powertrain that would provide a shorter range of about 200 miles (320 kilometers).
To validate the design of its propulsion system, Whisper has conducted an extensive flight test campaign using five different drones equipped with various prototypes of the company’s electric ducted fans. Most of that testing has been done with a 55-pound (25 kilograms) demonstrator drone, which Whisper says is inaudible when flying overhead at an altitude of about 200 feet.
Although Whisper has developed multiple technology demonstrators and crafted a clean-sheet design for the Whisper Jet, the company says it has no plans to actually produce its own aircraft. Rather, the company hopes its Whisper Jet concept will attract the attention of aircraft designers who may want to integrate its propulsion system into their new aircraft designs, particularly those intended for regional air mobility.
A Solution for Regional Air Mobility
While eVTOL air taxis and urban air mobility (UAM) have attracted significantly more attention and financial investments in recent years, regional air mobility has just as much if not more market potential and might even be more feasible in the short term.
According to analysts at the global management consulting firm McKinsey & Company, technological advancements like new electric and hydrogen-powered aircraft will soon make regional flights more economically viable. While UAM will require new infrastructure to support flight operations in densely packed urban areas, regional aircraft can make use of the thousands of small and often underutilized airports that already exist today.
Whisper says the electric ducted fans it is developing could help to unlock the regional air mobility market by enabling quiet and low-cost flights in areas that are currently underserved by aviation—especially in places where noise pollution from aviation has been a concern to local residents.
“You’ve got to be quiet, and you’ve got to be economical, and it all starts with propulsion,” Villa told FutureFlight during a recent media briefing on regional air mobility organized by McKinsey & Company. “The biggest step changes in aviation start with propulsion,” he added.
Regional flights on next-generation regional aircraft such as the Whisper Jet would cost each passenger around $0.69 per mile. In contrast, the U.S. federal government reimbursement rate for driving a car on business trips is about $0.66 per mile. At just a few extra cents per mile, passengers can reach their destinations in a fraction of the time.