Tipped for success: Whisper Aero reveals its Whisper Jet

Whisper Aero has revealed details of its hybrid-electric regional aircraft, the Whisper Jet, after a two year stealth period following its founding in 2020. 

The aircraft includes a variety of technologies the industry has seen integrated into other AAM applications such as NASA’s X-57 and the Lilium Jet, including the firm’s proprietary electric ducted fans (EDF). Whisper’s team is in San Diego this week at the American Institute of Aeronautics and Astronautics (AIAA) forum sharing more details about its electric propulsors and its nine-passenger, fixed-wing eCTOL. 

Whisper Aero, like much of the AAM sector, is trying to develop technology to achieve the “quietest and most efficient high speed integrated propulsion solutions” for converting electricity into thrust. Due to weight sensitivity being so high, the Whisper team found the method through which propulsors are integrated has “significant implications” on overall aircraft aerodynamics, thermal management, acoustics, structures, and control. 

“From the earliest days of development, scale independence has been highlighted as a key attribute to enable aircraft developers to design aircraft with greater freedom to embrace distributed architectures. Distribution offers the potential to increase design robustness and increased inter-disciplinary connectivity while minimising criticality, at the cost of increased design complexity,” stated Whisper in a paper published this week on its EDF propulsion technology. 

The OEM decided on a high blade count, high solidity propulsor disc method to achieve the low noise and tip speed required. With a high enough blade count, Whisper found the blade passage frequency (BPF) could be pushed into the ultrasonic range therefore becoming inaudible to humans. But to achieve this, high RPM is also required — a frequency of around 16,000 Hz is required, the typical limit to human audibility. But, high RPM does not necessarily mean high tip speed and low noise requires low tip speeds (in the range of 400-600 ft/s). High blade count means smaller fans. It is because of these small diameter ducted fans that high RPM and low tip speed can mutually exist, which in turn provides the added value of achieving high specific power since electric motors achieve their best performance at a very high RPM. 

NASA had previously attempted a high blade count for a turbofan application in the early 2000s. Despite wind tunnel tests proving noise reduction, the administration decided the approach was “structurally unfeasible” due to the fan blades potential to bend, flap, and aeroelastically deform under strain. However, Whisper, unlike NASA, had acoustic reasons for the approach that embraced low tip speeds instead of a performance bias that demanded the opposite. Low tip speeds mean Whisper has been able to integrate a shroud which connects the blade tips — this is impossible at higher RPM. Like spokes on a bicycle wheel, the tip shroud enables both statically and centrifugal tensioning of each blade.

Whisper Aero wants to supply its propulsors to a variety of platforms and no plans to become an aircraft OEM. However the firm has developed its Whisper Jet concept to best show hows its EDF can be integrated into an airframe. The aircraft is marketed a nine-seat, hybrid-electric aircraft capable of Part 23 operations. The most striking feature of the concept is the “jetfoil” which looks similar to the design of the Lilium Jet eVTOL. The difference between the two is that Whisper integrates their array of fans into the leading edge of the wing rather than the trailing edge. According to Whisper, this approach minimises any aerodynamic and noise challenges. The firm said its jetfoil avoids drag related to swirl effects because the EDF exhausts are de-swirled through a stator and merged into one constant jet sheet.

There are some challenges related to the jetfoil however. The Whisper team discovered the aircraft experiences a strong downwash field during takeoff and landing. This downwash resulted in poor conventional horizontal tail performance, so the OEM employed an outboard horizontal tail configuration instead first developed in the Second World War. 

Performance wise, Whisper Aero forecasts a range of 161 to 193 miles on just its battery power with a payload of 2,400lbs. A proposed hybrid range extender could increase that to upwards of 400 miles. Thanks to the EDFs the Whisper Jet should be able to achieve a top speed of 250 knots and maintain a stall speed of 61 knots. With a potential list price of $3m, Whisper also has plans to develop a larger, Part 25-capable aircraft in the future. 

As one industry stakeholder put it: “This is some new groundbreaking stuff. It is about as much of a step forward as putting dimples on a golf ball. It will not work well for full VTOL, but for flying the fat lady has sung. This is ‘The Nibelung’ of electric ducted fans for eCTOL.”

Meanwhile, last month Whisper announced a $32m Series A funding round. The investment, led by a syndicate including Menlo Ventures and EVE Atlas, will help to expand the team and establish production — aimed initially at the US market.

“Most companies only think about designing for low noise by reducing disk loading and tip speed with a willingness to sacrifice efficiency. We never accepted this premise,” said CEO Moore. “Truly designing for low noise means examining all of the sources of noise critically and then tailoring each source’s acoustical content to the way that humans hear so that it can blend in seamlessly into the soundscapes of our everyday lives.”