The sound of silence

Not far from the super computers at Oak Ridge, Moore has assembled a team of engineers on the shore of a Tennessee lake. The former resort hotel is now home to Whisper Aero, an R&D operation Moore formed with Ian Villa. The two met at Uber, the ride share business, where they helped create the urban air mobility concept, better known as flying taxis.

Also known as eVTOL, (electric vertical takeoff or landing), the flying taxis are a new class of aircraft that Moore helped birth through 30 years of NASA research in distributed electric propulsion. Some will even tell you he is the father of eVTOL. But where eVTOLS have typical ranges of 150 miles or less, Moore and his team are focused on regional travel. They target trips between 50 and 500 miles using fixed-wing, electric-powered conventional takeoff or landing airframes. Beyond 500 miles, current fossil-fueled engines are still the most efficient way to travel.

The engineers at Whisper Aero have reinvented the electric ducted fan. Their electric ducted fans, or propulsors, look very much like miniature versions of the massive, high-bypass-ratio turbofans engines on large airliners. According to Moore, the real beauty of the Whisper Aero propulsion technology “is we’re able to shrink our propulsor size down to fit inside the wing system. And we don’t give up any [ducted fan] efficiency.”

The small size of the propulsors means blade tip speeds are low compared to large diameter fans. At the same rpm, the larger the diameter of the fan—or propeller—the higher the blade tip speed. As tip speeds approach Mach 1 they encounter compression issues and generate significant noise, an undesirable trait for Whisper Aero. Making aircraft quieter is a primary goal for Moore and his team.

As part of its sound reduction efforts, Whisper Aero attaches propulsor blades to a shroud in a manner similar to spokes on a bicycle. That closes the air gap between the blade tips and interior of the propulsor and reduces the sound signature. The propulsors also have more blades than other electric ducted fans. The additional blades create harmonics that move certain sound frequencies above the range of human hearing. So, to observers, Whisper Aero propulsors are vastly quieter than turbine or reciprocating aircraft engines.

In a test, a 55-pound, fixed-wing drone powered by one propulsor overflew microphones at cruise speed and 200 feet agl. The drone was so quiet it could not be detected by sound-measuring equipment.

Whisper Aero’s current propulsors generate 11 or 72 pounds of thrust but they are scalable from 5 to 500-plus pounds. Moore says scalability will allow the propulsors to power an aircraft the size of a Boeing 737 and reach speeds of 0.7 to 0.8 Mach.

Whisper Aero has created an aircraft concept called “Whisper Jet” to illustrate the company’s propulsion technology. They don’t want to build the aircraft themselves because they aren’t aircraft builders. But they’re happy to help anyone who wants to.

The Whisper jet concept creates a unique aircraft with takeoff weight, passenger load, and performance similar to a Beech King Air B200. Its 22 propulsors should push the 12,500-pound Whisper jet along at 250 knots with nine passengers. Flying on batteries, range is listed at 200 nm and a hybrid power version should reach 500 nm.

Whisper Aero’s technology, says Moore, “is what aerospace engineers have always wanted to do. And that is: completely couple the wing lift and the propulsion system so that it is one integrated system that is much more effective than either by themselves.”

The Whisper jet propulsors will be built as part of the leading edge of the 50-foot wingspan. Each propulsor ingests smooth, undisturbed air while maintaining wing camber. They exhaust over the wing in a jetfoil that creates a near continuous flow of compressed air across the wing from wing root to wing tip. With the propulsor internal to the wing, and with the over wing exhausting, the setup further reduces aircraft noise while increasing lift.

Ganged together, the Whisper Aero propulsors are married to aerodynamics that are at once old and new. “What Whisper [Aero] has come up with,” said Moore, “is the idealized equivalent of a Custer Wing with circulation control without any of the wasted structural weight and drag of the side walls of the duct.”

Thanks to the Coanda effect—where a fluid jet (i.e., air) tends to stay attached to a convex surface—the propulsor thrust follows the camber of the wing and the mid-wing flow creates a high coefficient of lift, up to 10 times more lift than a conventional wing. As an edge of excitement creeps into his voice, Moore says Whisper Jet “gets such a fantastic CL MAX that the wing loading can go up to 60 to 80 pounds per square foot and still be able to take off in 2,500 feet. That means, for the exact same amount of power, you can cruise about 50 knots faster because with higher wing loading your wing wetted area decreases and you are able to cruise at your L/DMAX. So, you get high speed cruise efficiency, you get great ride quality, and in terms of safety, you get improved pattern safety speeds.”

One of the advantages of the Coanda Effect is at extreme angles of attack. Moore says the wing has been tested to nearly 45 degrees “and still not stalled because the propulsion system is keeping the flow attached. There is no stall until you are at a ridiculous angle! So, all three of those things are just enormous breakthroughs for a GA pilot: safety, performance, and ride quality are enormously improved.”

And for the folks on the ground, there is the joy of silence.

Tim Wright is a pilot and freelance aviation writer from Virginia.