The eVTOL S4 hexacopter diminishes its decibels one rotor blade at a time
Flying cars have been a staple of science fiction for decades. Soon, they might be whisking wealthy city dwellers from point to point while common folk idle in traffic jams below.
California-based Joby Aviation hopes to obtain a license from the Federal Aviation Administration (FAA) for its hexacopter-like vehicle to shuttle paying passengers around America’s busiest cities as early as next year.
The electric vertical take-off and landing (eVTOL) aircraft can carry four passengers and a pilot, fly 100 miles on one battery charge and reach speeds of 200 mph. During the aircraft’s 15 years of development, Joby engineers had a major problem: making the aircraft so quiet that it wouldn’t bother city dwellers.
“From day one, we considered a low acoustic footprint to be a core design requirement for the Joby aircraft,” says Didier Papadopoulos, president of Joby’s Aircraft OEM division. “We knew that in order for electric flight to become an accepted mobility tool in and around cities, our aircraft would have to be orders of magnitude quieter than today’s helicopters – and that is exactly what we’ve achieved.”
As quiet as a conversation
There’s no missing a helicopter hovering above a city. The noise of its rotor blades interacting with the surrounding air thunders compared to the background noise of an urban environment.
The sound of an air ambulance rushing to save a life, a police chopper on the hunt for a fugitive or the occasional politician on their way to a high-profile event is considered a fact of life in the city. But hundreds of helicopters carrying people across the city solely for convenience will almost surely spark public outrage. Furthermore, transport noise pollution is a significant health hazard, with studies showing that exposure to consistently elevated levels of noise frequently causes disturbed sleep and stress, increasing the risk of heart disease and diabetes.
Joby, therefore, set out to make its eVTOLs barely perceptible “against the ambient environment of cities.” That goal was achieved two years ago when a NASA-led test campaign proved that the sleek air produces sound of about 45 weighted decibels (the relative loudness perceived by a human ear) when flying overhead at a distance of 500 meters. That’s quieter than a regular human conversation. An average helicopter hovering 300 meters away generates nearly 80 dB, nearly a thousand times louder than the Joby aircraft. Papadopoulos says Joby’s aircraft is 100 times quieter than a helicopter of a similar weight. The company compares its sound to the rustle of leaves in a breeze.
“Our aircraft’s low noise profile is the result of more than 10 years of hard engineering focused on acoustics, ranging from the design of our motors and propellers to the overall architecture of the aircraft,” Papadopoulos says. “There are numerous aspects of the aircraft’s design that have been selected and optimized to both reduce the magnitude of the sound produced during flight and improve the quality of that sound.”
Minimizing blade-vortex interaction
The Joby aircraft can be piloted remotely, but the company plans to fly passengers with a pilot on board to improve comfort and safety. The eVTOL’s six propellers are arranged roughly hexagonally and draw power from four battery packs similar to those used in electric cars, with each motor drawing power from two of the packs to reduce the chance of failure.
But Papadopoulos says that swapping an internal combustion engine for an electric one doesn’t solve the helicopter noise problem because most noise results from the rotor blades moving through the air. Joby’s engineering team countered this by perfecting the design and positioning of the six propellers and each of their blades.
“Many aspects of Joby’s configuration were carefully selected to minimize or eliminate noisy adverse interactions between air flows produced by our six propellers in all stages of flight,” says Papadopoulos. “This includes maximizing the distance between each propeller, the placement of propellers relative to the airframe, and the raised tail rotors.”
Each propeller can also independently adjust its rotation speed, tilt, and the pitch of its blades. That, Papadopoulos explains, enables the aircraft to minimize the interactions between the blade tips of one propeller and the aerial vortices caused by the blade tips of another. Since the rotor blade tips slice the air at half the speed of sound, suppressing these vortices is extremely important.
The amount of noise each blade produces is also proportional to the weight it lifts. “We made careful decisions around the number of propellers our aircraft has — six larger propellers instead of more, smaller propellers — and five large-area blades per propeller,” Papadopoulos says. “This directly correlates to noise through the tip speeds of the rotor blades.”
However, the Joby vehicle is not purely a giant drone. The aircraft also features an 11.6-meter wing that takes over most of the lifting work once the vehicle is aloft. According to Papadopoulos, that means the propellers must provide only about a tenth of the force needed by a helicopter of a similar size when in cruise mode.
NASA and DARPA tools aided blade design
Joby has been flight-testing its first full-scale prototype aircraft since 2017. At that time, the noise generated by the prototype was considerably lower than that by a conventional helicopter – but the engineers believed it could be lower still. They began fine-tuning the propulsion system, focusing especially on the shape of the blades.
“We started with a CAD design of the blades,” Papadopoulos says. “We then used rapid composite prototyping to build the blades, test them, process the results, and design a new blade. We build many blades to improve our understanding of the noise sources to design new, quieter blades.”
The team used a range of high-fidelity tools developed by NASA and the U.S. Defense Advanced Research Projects Agency (DARPA) to model the acoustic behavior of the propellers. They then tested the prototypes in a wind tunnel and on a special test circuit they built near Joby’s headquarters in Santa Cruz, Calif. Nicknamed the Whirly, the test track allows the engineers to send a propeller attached to a rig around at a speed similar to the Joby aircraft’s cruising speed and measure its aerodynamic behavior in conditions akin to those of an actual flight.
The experiments showed, for example, that wide blades produce less noise than narrow ones. Subsequent flight tests showed the redesign further reduced the loudness of the propulsion system by 3 dB while delivering the same thrust.
“Generally speaking, our blades have much more surface area than helicopter blades, allowing for lower tip speeds, but not too much area, which can increase other noise sources,” says Papadopoulos. “We have found that careful optimization of the airfoil design and blade shape can balance the noise sources, allowing for a sound that more closely resembles leaves in a breeze than the strong whop-whop of a helicopter.
In May, Joby announced the completion of more than 1,500 flights covering more than 33,000 miles, including 31 flights in partnership with the FAA to demonstrate “the aircraft’s operational characteristics and precision landing capabilities.” Pilots were on board for more than 100 of those flights.
The company plans to commence further testing with the FAA soon to obtain final certification to start commercial operations. If all goes well, Joby could ferry its first paying customers in 2025.
