The Future of Advanced Air Mobility

Quiet eVTOL Flights Will Be the Benchmark For Olympic Gold at Paris 2024 Games

The 2024 Paris Olympic Games should open to loud fanfare in late July of that year. If the preparations being made by the 30 partners in the Re.Invent Air Mobility Challenge go as well as those being undertaken by the world’s athletes, the event could feature eVTOL vehicles providing transportation between Olympic sites across the French capital.

The program’s key backers—Choose Paris Region, Aéroports de Paris (ADP), and ground transportation group RATP—clearly hope the exercise will have a big impact on public attitudes toward advanced air mobility (AAM) services, but for this to be positive, the flights will need to be very visible but barely audible. Given the often-negative public perception of helicopter noise in European cities, the noise output from the eVTOL air taxis will be as closely monitored as the start of the 100-meter sprint final.

The need for quiet operations is what brought Volocopter’s engineering team to the Pontoise-Cormeilles-en-Vexin airfield, about 22 miles northwest of Paris. They have been using the general aviation airport, which is part of the ADP group, to conduct noise trials using its VC2X technology demonstrator aircraft. This is slightly smaller than the two-seat VoloCity model that the German company aims to bring to market in 2024 but uses a fundamentally similar configuration of 18 motor/rotor units, each of which is a source of noise.

The VoloCity itself started flight testing from the company’s Bruschal headquarters on Dec. 22, 2021. (The short hover flight on that date can be seen in a video released on April 13.) The prototype has been used for further flight trials since then but Volocopter has declined to provide details on progress, beyond reporting that it has already logged more than 1,500 flight tests with the 2X aircraft.

The process for measuring and evaluating aircraft noise might seem straightforward, but for new models like the eVTOL vehicles, it is complicated because there is no baseline data to work from and accurate acoustic noise prediction depends on having absolute values as a starting point. According to Ulrich Schäferlein, Volocopter’s head of flight physics, the flight test program has been used to assess all operational aspects of the aircraft’s noise profile, as well as the impact of ambient noise.

In a joint test campaign with EASA and aviation noise specialist Anotec, Volocopter has been seeking to evaluate how the existing ICAO Annex 16 Chapter 8 standards might be applicable to eVTOL aircraft certified under the European regulator’s new Special Condition VTOL requirements. It will now start incorporating data from the VoloCity, which has already established that, as expected, it is quieter than the 2X, mainly due to having a lower frequency band that makes the noise generated “more comfortable,” according to Schäferlein.

Volocopter has conducted several public flight demonstrations around the world with the 2X, and these have provided opportunities to assess public attitudes toward eVTOL operations. In the German city of Stuttgart, the company surveyed some of the 12,000 visitors who attended a demonstration event and the majority indicated that they found the flights to be quieter than they had anticipated. Similar results were collected in Singapore, with visitors saying that for the most part, they couldn’t hear anything from the aircraft.

In one of its acoustic noise measurement trials, Volocopter compared the 2X (which is also referred to as the VC200-2) with the similarly-sized Robinson R22 piston-powered helicopter. These tests found the eVTOL model to be 10 dB quieter than the R22 while climbing and 17 dB quieter when hovering at 75 meters (246 feet).

The company claimed that at 75 meters the 2X has the same noise profile as the two-seat helicopter at 500 meters (1,640 feet). For the R22, the main noise signature is based on its main rotor, tail rotor, and 131-hp Lycoming engine, while the eVTOL vehicle has 18 independent noise sources in the rotor/motor units installed across a circular frame above the fuselage.

Volocopter estimates that when it enters service the VoloCity will have only about 25 percent as much noise emissions as the larger Bell 407 helicopter, which is a widely used workhorse for passenger flights, such as sightseeing over New York City. Flying over 120 meters (394 feet), the eVTOL is expected to have a 65 dB(A) profile compared with 87 dB(A). During landing, the VoloCity will likely peak at 76 dB(A). By comparison, normal speech is generally assessed at around 50-60 dB(A) and constant road noise in a major city at 78 dB(A).

Last year, in California, Joby Aviation conducted a two-week flight trial with its eVTOL prototype, which uses tilting propellers, as part of NASA’s Advanced Air Mobility National Campaign. This assessed the aircraft's acoustic noise signature in takeoff, hover, and overhead flight.

NASA deployed its mobile acoustics facility and has installed more than 40 pressure ground-plate microphones. This grid array of microphones will support multidimensional measurement of sound levels from the Joby aircraft. In August 2021, Joby released a video that explained the differences in noise levels it recorded while testing five different fixed-wing aircraft and helicopters alongside its eVTOL model.

For Volocopter, the key question its engineering team is seeking to resolve isn’t whether the VoloCity will meet ICAO noise limits, which they expect it to do comfortably. “The main question for us is how do we measure noise in a repeatable way, and what does the noise mean for operations and how the aircraft can be integrated [into existing airspace], because individual locations could have their own noise limits,” Schäferlein stated.

In Paris, RATP, which runs the city's trains, subways, buses, and trams, has a particular interest in how eVTOL aircraft could be part of this mix. The company has been measuring noise levels throughout its transportation network for decades and is closely involved in the urban air mobility sandbox trials now getting underway in Pontoise, along with the French noise assessment center Bruitparif.

Joran Le Nabat, an acoustical engineer working on acoustic and vibratory studies for RATP, explained that his team is preparing noise maps around specific potential routes across the city and possible sites for vertiports. One of these sites is the Gare d’Austerlitz railway station on the east side of the city, and RATP is exploring the potential for eVTOL flights to shuttle passengers to other transport hubs from there. RATP is already using autonomous cars to carry passengers to the Gare de Lyon station, which is just over 600 yards away.

Le Nabat told reporters that the challenge is not simply a case of achieving a specific “magic number” of decibels from the eVTOL aircraft. Instead, public acceptance of the new mode of transportation involves a complex matrix of considerations that also includes factors such as operational safety, economic impact, and contribution to the good of the wider society.

Another Re.Invent Air Mobility partner, Skyports, expects to open a vertiport at Pontoise in June that will be used to evaluate other aspects of eVTOL aircraft operations in the build-up to the 2024 Olympics. During the games, flights are expected to connect Paris’s main Charles de Gaulle Airport with Le Bourget Airport (which will host the Olympic media enclave), and also the main Olympic village and at least three other vertiports around the metropolitan area.

Several other aircraft manufacturers are involved in the program, including Airbus, EHang, Vertical Aerospace, and Ascendance Flight Technologies, as well as drone makers Safran Electronics & Defense and Zipline. Other partners include Air France, flight training group CAE, Dassault Falcon Services, helicopter providers Helifirst and Helipass, avionics group Thales, and French aerospace research agency Onera.