In a public interview hosted by the Vertical Flight Society, Dufour Aerospace elaborated on the design process and capabilities of their upcoming aEro 3 eVTOL aircraft. eFLYING.news was among the participants and was able to ask a few questions.
The interviewees were Thomas Pfammatter, CEO of the company, and Jasmine Kent, CTO of Dufour. Thomas has over 25 years of flight experience as a helicopter pilot with 3,000 rescue operation accumulating over 10,000 flight hours. He is responsible for the airframe configuration, certification process and commercial operation of the company. Jasmine co-founded Daedalean, a company in autonomous aircraft systems, after working at Google for 8 years. As an expert in software development, she is responsible for design evaluations, simulations, control algorithms, software and automation. The interview was moderated by Nicolas Zart from the Vertical Flight Society, the professional technical society for vertical flight that was founded as the American Helicopter Society in 1943.
Thomas and Jasmine started by introducing the aEro 3, the eVTOL with which they wish to revolutionize the way emergency rescue missions are flown as well as the regional urban air mobility market. The aEro 3 will feature four rotors mounted in front of the leading edge of a single main tilt-wing as inspired by the Canadair CL-84. The whole wing tilts to a vertical position for hover as opposed to tilting rotors in combination with a fixed wing, a design choice many other eVTOL concepts go for. The cabin features room for one pilot and four passengers, and can be configured to hold a patient in a stretcher fully horizontally throughout its transition from hover to horizontal flight. Dufour showed the audience the video that was released upon completion of the first flight phase with their scaled demonstrator. They performed a number of flights including transition phases to analyze the aerodynamic performance and stability & control characteristics throughout the flight envelope.
The team showed a series of scenarios in which the advantage of the aEro 3 as compared to ground transportation became clear. Being based in Switzerland, they have a lot of experience with hard to reach hospitals, where ambulances sometimes have to drive around the mountains to reach a more specialized facility. The aEro 3 can do this journey much quicker and cheaper. The main cost savings come from its high cruise speed. The operating cost per hour is 1,400CHF (roughly $1,540), which at a cruise speed of 350km/h results in 4CHF or roughly $4.4 per kilometer. Dufour compares this to an operating cost for a helicopter of 9.5CHF or $10.5 per kilometer for an Airbus Helicopters H125 model. They claim their eVTOL is 1.1x cheaper and 4x faster than a regular ambulance, and 3x cheaper and 1.6x faster when compared to a helicopter.
A regional urban air mobility simulation was presented in which the goal was to reach MIT in Boston, Massachusetts as quickly as possible, using either a car as ground transportation, the aEro 3 eVTOL for point to point travel, or an airplane for which an airport first had to be reached. This had to be done from any location shown on the map below. It shows that within a radius of around 30 miles around Boston, the car would be the quickest mode of transportation. However, outside of that, the aEro 3 should prove to be the quickest, apart from some zones far away from the city.
After the general presentation, the virtual floor was opened to questions, and eFLYING.news was able to ask the following. Note that the answers are paraphrased.
Q: Thomas, as an expert in helicopter-based rescue missions, do you foresee a future for eVTOL aircraft in these scenarios? Will hover in windy conditions ever be stable enough to do this in such dangerous terrain?
80-90% of EMS missions can be replaced by the aEro, with around 10% being too specialized and left for helicopters. It is important to know that the aEro is more plane than helicopter. It can hover, but the pilot still has to think of a proper approach. The tilted wing is actually beneficial for gusty conditions. When the wing is vertical, the ailerons allow for a lot of yaw control. The wing remains pressurized due to the prop wash, and hence is not so affected by gust conditions.
Q: Why did you decide to go with a tilt-wing, as opposed to e.g. tilting rotors with a fixed wing? Is this because of the beneficial properties in hover?
Tilt rotors pose the issue of inducing a lot of download onto the fixed wing. A part of their area of effect is blocked by the wing, causing them to lose some thrust, and a lot of wasteful vortices are generated. In the transition phase of tilting rotors with a fixed wing, the wing never makes a good incidence angle with the oncoming air to generate enough lift, so all the lift has to come from the rotors. This becomes increasingly difficult as they transition from hover to forward flight and approach a horizontal position. With a tilt-wing, the prop wash is always in line with the wing. This way, the wing experiences smooth airflow, and is always pressurized and hence less vulnerable to gust.
Q: You want today’s pilots to be able to easily fly your eVTOL aircraft. How do you foresee the transfer from a helicopter pilot license or fixed wing pilot license to flying your eVTOL? Will it be easier for helicopter pilots or would you say the controls are different enough so that having helicopter experience does not pose a real advantage?
The control system will be made such that you can fly it easily with a fixed wing pilot license. It is much more plane than helicopter. The pilot needs to think of a proper landing trajectory and final approach. There is just a transition to hover in the end before touching down.
Q: If one motor fails, how do you counter the reaction torque from an uneven number of motors? Can you hover in this condition or do you need forward velocity to counter the moment with the rudder?
The aircraft is designed to be able to cope with a failure of any of its motors. Similarly to twin helicopters, there will be a decision point on take-off at which the pilot has to decide whether to transition or come back in for a landing in case of an engine failure.
Q: Can you tell us a bit more about the simulation models and aerodynamic tools you used in trading off conceptual designs?
X-Plane, a commercially available flight simulator, actually proved very meaningful in quickly assessing the flight characteristics of new concepts.
Other people in the audience were also able to ask questions. We learned that the manufacturing of the full-scale model has not yet started, but Dufour hopes to start working on the experimental project of that next year, with the certification process taking a few more years after that. While the company considered more or less rotors, they settled on four because in their eyes this was the optimal trade-off between redundancy and efficiency. The range or payload of the aircraft can be increased if it is operated in short take-off and landing mode, where it will not be required to take off or land vertically. The fuselage will stay completely level throughout transition, which is important when carrying a patient on a stretcher. When asked what makes Dufour Aerospace stand out from all other eVTOL companies, Jasmine didn’t hesitate to point to Thomas’ operational experience, causing him to know what a good aircraft needs to be able to do. Also, they are proud of having shown many transitions in their flight tests as opposed to some other manufacturers just showing hovering flight. Dufour also says they are cost efficient due to few mistakes made by the team which has a lot of seniority.
A recording of the event can be viewed on the VFS YouTube channel.