Swiss eVTOL aircraft developer Dufour Aerospace released the final design for its Aero 2 cargo drone today after successfully flying its third-generation prototype for the first time. The prototype, which Dufour calls the X2.2, made its first short hover flights at Dübendorf airfield near Zurich last week.

Dufour has been developing its Aero family of electric and hybrid-electric aircraft since 2015, starting with a fully electric aerobatic model called the Aero 1. The Aero 2, which is the first product that Dufour intends to bring to market, will be a hybrid-electric, uncrewed cargo freighter with a 40-kilogram (88-pound) payload, an anticipated range of 400 kilometers (216 nm), and a 92-knot cruise speed.

The planned Aero 3 model, Dufour’s flagship aircraft, will be a larger version of the Aero 2, capable of carrying up to 750 kilograms (1,650 pounds) of payload, or eight passengers plus a pilot, and will have a range of 551 nm (1,020 kilometers) with a hybrid-electric powertrain. Both the Aero 2 and Aero 3 feature a tiltwing design, with stationary propellers mounted onto the wing.

Aero2 X2.2 first take-off in Dübendorf

Dufour’s Unique Tiltwing Design

While the tiltwing is not exactly a new concept in aviation, this feature makes Dufour somewhat unique among eVTOL aircraft developers, most of which are working with fixed wings and tilting rotors. Dufour chief commercial officer Sascha Hardegger told FutureFlight that the company chose to use a tiltwing rather than tiltrotors because it’s a less complex approach with fewer moving parts. Whereas a tiltrotor aircraft has multiple rotors that must be actuated individually, with a tiltwing aircraft all the propellers are fixed to a single moving structure.

According to Hardegger, the tiltwing configuration also provides some aerodynamic advantages. During hover, when the wing is tilted all the way into a vertical position, the lift produced by the propellers creates a slipstream over the wing, “so this wing already wants to fly while the aircraft is still on the ground,” he said.

“With a tiltrotor approach, when you start activating your rotors, they produce lift, but they also produce downwash,” Hardegger explained. Downwash occurs when the air flowing from the rotors is deflected by the top of the wing, which pushes the wing down and reduces the efficiency of the motors. With a tiltwing, “you can immediately reduce the amount of energy needed by your propellers because the wing starts helping you very early to lift,” he said. “This is a big advantage of a tiltwing concept if you compare it to other concepts that are in the market.”

Aero 2 Passes Preliminary Design Review

A digital rendering of the final design of Dufour's Aero 2 — Following the completion of the preliminary design review for the Aero 2, Dufour released this digital rendering of the aircraft's final design. The company is now working toward the completion of the critical design review but does not expect significant changes to the airframe or other structures. (Image: Dufour Aerospace)

Hardegger said Dufour recently completed the preliminary design review for the Aero 2 model and is now preparing for the critical design review, during which the company will fine-tune the aircraft’s design using data from the test campaign with its latest prototype, the X2.2.

With the successful hover flight tests in the books, Dufour will spend the coming weeks working toward a full transition flight, in which the aircraft will transition from vertical lift to horizontal, fully wing-borne flight. The company has already achieved transition flights with its second-generation prototype, “but that's something you restart with every new prototype,” Hardegger said. “We need to see if X2.2 is behaving as we expected based on the data of X2.1.”

Compared with the earlier X2.1 prototype, the X2.2 has a larger fuselage and wingspan. The X2.2 also features an H-tail, while the X2.1 has a conventional tail. Dufour says it has also improved the hardware and software control systems architecture. According to Hardegger, the company plans to make at least one more Aero 2 prototype, the X2.3, before building the first production version of the aircraft.

While Dufour will conduct flight tests with the X2.2 on almost a daily basis, the company is also doing extensive ground tests with the hybrid-electric powertrain that will be installed on the next prototype. A key component of the powertrain is the 17.4-kilowatt TOA288 engine provided by Suter Industries. This engine will run on gasoline or avgas and work in tandem with the electric motors and batteries.

Dufour Aerospace: Introducing our hybrid propulsion system

Dufour says it is now ordering the materials it will need to build the X2.3 prototype. That prototype is expected to begin flight testing in 2024, and Dufour plans to start series production of the Aero 2 in 2025. The company expects to have the Aero 2 certified in Europe by 2024 under EASA’s rules covering light uncrewed air vehicles, with the expectation that the FAA will validate this approval for the U.S. market.

Initial certification efforts are based on the European aviation safety agency’s Specific Assurance and Integrity Level IV (SAIL IV) requirements for uncrewed air systems, with the scope to progress to the more advanced SAIL VI certification. “Once EASA has defined requirements and means of compliance for SAIL VI certification, Dufour plans to apply to this standard, which will enable operations over populated areas,” the company said in a statement.

In November, Dufour announced that its launch customer will be Spright, the drone division of major U.S. helicopter operator Air Methods, which aims to use the aircraft to transport medical supplies. Spright has agreed to purchase 40 of Dufour’s Aero 2 cargo drones with the option to add up to 100 additional units. According to Dufour, this deal represents the largest purchase of UAVs in U.S. history. A Dufour spokesman told FutureFlight that Spright’s order has already been backed with down payments, which is helping to fund the development process for the Aero 2 vehicle.

In addition to Spright, several other companies have committed to either operating or distributing the Aero 2 and Aero 3. In March, Swiss Helicopter AG signed a letter of intent for the purchase of two Aero 2 and one Aero 3 aircraft. Global commercial aircraft and helicopter marketing specialist Blueberry Aviation plans to be a distributor for Dufour's Aero 2 and Aero 3 vehicles and has agreed to acquire 100 of each model. Scandinavian rotorcraft sales and marketing group Savback Helicopters also plans to be a distributor for Dufour in northern Europe, while V-Star Powered Lift Aviation has expressed interest in operating the aircraft in Australia, and Tokyo-based AirMobility has agreed to purchase an unspecified number of the Aero 3.

Aero 2: Skids or Wheels?

Dufour’s aircraft is designed to have vertical takeoff and landing (VTOL) capabilities, but it could also take off and land on a runway like an airplane. Conventional takeoff and landing (CTOL) requires significantly less energy than vertical flight, and the tiltwing design also enables it to utilize shorter runways.

However, when Dufour starts manufacturing the Aero 2, the initial production version of the aircraft will have helicopter-style skids and no wheels for landing gear. This means that it will be able to take off and land only vertically like a helicopter, even though the rest of the aircraft is otherwise capable of short-takeoff-and-landing (STOL) and CTOL operations.

Hardegger said that Dufour opted to go with skids rather than wheels “for the sake of simplicity,” and because Spright intends to use the aircraft specifically for VTOL operations anyway. Wheeled landing gear adds more mechanical complexity and weight to the aircraft, and that feature won’t be necessary for Spright’s intended use cases. However, Hardegger added that wheeled landing gear is on Dufour’s development roadmap for future iterations of the aircraft. “As soon as we see that the market demands this—and this is to be expected—we will come up with the solution.”

This graphic depicts the landing of Dufour's planned Aero 3 VTOL aircraft, which will likely have wheeled landing gear rather than the skids featured in the initial production version of the smaller Aero 2 cargo drone. (Image: Dufour Aerospace)

It’s worth noting that when Dufour began developing the Aero 2, the company did not intend for it to become a product. Rather, the Aero 2—much like its predecessor, the Aero 1—was initially intended to serve as a technology demonstrator. As a proof-of-concept aircraft, it would help to validate the aerodynamics of Dufour’s tiltwing design and inform the subsequent development of the larger Aero 3 model. However, during the Aero 2 development process, several potential customers approached Dufour to express their interest in the smaller, uncrewed cargo drone, Hardegger said. So, Dufour decided to make the Aero 2 into a product while continuing work on the Aero 3.

“Everything we invest into Aero 2 also benefits the development and future development of Aero 3, so it's not a sort of detour,” Hardegger said. “It's really straight to the point, and the two aircraft are really very similar.” Making a product of the Aero 2 was also a logical financial decision, he explained, as having an early product on the market will allow Dufour to begin generating revenue sooner.

Hardegger says the Aero 2 will be Dufour's “principal strategy” for the next two years. In the meantime, the company will wait to decide exactly what it wants to make of the Aero 3 model. Originally Dufour intended for the Aero 3 to be a piloted, passenger-carrying eVTOL aircraft with eight seats. But now the company is also considering making it into an uncrewed cargo aircraft that would essentially be a scaled-up version of the Aero 2 cargo drone.

“At this stage, [both of] these options are clearly on the table, and we have the opportunity to see what markets do—how they evaluate, how they evolve, how the regulations evolve around all of this space—and then we can make a clear strategic decision,” Hardegger said. “We don't have to make this decision right now because we are still working toward both of these paths with the Aero 2.”

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