For fixed-wing eVTOL aircraft developers, making the transition from vertical lift to horizontal flight is one of the most crucial phases of flight testing.
In fact, it’s what some experts call the “highest risk” phase of flight. This is “due to changes of configuration, which involve aerodynamic characteristics, propulsion system performance, and control mechanics,” said an Aerofugia engineer. “On the contrary, the hovering and fixed-wing flight phases are actually very safe because of sufficient data, experience and standards.”
Sergio Cecutta, founder of SMG Consulting, noted that as of mid-June, mastering transition of a full-scale eVTOL has only been accomplished by five original equipment manufacturers (OEMs): Joby Aviation, AutoFlight, Beta Technologies, Archer Aviation, and Aerofugia. Wisk transitioned last year, but it will count, Cecutta said, when they transition their sixth-generation version.
Dufour Aerospace, with its X2.2 prototype of the uncrewed Aero2, and the now-defunct Kitty Hawk, with its Heaviside prototype, have also demonstrated transition with their much smaller models.
“Transition is a big milestone,” said Ron Epstein, securities research analyst at Bank of America and frequent aerospace industries commentator. “It must not only be demonstrated under perfect lab conditions but also under real-life conditions that are far less than optimal, including loss of power. And to demonstrate with a pilot is far more significant than remote operation.”
Of the full-sized eVTOL models currently seeking certification, only Beta has done it with a pilot onboard.
When asked about its achievement of full, manned transitions, Beta called it “a leap forward, as it provides us with additional data to support design decisions as we continue to work toward certifying our Alia-250 aircraft.”
“This is an entirely new flight regime,” said a company spokesperson, “so the team worked diligently for a long time, taking a step-by-step approach to close the gap between the many test points we’d completed in order to execute full, manned transitions.”
The achievement followed years of conducting transitions with sub-scale aircraft and use of the company’s simulator, “collecting a lot of data to ensure the safety and accuracy of our engineering as we worked up to this step. It’s been exciting to unlock this next phase of flight and be able to continue working toward bringing the technology to the market.”
In Epstein’s view, Beta is among the companies that have been more realistic than others when it comes to timelines to reach the transition milestone and other milestones, and he believes being realistic is better for everyone involved.
More realistic timelines benefit the industry in terms of maintaining investor confidence, keeping overall industry pressure to reach the commercial stage down to a reasonable level, and so on, he said.
Archer’s flight test journey
Archer successfully completed uncrewed transition in June with its Midnight aircraft, flying at a speed of over 100 miles per hour (160 kilometers per hour). At about 6,500 pounds (3,000 kilograms), Midnight is among the largest eVTOL aircraft to complete transition — achieving this milestone just seven months after Midnight’s first flight.
Archer had already successfully achieved transition of its first-generation full-scale Maker eVTOL aircraft in November 2022, 11 months after its first flight. It still flies regularly in the firm’s test program.
As the company inches closer to commercialization, Archer has already received its part 135 and part 145 certificates from the Federal Aviation Administration (FAA), and is in the final “implementation” phase of its type certification program for Midnight, the company said.
Archer said piloted flight testing is on track to begin later this year. Meanwhile, the Midnight test program includes flying simulated commercial routes to demonstrate the aircraft’s operational readiness, executing high-rate flight operations, and testing additional flight maneuvers that will be used in commercial settings. It will also continue to expand its speed and endurance flight envelope.
Archer’s chief engineer Geoff Bower said that while Midnight has progressed through its flight envelope extremely efficiently, “we are taking a very incremental approach to our flight test program with each new aircraft.”
Bower explained that the first challenge that all eVTOL developers face in working toward transition relates to the significant changes of flight dynamics through the transition maneuver.
“In other words, the rigid body dynamic modes go from being open loop unstable in hover to the typical stable aircraft modes — phugoid, short period, spiral, dutch roll and roll subsistence modes — in forward flight,” he said. “We have models of how the vehicle is expected to behave, but we take an incremental envelope expansion approach to clearing the transition envelope.”
Bower added, “we flew multiple flights at each 10-knot speed increments where we conducted system identification test points to quantitatively confirm the stability margins — gain and phase margins — in each control axis. This methodical approach gave us confidence in our models and allowed continued expansion to the next test point.”
At the program level, Bower said the biggest challenge was executing the transition envelope expansion campaign on the automated production prototype with a small team.
“The vast majority of the company stayed focused on our first manned production aircraft, which is on track to finish final assembly and begin piloted flight later this year.”
Three piloted Midnight aircraft will conform to the intended type design for FAA certification, the company said.
As reported by Archer in February 2024, component manufacturing is well underway and final assembly is occurring at the firm’s manufacturing facility in San Jose, California. This initial fleet of piloted aircraft will be used in “for credit” testing with the FAA as the company progresses toward its goal of commercialization in 2025.
Aerofugia aims for manned test flights in summer 2025
For Aerofugia, achieving unmanned transition started with hover testing. The company gradually expanded the envelope by smaller tilt angle and higher flight speed to gain a clear understanding of aerodynamics, electric propulsion characteristics, and structural response, among other characteristics.
“The entire transition flight was very precise and smooth, with the outer hovering propeller completely stopped, and the aircraft entered cruising solely relying on the forward propeller,” said a company engineer. “With more than 55 percent power margin, it easily reached level flight speed of 210 km/h [130 mph], then several maneuvers like acceleration, climbing, descending, loitering and successfully slowing down and transitioning to hover and then landing.”
Among the technical challenges to achieve transition in a tilting aircraft with a distributed electric propulsion (DEP) configuration are the complex flow field interference, tilting transition corridor design, flight control allocation, aerodynamic-structure-servo coupling, and continued flight in fault modes.
The AE200 engineering team “solved these problems one by one in a limited time.” Scientific methods, the engineer added, are essential for eVTOL research and development.
In the area of aerodynamic-structure-servo coupling, the engineer explained that while DEP technology brings higher safety margins and more flexible control methods for eVTOLs, it also causes a greater spatial mass dispersion and corresponding inertia. In addition, these aircraft use DEP along with aerodynamic control surfaces as the actuator for attitude control.
“Meanwhile, to maximize performance, eVTOLs need a really low structural coefficient,” the engineer explained. “These reasons lead to significant differences in the structural mode spectrum compared to traditional aircraft. There will be more low-frequency points and denser distribution of pitch, bending and torsion frequency points together. Additionally, the up and down vibrations of the rotor installed at the far end of the wing also generate additional angles of attack for the rotors during flight. These initial-elastic-aerodynamic forces will mutually excite each other and ultimately enter the servo loop through the fly-by-wire flight control and rotor speed closed-loop control, leading to the so-called aerodynamic-structure-servo coupling oscillation problem of the whole aircraft and local components during the whole flight profile.”
These coupled oscillations can affect the control quality of the aircraft and also lead to fatigue and shortened lifespan of the blades and structure.
“In response, our team used first principles, seeking the nature of the problem, and collaborated with aerodynamic, structural and control experts to work together to design and optimize, then conducted thousands of detailed CAE simulation models, hundreds of component and aircraft GVT/CT tests, and flight verification,” the engineer said. “Finally, without increasing the overall weight of the aircraft, we solved the problem and developed a complete set of requirements analysis, design, evaluation, experimental verification and compliance methods. We are currently writing technical standards and hope to help with the design of eVTOL with DEP in the future.”
The second example of a transition challenge is continued flight in fault modes. The engineer explained that since the AE200 is a piloted aircraft based on visual flight rules regulation, the design and validation of human-aircraft interaction are particularly important.
“Although our pilot currently uses a ground control station for the test flight, the entire control logic and emergency operations were the same as the future manned piloting way. Through failure mode and effects analysis of the onboard system for all stages of flight, we have identified hundreds of failure modes and their combinations. Preliminary analysis shows that in some cases, automatic emergency control strategies are better than manual way, while in others, it is safer to leave decision-making and control to pilots.”
To deal with this challenge the team introduced an innovative experimental evaluation method process based on systems engineering and safety analysis — Aerofugia’s own integrated simulation and verification platform.
With this platform, they executed 10,000 software-in-loop simulations, thousands of hardware-in-loop simulations, and hundreds of pilot-in-loop training and subscale flight experiments.
“We completed parametric stress, Monte Carlo, fault injection, expert experience cases for evaluation and verifications, thus verifying the design results to the maximum extent with the fastest speed and minimum resources, ensuring that we have qualified emergency operation procedures and sufficient confidence for the test flights,” said the engineer. “With the highest level of safety preparation, we completed dozens of tilt flight tests.”