Spanish company describes its recipe for a smoother-riding, longer-range air taxi

By Paul Brinkmann|September 19, 2024

Flights of six-seat prototype are scheduled to start in 2026

Among the many emerging airframe designs in the fledgling electric air taxi industry, it’s unusual to come across a claim of fundamental uniqueness.

Crisalion Mobility, an 8-year-old company located in Madrid, is attempting to beat other multirotor designs on range while beating tiltorotor designs on smoothness of the ride by isolating the airframe from unwanted forces produced by its rotors.

The company propelled a two-seat demonstrator with this patented FlyFree propulsion structure during a series of remotely piloted flights. The airframe was connected to the rotor assemblies with passive joints, meaning they didn’t have actuators like those that would shift the position of propellers on a tiltrotor. These joints flexed in a manner similar to shock absorbers. Control of the aircraft was derived from groups of four rotors, with each group mounted on a frame. The rotors’ speed of rotation was adjusted to flex the passive joint and position the rotor groups to either make the aircraft climb, descend or fly horizontally.

Specifically, FlyFree consists of two booms perpendicular to a main wing, each boom with one of the rotor assemblies on each end. The four rotors in each group act as independent quadcopters, which the company calls UPMs, or unit propulsion motors. The passive joints that connect the UPM assemblies to the booms absorb unwanted force from the rotors to produce a smooth ride for those in the passenger cabin. Each UPM can shift its orientation independently of the others because of the flexible passive joints that support each UPM.

Having flown the demonstrator, the company is now building a winged, six-seat piloted electric air taxi called Integrity that will be propelled by the FlyFree method. Crisalion expects to fly this production prototype by 2026. The goal is to have Integrity in commercial service by 2030 after certification by the European Union Aviation Safety Agency.

The aircraft achieves “exceptional safety, stability and agility in all phases of flight, even in adverse weather conditions,” the company claims.

FlyFree was developed by Tecnalia, a nonprofit research and development center based in Donostia, Spain. Crisalion, then called UMILES Next, bought the rights to the technology and patented it in 2019. Crisalion has received funding from the Spanish Ministry of Science, Innovation and Universities. Additional investors include two wealthy Spanish families.

Oscar Rapp, Crisalion’s chief operating officer, tells me the problem with most electric air taxis is that vibrations are passed to the airframe and to the passenger cabin by the rigid connections between the airframe and the propulsion units. This results in a bumpy ride for passengers.

While several companies are developing tilt-propeller air taxis, Rapp pointed out that the only tiltrotor design in operation today is the V-22 Osprey, which is certified only for military use. He noted that the Osprey has repeatedly been grounded after accidents, most recently in 2023 after a crash off the coast of Japan that killed eight U.S. Air Force airmen.

“The tiltrotor or tilt-propeller design, we don’t like it too much. Its complexity causes problems for certification,” Rapp said. “What Crisalion has instead is a passive joint, a less complex structure, so that we are able to control the aircraft in a much simpler way. We are minimizing the coupling between the motors, the propellers and the cabin.”

He continued: “And this is important, because when you have somebody inside, you will be able to correct for wind gusts or turbulence, just by moving your power plants” — the UPMs — “and minimizing the movement of your cabin.”

Rapp said each UPM can adjust immediately to counter or control for turbulence in the atmosphere. “What happens in a normal plane or helicopter is that your engine is moving together with the cabin. So when you have turbulence, the whole aircraft is going to shake up and down,” he said.

But the Flyfree structure, due to its passive joints, will respond to turbulence without transmitting it to the cabin, he said. “So we think that we are really solving a problem from the point of view of passengers and potential customers — they will have a much more pleasant experience because they are not going to be moving up and down, shaking inside.”

Researchers from Tecnalia published several papers describing FlyFree. One of them states that the structure enables “independent control of the six degrees of freedom of the airframe without having fixed propellers at inefficient configurations or making use of dedicated rotor-tilting actuators.”

The production model of Integrity will be 3.5 meters high with a 15-meter wingspan, and have a range of 130 kilometers at speed of 180 kph, according to Crisalion.

U.S. observers of the industry that I spoke to said they were intrigued by the FlyFree concept but wanted to know more about it.

Mike Hirschberg, director of strategy for the Vertical Flight Society, the Virginia-based non-profit technical society, said he doesn’t know what to make of the boom structure on Integrity, and he’s hoping Crisalion will present at an upcoming meeting of the group.

Daniel Smith, a helicopter pilot and assistant professor of aviation at University of Nebraska at Kearney, reviewed papers about FlyFree at my request. He said he was intrigued and would like to know more about Integrity.

Smith said the structure would eliminate the need for actuators, but he thought some kind of motor control would still be required for the joints connecting the rotors to the airframe.

“This is definitely an interesting concept. I am not sure that it will increase stability, nor am I sure of the efficiency and durability of this design,” he said. “But we haven’t seen many advances in rotorcraft technology, so this would be an exciting technological improvement.”