With backing from KLM and Airbus, the Technical University of Delft in the Netherlands has tested its Flying-V concept for an ultra-efficient airliner by conducting the first flight of a subscale model.
Built to test low-speed stability and control and powered by two small electric ducted fans, the 4.7%-scale, 10-ft.-span model made the five-minute remotely piloted flight in July at an air force base in Germany.
The Flying-V is a V-shaped concept aircraft that has the same wing span and passenger capacity as the Airbus A350 but is predicted to use 20% less fuel. This is because the aircraft has less surface area, resulting in lower drag.
The design is a tailless flying wing. Instead of a single wide, twin-aisle cabin, the Flying-V has two A320-size single-aisle cabins integrated into the V-shaped airframe along with the cargo holds and fuel tanks.
Despite stability issues which culminated in the model being damaged on landing, “overall, we can say that this was a successful first flight test,” said Roelof Vos, Flying-V project leader at TU Delft.
The primary goal of flying the subscale model was “to convince ourselves and the world that this is not just a paper airplane, and can not only fly but also take off and land,” he said. The second goal was to collect data on the flight dynamics of the aircraft for use in the remainder of TU Delft’s Flying-V project.
The concept originated at Airbus in Germany and the manufacturer continues to be involved in the project with an eye to the configuration’s potential as a future sustainable long-haul airliner.
“If we are required to think about disruptive changes in our energy infrastructure for aircraft, something that is not drop-in like sustainable kerosene but a different energy carrier like hydrogen, we would need to architect the aircraft differently,” said Daniel Reckzeh, senior manager for research and technology at Airbus.
“The tanks would look different. They would need to be integrated into the aircraft in a completely different way,” Reckzeh said. “The Flying-V is showing interesting degrees of freedom, where a combination of the need to think about the integration of novel energy carriers and disruptive concepts may open up different paths to take.”
The Flying-V model proved controllable on the ground and during takeoff. The aircraft took off at 80 kph (43 kt.), rotating to a high angle of attack and climbing steeply at first. “You could see a strong climb. It rotated quite fast, but it never stalled. And that was also something we were wondering about. In the air, it was controllable and responsive to input on the throttle and control surfaces,” Vos said.
“But as with any test, not everything goes perfectly. So there were things that could still be further improved,” Vos said. The center of gravity of the model proved to be too far rearward. This is important for stability. “You could see that the airplane was neutrally stable or maybe a little bit unstable in pitch. And that meant the pilot had to work hard to keep the airplane flying steadily,” he said.
The second issue, predicted by TU Delft’s modeling, was that the aircraft “wobbled a bit,” a combination of roll and yaw called Dutch roll. “It is not an uncommon phenomenon in airplanes. We predicted it and we saw it in flight. It’s also something that we can cure with system like a yaw damper,” Vos said.
“Because of this the airplane was a little bit unstable. The pilot had to put some work in to keep it level, and it was difficult to land,” he said. Instead of the main gear, it touched down on its nose gear, which broke off during the landing. The damage will be repaired, and the model is expected to fly again in 2021.
The data collected during the short flight remains to be analyzed and incorporated into TU Delta’s flight dynamics model for the unconventional Flying-V. “The good thing about this flight is that the amount of controlling the pilot put into this airplane means that we have a lot of data to work with,” Vos said.
“But the first thing we see immediately is that the central gravity position needs to be shifted forward. The second thing is that the landing gear probably also needs to shift forward a bit,” he said.
“This is a very unconventional airplane, so we cannot rely on any of the statistical data that we have from tube-and-wing airplanes to predict how this will behave in the air,” Vos said. “We are quite confident that even with this five-minute test we can already build a decent flight dynamics model.”
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