A radically designed "flying wing" remotely piloted aircraft at NASA's Armstrong Flight Research Center in California, inspired by birds in flight, is getting ready for a new round of testing that could help to a major rethink of aircraft design to increase fuel efficiency and emissions control.
The Prandtl-D No.3 is being equipped for new test flights this summer, following work over the past three years. Engineers working on the project say fuel efficiency could improve by up to 11% thanks to the aircraft's unique profile.
The wing design was originally proposed by German engineer Ludwig Prandtl in the 1930s. It is flat at the centre, and then it tapers to the tip and flattens – the curve is absolutely flat when it gets to the wing tip. This is known as a bell-shaped span load due to the shape of the curve.
Previous tests have shown that this can provide thrust, rather than drag, at the wingtips, removing the need for a tail and improving efficiency.
"The bell-shaped span load pushes the air down in the center. But in the wing tips, the flow goes up, and you have upwash at the wing tips," explains Al Bowers, Prandtl-D project manager and NASA Armstrong chief scientist.
"This is what birds have been doing naturally. That’s exactly what the load needs to be."
In fact Bowers has taken much of his inspiration from watching birds, such as the albatross, in flight.
The design of his wing does away with the need for a tail – exactly as birds have done, he says. "The result is greater efficiency and the potential to reimagine airplane design."
Testing on the Prandtl-D series has also shone light on potentially more efficient ways for aircraft to fly in formation.
Aircraft with a standard elliptical spanload achieves the best efficiency in formation flight by trailing one wingtip directly behind another, whereas birds fly in formation with their wingtips overlapped. Prandtl-D has demonstrated that the overlapping of birds in flight is the optimal aerodynamic answer.
But still Bowers's ideas have been slow to catch on, as thrust at the wingtips goes against traditional aeronautical design.
"No one has criticised the math in our paper," Bowers said of peer reviewers. "No one has found any sort of defect in the logic.
"The revolutionary part of this is it allows aircraft designers to completely eliminate the tail on an aircraft and you end up with a flying wing. All of the problems that have traditionally been associated with flying wings, and the reason we put tails on airplanes appear to be solved by doing this.
"The Prandtl-D solution contends that once you pick a certain size of aircraft that the purely aerodynamic answer is no longer sufficient. The amount of payload the airplane is going to carry and the amount of structure necessary to carry that payload changes the answer."
But Bowers says the new research does not preclude traditional methods.
"There are still many situations where the old solution is the correct one," he says. "If you have a certain constraint on wingspan, the old solution is the right one. Very large aircraft that barely fit within the current infrastructure we have would still want to solve those problems the exact same way we are now."
The NASA technical publication is entitled, "On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds," NASA/TP – 2016-219072 and you can read an interview with Bowers about the design here.
Bill Condie is a science journalist based in Adelaide, Australia.
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