These days computers handle a lot of the work for designing an airplane, but when it comes to seeing how well the new design will really handle in the air, there’s nothing like a real-life test in a wind tunnel. NASA’s National Transonic Facility is one of the best—and when the Agency’s engineers aren’t using it to test their own aeronautic innovations, private companies are sometimes able to use the available space by reimbursing NASA for time not already scheduled.
That’s just what Honda did when it was designing a new business jet. The plane body and its over-the-wing engine mount were designed after conducting “extensive research, using computational fluid dynamics, or CFD,” explains Honda Aircraft Company CEO and President Michimasa Fujino. “We found that if we put the engine at the optimum location relative to the wing, we find a sweet spot to reduce wave draft.”
But before Honda was ready to invest in a new company offshoot and product line, it needed to test design variables in real-world conditions. “In order to experimentally validate our concept, we were searching for a wind tunnel all over the world,” Fujino recalls. The National Transonic Facility, or NTF, at Langley Research Center, was the clear choice.
The NTF “was the first of its kind in the world, and there are only two like it in the world now,” explains Richard Wahls, senior technical advisor to the director of NASA’s Advanced Air Vehicles Program, where the facility is housed. Opened in 1983, the NTF aimed to help design the bigger airplanes that were being developed.
“In the 1960s and ’70s, there were always surprises during initial flight tests. For example, the aerodynamic loading might be different than expected if the shockwave over the wing was in a different location than expected,” Wahls says.
“But by that stage of the development process, you couldn’t start from scratch, so you had to figure out how to make a fix.” Those kinds of fixes tended to add weight to the plane or were in other ways non-optimal.
It wasn’t that nobody had wind tunnels. They did, including NASA. But none of them corrected for an important problem: the models of aircraft used for testing were much smaller than the real thing, often as much as 50 times smaller, while the molecules in the surrounding air were the same size.
“There’s a certain kind of spacing, which you can’t see with your eye, characteristic to air,” Wahls says. That air moves across the curves of an aircraft in specific ways. “But the air will act differently over that small-scale model, because the molecules are spaced the same way, but now they’re flowing over a smaller geometry.”
The mathematical term that expresses that relationship is called the Reynolds number. To partially correct for these Reynolds-number disparities, the NTF, like several other wind tunnels, is pressurized, so the air can be compressed proportionally to the scale of the aircraft. However, to simulate large aircraft, this pressurization is not enough to match flight conditions. To finish the job the NTF can also pump in pure nitrogen gas at temperatures as low as -250 °F, which further increases air density to simulate the real conditions airplanes experience at altitude.
NASA has used the NTF for many projects, including foundational research on how air flows over simple shapes at different Reynolds numbers, which can improve computational models. The Agency has also tested innovations in aircraft design there—some of which, like the winglet, have become standard in modern jets.
Today NASA uses the NTF to test advanced aircraft concepts like the blended wing body, a flattened, triangular design that could one day replace the more familiar airplane shape of a tube fuselage with wings sticking out.
When the Agency doesn’t need the NTF, private companies can step in to take advantage. These contracts can be cooperative, where NASA and the company share the costs and the resulting data, or fully reimbursable, where the company pays for the wind tunnel time in full and retains full rights to its data.
Honda Aircraft negotiated the second type of contract, Wahls says. Fujino and his team had already mostly designed their aircraft but needed to do final testing.
“In order to have both high Reynolds and high Mach number conditions simultaneously, NASA is the best facility to give that condition,” Fujino remembers. “And also NASA’s wind tunnel gave us very high accuracy of test results because its instruments are capable of very sophisticated tabulation and data collection.”
The team worked with NASA wind tunnel experts in advance to ensure their model, 10 percent the size of a full-scale jet, was properly designed to withstand the highspeed testing and could interface with all the instruments. Then they underwent a week of testing, from which they gathered “several tons of data from the very efficient NASA experiment process,” he says.
Fujino says the test allowed him to choose the best among several options for the fairing of the over-the-wing engine mount configuration, but mainly it proved the existing design worked as expected.
“Because I could confirm my concept from test results, I had more confidence to go into the commercial phase,” Fujino says. “The experiment was proven: not only theoretically proven but experimentally proven.”