Why NASA tested a plane with a pivoting wing

By Jacopo Prisco, CNN

Not many aircraft can claim to be truly one of a kind, but the NASA AD-1 is undoubtedly one of them. A slim pointed cigar with a single wing that pivoted around a central point, which led to a jarring asymmetry in flight. No other piloted plane has ever been built with a wing that could turn to a lopsided 20 to 2 angle — but the fascinating story is why it did at all.

The concept is known as an “oblique wing,” a subset of the “variable-sweep wing” or “swing wing.” The idea has been around since the 1940s but it wasn’t until the NASA project came along in the 1970s that the technology was put to the test.

It successfully proved that the oblique wing concept had potential for the development of highly efficient supersonic passenger planes, as well as military applications, but more than 40 years after the experimental plane last flew, there have been no others to follow suit.

Its inventor — aeronautical engineer Robert T. Jones from NASA’s Ames Research Center in California — was a pioneer who wanted to challenge conventions. “One of the unspoken assumptions in aircraft design is that of bilateral or mirror symmetry,” he wrote in a 1972 scientific study on oblique wings. The idea that a pivoting wing would lead to better supersonic planes was “surprising,” he admitted, but he hoped that he could demonstrate its merits.

Before building the AD-1, Jones tested a model in a wind tunnel. The results showed that a supersonic plane with an oblique wing would have twice the fuel economy of a traditional wing. It would also make less noise during takeoff, have a quieter sonic boom, and have an increased range. With this encouraging data, Jones obtained the funding to go full size.

79 flights

The AD-1 was a frugally budgeted craft, costing about $240,000 in total, or just under $1 million in today’s money. The figure was so low that some of the agency’s personnel thought they were approving a remotely controlled plane, rather than one with a pilot — as Bruce I. Larrimer narrates in “Thinking Obliquely,” a NASA book about the AD-1 program.

The design was by aviation legend Burt Rutan, known for his bold and often maverick creations. Just over 38 feet long, the single-seat aircraft sat comically low to the ground — due to a short landing gear optimized for less drag — and was just 6.75 feet high. It was powered by two small turbojet engines and its top speed was only around 200 miles per hour, in the interest of safety. Most of all it was light, with an empty weight of under 1,500 pounds thanks to a structure made of plastic reinforced with fiberglass. There were no hydraulics at all.

Its main structural curiosity, the pivoting wing, was attached to the fuselage just in front of the engines and powered by electric motors activated by a switch in the cockpit. During takeoff and landing, the wing was always in the neutral, or perpendicular, position. It was only activated during cruise, and in slow increments over the course of the program’s 79 flights.

Proof of concept

The plane first took to the air on December 21, 1979, with NASA research pilot Thomas McMurtry at the controls: “He was anxious about how it would behave,” says Christian Gelzer, chief historian at NASA’s Armstrong Flight Research Center. “The wing could pivot back [to the traditional] 90 degrees to the fuselage to be able to land, and he found out that you’d have to do a very gentle, slow descent, but you’d get what you needed and it would be okay.”

The wing’s maximum sweep of 60 degrees was reached in April 1981, after which the aircraft was flown for another year of further testing. All pilots involved with the program were asked to rate its handling, and the general consensus was that the AD-1’s performance was acceptable up to 50 degrees of sweep, or just shy of the maximum. Some degradation occurred beyond that — described by NASA as “unpleasant flying characteristics and poor handling qualities” — but which the agency believed could have been improved with more sophisticated materials and construction.

What mattered most, however, was proving the concept that the plane could fly safely and with reduced drag, confirming Jones’ wind tunnel results: “The principle worked,” says Gelzer, “and I think the AD-1 was like other experimental NASA planes, in that how it behaved was less concerning compared to whether or not it did what it was meant to do.”

Oblique future

During the program, Boeing and Lockheed conducted design studies on potential supersonic passenger planes with an oblique wing design, to be ready to build one by the time the AD-1 had proved the concept.

One proposed plane, the Boeing 5-7, could carry 190 passengers and cruise at Mach 1.2, faster than sound, using four turbofan engines. It would have been 287 feet long, with a wingspan of 202 feet in the unswept position, reducing to just 130 feet at maximum sweep.

But the Boeing 5-7 never progressed beyond an idea on paper, nor did any other oblique-wing plane except the AD-1 itself, which made its final flight way back in 1982. The reason is that a pivoting wing was just too mechanically complicated compared to simply shaping the wings for supersonic speeds and accepting the compromise of lower efficiency when flying subsonic. This design could take the form of a delta wing — a triangular shape used by Concorde, among others — or simply a swept wing, at an angle optimized for faster-than-sound travel.

Some military planes, such as the 1980s B-1B Lancer or the 1960s F-111 Aardvark, had wings with variable geometry, which would stay fully open at subsonic speeds, and then pivot closer to the fuselage when flying supersonic, offering the best possible handling and fuel efficiency. But their complicated engineering and moving parts added complexity, weight and the possibility of mechanical failures: “In the case of the F-111, there were two gigantic titanium gears moving the wings. Titanium is expensive, hard to work with and heavy,” says Gelzer.

The AD-1, with just one pivoting wing rather than two, was in part meant to achieve the same benefits with less complication, but ultimately still wouldn’t best a simple swept wing design: “Nobody builds [variable geometry] airplanes anymore, even if they’re trying to go supersonic — they just sweep the wings and they fly it that way. It may not be as efficient as you want it, but it saves the headache of the mechanism and it saves the headache of the weight,” Gelzer adds.

In the end, the AD-1 program showed potential, but not enough to warrant investing in a complicated system that modern design had rendered superfluous. However, the data gathered during those 79 flights has been useful — and we can’t rule out that it could become useful again sometime in the future.

“I would never say that the concept is never going to come back,” says Gelzer. “But I don’t see the application right now, because we’ve got a way around what we were trying to fix.”

Top image: The NASA AD-1 oblique wing research aircraft performs a wing sweep test flight. (NASA).

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