NASA's AD-1 slant-wing research aircraft performs a wing-sweep test flight.
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(CNN) --
There aren't many planes that can claim to be truly one-of-a-kind, but NASA's AD-1 is certainly one of them.
A slender, pointed cigar with a single wing that rotates on a central axis, causing surprising asymmetry in flight.
Another piloted aircraft with a wing capable of turning 20 to 2 has never been built, but what's fascinating is why it was invented in the first place.
The concept is known as a "skew wing", a subset of the "variable sweep wing" or "pencil wing".
The idea has been around since the 1940s, but it wasn't until a NASA project in the 1970s that the technology was put to the test.
It successfully demonstrated that the oblique wing concept had potential for the development of highly efficient supersonic passenger aircraft as well as military applications, but more than 40 years after the experimental aircraft last flew, there have been no others to follow its lead. example.
Its inventor, aeronautical engineer Robert T. Jones of the NASA Ames Research Center in California, was a pioneer who wanted to challenge convention.
"One of the tacit assumptions in aircraft design is that of bilateral or mirror symmetry," he wrote in 1972 in a scientific study on oblique wings.
The idea that a pivot wing would lead to better supersonic aircraft was "surprising", he admitted, but he hoped he could demonstrate its merits.
Before building the AD-1, Jones tested a model in a wind tunnel.
The results showed that a supersonic aircraft with a slant 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 longer range.
With this encouraging data, Jones secured the necessary funding to go full-size.
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79 flights
The AD-1 was a modest budget device, costing about $240,000 in total, or just under $1 million today.
The number was so low that some agency staff thought they were approving a remote-controlled aircraft, rather than one with a pilot, as Bruce I. Larrimer recounts in "Thinking Obliquely," a NASA book about the AD program. -one.
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It was designed by aviation legend Burt Rutan, known for his bold and often maverick creations.
At just over 11.5 meters long, the single-seater aircraft sat comically low to the ground due to short landing gear optimized to reduce aerodynamic drag and was only 2 meters tall.
It was powered by two small turbojet engines and its top speed was only about 320 km/h, for the sake of safety.
Above all, it was lightweight, with an empty weight of less than 680 kg thanks to a fiberglass-reinforced plastic structure.
It had no hydraulic system.
Its main structural curiosity, the pivot wing, was attached to the fuselage just forward of the engines and was 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 activated only during the cruise flight, and in slow increments throughout the program's 79 flights.
Multiple exposure image showing wing movement on the AD-1.
Credit: NASA
proof of concept
The plane took off for the first time on December 21, 1979, with NASA research pilot Thomas McMurtry at the controls: "I was anxious to see how it would perform," says Christian Gelzer, chief historian at the Armstrong Flight Research Center. from NASA.
"The wing could pivot back [to the traditional] 90 degrees to the fuselage in order to land, and he found that you would have to do a very smooth, slow descent, but you would get what you needed and nothing would happen."
The maximum wing sweep, 60 degrees, was reached in April 1981, after which the aircraft flew for another year of testing.
All pilots involved in the program were asked to rate their handling, and the general consensus was that the AD-1's performance was acceptable up to 50 degrees of sweep, or just below maximum.
There was some degradation from there, described by NASA as "unpleasant flight characteristics and poor handling qualities", but which the agency believed could have been improved with more sophisticated materials and construction.
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Most important, however, was to demonstrate 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 NASA experimental aircraft, in the sense that how it behaved was less of a concern compared to whether or not it did what it was supposed to do."
Robert T. Jones poses with the AD-1.
Credit: NASA
oblique future
During the program, Boeing and Lockheed conducted design studies on possible supersonic airliners with a raked-wing design, to be ready to build one when the AD-1 had proven the concept.
One of the proposed planes, the Boeing 5-7, could carry 190 passengers and fly at Mach 1.2, faster than sound, using four turbofan engines.
It would have been 87 meters long, with a wingspan of 61.5 m in the ungoverned position, reducing to 40 m at maximum sweep.
But the Boeing 5-7 never got beyond an idea on paper, and neither did any other oblique-wing aircraft, except for the AD-1 itself, which made its last flight back in 1982. The reason is that a pivot wing it was too complicated mechanically compared to simply configuring 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, with an angle optimized for faster-than-sound travel.
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Some military aircraft, such as the B-1B Lancer of the 1980s or the F-111 Aardvark of the 1970s, had variable geometry wings, which stayed fully open at subsonic speeds and then pivoted closer to the fuselage when flying supersonic, offering the best possible handling and fuel efficiency.
But its complicated engineering and moving parts added complexity, weight, and the possibility of mechanical failure: "In the case of the F-111, there were two gigantic titanium gears that moved the wings. Titanium is expensive, difficult to work with, and heavy." Gelzer says.
The AD-1, with a single pivot wing instead of two, was partly intended to achieve the same advantages with fewer complications, but ultimately still failed to beat a simple swept-wing design: "No one builds [low-geometry] planes anymore." variable], even if they try to reach supersonic speeds: they just sweep the wings and make them fly like that. It may not be as efficient as you want, but it saves you the headache of the mechanism and the weight," adds Gelzer.
In the end, the AD-1 program proved to have potential, but not enough to justify investment in a complicated system that modern design had rendered redundant.
However, the data collected during those 79 flights has been useful, and we cannot rule out that it will be useful again in the future.
"I would never say that the concept is never coming back," says Gelzer.
"But I don't see the app right now, because we have a way around what we were trying to fix."
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