(CNN) — There aren’t many airplanes that can truly be said to be one-of-a-kind, but NASA’s AD-1 is certainly one of them. A thin, pointed cigar with a single wing rotates about a central axis, causing surprising asymmetry in flight. Another pilot aircraft capable of turning 20 to 2 has never been built, but the fascinating thing is why it was invented in the first place.
This concept is known as “Skew Wing”, a subset of “Variable Sweep Wing” or “Pencil Wing”. The idea has been around since the 1940s, but it wasn’t until the 1970s that a NASA project put the technology to the test.
It successfully demonstrated that the pitch-wing concept had the potential to develop highly efficient supersonic passenger aircraft and military applications.
Its inventor, aeronautical engineer Robert D. of the NASA Ames Research Center in California. Jones was a pioneer who wanted to challenge convention. “One of the implicit assumptions in aircraft design is bilateral or mirror symmetry,” he wrote in a 1972 scientific review of swept wings. He admitted that the idea that a pivoting wing could lead to better supersonic flight was “surprising” but hoped he could prove its worth.
Before developing the AD-1, Jones tested a prototype in a wind tunnel. The results show that a supersonic aircraft with a swept wing can have twice the fuel economy of a traditional wing. It produces less noise during take-off, has a quieter boom and longer range. With this encouraging data, Jones secured the funding needed to go full scale.
The AD-1 was a modest budget device, costing around $240,000 in total, or less than $1 million today. As Bruce I. Larrimer describes in “Thinking Obliquely,” the NASA book about the AD program, some agency staff considered approving remote-controlled aircraft rather than manned aircraft.
The design was carried out by the aviation legend Burt Rutten, known for his bold and often controversial works. At just over 11.5 meters long, the single-seater aircraft sat comically on the ground thanks to the narrow landing gear optimized to reduce aerodynamic drag and was only 2 meters high. It was powered by two small turbojet engines and its top speed was limited to 320 km/h for safety reasons. Above all, it was lightweight, with an empty weight of less than 680 kg, thanks to its fiberglass-reinforced plastic construction. It has no hydraulic system.
Its main structural interest, the pivot wing, was attached to the fuselage just forward of the engines and powered by electric motors activated by a switch in the cockpit. During takeoff and landing, the wing is always neutral or vertical. It was implemented only during the cruise, and in slow increments throughout the program’s 79 flights.
Proof of concept
It took off for the first time on December 21, 1979 with NASA research pilot Thomas McMurtry on board. NASA says “the wing can retract [a los tradicionales] 90 degrees to land, and found that you have a very smooth, slow descent, but you get what you need and nothing happens.”
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 project were asked to rate their handling, and the general consensus was that the AD-1’s performance was up to a 50-degree sweep or less at most. There have been some degradations, described by NASA as “undesirable flight characteristics and poor handling qualities”, but the agency believes that could have been improved with more sophisticated materials and construction.
Most important, however, was proving that the plane could fly safely and with low drag, confirming Jones’ wind tunnel results: “The theory worked,” says Kelzer, “and the AD-1 was similar to other NASA test flights. In terms of how it behaved, it did what it was supposed to do.” It’s not a concern compared to whether or not it’s done.”
During the program, Boeing and Lockheed were conducting design studies of supersonic aircraft with a rake-wing design, to be ready to build one when the AD-1 proved the concept.
One of the proposed aircraft, the Boeing 5-7, could carry 190 passengers, use four turbofan engines and fly at Mach 1.2, faster than sound. It would have been 87 meters long, with a wingspan of 61.5 m unruled, reducing to 40 meters at maximum sweep.
But Boeing went beyond the 5-7 as an idea on paper, and never made any other swept-wing aircraft except the AD-1, which made its last flight back in 1982. Because it was a central division. Mechanically more complex compared to the simple configuration of wings for supersonic speeds and compromising low performance during subsonic flight. This design could take the form of a delta wing – the triangular shape used by Concorde among others – or a swept wing with an angle optimized for supersonic travel.
Some military aircraft, such as the B-1B Lancer of the 1980s or the F-111 Aardvark of the 1970s, had wings of varying geometry, which were fully open at subsonic speeds and then retracted closer to the fuselage during supersonic flight. Potential handling and fuel efficiency. But its complicated engineering and complex moving parts added weight and potential for mechanical failure: “In the case of the F-111, there were two gigantic titanium gears to move the wings. Titanium is expensive, difficult to work with, and heavy.” Kelzer says.
The AD-1, with a single pivot wing instead of two, was intended to achieve the same advantages with fewer complications, but ultimately failed to beat the still simple swept-wing design: “Nobody makes airplanes anymore. [de geometría variable], even if they try to reach supersonic speeds: they sweep the wings and make them fly like that. It may not be as efficient as you’d like, but it saves the headache and weight of the mechanism,” says Kelzer.
Ultimately, the AD-1 program proved capable, but not enough to justify the investment in a complex system where modern design was unnecessary. However, the data collected during those 79 flights was useful and we cannot rule out that it will be useful again in the future.
“I would never say that the concept won’t come back,” says Kelzer. “But I don’t see the application right now because we have a way around what we tried to fix.”
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