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The NASA AD-1 in flight with its wing swept at 60 degrees, the maximum sweep angle.

Flying at an Oblique Angle: The NASA AD-1

By Mark Wolverton
3/9/2017 • Aviation History Magazine

NASA’s AD-1 proved that oblique-wing aircraft are practical, despite their odd appearance.

In a century of powered flight, airplanes have varied in size and shape from Sopwiths to 787s, from Cessnas to C-5s, from F-35s to Super Guppys. But almost all had one thing in common: bilateral symmetry. In other words, the left side of the aircraft is a mirror image of the right. We’re so used to the idea that it seems completely natural and logical, and anything else just looks wrong. After all, how could a plane possibly fly with, for example, a wing skewed at some radically oblique angle?

But it can, as demonstrated by one of the most influential and creative aeronautical engineers of all time, Robert Thomas “R.T.” Jones. For almost three years in the early 1980s, the NASA AD-1 aircraft that he proposed and helped design and test was plying the skies, mostly over Dryden Flight Research Center in California. The AD-1 was nothing flashy, just a humble affair of fiberglass and plastic with two tiny turbojet engines that could take it to speeds of about 180 knots and altitudes of roughly 15,000 feet. What made it unique was its wing: a single airfoil that could pivot on the fuselage from zero to a full 60 degrees in flight.

The AD-1 remains the first and only manned aircraft to fly with an oblique wing, a concept so radical and counter intuitive that decades passed from its inception on paper to realization in flight. And despite its proven advantages and superiority for certain aeronautical purposes, it has yet to fully catch on, even with 21st-century technical advances that make it even more practical and feasible.

For a visionary engineer such as Jones, the idea of an oblique wing evolved naturally from the sweptwing concept, which he hit on while working at NACA’s Langley Aeronautical Laboratory during World War II. As military aircraft became faster and more powerful, aerodynamic problems multiplied, causing pilots to lose control as they approached the speed of sound. Jones calculated that abandoning the straight wing perpendicular to the fuselage in favor of sweeping each side of the wing back toward the tail would greatly reduce drag and increase efficiency at transonic speeds.

Jones’ first paper on the sweptwing was rejected by NACA as too far out. He wasn’t vindicated until the war in Europe ended, when it was found that German engineer Adolf Busemann had proposed the sweptwing as far back as 1935. Further analysis and experiments confirmed the sweptwing theory, and most historians give Busemann and Jones shared credit.

It’s unclear whether the oblique-wing concept actually originated with Jones. German engineer Richard Vogt proposed the first known design for such a craft, the Blohm & Voss P.202, in 1942, though that airplane was never built or tested. But as Jones continued to work on the sweptwing into the 1950s, the oblique wing seemed to him to be a natural progression. After all, since wings swept at 45, 50 or 60 degrees were so efficient at high speeds, why not simply sweep them back completely—or more specifically, pivot the entire wing so that it could be adjusted for different flight regimes? He envisioned a supersonic passenger aircraft with an obliquely pivoting wing resulting in increased speed and range while using less fuel.

He knew it was a wild idea. “Artifacts created by humans show a nearly irresistible tendency for bilateral symmetry,” he admitted wryly in a paper. Yet he was sure it would work. When skeptics pointed out that there were no birds with oblique wings, Jones would agree, then counter that birds don’t fly at supersonic speeds.

Jones tested the notion with wind tunnel models at Langley and published some papers, but his vision inspired only theoretical interest in the late 1940s. As supersonic flight came of age, however, any crazy idea that could reduce drag and increase aerodynamic efficiency past Mach 1 became of practical interest. In 1958 supersonic wind tunnel testing at NASA Ames Research Center (where Jones was transferred after WWII) proved the oblique wing was much more efficient than even the now-common sweptwings at transonic and especially supersonic speeds. He presented the idea at the first International Congress in the Aeronautical Sciences later that year, and several designers, notably Sir Godfrey H. Lee of Handley-Page Ltd., began toying with oblique-wing aircraft concepts.

But the inherent oddness of the design made it anathema to more conventionally minded engineers. When the restless Jones left aeronautics for several years in the early 1960s to work on other ideas (including research for the first artificial heart), the oblique wing languished, seen as little more than an intriguing curiosity.

By 1970, however, Jones was back at Ames working on high-speed aerodynamics, including the oblique wing. He spent the next few years studying various configurations, performing more wind tunnel experiments, writing more papers, conducting computer simulations and building radio-controlled models. Finally it was time to take the next step by actually building a manned test aircraft.

Dubbed the NASA AD-1 (Ames-Dryden-1), it would be a modest, low-cost affair as experimental aircraft go. The overall configuration was based on an oblique-wing transport plane study by Boeing, along with detailed design and load analyses by Burt Rutan. The one-of-a-kind craft was built by the Ames Industrial Company of upstate New York (no relation to NASA Ames) for just under $250,000, and delivered to NASA Dryden in February 1979. Although Jones’ research had shown the

oblique wing could be permanently fixed at a given angle, the AD-1 used a pivoting wing so tests could be conducted in various configurations. With its 2,100 pounds resting on fixed tricycle landing gear, the airplane was remarkably tiny, just over 38 feet long with a wingspan of barely 32 feet, constructed of lightweight fiberglass-reinforced plastic. Two Microturbo TRS-108-045 engines, each producing slightly more than 200 pounds of thrust, were mounted on either side of the mid-fuselage, just under the pivoting wing. There was only the bare minimum of instrumentation and electrical system, and no hydraulics or ejection seat.

Unlike many of the famous experimental planes that had flown over the Southern California desert in recent decades, the AD-1 was not about to break any records. But Jones would be happy as long as it achieved its purpose: to prove the vi ability of the oblique wing on a manned aircraft and to give pilots a chance to explore its handling and control qualities. On December 21, 1979, NASA test pilot Thomas McMurtry took the AD-1 up for the first time—with the wing firmly fixed at the traditional perpendicular angle. It wasn’t until the following May that wings-weep flights began, starting with a conservative 15 degrees and gradually working up to flights at 60 degrees obliquity.

Pilots found that the oblique wing behaved almost exactly as Jones had predicted. Minor problems with lateral stability and oscillations didn’t faze these experienced hands. “It was easy to fly,” recalled NASA test pilot Warren Hall. “Takeoff and landing, you just did it, you didn’t think about it. Because you worked your way up from 10 to 60, you learned real quick to go left and right going whichever way you want to go, up and down, and there wasn’t any problem at all. There wasn’t anything that said, hey, this is a bad flying airplane. I would have liked to put it in my pocket and bring it home.”

The AD-1 made its final flight at Oshkosh, Wis., at the Experimental Aircraft Association’s annual airshow on August 7, 1982. After further wind tunnel testing at NASA Langley Research Center, the experimental plane was returned to the West Coast, first placed in storage at Dryden, then put on display at NASA Ames before finding a permanent home at nearby Hiller Aviation Museum in San Carlos, Calif.

The 79 tests of the AD-1 remain the only piloted flights of the oblique wing, but the concept stubbornly persists. Jones continued to champion the concept until his death in 1999, proposing new designs such as a full-scale supersonic oblique-flying-wing transport. The Navy was quite enamored of the prospects of the oblique wing for carrier use, and planned to build a test aircraft with a pivoting wing based on the Vought F-8 Crusader fighter. But cost overruns kept the project from getting out of the wind tunnel.

Other projects, conceived by entities including NASA, the military and private industry, continue to explore the promise of the oblique wing. While the human prejudice for bilateral symmetry ruefully noted by Jones continues to hamper the realization of oblique-wing passenger aircraft, it has not escaped the Pentagon’s no tice that the oblique wing may prove to be just the thing for supersonic unmanned drones. Oblique-wing aircraft could someday seem far less outlandish than when R.T. Jones’ asymmetrical brainchild first took to the air over California 35 years ago.

 

Originally published in the March 2014 issue of Aviation History. To subscribe, click here.

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