To anyone familiar with aircraft, they will not help but notice that one design trait unifies them all — their symmetry. But, in the second half of the 20th-century, one NASA engineer decided to break the mold, ultimately leading to the development of the NASA AD-1, an asymmetrical pivot winged jet plane.
While ultimately the aircraft would prove to be a dead-end, it did show that thinking outside of the box, can, and often will yield extraordinary results.
What was the NASA AD-1?
The NASA AD-1 was an experimental aircraft (and program) developed by researchers at NASA during the late 1970s and early 1980s. It was developed, built, and tested at NASA's Dryden Flight Research Center in Edwards, California, and demonstrated the concept of using oblique wings (or "scissor wings") on aircraft.
The AD-1 (Ames-Dryden-1) prototype plane was a relatively small, subsonic jet-powered aircraft and was flown 79 times over a period of 3 years between 1979 and 1982.
In-flight, the wing could be rotated around its center pivot to provide the most effective angle for the aircraft's airspeed at any given time.
Test flights extensively tested the efficacy of a pivot-wing concept and gathered information on this kind of wing setup's aerodynamics and handling at a variety of speeds and oblique wing angles.
The principles behind the AD-1 were first conceived by Robert T. Jones, an aeronautical engineer who worked at NASA's Ames Research Center at Moffett Field in California. He combined his own independent work with research conducted by NASA (then NACA) on the variable-sweep-wing concept, which was investigated during the X-5 research plane program in the 1950s.
Jones proposed the idea of sweeping wings rearward in order to delay the shock waves and compressibility as an aircraft neared the speed of sound, allowing aircraft to fly more efficiently at high subsonic speeds.
Variable-sweep wings, like those on the F-14, F-111, B-1, and the British Panavia Tornado, combine the best of both worlds of swept and fixed wings during takeoff and cruise, and high-speed travel.
While not the first oblique-winged aircraft proposal (another had been explored during the 1940s), the AD-1 was the first to have a working prototype built and tested.
Jones' initial studies using wind tunnels, indicated that if a supersonic transport-sized oblique winged aircraft could be built, it should have much higher aerodynamic performance than conventional fixed winged planes. In fact, it should prove to be at least twice as fuel-efficient!
On the strength of these studies, a working, scaled-down prototype was built and delivered to Dryden in the early-1970s. Built by the Ames Industrial Company, Bohemia, New York, it cost about $240,000 to build.
It was powered by two Microturbo TRS18-046 turbojet engines, each capable of producing around 0.98kN (220 lb) of thrust at sea level, and the AD-1 was capable of a top speed of around 200 mph (322 km/h) but, for safety purposes, was limited to mph 170 mph (270 km/h). It also had a conventional rudder.
The prototype had a total length of 38.8 feet (11.8m), and a wingspan of 32.3 feet (9.8 mt) when in their horizontal configuration. Its main construction was composite of fiberglass reinforced plastic coating to a rigid foam core.
Since it was purely a testbed for the unconventional wing design, this AD-1 prototype lacked any form of armament.
The AD-1 weighed in at a total of 2,145lbs (973 kg) gross and had a fixed tricycle configuration landing gear. This gear was very short and held close to the fuselage to reduce drag giving the aircraft a total height to the top of the tail stabilizer of 6.75 feet (2.06 mt).
The aircraft's unique wing was pivoted using an electrically-driven mechanism, located just in front of the engines within the fuselage.
What are the benefits of the NASA AD-1's scissor wings?
Aircraft wings are generally optimized to perform one of two main tasks — lift (during takeoff and landing) and maximizing speed when airborne. Aircraft designers usually need to prioritize one of these.
Maximizing one will generally mean the other will suffer to some extent — it is a tradeoff. For example, fixed horizontal wings are best at low speeds, especially when getting airborne, and swept wings excel at high speeds.
This has led some aircraft designers to ponder if it would be possible to get the "best of both worlds". Enter the oblique variable-sweep wing, otherwise known as "scissor wing".
The AD-1's ultimate inspiration began with Jones' work at NASA's Langley facility in the 1940s. Here he realized that as aircraft reached high speeds, around Mach 1.0, they became more unwieldy and inefficient. Faster airflows over the wing produces shockwaves that increase drag and, subsequently reduce lift.
Experimentation later found that by sweeping wings backward, aircraft performance at high speed would greatly improve. This is because this configuration of wing helps reduce something called wave drag.
This is a force (drag) that slows down the forward movement of an airplane in supersonic and transonic flight as a result of the formation of shock waves.
Seems reasonable, but he would go further.
Jones proposed that similar benefits could be achieved if only one wing was swept backward, rather than both.
The basic idea is that by moving the wingtips towards the tail, shockwaves could be delayed, allowing the aircraft to fly with greater efficiency. Jones showed his superiors his concept, but it was rejected for appearing to be too complicated to actualize — at least at that time.
Post-war, Jones stumbled upon some similar German designs and continued his research. He found that oblique wing designs could greatly improve fuel efficiency and performance allowing aircraft to have lower takeoff speeds and a much-improved range at cruising speed.
Further research in the 1950s using wind tunnels proved that Jones' conclusions were indeed correct. However, it would not be until the 1970s that an actual working prototype could be attempted with the joint NASA and Boeing aircraft the AD-1.
Although Jones had originally proposed the development of a fixed-oblique winged aircraft, the AD-1 had a wing that could be variably swept using electrical motors.
This setup allowed different wing angles to be tested mid-flight. The wing's angle could be altered using a switch in the cockpit.
Ground and shake testing of the airframe prior to in-flight testing showed that the wings were 15% more robust than previously indicated in their designs.
With the airframe given the green light, its maiden test flight was December 21st, 1979 at Edwards Airforce Base in California. This first flight used the wings in their unswept, zero degrees, configuration.
Future flights were then conducted by increasing the wing angles in increments to assess the effects on aircraft handling and performance. It wasn't until 1982 that the aircraft was tested with its wings at their maximum sixty-degree angle.
This was not only interesting from an engineering point of view but also made history, as the first flight of an oblique-winged aircraft. All-in-all, the AD-1 would complete 79 flights between 1979 and 1982, with 17 different pilots, two of which were NASA test pilots Thomas C. McMurty and Peter Knight.
Once all the test flights had been completed, it was found that the aircraft performed as anticipated by its designer Jones. It did, however, come with some issues mid-flight, like decreased lateral stability (not surprising), and some oscillation at certain angles (especially when maneuvering with the wings at a full 60-degree angle).
Above 50-degrees, most test pilots also agreed that the aircraft did become a bit challenging to control.
The AD-1's final flight was on August the 7th, 1982, after which it was returned to NASA Dryden and was later displayed at the Hiller Aviation Museum in California.
While tests of the AD-1 proved successful, final reports from pilot evaluation suggest that future experiments be conducted at supersonic speeds using fly-by-wire control systems. At these sorts of speeds, it was anticipated that the aircraft should have peak performance and handling.
Other suggestions included considering the wing design for future carrier-based, anti-submarine aircraft. This is because they believed it would offer the aircraft lower approach and landing speeds, as well as, having supersonic dash capabilities.
What other examples of scissor wing aircraft are there?
As we have already touched on above, there have been some other forays of developing similar designs in the past. Here are some of the most interesting examples.
1. The Messerschmitt Me P1109 is one of the strangest aircraft you've ever seen
An earlier attempt to create an oblique-winged aircraft was the Messerschmitt ME P1109. This amazing concept fighter envisaged two sets of pivoting wings — one above and one below the fuselage.
Developed in 1944, the two sets of wings could be pivoted at 60-degree angles fore and aft. The theory was that this setup would drastically reduce aerodynamic drag allowing it to travel at higher speeds over much improved ranges, in theory.
The concept foresaw the aircraft being powered by two H3S 001 engines, placed side-by-side inside the fuselage. The wing pivoting mechanisms would have been located behind the aircraft's fuel tanks.
There are no known records of a wind-tunnel model ever being made, but the experimental fighter was part of a series of projects designed with the object of developing new and unproven aerodynamic theories.
The Me P1109 was, without doubt, one of the most bizarre aircraft designs ever created.
2. The Blohm und Voss P 202 was the first of its kind, as far as we know
The Blohm und Voss P202 was another early concept for a scissor winged aircraft. Designed in 1942 before the Me P1109, it was the first aircraft known to incorporate the scissor wing concept.
Designed by Richard Vogt, the aircraft's main wing could be swept up to 35 degrees during high-speed flight. The concept aircraft had a wingspan of around 39.4 feet (12 meters) unswept, and 32.8 feet (10 meters) when fully swept.
The P202 came was designed with a long undercarriage that could, theoretically, also be retracted into the wing, with a third retractable nose wheel also present. This amazing aircraft would have been powered by two turbojets that were slung under the fuselage.
The plane also would have come equipped with forward-firing cannons mounted in the nose. As far as we know, no test models were ever built or wind tunnel tested.
3. The Sikorsky "X-Wing" is another interesting unconventional aircraft design
While not technically speaking a "scissor wing", the Sikorsky X-Wing was based on a similar concept, except with a bit of a twist (quite literally). A blend of a helicopter and jet, this interesting concept aircraft was proposed as a way to combine the hovering capabilities of the helicopter with the speed potential of fixed-wing aircraft.
The aircraft's X-wing spins during takeoff (like a conventional helicopter), and then progressively slow down and lock into place as the aircraft increases in airspeed.
Development of the aircraft was conducted between 1983 and 1988 as a joint project between NASA, DARPA, and the helicopter manufacturer Sikorsky.
The project was ultimately canceled, with research in the field shifting to focus on tilt rotors on aircraft life the V-22 Osprey.
While ultimately asymmetrical oblique winged aircraft would prove to be a technological dead-end, aircraft like the AD-1 would lay the groundwork for swept winged designs in fast jets that common today.