Grumman X-29: The impossible fighter jet with inverted wings
There's no airplane quite like the Grumman X-29. Its astonishing forward-swept wings were just one of its many bold innovations.
Created at the height of the Cold War by a conglomerate of giants -- NASA, the US Air Force, the "men in black" at the Defense Advanced Research Projects Agency (DARPA) and aerospace behemoth Grumman -- it first flew in 1984 as part of a quest to build the ultimate fighter jet.
But its highly experimental design made it the most aerodynamically unstable aircraft ever built.
"It was unflyable -- literally -- without a digital flight computer on board, which made corrections to the flight path 40 times a second," said Christian Gelzer, chief historian at the NASA Armstrong Flight Research Center in southern California (where the plane was tested) in a phone interview.
"The engineers concluded that if all three flight computers had failed together, the airplane would have broken up around the pilot before the pilot had a chance to eject."
Planes with forward-swept wings, which are angled in the opposite direction of conventional wings, are rare, but the X-29 wasn't the first to employ them. The German Bomber Junkers Ju 287, a successful prototype with such design, first flew in 1944. The man who designed it, Hans Wocke, later applied what he learned to the Hansa HBF 320, a small business jet which took to the skies in 1964. A few dozen were built and some still fly -- the only commercial aircraft with forward-swept wings.
In the Hansa Jet, however, the wings are swept forward to make the most of the small fuselage and create more space for passengers in an otherwise cramped aircraft, since this position allows the wings to be mounted further back along the plane's body.
The Hansa Jet wings are also swept forward by just a few degrees, compared to 33 degrees in the X-29. Such a radical adjustment meant trading stability for maneuverability, because to maneuver quicker, a plane must be inherently unstable to start with.
"An F-18 fighter jet has an instability factor of only 5%. The X-29, on the other hand, was 35% unstable," said Gelzer.
But mounting the wings backwards has another immediate effect. Small appendages called ailerons (meaning "little wings" in French), a crucial component to control the plane, are mounted close to the wingtips. When a normal plane stalls -- a loss of lift that can lead to a crash -- ailerons are usually the first thing to stop working, because stalls tend to start at the wingtip due to the way the air flows over the wing. That means loss of control in an already dangerous situation.
Forward-swept wings, however, force air to flow the opposite way, moving inboard from the wingtips. Therefore, stalls tend to start closer to the fuselage, leaving the ailerons functional for longer and giving pilots much needed control.
"After a stall, some military fighter jets can keep flying when they should no longer be flying, through the gargantuan amount of thrust their engines provide. The problem is, can you control the airplane? Not until the X-29. It is the only aircraft that had forward-swept wings and explored this 'post stall' environment," said Gelzer.
"At the time, this maneuverability was believed to be absolutely essential to fighter superiority. If your airplane is going to stall before mine, I can shoot you out of the sky in a heartbeat."
When stealth came along
The unusual shape of the wings created another problem for the X-29's engineers: weight. Inverted wings are subject to an extreme twisting force that can break them, so they need to be reinforced. But the X-29's wings would have weighed too much if they were made of metal. Instead, the weight was kept down by using advance composite materials, which are now the standard across commercial and military aircraft.
The plane also had a futuristic, digital fly-by-wire system, which used an electronic interface to fly the aircraft instead of traditional manual controls -- another innovative feature that's now commonplace in aviation.
Two X-29s were built by Grumman, the defense contractor behind the successful F-14 Tomcat and the historic Apollo Lunar Module. Grumman won the contract, worth $87 million (around $245 million in today's money) thanks to cost-cutting measures such as using parts from existing fighter jets, including the F-5A and the F-16 Falcon. The project was commissioned by DARPA, the American Defense Advanced Research Projects Agency, and the USAF Flight Dynamics Laboratory at Wright-Patterson Air Force Base in Ohio. NASA handled the test flights.
The X-29s were flown in 422 research missions between 1984 and 1992. But in the end, its most radical features remain confined to history.
"The benefits did not outweigh the disadvantages. But also, stealth came along and became the must-have feature of a jet fighter," said Gelzer.
Stealth allows planes to be invisible to radar, and it offers such advantages that many modern fighters are not even optimized for aerial battles, or dogfights, which have become rare.
Forward-swept wings have also been made obsolete by thrust vectoring, or the ability to physically move an engine to control the direction of its thrust, ensuring maneuverability even in case of stall. "You can accomplish almost everything the X-29 was able to do in an F-22 with modern aerodynamics and thrust vectoring," said Gelzer.
Nevertheless, the plane is fondly remembered. "Those who participated in the program speak very highly of it. People are really enamored with this strange looking airplane. And NASA was critical to the program: it contributed pilots, engineers, maintenance and housing for the aircraft. It was integral right from the start."
A Russian clone?
The X-29 wasn't the last of its kind, however. On Sept. 25, 1997 -- five years after the X-29's final mission -- the Russian Air Force flew the Sukhoi Su-47, its own version of a fighter jet with forward-swept wings. Nicknamed "Berkut" (Russian for "golden eagle"), it was the result of a project that was launched in 1983, but was delayed due to the dissolution of the USSR.
Similarities in the design and timing of the plane suggest that it may have been directly inspired by the X-29. "It's hard not to draw that conclusion, that they saw this and they decided, 'We'd better find out if it works as well,'" said Gelzer.
However, the Su-47 was almost twice as big as the X-29 and was built more like a full-fledged fighter than an experimental testbed. Despite this, the aircraft never entered production and only one was built.
In 2015, a Russian firm started testing the SR-10, a small inverted wing fighter jet, with the intention of offering it to the Russian Air Force, signaling that the design might still be somewhat viable.
But will we ever see another fighter jet with forward-swept wings from NASA or the Air Force? Don't hold your breath, said Gelzer.
"I can see that happening from smaller firms, but not from the big military builders and designers. I think it would take something extraordinary for them to return to this concept."
Top image: An X-29 lifts off from the runway at Edwards Air Force base on a 1989 test flight.