Grumman X-29: The impossible fighter jet with inverted wings

by Editorial Team

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.”

Extreme maneuvers

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.Out of the blue: A look back at Air Force One’s classic designIn 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.Project Habbakuk: Britain’s secret attempt to build an ice warshipForward-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.

Read from source https://edition.cnn.com/style/article/grumman-x-29-nasa-darpa-fighter-plane/index.html

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