Saturday, November 26

Grumman X-29, the amazing fighter that dared to fly with inverted swept wings


In 1984 some of the most advanced US agencies gave the green light to a special project: the new Grumman X-29 fighter jet with which they sought to have the most advanced aircraft of its time (and it had its competitors).

That fighter was special for many reasons, but one of its characteristics stood out above the rest: it had an inverted swept wing design, which made it the most aerodynamically unstable aircraft ever built. That, surprisingly enough, had its advantages.

That plane was so unstable, we insist, that it was practically impossible to fly it without using the on-board computer, which was in charge of making corrections on those flights with an exaggerated frequency: 40 times per second.

This was highlighted by Christian Gelzer in a recent interview for CNN. This NASA historian also explained that if the three flight computers that made up the plane had failed at any time, the pilots would have been unable even to activate the ejection mechanism.

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The truth is that the X-29 was not the only aircraft of its kind: the prototype of the German Junkers Ju 287 bomber already flew successfully in 1944 with this design, and in fact the creator of that design, Hans Wocke, ended up applying it to the small commercial jet Hansa HBF 320** that began to fly in 1964 and that in fact still continues to do so with this design.

The Hansa HBF 320’s design was however less aggressive than Grumman’s X-29, which had a 33° angle. In this unique fighter jet that design provided a remarkable maneuverabilityalthough that compromised stability.

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The X-29 was a single-engine aircraft with a length of 14.7 meters, a wingspan of 8.29 meters and a General Electric F404 engine that delivered a thrust of 71.2 kN or 16,000 lbf. That same engine was used in other fighters such as the famous F/A-18 Hornet.

No fear of loss

There were other unique elements in that design, such as the ailerons that help stabilize the plane in case of stall and that in the X-29 were located in front of the wings and not in the tail, as is usual in most commercial and military aircraft.

This allowed the X-29 to have a certain ability to maneuver even in a stall, something that had not been seen before and that gave it a theoretical advantage to obtain superiority in the air.

In 1992 the United States Air Force began a program to study the so-called ‘vortex flow control’, with the aim of providing greater control in the aircraft. at notable angles of attack where conventional control systems were not effective. Test results were mixed, but they served to determine how far the risky X-29 design could go.

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However, this design introduced many problems such as the weight that they had to have to support the forces that would put stability in problems. The wings of the X-29 were made of composite materials which are now common in aircraft production, but which at the time represented another remarkable novelty.

Helping its operation were the aforementioned flight computers, which made use of an electronic interface (the so-called Fly-by-Wire or FBW) instead of traditional manual controls. The physical controls existed, but each action was translated into an electronic signal that was transmitted by cable to the flight computers to provide the commanded response.

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The plane that could have been and was not

Grumman ended up building just two of these models. He won the contract with a budget of 87 million dollars at the time that was achieved thanks to the reuse of existing fighter parts such as the F-5A or F-16.

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These models flew 437 missions between 1984 and 1992 to assess their performance, but were ultimately abandoned for an argument that didn’t even exist when they were developed: future US fighter jets should try to be undetectable, something that design didn’t. fulfilled.

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In fact many of its advantages they ended up reaching models like the F-22which with its aerodynamic design and the so-called ‘thrust vectoring’ -the ability to physically move the engine to control the direction of the propulsion- also allows access to that maneuverability even in a stall.

After the abandonment of that model, however, the “Russian version” appeared. The Sukhoi Su-47 was suspiciously similar to the X-29, but it ultimately did not go into production and only the initial prototype was developed. A recent Russian project, the SR-10 from 2015, was also exploring this option, but it seems that inverted wings have never succeeded in unseating the traditional design.

The X-29, yes, was close to doing it.

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