Moments and Milestones: Swept Forward

Moments and Milestones: Swept Forward

Pilots needed a computer to fly Grummans X-29
Hello HAL: Pilots needed a computer to fly Grumman’s X-29. NASA DFRC

At Mach 1, the speed of sound, wings develop shock waves that add drag and subtract lift. When the wings are swept, they look thinner to the passing air and the shock formation is delayed and occurs at a higher airspeed. It's the difference between slicing salami straight across or cutting it at an angle: A swept wing is like salami cut at an angle.

Swept wings are nothing new, but one lesser known fact about them is that when it comes to making the wing appear thin to the air, sweeping the wings forward is as good as sweeping them aft. And forward sweep has advantages. At high speed, the air passing over conventional aft-swept wings tends to flow out to the wingtips, a phenomenon called spanwise flow. The air piles up near the wingtips and makes the ailerons ineffective, so designers install wing fences and use a number of tricks to block this flow. Sweep the wings forward and the air flows in toward the fuselage. Problem solved.

Forward sweep also allows the spar box, to which the wings are anchored in the fuselage, to be located farther aft, where it's out of the way of the cabin or a pressure bulkhead; that's good. But few airplanes have been built with forward sweep. This is because forward sweep also has a disadvantage. When an airplane turns and applies high G-loads on forward-swept wings, their tips bend upward and, as they do, the leading edges twist upward too, increasing the angle of attack. If the twist goes too far, the wing fails structurally; that's bad. To counter that tendency, wings with forward sweep had to be exceedingly stiff and therefore heavy—until the X-29.

NASA and the Air Force conceived the X-29 as a way to explore forward sweep, and Grumman built two. First flight was 25 years ago, in December 1984. Development of the X-29 also marked the end of an era, as piloted X-aircraft for pure research—as distinguished from military prototypes—became scarce. (There have since been only three: the X-31, X-49, and X-53.) To build the X-29, Grumman modified a Northrop F-5's forward fuselage and mounted composite wings on it. The layers of composite fibers were tailored so that as the tips bent upward, the wing's leading edge twisted down, countering the nasty tendency of the wing to diverge and fail. The main gear came from an F-16, and its engine, a General Electric F404, powered the F/A-18, the stealthy F-117, and many others.

The X-29 also had a forward control surface, a canard, which rendered the airplane completely unstable. No human pilot could manage it without help, which came in the form of three computers that constantly adjusted the control surfaces. It happened so fast (about 40 Hertz) that the airplane appeared to have a bad case of coffee nerves, but it felt stable. Why three computers? If one malfunctioned, it was outvoted by the other two. And the computers were backed up by a three-channel analog system. The X-29 program, which ended in 1992 with the airplanes' retirement, succeeded in providing an engineering knowledge base and proved that forward sweep confers efficiency in level flight and good manners during extreme maneuvers. Despite its success, forward sweep has not been applied to any production military or civil aircraft except the HFB 320 Hansa Jet, a business jet that was built by Hamburger Flugzeugbau in Germany until 1973.

Both X-29s survive. One is at the National Museum of the U.S. Air Force in Dayton, Ohio, and the other at NASA's Dryden Flight Research Center in California. The National Air and Space Museum on the National Mall has a full-scale mockup in the Beyond the Limits gallery, a permanent exhibit that examines the role computers play in modern flight.

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