Whatever happened to the aerodynamic experiment done back in 1963 that could allow planes to improve its range by 50 percent and its payload by 75 percent?
By: Ringo Bones
The Northrop X-21A program was the now largely forgotten attempt of a conventional jet powered subsonic aircraft to be able to fly nonstop around the world. And unlike the Rutan designed Voyager carbon fiber composite propeller powered plane that successfully flew nonstop around the world in 1985, it would be able to fly nonstop around the world while carrying a few passengers and crew and other something useful thing called cargo. But is the Northrop X-21A program back in 1963 proved to be an aerodynamic engineering dead end?
Where the air is smooth, the drag force is considerably less than in the turbulent portion of the boundary layer. It is obvious, then, that if the boundary-layer flow could be kept laminar, there would accrue a tremendous benefit to airplane performance. The airplane’s engine would not have to expend so much of its power overcoming boundary-layer drag and available power could be better utilized more efficiently to provide greater speed, range or payload.
Control of the boundary layer has attracted considerable attention from flight researchers. One experiment devised to investigate possible reduction of drag through laminar-flow control involved the conversion of two weather reconnaissance planes; their twin jets were moved to mountings at the aft end of the fuselage and a pair of pumps was installed where the engines had hung beneath the wings. Razor-thin slot lines, running outward from the fuselage to the tips, were cut from the fuselage to the tips, were cut in both the upper and lower surfaces of the wing. In flight, the turbine-driven pumps drew the turbulent air into the wing, smoothing the flow; the “inhaled” air was then discharged from the rear of the pods. Test instrumentation showed that the suction system was able to maintain laminar flow over most of the wing, thus substantially lowering friction drag.
The advent of more powerful engines in the early 1960s, which in itself offered substantial improvement in range and payload, has lessened the interest in boundary layer research, but around 1963, some authorities have recommended renewed efforts to apply its efficiencies. Laminar-flow control, an experimental technique to reduce drag caused by air friction across the surface of a plane’s wings, was first successfully tested in 1963. A special wing was built with numerous razor-thin slots running its length and rigged with two turbine driven pumps underneath. As the layer of churning air swirled past the wing’s surfaces, it was sucked through the slots by the pumps then blown rearward from under the wing, resulting in a smooth flow. The system was mounted in an experimental aircraft made by Northrop called the X-21A. Many questions about the pumped-laminar-flow system remain to be answered; how does the system perform under operational conditions? How much of a problem is maintenance? How costly is it in routine use?
Even though the laminar-flow-system worked with flying colors in reducing drag that allowed substantial increase in the Northrop X-21A’s range and payload, the maintenance of the numerous razor-thin slots seems too much compared to conventionally winged planes as tiny insects, leaves and other plant debris are the main culprits in blocking the air intake slots. Given the unionized nature of commercial airline industry’s maintenance crews, such a system is not economically viable in real-world commercial civil aviation applications.
A competing “passive” boundary layer control / manipulation system invented before the “active” laminar-flow-control / LFC system of the Northrop X-21A called the vortex generator which was then used on the wings on the first generation of the Boeing 707 planes used by commercial airlines proved to be a better, cheaper and much more easier to maintain option. With advancements in supercritical wing designs and more efficient and quieter high-bypass ratio turbofan jet engines all aimed primarily to cut fuel consumption has since relegated the laminar flow control system like the one used in the Northrop X-21A to the dustbin of aviation history.