X-21 Laminar Flow Control

The X-21 program consisted of a pair of WB-66D's modified by Northrop to conduct Laminar Flow Control wing studies. Laminar-flow control is a technology that offers the potential for improvements in aircraft fuel usage, range or endurance that far exceed any known single aeronautical technology. In principle, if 80% of wing is laminar, then overall drag could be reduced by 25%. The frictional force between the air and the aircraft surface, known as viscous drag, is much larger in a turbulent boundary layer than in a laminar one. The principal type of active laminar-flow control is removal of a small amount of the boundary-layer air by suction through porous materials, multiple narrow surface slots, or small perforations.

The USAF Wright Air Development Division (WADD) proposed use of two WB-66D airplanes based on minimum cost, high degree of safety, and short development time. The Northrop Corporation, under sponsorship of the Air Force (with a monetary contribution from the Federal Aviation Administration), later modified these airplanes with slotted suction wings and designated them as experimental aircraft X-21A and X-21B. The B-66 fuselage was modified with a large hump on the top of the fuselage, with additional modifications to the wings, engines, laminar flow exhausts, and tail cone. Slots were incorporated in the wing's surface to inject air into the boundary layer, inducing non-turblent laminar air-flow.

Practical application of the concept proved unworkable, since rain, dirt, dust and other particulates clogged the slots. Northrop began flight research in April of 1963 at Edwards Air Force Base. Several problems arose early in the project that consumed significant periods for their solution. Principal among these were surface smoothness problems and an unexpected severity of a spanwise contamination problem. With respect to the smoothness problem, in spite of a concerted effort to design and build the slotted wings for the two airplanes to the close tolerances required, the resulting hardware was not good enough. Discontinuities in spanwise wing splices were large enough to cause premature transition to turbulent air flow. Putty, used to fair out these discontinuities, chipped during flight with resulting roughness large enough to trigger transition from laminar to turbulent flow.

The combination of X-21 wing geometry, flight altitudes, and Mach numbers was such that local turbulence at the attachment line, e.g., from the fuselage or induced by insect accumulation, caused turbulent flow over much of the wing span. With the large-scale X-21 flight tests and further wind-tunnel tests, Northrop developed methods for avoidance of spanwise contamination.

Another problem that was uncovered during the X-21 flight tests was associated with ice crystals in the atmosphere. Researchers noted that when the X-21 flew in or near visible cirrus clouds, laminar flow was lost but that upon emergence from the ice crystals, laminar flow was immediately regained. Northrop developed a theory to indicate when laminar flow would be lost as a function of atmospheric particle size and concentration.

By October of 1965, attainment of "service experience comparable to an operational aircraft," one of the program's principal objectives, had not even been initiated because of the effort absorbed by the previous problems. To proceed with this initiative, the advisors to the Air Force recommended that a major wing modification would be needed before meaningful data on service maintenance could be obtained. This, unfortunately, was never done because of various considerations at high levels of the Air Force, probably predominantly the resource needs of hostilities in Vietnam. Much extremely valuable information, however, was obtained during the X-21 flight program, supported by wind-tunnel and analytical studies. At the end of the program,38 flights attained laminar flow on a fairly large airplane over 95 percent of the area intended for laminarization. Unfortunately, top management in government and industry remembered the difficulties and time required to reach this point more than they did the accomplishment.