The Northrop X-21A

Northrop X-21A
Type	Experimental Aircraft
Manufacturer	Northrop
Maiden flight	18 April 1963
Introduced	experimental
Retired	1968
Primary user	National Aeronautics and Space Administration (NASA)
Number built	2

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Douglas WB-66D

The Northrop X-21A was an experimental aircraft designed to test wings with laminar flow control. It was based on the Douglas WB-66D airframe, with the wing-mounted engines moved to the rear fuselage and making space for air compressors. The aircraft first flew on 18 April 1963 with NASA test pilot Jack Wells at the controls.^[1] Although useful testing was accomplished, the extensive maintenance of the intricate laminar-flow system caused the end of the program.

Design & Development

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Northrop X-21A

Laminar-flow control is a technology that offers the potential for significant improvement in drag coefficient which would provide 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.

Two major modifications were required, the first involving the standard under wing podded J71 engines being removed and replaced by a pair of 9490 lb.s.t General Electric XJ79-GE-13 non-afterburning turbojets mounted in pods attached to the rear of the fuselage sides. Bleed air from the J79 engines was fed into a pair of underwing fairings, each of which housed a "bleed-burn" turbine which sucked the boundary layer air out through the wing slots.

The X-21A test vehicles (55-0408 and 55-0410) also incorporated sophisticated laminar flow control systems built into a completely new wing of increased span and area, with a sweep reduced from 35 ° to 30 °. The wing had a multiple series of span-wise slots (800,000 in total ^[2]) through which turbulent boundary-layer was "sucked in," resulting in a smoother laminar flow. Theoretically, reduced drag, better fuel economy and longer range could be achieved. ^[3]

The forward cockpit carried a pilot and two flight engineers while two additional flight test engineers were housed in a central fuselage bay underneath the wing.

Testing

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X-21A in testing

In initial testing there were significant problems with the porous materials and surface slots getting plugged with debris, bugs, even rain. In certain conditions, ice crystals would form due to the rapid cooling of air over those laminar surfaces abruptly disrupting laminar flow, causing rapid melting and rapid transition back to laminar flow. Maximum achievement was 95% laminar flow over those areas desired^[4]. However, the design effort was cancelled due to the plugging problems.

Nevertheless, pioneering data were obtained in the X-21 flight program, including the effects of surface irregularities, boundary-layer turbulence induced by three-dimensional span wise flow effects in the boundary layer (referred to as spanwise contamination) and degrading environmental effects such as ice crystals in the atmosphere.^[5]

Survivors

Both X-21As ended up in storage at Edwards Air Force Base, California where gradually, they became derelicts. The remains can still be viewed but no efforts have been made to recover a single example for restoration or display. ^[6]

Specifications (X-21A)

General characteristics

Crew: Five
Length: 75 ft 3 in (22.94 m)
Wingspan: 93 ft 6 in (28.51 m)
Height: 25 ft 7 in (7.8 m)
Wing area: 1,250 ft² (116.17 m²)
Empty weight: 45,828 lb (20,783 kg)
Loaded weight: 83,000 lb (37,727 kg)
Powerplant: 2× General Electric J79-GE-13 turbojets, 9,400 lbf (41.9 kN) each

Performance

Maximum speed: 487 kn (560 mph, 896 km/h)
Range: 4,156 NM (4,780 mi, 7,697 km)
Service ceiling: 42,500 ft (12,957 m)

References

American X-Vehicles: An Inventory. Access date: 13 February 2007.

Winchester 2005, p. 297.

Baugher,Joe. Northrop X-21A. [1] Access date: 14 February 2007.

Winchester 2005, p. 297.

http://www.history.nasa.gov/monograph39/mon39_a.pdf

Winchester 2005, p. 297.

Baugher,Joe. Northrop X-21A. [2] Access date: 14 February 2007.

"Northrop X-21A." X-Planes Detailed Data. [3] Access date: 14 February 2007.

Winchester, Jim. X-Planes and Prototypes. London: Amber Books Ltd., 2005. ISBN 1-904687-40-7.

Wikipedia

Two WB-66Ds (55-0408 and 55-0410) were extensively modified by the Northrop Corporation as test vehicles for laminar flow control systems. The aircraft was fitted with a completely new wing of increased span and area, with a sweep reduced from 35 degrees to 30 degrees. The wing has a series of span-wise slots through which turbulent boundary-layer was sucked away, resulting in a smoother laminar flow operation, hopefully resulting in reduced drag, better fuel economy, and longer range. The under wing podded J71 engines were removed and replaced by a pair of 9490 lb.s.t General Electric XJ79-GE-13 turbojets mounted in pods attached to the rear of the fuselage sides. Bleed air from the J79 engines was fed into a pair of underwing fairings, each of which housed a bleed-burn turbine which sucked the boundary layer air out through the wing slots.

The forward cockpit carried a pilot and two flight engineers. Two additional flight test engineers were housed in a central fuselage bay underneath the wing.

Testing proved that the overall concept was feasible, and a substantially improved range was obtained. However, it was found essential to keep the tiny wing slots spotlessly clean for effective operation, and this and other maintenance difficulties made the concept too costly for practical applications.

To the casual eye, Northrop Corp.'s brand-new X-21A airplane has the look of an already obsolescent bomber. It is a familiar twin-jet Douglas B66 fitted out with oversize, swept-back wings. But a close look shows a more significant change. There are hundreds of paper-thin slots slicing through the wings' metal skin. And those slots, if the calculations of Northrop's Norair Division scientists prove correct, may well revolutionize the aircraft industry.

Designed by Swiss-born Aerodynamicist Werner Pfenninger, the intricate tracery promises to be the first practical answer to a problem that is as old as airplanes: how to smooth out the turbulent air that burbles along the surface of a moving wing. Every airplane wastes some of its power overcoming the drag of that churning air, but not until modern planes moved up toward jet speeds did the drag demand a remedy. Slow planes can live with their own slight turbulence; a fast ship becomes a fuel-gulping monster as it fights the furious air waves that swirl and eddy over its wings.

Perfect Maze. The solution, surprisingly, has long been obvious. But while engineers knew that the laminar (smooth) airflow they wanted could be had by sucking any turbulent air into a wing's inner cavity, putting theory into practice proved a stubborn puzzle. Dr. Pfenninger worked on his LFC (laminar flow control) wing for 23 years before perfecting its closely packed slits that are only a few thousandths of an inch wide. Under each slit, a small chamber gathers the incoming air and channels it through pin-size holes into ducts that lead to streamlined nacelles hanging under each wing. Inside each of those nacelles, a pair of light, powerful gas turbines—one for the forward part of the wing, one for the more turbulent air in the rear—generate the suction that keeps the system operating.

Northrop engineers, who have run thousands of hours of wind-tunnel tests, say that once the suction is started, there is smooth, laminar flow over both top and bottom of their new wing. Up to 80% of the friction drag is eliminated—and this figure includes compensation for the drag caused by the nacelles and for the power needed to run the turbines. With drag so drastically reduced, an airplane uses much less fuel, thus can fly farther or carry more payload. The null will not have its first flight tests until next month, but Northrop is already making a joint study with Lockheed to apply LFC to Lockheed's EUR-141 jet cargo plane. Project Manager Don Warner is sure that the sucking slots can increase a C-141's payload by 74% or its nonstop range by 50%.

Loitering Platform. Extra payload and range are all-important in commercial aviation, but the brightest prospect for the LFC principle is probably military. Aware that modern detection svstems and ground-to-air missiles are too effective to let many ordinary bombers get close to important targets, the Pentagon is hopefully looking forward to flying missile platforms. And an ideal platform would be a plane, loitering aloft, just beyond reach of enemy interceptors, ready to launch long-range air-to-ground missiles at targets deep in enemy territory. Existing bombers have small talent for loitering; the big B-52s, backbone of the Strategic Air Command, can stay in the air little more than 20 hours. Even if drastically rebuilt with LFC wings, their flight time might increase at most to 33 hours. For really effective loitering, says Warner, an LFC missile platform should be designed from scratch. With economical new turboprop engines, the new plane would be able to stay in the air for three days, cruising almost anywhere on earth. One proposal is to arm these loitering ships with low-flying missiles capable of streaking to their targets under the searching beams of enemy radars. The mere existence of such deadly platforms would force an enemy into costly efforts to defend against them.

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.

General characteristics

Performance

References

Wikipedia

Sources: