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The Northrop X-21A was an experimental aircraft designed to test wings with American X-Vehicles: An Inventory, June 2003.
Retrieved: 13 February 2007.. Although useful testing was accomplished, the extensive maintenance of the intricate laminar-flow system caused the end of the program.
Northrop X-21A
Retrieved: 14 February 2007. 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.
In initial testing there were significant problems with the porous materials and surface slots getting plugged with debris, bugs, and even rain. In certain conditions, ice crystals would form due to the rapid cooling of air over the laminar surfaces. This would abruptly disrupt laminar flow, causing rapid melting and rapid transition back to turbulent flow. Maximum achievement of 95% laminar flow over those areas was desired. However, the design effort was canceled due to the plugging problems.
Pioneering data was 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 span-wise contamination) and degrading environmental effects such as ice crystals in the atmosphere.Chambers, Joseph R
Laminar-flow Control: The Holy Grail of Aerodynamics
NASA, p. 133–134.
"X-Planes Detailed Data – Northrop X-21A"
Retrieved: 15 November 2017.
*''Plattner, C. M.'
X-21 Tests Laminar Flow Control Theory
// ''Aviation Week & Space Technology'', June 24, 1963, v. 78, no. 25, pp. 52–62. {{Authority control Edwards Air Force Base X-21, Northrop X-021 High-wing aircraft Twinjets Aircraft first flown in 1963
laminar flow
In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mi ...
control. It was based on the Douglas
Douglas may refer to:
People
* Douglas (given name)
* Douglas (surname)
Animals
*Douglas (parrot), macaw that starred as the parrot ''Rosalinda'' in Pippi Longstocking
*Douglas the camel, a camel in the Confederate Army in the American Civil W ...
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
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
test pilot Jack Wells at the controls.Retrieved: 13 February 2007.. Although useful testing was accomplished, the extensive maintenance of the intricate laminar-flow system caused the end of the program.
Design and development
Laminar-flow control is a technology that offers the potential for significant improvement indrag coefficient
In fluid dynamics, the drag coefficient (commonly denoted as: c_\mathrm, c_x or c_) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equ ...
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
In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...
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 (boundary layer suction
Boundary layer suction is a boundary layer control technique in which an air pump is used to extract the boundary layer at the wing or the inlet of an aircraft. Improving the air flow can reduce drag. Improvements in fuel efficiency have been esti ...
).
Two major modifications were required, the first involving the standard underwing podded Allison J71
The Allison J71 was a single spool turbojet engine, designed and built in the United States. It began development in 1948 as a much modified J35, originally designated J35-A-23.
Operational history
The Allison J71 turbojet powered the Douglas B ...
engines being removed and replaced by a pair of static thrust 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)Winchester 2005 p. 297. 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.Baugher, JoeNorthrop X-21A
Retrieved: 14 February 2007. 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
In initial testing there were significant problems with the porous materials and surface slots getting plugged with debris, bugs, and even rain. In certain conditions, ice crystals would form due to the rapid cooling of air over the laminar surfaces. This would abruptly disrupt laminar flow, causing rapid melting and rapid transition back to turbulent flow. Maximum achievement of 95% laminar flow over those areas was desired. However, the design effort was canceled due to the plugging problems.
Pioneering data was 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 span-wise contamination) and degrading environmental effects such as ice crystals in the atmosphere.Chambers, Joseph RLaminar-flow Control: The Holy Grail of Aerodynamics
NASA, p. 133–134.
Disposition
Both X-21As ended up in storage atEdwards Air Force Base
Edwards Air Force Base (AFB) is a United States Air Force installation in California. Most of the base sits in Kern County, but its eastern end is in San Bernardino County and a southern arm is in Los Angeles County. The hub of the base is E ...
, California, where they gradually became derelicts, used primarily as photo targets. The remains can still be viewed, but no efforts have been made to recover either example for restoration or display.Jenkins, Dennis R. ''X-Planes Photo Scrapbook''. North Branch, Minnesota: Specialty Press, 2004.
Specifications (X-21A)
See also
References
;Notes ;Bibliography * Winchester, Jim. ''X-Planes and Prototypes''. London: Amber Books Ltd., 2005. ."X-Planes Detailed Data – Northrop X-21A"
Retrieved: 15 November 2017.
External links
*''Plattner, C. M.'
X-21 Tests Laminar Flow Control Theory
// ''Aviation Week & Space Technology'', June 24, 1963, v. 78, no. 25, pp. 52–62. {{Authority control Edwards Air Force Base X-21, Northrop X-021 High-wing aircraft Twinjets Aircraft first flown in 1963