X-21A

Specifications

Company- Northrop
Type- Flight demonstrator for laminar flow control testing.
Goals- Test full-scale boundary control on large aircraft.
Primary Testing Facility Research- Edwards AFB
Dimensions- Span- 93 ft, 6 in; Length- 75 ft, 3 in; Height: 25 ft, 7 in
Max Speed- 560 MPH
Range- 4,780 miles
Max Altitude- 42,500 ft
Power Plant- One General Electric J79-GE-13
Thrust- 18,800 lbf
Weights- Empty: 45,828 lbs; Fully Loaded- 83,000 lbs
Payload- N/A
Flights- Unknown
Number of Prototypes Built- 2
Project Tenure- 1963-1964
Project Status- Cancelled

Information

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. Although useful testing was accomplished, the extensive maintenance of the intricate laminar-flow system caused the end of the program.

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 underwing 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) 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.

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, 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. 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 spanwise flow effects in the boundary layer (referred to as spanwise contamination) and degrading environmental effects such as ice crystals in the atmosphere.

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