Northrop X-21A

Last revised April 28, 2001

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 underwing 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.


  1. Post World War II Bombers, Marcelle Size Knaack, Office of Air Force History, 1988.

  2. McDonnell Douglas Aircraft Since 1920, Volume 1. Rene Francillon, Naval Institute Press, 1988