The Northrop X -21A was an American experimental aircraft.
With their possibilities should be examined to influence the boundary layer flow on an airfoil in order to reduce the air resistance and thus to increase the range.
The two machines created by conversions from Douglas WB- 66D weather observation aircraft ( s / n 55-0408 and 55-0410 ). The existing jet engines of the type Allison J71 were exchanged J79 -GE -13 engines with General Electric and moved to the rear so as not to affect the air flow on the wings.
The newly constructed wing had a relation to the B -66 increased span and wing area. Over the entire span of tiny grooves were milled from which the air was sucked, to delay the envelope from laminar to turbulent boundary layer and thus to reduce resistance. The air extraction was carried out by two turbines, which were housed in panels under the wings and were driven by bleed air from the J79 engines. The extracted air was blown from nozzles at the wingtips and thus generated additional boost.
The crew consisted of a pilot and two flight engineers. Two other flight engineers were housed in a body shaft underneath the wings.
The first flight took place on 18 April 1963. Due to the extreme vulnerability of the grooves against pollution and icing the project was set back in 1964.