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 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.

In initial testing there were significant problems with the porous materials and surface slots getting plugged with debris, bugs, and even rain.

[4] Both X-21As ended up in storage at Edwards Air Force Base, California, where they gradually became derelicts, used primarily as photo targets.