Even the smoothest skins will still have flow separations leading to
While the problem can be addressed, it takes a bit more than dimples
to stick the boundary layer down tight over the entire wing.
Consider the X-21A program.
It worked exceptionally well, but unfortunately proved to be unmaintainable.
First Flight: April 18, 1963
Mission: Full sized test bed for testing "Laminar Flow Control" (also
referred to as boundary layer control) theory
Major Accomplishments: Proved that while Laminar flow control was
possible, it was not feasible with existing technology.
Power Source: General Electric J79-GE-13, 9,400 lb thrust max.
Wing Span: 93' 6"
Length: 75' 3'
Weight (Loaded): 83,000 lbs
Maximum Achieved Speed: 560 mph
Maximum Achieved Altitude: 42,500 ft
Additional Information: Only two X-21's were built, and were actually
highly modified Douglas WB-66D's. The X-21 was flown to test the
"Laminar Flow Control" theory. The basic concept is that the exterior
surface of the aircraft can be designed to create a slight suction
during flight. Slots are incorporated in the aircraft's surface to
produce the suction. Though the concept works, environmental
considerations including rain, dirt, dust and other particulates
required excessive maintenance on the aircraft.
The last known location of both X-21's was Edwards AFB, where they had
been gutted of most instrumentation and left out of doors to deteriorate.