Northrop X-21

The Northrop X-21A was an experimental aircraft built 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 to make space for air compressors, and a new wing with about 800,000 span-wise slots through which boundary-layer air was sucked in to keep the airflow smooth. Two General Electric XJ79 turbojets sat in pods on the rear fuselage sides. It first flew on 18 April 1963 with NASA test pilot Jack Wells. Testing produced pioneering data on surface irregularities, span-wise contamination and ice crystals, but slots and porous surfaces clogged with debris, insects and rain, and the intricate system needed so much maintenance that the program ended. Both aircraft were left derelict at Edwards Air Force Base.