Northrop X-21

Maximum speed: 560 mph (896 km/h)
Range: miles ( km)
Service ceiling: 42,500 ft (12,957 m)
Rate of climb: ft/min ( m/min)
Wing loading: lb/ft² ( kg/m²)
Power/Mass: hp/lb ( kW/kg)

From Wikipedia, the free encyclopedia

The Northrop X-21 was an experimental aircraft designed 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 and making space for air compressors.

Laminar-flow control is a technology that offers the potential for significant improvement in drag coefficient which would provide improvements in aircraft fuel usage, range or endurance that far exceed any known single aeronautical technology. In principle, if 80% of wing is laminar, then overall drag could be reduced by 25%. 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 principal type of active laminar-flow control is removal of a small amount of the boundary-layer air by suction through porous materials, multiple narrow surface slots, or small perforations.

In initial testing there were significant problems with the porous materials and surface slots getting plugged with debris, bugs, even rain. In certain conditions, ice crystals would form due to the rapid cooling of air over those laminar surfaces abruptly disrupting laminar flow, causing rapid melting and rapid transition back to laminar flow. Maximum achievement was 95% laminar flow over those areas desired. However, the design effort was cancelled due to the plugging problems.