NASA Aerofoils

A concerted effort within the National Aeronautics and Space Administration (NASA) during the 1960s and 1970s was directed toward developing practical aerofoils with two-dimensional transonic turbulent flow and improved drag divergence Mach numbers while retaining acceptable low-speed maximum lift and stall characteristics and focused on a concept referred to as the supercritical aerofoil. This distinctive aerofoil shape, based on the concept of local supersonic flow with isentropic recompression, was characterized by a large leading-edge radius, reduced curvature over the middle region of the upper surface, and substantial aft camber.

The early phase of this effort was successful in significantly extending drag-rise Mach numbers beyond those of conventional aerofoils such as the National Advisory Committee for Aeronautics (NACA) 6-series aerofoils. These early supercritical aerofoils (denoted by the SC (phase 1) prefix), however, experienced a gradual increase in drag at Mach numbers just preceding drag divergence (referred to as drag creep). This gradual buildup of drag was largely associated with an intermediate off-design second velocity peak (an acceleration of the flow over the rear upper-surface portion of the aerofoil just before the final recompression at the trailing edge) and relatively weak shock waves above the upper surface.

Improvements to these early, phase 1 aerofoils resulted in aerofoils with significantly reduced drag creep characteristics. These early, phase 1 aerofoils and the improved phase 1 aerofoils were developed before adequate theoretical analysis codes were available and resulted from iterative contour modifications during wind-tunnel testing. The process consisted of evaluating experimental pressure distributions at design and off-design conditions and physically altering the aerofoil profiles to yield the best drag characteristics over a range of experimental test conditions.

The insight gained and the design guidelines that were recognized during these early phase 1 investi­gations, together with transonic, viscous, aerofoil analysis codes developed during the same time period, resulted in the design of a matrix of family-related supercritical aerofoils (denoted by the SC (phase 2) prefix). Specific details about these profiles can be found in Reference 1.