What do the digits of a NACA four-digit airfoil represent?

• A. Maximum camber, location of maximum camber, and thickness as percent of chord
• B. Leading-edge radius, trailing-edge angle, span
• C. Surface roughness, viscosity, Mach number
• D. Thickness distribution only

Answer: (A)

NACA four-digit airfoils encode the shape directly in the digits. The first digit is the maximum camber as a percent of the chord, the second digit is the location of that maximum camber along the chord (in tenths of the chord from the leading edge), and the last two digits are the maximum thickness as a percent of the chord. Together, these numbers define the camber line and the thickness distribution of the airfoil.

For example, a 2412 airfoil has 2% camber with the maximum camber at 40% of the chord from the leading edge, and a maximum thickness of 12% of the chord. The other options describe parameters that aren’t part of the four-digit code (like leading-edge radius, span, surface roughness, viscosity, Mach number) or only mention thickness, which doesn’t capture the camber information.