English: Two different and incompatible definitions of airfoil thickness are used in the aerodynamics field (here illustrated on the same airfoil).

• In case A above the thickness is measured perpendicular to the foil camber line. This is sometimes described as the "American convention"^[1] and can be seen in Houghton & Carpenter 2003,^[1] Phillips 2010.^[2]

• In case B above the thickness is measured perpendicular to the foil chord line. This is sometimes described as the "British convention" and can be seen in Bertin & Cummings 2009^[3].

Note that each definition implies a different camber line geometry for a given airfoil. Since the camber line curvature is small on most aircraft, the effect of difference between the definitions is often small.

1. ↑ ^{a b} Houghton, E. L.; Carpenter, P.W. (2003) Butterworth Heinmann, ed. Aerodynamics for Engineering Students (5^th ed.) ISBN: 0750651113. p.17
2. ↑ Phillips, Warren F. (2010) Mechanics of Flight (2^nd ed.), Wiley & Sons ISBN: 9780470539750. p.27
3. ↑ Bertin, John J.; Cummings, Russel M. (2009) Pearson Prentice Hall, ed. Aerodynamics for Engineers (5^th ed.) ISBN: 9780132272681. p.199