1. Keenan, Joseph H., and Kaye, Joseph, Thermodynamic springer-Verlag, Berlin, 1943. Properties of Air, pp.36, 58;John Wiley & Sons, Inc., New York, 1945. (Continued on page 58) plest case, this would result in the use of a rectangular wing with a flat lower surface (very common 10 years ago), b u t it is also likely to affect thickness distribution and surface curvature. Peculiar twists and nonuniform curvature will frequently be so difficult to form t h a t the wing incorporating t h e m m a y be so distorted when it comes out of the shop as to definitely indicate some simpler shape would have given better aerodynamic results. This manufacturing problem m u s t not be overlooked in choosing airfoils.