On Airfoil Theory and Experiment

DURING THE EARLY YEARS of airfoil theory, lift curve slopes were so far below the thin airfoil value of 2TT per rad. t ha t it became customary to use t ha t criteria for thick airfoils as well, and both in writing m d conversation the 2-K value (0.1095 per deg.) became i s tandard tha t was broadly accepted. In a similar way the thin airfoil approximation, t ha t the aerodylamic center was a t the quarter-chord, became "standard," and, indeed, for a long period all airfoil tests reDorted demonstrated aerodynamic centers a small l istance ahead of the quarter-chord. When it became possible in the light of newer in'ormation to design airfoils with more advantageous Dressure distributions, disturbing results appeared. Lift curve slopes were higher than the first approxima;ion theory, and aerodynamic centers appeared behind ;he airfoil quarter-chord. I t became a t once necessary ;o re-examine the approximations previously made for ;hin airfoils in order to include the effect of thickness. To begin with, let us note the equation for the lift :urve slope as shown in reference 1 (with one addiional te rm of the series retained).