The initial lift and drag of an impulsively started airfoil of finite thickness

The method of apparent masses is utilized to compute the initial lift and drag of an airfoil that starts impulsively from rest. Analytical solutions are obtained for in viscid incompressible flow past a slightly cambered airfoil at a small angle of attack. For a Joukowski airfoil with a cusped trailing edge, it is found that increasing camber or angle of attack will cause increases in both initial lift and drag, whereas increasing thickness will result in an opposite effect. Effects of trailing-edge angle are examined by considering the symmetric Kármán-Trefftz airfoil. The result shows that both lift and drag vanish at the initial instant if the airfoil has a finite trailing-edge angle.