EXAMPLE 4.1 Consider an NACA 2412 airfoil with a chord of 0.64 m in an airstream at standard sea level conditions. The freestream velocity is 70 m/s. The lift per unit span is 1254 N/m. Calculate the angle of attack and the drag per unit span. #24 PART 2 Inviscid, Incompressible Flow Solution At standard sea level, p = 1.23 kg/m: 900 =P V = (1.23)(70) =3013.5 N/m L' L' C = = 1254 3013.5(0.64) = 0.65 900 S qooc(1) From Figure 4.10, for c = 0.65, we obtain a = 4 To obtain the drag per unit span, we must use the data in Figure 4.11. However, since cd = f(Re), let us calculate Re. At standard sea level, = 1.789 x 10-5 kg/(m-s). Hence, Re= Poo Voc foo 1.23(70) (0.64) 1.789 x 10-5 = 3.08 x 106 Therefore, using the data for Re = 3.1 x 10 in Figure 4.11, we find ca = 0.0068. Thus, D' = qoo Scd = qooc(1)cd=3013.5(0.64)(0.0068) = 13.1 N/m