In reality, the boundary layer on the airfoil discussed in prob. 5.37 is neither fully laminar nor fully turbulent. the boundary layer starts out as laminar, and then transitions to turbulent at some point downstream of the leading edge (see the discussion in sec. 4.19). assume that the critical reynolds number for transition is 650,000. calculate the skin friction drag coeffi cient on the naca 2415 airfoil, and compare your result with the experimental section drag coeffi cient in app. d. note: you will fi nd from the answer to this problem that 86 percent of the airfoil section drag coeffi cient is due to skin friction and 14 percent due to pressure drag from fl ow separation. comparing this answer with the result of prob. 5.36 , which pertains to a thinner airfoil, we fi nd that the pressure drag is a higher percentage for the thicker airfoil. however, for airfoils in general, the pressure drag is still a small percentage of the total drag. this drag breakdown is somewhat typical for airfoils at small angles of attack. by intent, the streamlined shape of airfoils results in small pressure drag, typically on the order of 15 percent of the total drag.