In most piston engine powered planes, the cooling air is somehow released by a slit and flows along the fuselage (e.g. in all classic cowlings with flaps of radial engines). With such a design, the warm (=high viscosity) and turbolent air is attached to the fuselage, which will quite certainly increase drag. I believe, a NACA duct might (or might not) also work in reverse and bring a seperation between the outflowing cooling air and the boundary layer. I haven’t seen such a design in any aircraft, but I’m sure someone has tested it, does anybody know?