Wetted Area Calculations
In order to compute the skin friction drag, it is necessary to multiply this coefficient by the wetted area. For wing-like surfaces, the wetted area is related to the exposed planform area. It is a bit more than twice the exposed area because the arc length over the upper and lower surfaces is a bit longer than the chord:
The exposed area is that portion of the wing planform that is exposed to the airflow. It does not include the part of the wing buried in the fuselage, but does include any chord extensions.
For bodies, the wetted area can be computed by adding the contribution of the nose section, constant section, and tapered tail cone. This requires knowledge of the actual fuselage shape, but for typical transport aircraft, the wetted area of the nose and tail cone may be approximated by:
where D is the diameter of the constant section and L is the length of the nose or tail cone. For elliptically-shaped fuselage cross-sections, of width W, and height H, an approximate formula for the perimeter may be used to estimate an effective diameter. One such expression is given below.
Deff = (W/2 + H/2) (64 - 3 R4) / (64-16 R2)
where: R = (H-W)/(H+W)