Computation of Wetted Areas
Computation of the wetted area, Aw, of the aircraft component is shown herein. Skin friction is generated on that part of the surface over which air flows, the so – called wetted area.
These are approximate to the flat surfaces, with the wetted area slightly more than twice the reference area due to some thickness. Care is needed in removing the areas at intersections, such as the wing area buried in the fuselage. A factor к is used to obtain the wetted area of lifting surfaces, as follows:
Aw = к x (reference area, S – the area buried in the body),
where к = 2.02 for t/c = 0.08%
= 2.04 for t/c = 0.12%
= 2.06 for t/c = 0.16%
The factor к may be interpolated linearly for other t/c ratios.
The fuselage is divided conveniently into sections – typically, for civil transport aircraft, into a constant cross-section mid-fuselage with varying cross-section front – and aft-fuselage closures. The constant cross-section mid-fuselage barrel has a wetted area of Awfmid = perimeter x length.
The forward- and aft-closure cones could be sectioned more finely, treating each thin section as a constant section “slice.” A military aircraft is unlikely to have a constant cross-section barrel, and its wetted area must be computed in this way. The wetted areas must be excluded where the wing and empennage join the fuselage or for any other considerations.
Only the external surface of the nacelle is considered the wetted area and it is computed in the same way as the fuselage, taking note of the pylon cutout area. (Internal drag within the intake duct is accounted for as installation effects in engine performance as a loss of thrust.)