ESDU 07002:2011

Viscous drag coefficient is defined in ESDU 07002 as the drag
coefficient arising from all the effects of viscosity. Equations
and graphs are provided for predicting the viscous drag coefficient
of wings with sweep angles not exceeding 45°, aspect ratios not
exceeding 12, maximum section thickness to chord ratios within the
range 0.1 to 0.2 and up to 10° wash-in or wash-out.

ESDU 07002 is applicable to values of lift coefficient at which
both upper and lower surface boundary layers are fully attached, in
flow fields where shockwaves are not present. The method is
applicable to both straight-tapered and cranked wing planforms and
where the section geometry, including camber, either remains
unaltered or varies with spanwise location. Knowledge of a constant
chordwise location of boundary-layer transition across the span is
required. The method is based on a semi-empirical investigation
using strip theory presented in ESDU 07003.

As a means of testing the validity of the method, polars of
predicted and measured total drag coefficient for five wing
geometries are compared. The predicted total drag coefficient was
obtained by adding values of inviscid trailing-vortex drag
coefficient (predicted using ESDU 10022) to values of viscous drag
coefficient obtained by the method of this Data Item. In most cases
there is agreement to within about 10 drag counts
(C_D = 0.001) between the predicted data and the
test data, over the ranges of lift coefficient where the boundary
layers are indicated as being fully attached. The method is
appropriate for initial project calculations, where an agreement to
within 10 drag counts might be considered reasonable if the effects
of configuration changes rather than absolute values are of
interest.