Abstract:

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.

Two worked examples illustrate the use of the method.