Vortex generator installation drag on an airplane near its cruise condition

A method is discussed for predicting the drag increment caused by the installation of a blade-type vortex generator (VG) on a transonic-transport airplane. The original Nash and Bradshaw magnification concept of roughness drag is extended to cover compressible flows and then is applied to VG blades to estimate the VG installation drag on an airplane. The drag of a VG blade placed on a wing will be amplified due to the growth of the boundary layer with distance along the wing surface. Nash and Bradshaw showed that the degree of magnification cannot be approximated simply by the ratio of local to freestream dynamic pressure (q effect). To demonstrate the magnification effects, some VG installation drag analyses for transonic-transport airplane models are performed using the new magnification factor formula. It can be seen that the agreement between these predicted drag increments and the wind-tunnel test results is good, but the drag increment based on the q effect is seriously underestimated.