DRAG MEASUREMENT IN TRANSONIC WIND TUNNELS.

The basic difficulties in assessing the drag of slender bodies by transonic wind tunnel testing are described. It is noted that, on the one hand, drag measurement on part models is very sensitive even to small errors in the pressure level of the tunnel and that, on the other hand, the high accuracies required for this type of measurement usually are not achieved. In order to increase the accuracy it is recommended that the simultaneously measured wall pressures be taken into account. By linking these wall pressures with theoretical wall interference computations, it seems possible to approach the ″absolute″ limit of accuracy. However this also requires consideration of the axial pressure gradients produced by the tunnel wall or by inappropriate model suspensions. An example shows that these pressure gradients can cause errors of more than 100% in the absolute pressure drag and about 20% in the drag differences. The influence of the Reynolds number on afterbody drag and on wing shock location is critically reviewed, and the variation of the wind tunnel boundary layer is suggested as the prime cause for these effects. Lastly, unsteady flow separation problems are briefly touched upon, and general recommendations for improved drag assessment are made.