Aerodynamic Drag of Engine-Cooling Airflow With External Interference

This report examines the aerodynamic drag and external interference of engine cooling airflow. Much of the report is on inlet interference, a subject that has not been discussed in automotive technical literature. It is called inlet spillage drag, a term used in the aircraft industry to describe the change in inlet drag with engine airflow. The analysis shows that the reduction in inlet spillage drag, from the closed front-end reference condition, is the primary reason why cooling drag measurements are lower than would be expected from free stream momentum considerations. In general, the free stream momentum (or ram drag) is the upper limit and overstates the cooling drag penalty. An analytical expression for cooling drag is introduced to help the understanding and interpretation of cooling drag measurements, particularly the interference at the inlet and exit. Empirical thrust coefficients, which represent the interferences, are introduced as a practical representation of the interaction to the exterior pressure distribution. Comparisons to experimental measurements on three vehicles are presented to illustrate the concepts.