Nacelle Base Drag

Among the many items that are included in an explicit manner, one usually estimates the drag increment associated with a small gap between the engine nozzle and the nacelle. Nacelle base drag is a small item, but is representative of the types of drag components that one tries to model in a realistic manner.

Most turbofan engines maintain a gap between the engine nozzle and nacelle of about 1/2 inch. Flow separates and creates a base drag area that may be estimated as the base area (.5 inch times the circumference at the nozzle exit) multiplied by a drag coefficient of about 0.2.

So C_{D_{nacelle_base}} = 0.5/12 * p * D_exit * 0.2 / S_ref

with the nozzle exit diameter measured in feet.