To reduce drag, an ideal inlet system takes the cooling air on board and slows it down without losing energy to flow separation and turbulence.

The cooling air inlet serves two functions: First, and most important, the inlet must ingest enough air to properly cool the engine and accessories over the entire range of airspeed and power settings at which the airplane will fly. Secondly, the inlet should be designed to ingest the cooling air efficiently and minimize the drag penalty due to cooling flow. To do this the inlet should manage the airflow in a way that preserves the energy in the cooling airflow and disturbs the external flow over the airplane as little as possible.