A high bypass engine results in which of the following?

• A. overall slower airflow and greater propulsive efficiency.
• B. overall faster airflow.
• C. greater propulsive efficiency.
• D. No change in airflow.

Answer: (D)

A high bypass turbofan improves propulsive efficiency by moving a large mass of air with a relatively small increase in jet velocity. The big fan pulls in a lot of air, and most of it bypasses the core and exits through the nozzle at a much lower speed than the core exhaust would in a turbojet. This lower jet velocity means less kinetic energy is wasted in the exhaust, so for the same thrust you get more useful propulsion energy—i.e., higher propulsive efficiency.

Note that the overall airflow into the engine does increase because the fan is moving more air, but the important point is the balance: more mass flow with a slower exhaust improves efficiency. The statement that there is no change in airflow isn’t correct in this context; the air moved by the fan is significantly larger, while the exhaust velocity is reduced, yielding the higher propulsive efficiency.