Compared with turbojets, turbofans have which characteristic?

• A. Higher thrust for a given mass of air
• B. Higher exhaust velocity for the same thrust
• C. Lower thrust for a given mass of air
• D. Similar thrust and efficiency

Answer: (C)

The idea being tested is how a turbofan’s bypass air affects thrust compared with a turbojet. Thrust comes from changing the momentum of the air that leaves the engine, roughly F ≈ mdot × (V_exit − V_in). In a turbojet, almost all the incoming air goes through the core and is expelled at a high velocity, so the air’s velocity change is large and the thrust per unit mass of air is high.

In a turbofan, a large portion of the incoming air is diverted around the core (bypass air) and accelerated by the fan to a much lower exit velocity. The core still produces high-velocity exhaust, but the overall effect is that the average velocity change per unit mass of air leaving the engine is smaller, even though more total air is being pushed out. Therefore, for the same mass of air entering the engine, the turbofan yields a smaller thrust than a turbojet.

That’s why turbofans are valued for higher efficiency and lower noise, especially at subsonic speeds, even though the thrust per unit mass of air is lower than a turbojet.