Compared to turbojets, turbofans provide what relationship of thrust to air mass?

• A. Higher thrust for a given mass of air
• B. Comparable thrust for a given mass of air
• C. Lower thrust for a given mass of air
• D. Thrust is independent of air mass

Answer: (C)

Think in terms of specific thrust—the thrust you get for each unit of air flowing through the engine. A turbojet accelerates a relatively small mass of air to a high exit speed, so the velocity change per unit mass is large and the thrust per unit air mass is high. A turbofan, on the other hand, moves a much larger mass of air because of the big bypass flow, but the air in that bypass is accelerated to a much lower exit speed. The momentum change per unit mass is therefore smaller, so the thrust you get per unit mass of air is lower.

So, while a turbofan can produce a lot of total thrust thanks to the huge bypass mass flow, the thrust per mass of air is less than that of a turbojet.