Question

In a high-bypass turbofan engine, how does increasing the bypass ratio affect the total thrust generated during takeoff?

Accepted Answer

In a high-bypass turbofan engine, increasing the bypass ratio significantly increases the total thrust generated during takeoff. The bypass ratio is the ratio of the mass of air that flows around the engine core to the mass of air that passes through the engine core. A high-bypass engine works by using a small gas turbine core to spin a large fan at the front. This fan pushes a vast amount of air around the outside of the core. Thrust is determined by the formula for momentum change, which is the mass of the air multiplied by its change in velocity. By increasing the bypass ratio, the engine processes a much larger mass of air. While this bypass air is accelerated to a lower velocity than the hot exhaust air exiting the core, the massive increase in the volume of air moved outweighs the lower velocity, resulting in a net increase in total thrust. This process is more efficient at lower speeds, such as during takeoff, because moving a large mass of air slowly is more effective at generating propulsive force than moving a small mass of air very quickly. Consequently, increasing the bypass ratio allows the engine to produce more takeoff thrust while simultaneously reducing fuel consumption and noise.

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In a high-bypass turbofan engine, how does increasing the bypass ratio affect the total thrust generated during takeoff?