bypassratios

Bypass ratio is a characteristic of turbofan engines that describes how much air drawn in by the fan bypasses the engine core relative to the air that passes through the core. In a turbofan, the incoming air splits into bypass flow and core flow. The bypass flow is accelerated by the fan and provides most of the thrust in high-bypass engines, while the core flow passes into the compressor, combustor, and turbine to generate additional thrust.

The bypass ratio (BPR) is typically defined as BPR = m_dot_bypass / m_dot_core, where m_dot_bypass is the mass flow of air that bypasses the core and m_dot_core is the mass flow through the core. Since the total mass flow equals the sum of both streams, m_dot0 = m_dot_bypass + m_dot_core.

Implications: Higher bypass ratios generally yield greater propulsive efficiency and lower specific fuel consumption at subsonic cruise, along with reduced noise, because a larger mass of air is accelerated by the fan at a lower velocity. However, a higher BPR usually means a larger, heavier, and more complex engine, with lower thrust-to-weight in some regimes. Lower bypass ratios produce higher exhaust velocities and can deliver more thrust in a compact package, which is common in many military or high-thrust applications.

Typical ranges: Modern civil turbofans commonly have BPR in the range of about 6 to 12, with some engines surpassing 12 for efficiency at cruising speed. Older or military engines may operate with BPR around 2 to 4 or be turbojets with effectively no bypass.

Variations: Bypass ratio can change with throttle, altitude, and engine design, and some modern engines employ variable or multiple-bypass architectures to optimize performance across flight regimes.