Bypass ratio

The normal definition for the bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for every 1 kg of air passing through the core. Note that in an aft fan engine, like the General Electric CJ805-23, all of the fan air enters the bypass stream, whereas on most turbofans only the air entering the outer section of the fan passes to the bypass duct. Another special case is the General Electric TF39 where most of the fan air plus some of the low pressure compressor air enter the bypass duct.

Turbofan engines are usually described in terms of bpr, which together with overall pressure ratio, turbine inlet temperature and fan pressure ratio are important design parameters. In addition bpr is quoted for turboprop and unducted fan installations because their high propulsive efficiency gives them the overall efficiency characteristics of very high bypass turbofans. This allows them to be shown together with turbofans on plots which show trends of reducing sfc with increasing bpr. Bpr is also quoted for lift fan installations where the fan airflow is remote from the engine and doesn't physically touch the engine core.

Bypass provides a lower fuel consumption for the same thrust, measured as thrust specific fuel consumption (grams/second fuel per unit of thrust in kN using SI units). Lower fuel consumption that comes with high bypass ratios applies to turboprops, using a propeller rather than a ducted fan. High bypass designs are the dominant type for commercial passenger aircraft and both civilian and military jet transports.

Business jets use medium bpr engines.

Combat aircraft use engines with low bypass ratios to compromise between fuel economy and the requirements of combat: high power-to-weight ratios, supersonic performance, and the ability to use afterburners.

If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle the aircraft is best suited to high supersonic speeds. If it is all transferred to a separate big mass of air with low kinetic energy the aircraft is best suited to zero speed (hovering). For speeds in between, the gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required. The first jet aircraft were subsonic and the poor suitability of the propelling nozzle for these speeds due to high fuel consumption was understood, and bypass proposed, as early as 1936 (U.K. Patent 471,368). The underlying principle behind bypass is trading exhaust velocity for extra mass flow which still gives the required thrust but uses less fuel. Whittle called it "gearing down the flow". Power is transferred from the gas generator to an extra mass of air, i.e. a bigger diameter propelling jet, moving more slowly. The bypass spreads the available mechanical power across more air to reduce the velocity of the jet. The trade off between mass flow and velocity is also seen with propellers and helicopter rotors by comparing disc loading and power loading. For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor.

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Wikipedia