Bypass ratio

This allows them to be shown together with turbofans on plots which show trends of reducing specific fuel consumption (SFC) with increasing BPR.

[2][3][4][5] High bypass designs are the dominant type for commercial passenger aircraft and both civilian and military jet transports.

If it is all transferred to a separate large mass of air with low kinetic energy, the aircraft is best suited to zero speed (hovering).

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.

[7] Power is transferred from the gas generator to an extra mass of air, i.e. a larger diameter propelling jet, moving more slowly.

In a bypass design, extra turbines drive a ducted fan that accelerates air rearward from the front of the engine.

[13] Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency.

Nevertheless, high-bypass engines have a high propulsive efficiency because even slightly increasing the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through the engine) multiplied by the difference between the inlet and exhaust velocities in—a linear relationship—but the kinetic energy of the exhaust is the mass flow multiplied by one-half the square of the difference in velocities.