# What is this number in the bypass ratio?

Here is an example of a bypass ratio 9:6:1. 9kg goes around the core and 6kg goes into the core, but what is the 1 for.

• By the way this is the bypass ratio for the General Electric GEnx-1B engines on the 787 Dreamliner. – Ethan Sep 4 '15 at 22:49

The bypass ratio of a turbofan engine is the ratio of the mass flow rate of the air that bypasses the engine core (i.e. it doesn't undergo combustion),$\dot{m_{f}}$ to the mass flow rate of the air that passes through the core $\dot{m_{c}}$ and undergoes combustion.

The bypass ratio is usually given as the mass flow of fan by the mass flow of core i.e. $\frac{\dot{m{f}}}{\dot{m_{c}}}:1$. In some cases, it is given as the ratio of cold air to hot air, $\frac{\dot{m{c}}}{\dot{m_{h}}}:1$. So, a bypass ratio of 9.6:1 means that 9.6 kg of air passes through the bypass fan for every 1 kg of air passing through the core.

It is 9.6:1 not 9:6:1.

9.6 parts air are bypassed for each part that goes through the core.

• That makes more sense – Ethan Sep 4 '15 at 23:26