Expert: Paul Soderman - 10/27/2009

Question
QUESTION: What basis Bypass ratio has chosen for turbofan and turbojet engine? Bypass ratio varies from 5:1 to recent Ge 90 115B1 has 9.4:1. how to choose bypass ratio for various thrust range? what are the parameters involves in choosing bypass ratio?



ANSWER: A turbofan is a much quieter, more efficient engine than a turbojet.  So you would like to achieve the highest bypass ratio possible.  But there are constraints such as geometry, weight and drag. I assume, therefore, that engine developers have designed families of turbofans to fit different aircraft and different missions.  For example, a small business jet with low wing could not support a high-bypass turbofan that would hit the ground.  A large 747 transport could support a much larger engine.  So aircraft and engines, including bypass ratio, are designed after a thorough system analysis involving all possible factors.

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QUESTION: Yes sir i accept your answer. if bypass ratio increases, the size and weight of the engine also increases
i have following questions

1.is there any relation between thrust(net thrust - Fan + Core) and bypass ratio?
2. is there any relation between core thrust and bypass ratio?
3. is there any relation between turbine power( to drive the compressor fan) and bypass ratio?  

Answer
An engine with a bypass ratio of 2 will pass 2 kg/s of air through the fan for every 1 kg/s through the core.  If you know the density and velocity of the two exhausts you could compute the thrust as:

T = (rho A V) V  where rho = density, A = exhaust area, V = exhaust velocity.  Density varies with temperature and fuel/air mixture, so the computation is not trivial.  The velocity of the core depends on the combustor design and is reduced by the energy lost to drive the turbine. The number of turbine stages depends on design. The velocity of the fan depends on the fan design.  So each engine will have a unique mix of velocities, temperatures, densities, etc., so there is no general equation that depends only on bypass ratio that I know of.