Re: Some Turbofan Questions

From:         Robert & Linda Wilson <>
Date:         03 Jul 96 01:23:54 
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  or MIME structure wrote:
>    I have some questions about high bypass turbofan engines:
>    What proportion of the thrust is generated by the
> bypass air and what proportion comes from the exhaust
> gas generated in the core of the engine?

As a first approximation you can assume that the thrust ratio is in
proportion to the bypass ratio. i.e. If you have a 4:1 bypass ratio 80%
will come from the fan and 20 % from the core.

> (next question, how much does it mix?) with the exhaust gas,
> but give me an estimate anyway.

If the engine has separate hot and cold nozzles  like on the P&W JT9D or
ROLLS RB211-524 you will have zero mixing. On engines with mixers, i.e.
the fan and core streams are mixed and ejected through a common final
nozzle like ROLLS RB211-535E4 mixing numbers can be of the order of 70%
with a well designed mixer. This gives a worthwhile reduction in noise
and specific fuel consumption

>    Also, are there any turbofans with more than one fan (not
> compressor) stages?

The Pegasus engine (Harrier Jump Jet) has a multi stage fan.

> And while I'm still in question mode,
> did early commercial jet engines (Comet, 707 etc) use
> any bypass air, or did it all go through the core?

Comets were originally engined with RR Avons - zero bypass. The 707's
bought by BOAC were engined with RR Conways. These, I think, were the
first bypass engines in comercial service, however the bypass level was
very low by today's standards. Less tha 0.5 if my memory serves me right.


Rob Wilson

ex RR and P&W performance.