Re: B747 technical questions

Date:         13 Jul 97 19:52:40 
From:         Wolfgang Decker <wdecker@flash.net>
Organization: Flashnet Communications, http://www.flash.net
References:   1
Next article
View raw article
  or MIME structure

187 wrote:
>
> Some questions:-
>
> 2) What is the best EPR number to be attained?  Is it the smaller the
> number the better or the greater the better?  Since EPR=turbine
> discharge pressure: compressor inlet pressure.  And why?

Basically thrust is generated by accelerating mass into the opposite
direction you are going. This is, as Ken Ishiguro wrote, like standing
on a skateboard and throwing a basketball. Mathematically this looks
like

F = (m/t) * (v2 - v1)

where F is the thrust, (m/t) is the mass of air per time (t) unit, also
called mass flow, v2 is the outlet speed and v1 the inlet speed, which
is  lowest when standing on the ground. The outlet speed of course is
determined by the pressure in the nozzle.

Now one can see that increasing the thrust is possible either by
increasing the mass flow or the speed gradient. Latter was done in the
very first time of jet engines, where no bypass was used. Looking at the
efficiency coefficient (unfortunately I don't remember the derivation
and the formula exactly, but it basically has the form e = v1 / (v2-v1)
), the efficieny of a jet egine is higher, if you maximize the mass flow
by minimizing the speed gradient. This is why modern engines have this
very high bypass ratio of larger than 6:1.

Now, the question which epr is best can't be answered that easy. It is
first determined by the engine type, and secondly by the current flight
condition, which includes air density (therefor altitude), inlet air
temperature and speed of aircraft.

I hope, this gives some additional aspects

Wolfgang Decker
wdecker@flash.net

I prefer to fly without engines!