Re: B747 technical questions

Date:         11 Jul 97 02:09:24 
From:         k_ish <>
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187 wrote:
> 1) For the 5th engine which can be installed on the pod under the wings,
> can it be used as part of the aircraft's engine thereby making the
> aircraft run on five engines?

No, this is simply a "hardpoint" to haul spare engines around.  I
personally have never seen this feature actually used.  The engine
provides no thrust (and some amount of extra drag).  FYI, the 747 does
contain a fifth engine, called an Auxiliary Power Unit (APU).  It is a
small gas turbine (jet) engine mounted in the tailcone, and provides
electrical power, climate control, engine starter air, etc. to the
aircraft, especially when parked on the ground.  It provides a small
amount of additional thrust, but this is just incidental to its main
function.  Most airliner have APUs; you can see the exhaust tube
sticking out of the tail.

> 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?

Don't know if bigger or smaller is better. A jet engine works by taking
a mass of air, adding energy in the form of pressure and temperature,
and accelerating it rearward. (like standing on a skateboard and
throwing a will roll in the opposite direction).  The
more the air can be compressed, the more energy it will contain, and the
more thrust that can be acheived.

> 3) What is in the inside of an aircraft tyre?  I mean is it
> filled/pumped up with air like that of car tyres or is it built as a
> solid whole piece??

Aircraft tires are filled with gas; air on smaller aircraft, and dry
nitrogen on large aircraft.  The wheel/rim assemblies are built up out
of several pieces, like a truck wheel.  The wheel assembly also contains
disk brake assemblies; there can be 1-8 disks per wheel depending upon
the aircraft.  Tire pressure ranges from 30 psi on small aircraft such
as Cessnas, up to 180-200 psi on large airliners.  Tires are inflated in
heavy metal "cages"- if a wheel breaks apart during inflation, it is
like a bomb going off!

Hope this answers your questions (except for EPR).

Ken Ishiguro