Re: 747 hump

Date:         16 Sep 97 02:35:26 
From:         Andrew Cruickshank <andrew@openkast.demon.co.uk>
Organization: OpenKast
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Agtabby wrote:
>
> Maybe a related question, why do 747's fly faster than other jets?  I've
> heard they can make a 12 hour flight about an hour shorter, is this true?

There are several aspects to this:

 a) the 747 is a 60's design - pre-fuel crisis
    and older turbofan technology

 b) it's got four engines - less engine drag
    than an equivalent sized twin

    a twin version of the 747's size has to
    carry a lot of excess engine (big wide fans)
    to handle engine failure on takeoff

 c) it's big and heavy so a bit of extra speed
    at the beginning of a cruise to generate
    more lift is quite useful until the fuel
    weight drops

 d) long range flight times are affected by
    winds - faster speed into a headwind
    proportionally reduces the time you burn
    fuel so your range increases

The 777 may contradict (b) but I've seen figures of
mach 0.82 and 0.86 quoted for the A330 and A340
respectively for engine configurations that have
the same combined thrust totals.

---------------------
Andrew Cruickshank