Re: A340 speed

From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
Date:         12 Jul 96 11:49:52 
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In article <airliners.1996.1214@ohare.Chicago.COM>, petercoe@netcom.com
(Pete Coe) wrote:

> I thought the 747 was historically the fastest of the current airliners
> anyway.  More recent design practice has decided that it's cruising
> speed is a little to high, so one would expect the A340 to be slower.

There is no such thing as too high a cruise speed.  The faster the plane
gets there, the better.  HOWEVER, when increasing the cruise speed
increases the fuel burn to the point where you sacrifice range or burn
more fuel than the extra speed is worth, then speed becomes a factor.
This is the case with the A-340.  The 747 meets its range, fuel burn, and
payload targets at a relatively high cruise speed.  The A-340 can cruise
every bit as fast as a 747 with no problems as far as flying is concerned.

The problem is drag, which as you know, goes up as the speed goes up.  In
order for the A-340 to achieve the very long ranges of which it is
capable, the cruise speed must be lowered to reduce the fuel burn.  Part
of the problem is that the A-340 apparently ended up with more drag from
the outboard engines and pylons than was expected.  When the cruise speed
is pushed up, this extra parasite drag, plus the increased induced drag
you're going to get anyway, increased the fuel burn to the point where the
plane could not make the ranges it was supposed to with the advertised
payload.  Thus the lower cruise speed in comparison to the 747.

To my knowledge, Boeing has never said the A-340 CAN"T cruise at whatever
figure is claimed for it by Airbus, but that if it does, its range or
payload will have to be reduced accordingly.  If you look at the
comparison of flight times on longer flights between the A-340 and the
747-400, the difference in cruise speed becomes very apparent.  The 747
arrives a full 45 minutes sooner on some flights like Narita to Heathrow.

C. Marin Faure
   author, Flying a Floatplane