Re: Three out of four on a -747?

From:         Joe Diederichs <joed@lsid.hp.com>
Organization: Hewlett-Packard
Date:         08 Aug 95 02:18:24 
References:   1 2
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"M.A. van der Eijk" <M.vanderEijk@LR.TUDelft.NL> wrote:
>Keith Edmunds <kae@itworks.demon.co.uk> wrote:
>>On more than one occasion I've noticed three contrails from a B-747.
>>Is it likely that one engine would be shut down?

>Yes, it is possible. It of course does not happen often, but ferry flights
>(from one, smaller, airport to another, bigger, to perform maintenance on
>an engine) can be flown with 3 in stead of four engines.
>Another possibility is that one engine's temperature differs from the
>other ones.

I think this is an asymmetry in the airflow around the plane, or
perhaps it is due to the direction of rotation of the engines.  I've
noticed this for years on 747's and DC-10's.  On DC-10's, the middle
trail seems to join up with one side right behind the plane, leaving
sort-of two trails behind.  On the 747, one side seems to blend
together right behind the plane, leaving what looks like three.

There is an interesting photo in an ad in an electronics design
magazine I get at work relating to this.  "EDN" magazine, July 20,
1995, page 30, has an ad from "Vicor" which uses a telephoto shot of a
747 at altitude.  This photo clearly shows the con-trails on the right
side of the plane appearing as one about three plane-lengths behind the
plane, whereas on the left side, the trails are distinct.

Joe Diederichs
joed@lsid.hp.com