Re: 2 vs 4 engines: R&D costs too much ?

Date:         21 Nov 99 23:56:49 
From:         "Robert Wright" <kdol97@home.com>
References:   1 2 3 4
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>
>I've been wondering if anybody ever considered building a three
>engine airplane, with all of the engines on the wing.  Now, I
>recognize that this is going to be funny looking, but work
>with me for a minute.

    Actually, we did take a peak at this one!

>You'd mount the single engine substantially farther out the
>wing than either of the two engines on the other side, so
>the thrust and weight moments could be not too far off.  You'd

    Until you lost an engine.  You'd have to design for a worst-case
scenario - losing the one engine set way out on one waing, so you'd need a
huge tail to deal with that.  If you put two somewhat larger engines in
close to the fuselage you could have a smaller tail and therefore less drag
and weight.

    Then there's manufacturing.  Having one engine hung way out presents a
very different set of vibration responses and structural characteristics
than two engine places evenly along the span, if I understand correctly.
(Anyone here work on the A330/340 wing?  That would be the best example.)
You'd either have to design and manufacture two totally different wings,
leading to more complexity, higher part count, and spare parts headaches, or
you'd have to design a win to handle either two engine in one set of mounts
or a single engine in a third location.  This would be an interesting
challenge, but not conducive to minimal weight and expense.  Also, you'd
then have to design your wing profile differently, since the engine nacelle
interacts quite a bit with the wing in modern jets which have the engines
hung up very close to the wings.

>Perhaps there would be some odd responses in different
>corners of the flight envelope, but modern computerized
>flight controls could hide those.

    There's a limit to what you can do with computers.

>You'd have a plane with more than two engines, but with
>less weight and complexity than a four engine plane, and
>you'd have all the engines on the wings (where they belong).
>You might place the single engine on the port side, so that
>there'd only be one engine on the side where jetways attach
>to the airplanes.

    I think in the end it would just be cheaper and easier to go with four
engines, or else bite the bullet and go for ETOPS.

    Before anyone asks, yes, we did look at twin fuselages with a third
engine between them.

Have a nice day.

RSW