Wing vs. tail-mounted engines?

Date:         23 Mar 2001 17:40:08 
From:         Wolfgang Keller <w_keller@gmx.de>
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Hello,

as a passenger, I've always preferred aircraft with tail-mounted engines
(German BA's F100, for example) over those with wing-mounted engines like
the 737 - because they are usually less noisy, because I felt a lot more
safe with those burning chambers far behind me, away from the ground and
from the fuel tanks (not that flying makes me frightened) and...

[tomato-shield on]

...because they look so much more elegant. :-)

[tomato-shield off]

As an engineer who had also some classes about aerodynamics, structural
and engine design at university, I always thought that the relevant
tradeoff between those two options is basically aerodynamics vs.
structural weight. Wing-mounted engines should result in a lower bending
moment on the wing and the hull (but they need longer and heavier landing
gear) while tail-mounted engines should result in 'cleaner' wing
aerodynamics (and possibly a smaller wing for the same takeoff weight),
thus less drag. But the numbers I have seen, for example, for the Fokker
F100 and the Fairchild-Dornier 928 don't support this at all. In fact the
928 seems to be even a bit heavier (empty weight) for nearly the same
payload (but less range, maybe due to more efficient engines). Am I
missing something here or are my numbers simply incorrect?

928:  empty weight 25202 kg
F100: empty weight 24375 kg

It seems that today all manufacturers prefer wing-mounted engines over
tail-mounted ones for new designs, in fact the Embraer 145 seems to be the
largest new design (in the 'west') for an airliner with tail-mounted
engines in the last decades - Boeing 717 and MD 90 are just 'evolutions'
of an existing design, the Fokker F70/100 as well, the CRJ 50 is a
derivative of a business jet, so all these don't really count. Some time
ago, the situation was quite different, with the 727, DC 9 and VC 10 etc.

So here's (finally :-) the question:

What are the actual design tradeoffs today and how did the situation
evolve over the past decades to favor wing-mounted engines that much? What
would be the impact of, for example, drastically increased fuel prices
and/or significantly more strict noise regulations (=> engines with bypass
ratio >>10)?

Regards,

--
Wolfgang Keller

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