Re: Wing vs. tail-mounted engines?

Date:         04 Apr 2001 16:41:19 
From:         "Tim J Lee" <tim.j.lee@btinternet.com>
Organization: BT Internet
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"ME Incorporated" <whyyou@boddah.me> wrote in message
news:airliners.2001.65@ditka.Chicago.COM...
> "Wolfgang Keller" <w_keller@gmx.de> wrote in message
>
> > 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)?

> And so, with more powerful engines, and the complaints/requests of big
> clients such as American, United, etc, these tail mounted engines are
> becoming a non desired design.  Hence, the design of the 777.  Very much
an
> airplane designed by and with the airlines, and not just the plane makers
> designing around a spec.  You can bet that Boeing wanted more engines, and
> maybe in the tail, but because of that maintenance thing, two is cheaper,
> and wing mounted cheaper still.

An engineer writes.....

Structurally, underwing engines are a better bet.  Reason?  If you put the
weight of the engines on the fuselage (a la DC9), this adds to the weight
that the wing root has to transmit - leading to a more complex wing/fuselage
interface and carry-through structure.  The object is to make the wing as
heavy as possible (so it carries it's own weight) and the fuse as light as
possible. It's the same reason that on aircraft with centre fuel tanks, they
use this first (usually for take off) - get the fuse light, keep the weight
on the wings.  As an aside, it also makes maintenance easier, but this is a
secondary consideration against airframe efficiency.

There are good reasons for tail mounted engines - the VC10 had the best - it
was designed with the hot and high airports that BOAC were serving in East
Africa in mind, and so an efficient and highly flapped wing was created.
Fouling the trailing edge up with engine exhaust was not considered a good
bet, due to the loss of flap section.

As for containment - the bit of the engine most likely to go bang is the
large fan at the front - which in any case is mounted ahead of the wing and
fuel.

T