Re: Wing vs. tail-mounted engines?

Date:         31 Mar 2001 16:43:03 
From:         johnmcgrew@aol.com (JohnMcGrew)
Organization: AOL http://www.aol.com
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In article <airliners.2001.58@ditka.Chicago.COM>, Wolfgang Keller
<w_keller@gmx.de> writes:

>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)?

Weight:  Putting engines in the back requires extra load carrying structure,
both for the thrust and the weight of the engine itself.  On the other hand,
little extra structure is required for the wing mounted engine since there's
structure already there; the wing is mostly structure.  Advantage: Wing Mount.

Noise:  Clearly, advantage rear, except perhaps for those sitting in the back.
Advantage: Rear Mount.

Maintenance:  Most maintenance on wing-mounted engines can be performed at
ground level without lifts or scaffolding.  Significant appeal to maintenance
staffs.  Advantage: Wing.

Engine-Out Performance:  Single-engine control is much better with rear-mounted
engines over wing, simply because thrust is far less asymmetrical because the
engines are so much closer towards the center.  Advantage: Rear

Aerodynamics:  Rear-mount allows for a cleaner wing design, and less conflicts
with airflow and mechanics for slats and flaps.  Advantage: Rear.

However, most rear engine configurations necessitate a T-Tail configuration for
the horizontal stabilizer, which requires extra weight for structure at the top
of the vertical stabilizer, as opposed to conventional horizontal stabilizers
attached to existing structure at the end of the fuselage.

CG Considerations:  There are differences in handling and stall characteristics
that tend to favor wing with the engine weight near the CG vs the increased
moment as a result of having a longer nose and all that weight in the rear.
However, advanced control systems and pilot training tend to negate these
differences.

Ground Handling:  Many rear-mounted designs allow the use of reverse thrust for
backing up the aircraft with limited danger of FOD ingestation, thus
eliminating the need for a push-back at the gate.  This is appealing to
airlines as it means that less ground support required.  Advantage: Rear.

That's all I can think of at the moment.

Personally, as a passenger, I prefer the MD-80 to the 737, mostly because of
less noise, the 2x3 seating, and the 737 is "bumpier".  But the reality is that
what really determines what plane you will be flying is the economics.

John