Re: Wing vs. tail-mounted engines?

Date:         04 Apr 2001 16:41:12 
From:         Ian Barclay <ian@ibarclay.demon.co.uk>
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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?

Others have covered most of the main issues (wing bending relief, clean
wing aerodynamics, ground clearance, FOD ingestion, etc) but a few other
considerations are :-

Wing mounted engines put mass in front of the flexural axis of the wing.
This damps out the aeroelastic forces & gives greater flutter margins or
allows lighter wing structures.

Rear mounting moves the cg aft reducing tail moment arms thus requiring
a larger tailplane and maybe even fin.

Rear mounted engines require heavier rear fuselage structure.

Rear mounted engines can be fitted with bucket reversers which tend to
be cheaper and more effective than cascade or 4 door reversers used on
wing mounted engines.

External noise levels are generally less for rear engine installations
as the fuselage partially shields one engine for sideline noise
measurements and the wings partially shield both engines for approach
measurements.

Wing mounted engines can easily be struck and damaged in a misjudged
crosswind landing.

The downwash from the wing varies with angle of attack and tends to
result in a relatively small variation in AoA at the engine inlet of
rear mounted engines making the inlet aerodynamics more closely
optimised across the flight envelope.

Rear mounted engines often require soft (rubber/fluid) engine mounts to
absorb vibration and blade off loads. For wing mounted engines the
flexible wings act as effective dampers thus allowing engines to use
cheaper hard mount arrangements.

Wing mounted engines are ideally located to supply bleed air for wing LE
anti-ice.

If the APU is tail mounted, bleed piping can be simplified with rear
mounted engines.

The length of fuel lines is minimised for wing mounted engines.

Ice shed from the wing can be ingested into rear mounted engines.

Wing mounting may limit the flap span to allow for the exhaust stream.

There is the possibility of high drag from the convergent/divergent
channel formed between the nacelle and the fuselage wall on rear mounted
engine installations.

More available fuel volume for rear mounted engines as no dry bays in
the wing fuel tanks to cater for disc bursts are required.

It may be possible for engine nacelles to be common port and starboard
for wing mounted engines.

It is easier to make accessories, piping & harnesses common on port and
starboard sides for wing mounted engines.

    At the end of the day its pretty evenly balanced. Hence the
popularity of both configurations. I think the two factors that tend to
swing the decision are the manufacturers experience (e.g. Boeing &
Airbus go for wing mounted, McDonnell Douglas used to go for rear
mounted because that was what they had done before, they had the
engineers with that design knowledge, it was the low risk approach) and
ground clearance (e.g. regional jets want to be able to load baggage
holds from the ground and passengers without jetways so rear mounted is
better for this case).

--
Ian Barclay
Salwick, UK