Re: T-tail

Date:         10 Sep 97 19:38:44 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>I understood one of the main disadvantages of a T tail aft engine mount
>config is weight.

While a T-tail and aft-mounted engines often go together, neither
implies the other.  There are airliners with a T-tail which do not
have aft-mounted engines (e.g., ATR-72) and airliners which have
aft-mounted engines which do not have a T-tail (e.g., Caravelle,
with the horizontal stabilizers mounted midway up the tail).

Moving the horizontal stabilizers to the top of the vertical stab
adds some weight, but not nearly as much as aft-mounted engines.

>Most power generated by the engines is dissipated or absorbed by the
>wings so a heavier rear fuselage structure is needed to carry the
>engines and tail structure and transmit the thrust forward to the wings.

I've always heard that the sheer weight of the engines was the big
issue -- the wings obviously are what's holding up the aircraft while
it's in the air, and the wing-box/landing gear is where the support
is while on the ground.

>Another point is large fuel and air ducts being taken through a
>pressurised fuselage, always a weak point.

The fuel pipes don't have to be that large, and many aircraft with
wing-mounted engines have them anyway due to fuel storage in the
horizontal and/or vertical tail -- optional on the 747-400, and
available (optional or possibly standard) on the A310 and I think
the A330 and A340, to name just a few.

Air ducts for what?  If bleed air for engine starting and/or AC PACKs,
you have that in aircraft with wing-mounted engines and an APU in the
tail -- all current Boeings (not 100% certain about the 757 and 767
but I think so) and all Airbuses.  Conversely, I think the DC-9 has
its AC PACKs in the back, so I don't see why it would need those air
ducts except for cabin pressurization (needed regardless), despite
having a T-tail (irrelevant) and aft-mounted engines.

--
Karl Swartz	|Home	kls@chicago.com
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