Re: Fourth landing gear on MD aircraft ?

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
Date:         05 May 95 03:27:12 
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>I do not know what model of MD aircraft it is, but it has the following
>	2 engines on wings
>	1 engine in tail

Based on this, it's clearly a DC-10 or one of its derivatives, the
KC-10 and MD-11 ...

>	winglets (up only)

... and this incicates it's an MD-11.

>	Is the 4th gear a feature that only MD now uses (as is the tail
>	mounted 3rd engine) or will that feature also be used by other
>	major manufacturers ?

The A340 also has a center main gear; the 747 has *two* center mains
in addition to the two outer mains and the nose gear.  The feature is
already used by other manufacturers where needed.

>	Is the purpose of that gear to provide additional breaking power
>	or as a structural feature to support the weight of aircraft during
>	touchdown ?

Speaking about a third (or fourth) main in a generic sense, it could
be to provide sufficient support for a heavy aircraft, or it could be
to distribute the weight over a larger area, which might be required
on a heavy aircraft to keep from punching a hole in a runway.  (Think
of a 200 lb man wearing 12EEE sneakers vs. a 100 lb woman wearing
spike heels!)

In the case of the MD-11, you have to look back to the DC-10.  First
came the DC-10-10, which had only two mains.  When MD built a longer-
range version, the DC-10-30, with a higher weight capability to allow
added fuel without reducing payload, they needed to support greater
weight than the two mains of the DC-10-10 could handle.  Therefore,
they added the third, center main.  (The DC-10-40 is essentially a
-30 with Pratt and Whitney engines and thus, ignoring some domestic
variants for Japan, also has the center main.)

Lockheed's approach on the long-range L-1011 was to keep two mains but
to strengthen them.  This didn't increase the airframe weight as much
as adding a third main would have (landing gear is very heavy!) but it
meant a loss of parts commonality, whereas a DC-10-30 operator would
not require an additional landing gear parts for a DC-10-10.

The MD-11 airframe is basically a stretched DC-10-30 with even higher
weight capability, and MD continued using the same landing gear as on
the DC-10-30 so the MD-11, too, has the third main gear.

Airbus presumably followed similar logic with the A330 and A340, which
are really the same airframe except for the wing in the vicinity of
the engine attachment points.  (The A330 has two large engines whereas
the A340 has four smaller ones.)  The A330 has only two mains, but the
heavier A340 has a third, center main, just like the MD-11.

The 747 has four mains in all its variants, so there never was any
concern about parts commonality.  Two huge main landing gear might
well have been prohibitively heavy, or too large to fit in the belly
of the aircraft when retracted, though footprint weight was a major
concern when the 747 was being designed.  The result was that the
747-100 put no more weight on each tire than a 707-320.

The 777 demonstrates yet another tradeoff.  Whereas all the aircraft
described above have four tires per main landing gear post, or two
for the center gear of a DC-10-30, MD-11, or A340, the 777 has only
two main landing gear but with *six* tires each.  A design with fewer
tires and a center main was considered, but the center main would have
consumed valuable space in the belly of the aircraft when retracted.
Boeing chose instead to stick with two mains (with extra tires) and
use the space saved for extra cargo or fuel.  (Probably fuel, but I
can't remember for sure.)

Karl Swartz	|INet
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		|Snail	2144 Sand Hill Rd., Menlo Park CA 94025, USA
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