Re: Boeing 767 main gear tilt

Date:         01 Aug 97 04:04:17 
From:         mikeh@zeta.org.au (Michael Hore)
Organization: Kralizec Internet, http://www.zeta.org.au/
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Whoops, I meant to reply on this before, and forgot...

>Could someone explain why the B767 gear is tilted the "wrong" way, i.e.
>front wheels down, instead of the seemingly more natural rear wheels
>down design like the 747, 777 or A340 ?

The landing gear layout is, like everything else, a rather complicated
compromise.  (I'm only an amateur here, but I've had a long-term
interest.)

The main gear bogie pivot has to be the "right" distance behind the
CG on the ground, but the leg attachment to the wing structure has
to be in front of the flaps, and at a position that makes sense
structurally.  On the 767 (and on a number of other types), this
is actually forward of where the bogie pivot needs to be, so there
has to be a backward angle on the main gear leg, when the gear is
down.

Then when retracted, the leg has to be clear of the flaps, and on
the 767 this requires a *forward* slant in this position (as
Allan.Howard mentioned in his posting).  The particular angle of
the main gear leg pivot looks after this change of angle in the
main gear leg between the retracted and extended position.

As Allen also mentioned, the main gear bogie is parallel with
the center line when retracted, and there's no special extra
positioning applied, so that the bogie is somewhat nose down
with respect to the leg.  Then when extended, the leg itself
has a backward tilt, and this means that the bogie is even more
nose-down with respect to a/c level.

On the 777, the same layout is used, but as C. Marin Faure
mentioned, the bogie is powered into a nose-up position when
the gear is extended, which allows a better absorption of
landing energy.  When the gear retracts, the bogie first tilts
into a nose-down position which is similar to the normal 767
position.  From photos (I haven't had a chance to see one
for real yet), this repositioning happens at the same time
as the gear doors open.

And Darren Rhodes <D.P.Rhodes@lboro.ac.uk> wrote:

>The wheels tilt the 'other' way on the A340/A330 as this helps to spring
>the aircraft into the air on rotation. I have heard Airbus claim it saves
>100ft or so on the takeoff run.

Maybe it makes a difference there (tho 100 ft isn't much), but the
main reason for the prominent nose-up positioning on the A330/340
bogies is to better absorb the landing energy, while allowing a
stiffer oleo arrangement in the main legs, which will minimize
height changes on the ground.  When the A330/340 gear retracts,
there's a shortening mechanism operating in the leg that also pulls
the bogie into a nose-down position with respect to the leg.  I
think this is mechanical, not hydraulic, and thus operates *during*
retraction, not as a separate preliminary movement as on the 777.

The gear position when retracted is very similar to the 767 and
777, with the wheels parallel to the a/c c/l.  This also,
ncidentally, leaves enough room for the center gear on the A340.

All these aircraft thus have a main leg which is angled forward
when retracted, but backward when extended.  This reflects the
similar engineering constraints applying to all these types.

Cheers,  Mike.

--
Mike Hore          mikeh@zeta.org.au