Landing Gear Loads

Date:         25 Aug 98 00:53:31 
From:         mike@dizzydev.com (Mike Schmitt)
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Howdy all --

I was talking with a friend about heavy aircraft and runway/taxiway
loading, and came up with a couple questions y'all might be able to
answer.

Take a friendly B744 taxiing towards takeoff as an example.  I know
for a couple of facts that there are 16 rear tires and 2 front tires,
and that the plane weights 875,000 pounds at MGTOW.  I'm assuming the
front tires hold 1/2 the weight of the rear tires each.  Using these
numbers, you end up with about 51,500lbs per rear tire and around half
that for the front ones of dead-weight "compression" load.

I also understand that there is a taxi length limit for heavily-loaded
aircraft to control the amount of heat and flex the tires go through
prior to the takeoff run, to reduce the probability of a blowout on
takeoff.

So our handy mythical plane flies halfway around the world, burning
off 300,000 pounds of fuel in the process, leaving us with a 575,000
pound plane about to land.  This would imply roughly 33,800 pounds per
rear tire if the aircraft were sitting on the ground.  However, the
plane -- despite every pilot's best effort to grease the landing --
will be descending at somewhere between zero and, say, 500 feet per
second when it touches down; the oleos will compress upon touchdown,
lessening the momentary impact load on the tires; and there is also a
shear effect from sideways loading (crosswind/not-quite-straight
landing) and, initially, spinning the tires up to ~150knots, then
hammering them with the brakes to slow our trusty airplane back down
to zero.

My questions revolve around the points during that cycle when the
tires are actually experiencing the maximum weight load (meaning
compression only), and the maximum overall load, including both weight
and the shear effects.  I'd also like to know, if the numbers are in
anybody's head, the approximate values for a given airplane type &
weight at both points.  Also, is the reason there is a taxi limit
because the tires actually experience higher loading/heating during
takeoff than landing?

It's one thing to say the tires hold 50,000lbs; it's another to
describe what they go through on each cycle.

Any information appreciated.

Cheers,

Mike