Re: Why is an airplane stable on the ground?

   That is a great question.  It does seem hard to believe that something 
as massive as an airliner can come slamming to the ground at 150 mph onto 
those little wheels.  Personally I sometimes find it even harder that 
something as massive as a 747 can fly at all!  It astounds me every time I 
think about it.  But we see them fly, and land on those small wheels, so 
we know it's possible.  Even though it seems like it, we also know that 
it's not some freak of nature.  The teams of engineers that design these 
technological marvels have calculated the forces, stresses, material and 
design strengths so that it all works.  So I guess that is the answer to 
your question - because the engineers say so.  :-)

   There are a couple of things I might point out that may make it easier 
to swallow.  First, airliner wheels are anything but small.   As tall as a 
person, the wheels only look small because they are next to the really big 
plane.  Second, the wheels carry the plane for only a short distance.  The 
wheels only make turns when the aircraft is going very slowly, this 
prevents lateral forces from being transmitted through the wheels.  When 
the aircraft is going at its fastest ground speeds, just before take off 
and after touch down, most of the aircraft's weight is being supported by 
the wings.  Except for taxi, direction control of the aircraft is mostly 
via the flying surfaces and the wheels are only along to support some of 
the weight.

   This is not to say that the wheels do nothing at all.  The concrete in 
the landing zone of the largest airports is 12 feet deep!  You are also 
right that in a cross wind the wheels will bear much of the lateral load  
caused by the wind.  This is part of what sets the limits for maximum 
cross wind that all planes carry.  The giant C-5 Galaxy has addressed this 
problem by using gear struts that rotate so that the aircraft can land 
crabbed into to wind.

   I hope I have answered you question.  Thanks for asking.

Dave Leonard
Navy Flight Surgeon