"Bob F." wrote in
:

"Bertie the Bunyip" wrote in message
...
wrote in news:l5f6n5-hp7.ln1
@mail.specsol.com:

Tman x@x wrote:
OK, these are trivial, but nonetheless been bugging me and the guys
at the airport don't have a satisfying answer;

1. Why do wing tip vortices sink? They're not made out of lead.
2. 747 overflies at FL 310 (or whatever). Is the weight of the
airplane ultimately supported by the earth's crust? How?


FWIW, my opinions on these: #1: because of a general downwards
movement of air in the wake, induced by lift. I've also heard that
the turbulence causes locally increased pressure and thus density,
but I s'pose it was also create locally reduced pressure -- so I
don't believe that factor.

The tip vortices do have a different pressure/temperature which can
be seen on humid days when vapor condenses and they become visible.


Uh, no, that's not it. The vector as the air leaves th ewingtip is
generally downwards, that's why the travel is down. Also the reason
they
travel out.

They travel outward because that's where the lower pressure is.


Well, in a way, yes, but they travel outward molecule by molecule so
it's a bit lie saying the wind blows because the trees wave back and
forth.
The spanwise flow is stronger on the bottom than the top of the wing and
the tendency is for the flow to travel around the edge of the wingtip.
It's all down to the force applied on them as they stream off the end of
the wing. all of the componenets of lift add up to them traveling out
and down, ultimately.


Bertie