Two questions that I've been wondering about -- w/t vortices andlift.

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.

#2: Yes, the wake of a overflying aircraft will cause a downward
acceleration of the air, which reacts with the surface of the ground
creating a locally higher pressure when it decelerates. Of course the
effect at FL310 is spread over such a large area that the effect at any
point is incredibly small, but it is spread over such a large area that
it in effect is supporting the weight of the A/C.

Ground effect at FL310 anyone?
T