wrote in :

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.


Though which effect is the dominate one that causes them to go down
I haven't a clue.

#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?

Nope, that's the old rockets can't work in space because they have
nothing to push against arguement redressed.

F=ma doesn't require the accelerated air to bounce off of anything.

It's not bouncing off anything, but the wieght of the airplane is still
borne by the air, ultimatley, though some of that is traded as drag.

Is a fly buzzing around the inside of th same ariplane contributing more
weight wehn it lands on the edge of your coffee cup?


Bertie