visualisation of the lift distribution over a wing

In article ,
cavelamb wrote:

Jim Logajan wrote:
Alan Baker wrote:
Look if you think that conservation of *mass* plays any role in this,
you're missing out from the start. It's conservation of *momentum*
that's in play here.

It appears you have never studied fluid dynamics (maybe elementary fluid
statics?) and I doubt that you own any books on the subject.

The aircraft has a force exerted on it equal to its weight. That means
that the aircraft must be exerting a force on the air in the opposite
direction.

In other news, 1 + 1 = 2.

That means that there is a constant change of momentum being done on
the air by the aircraft. That means air *must* be moving down (net)
after the aircraft has passed.

You must have a devil of a time figuring out what keeps balloons afloat,
what with no handy downward moving air!

(Just FYI, imagine a ~957 kg (Fg ~= 9379 N) helicopter dropped from a
balloon from 3,000 m altitude (rho ~= 0.83 kg/m^3) and it's engine
immediately started. After a small drop it levels out and maintains a
downwash of air moving through its 6 m diameter disk (A ~= 28 m^2)
at, say, 20 m/s. (So m_dot ~= 469 kg/s and hence Fe = Fg.)

It would take ~150 s for that downwash to reach the ground if it
maintained that speed. In the mean time, once the helicopter stopped
descending, conservation of mass in an incompressible fluid seems to
require an equal volume of air to have an upward vector of 20 m/s. So
the surface of earth appears to be irrelevant for over two minutes.)
Nope.

Dang - I try to use real numbers to establish a baseline example, and you
manage to use a single word to demolish my attempts! Really helpful
mathematical counter-example you produced - not.

The conservation of momentum says that there cannot be an equal amount
of air moving upward at an equal speed.

I don't know what your problem is - maybe you are thinking this is a
rocket problem where no external fluids are involved and you can't get
your mind around the fact that THIS ISN'T A BLOODY ROCKET PROBLEM.
Whatever the case, you seem to be fixated on applying one conservation
law to one element in the entire system to the exclusion of everything
else.

Best of luck to you.



Two dimensional Newtonian thinking in a three dimensional non-Newtonian world.

LOL

Sorry, caveman....


But conservation of momentum works well enough at the speeds at which
aircraft operate.

And Newton's laws tell us all we need to know.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg