Lift - Newton/Bernoulli ratio...

In article 6def6450-011d-4f9f-8595-8c455f6b6ea0
@hd10g2000vbb.googlegroups.com, , a says...

On May 22, 10:02*pm, brian whatcott wrote:
On 5/22/2011 5:15 AM, Dave Doe wrote:

Does anyone have any figures and references for about what ratio lift is
produced by Newton's Laws and Bernoulli's Laws?

I appreciate this is not a static figure - but say a yer average C-172,
or perhaps a 737.

I would hazard a semi-educated guess that lift is *primarily* produced
by angle of attack (or deflection if you like) - Newton's Laws - and by
a much lesser degree by Bernoulli's Law. *I would guess that Bernoulli's
principle might create 20% of the lift a wing generates. *A friend
believes it would be much lesser - about 5%.

Think of it this way:
Newton: force is proportional to the mass and its acceleration.

In this context, the meaning is, to produce the aircraft's weight in
lift i.e. upwards , an airmass has to move *with sufficent acceleration
to provide that up force.

Bernoulii: the mass of air flowing through a channel times its speed
gives the same product even if the channel then narrows to a waist:
the air mass has to flow faster, but its pressure drops..

In this context: air flowing in an airstream over a wing sees it bulging
(or waisting) and so that it needs to speed up, and pressure drops over
the upper wing. Arguments of this type can be used as evidence that 2/3
of the wing lift is produced at the upper surface, and 1/3 at the lower
wing surface.

The larger truth: air pressure drops over the upper surface of a wing,
and increases over the lower surface of a wing, and the resultant
downflow balances the lift on the wing.

Brian W

Does it matter to anyone posting here that the fluid flow described
by Berboulli's equation assumes the fluid is incompressible? Does
anyone here really believe there is no change in air density as if
flows at speeds of a hundred miles an hour past an airfoil? The
equation works well for water flow in pipes and around boat hulls. It
does not do such a good job of predicting pressures along an airfoil.
Stick with Newtonian Physics and the gas laws.

I think you are right - and I think most here would agree. Hoewver it
is, for practicle purposes, considered (or treated as) incompressible at
speeds under Mach 0.3 (something I read somewhere in my research).

In reality, air is of course *very* compressible, compared to say water.

--
Duncan.