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Old Nov 17, 2012, 12:49 PM
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Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
525 Posts
Crossplot,

Thank you for giving something tangible to talk about in this discussion.
I read the whole article and found many problems with it. They gave the well known equation for centripetal force but fail to offer any way of calculating the lift from it. There is no way to show that they are right or wrong because they do not provide any useful equations. To lead into this discussion they say
"The
air flow near the aerofoil follows the geometrical
shape of the upper surface generating a pressure
gradient and acceleration, both perpendicular to
the streamlines and directed to the centre of the
flow trajectory."
The acceration is most certainly not entirely perpendicular to the streamlines. The flow near the surface accelerates rapidly past the leading edge and then. Yes some acceleration will be perpindicular to the surface, but consider the face that the change in the length of the vectors they show in the Fig. 4 indicates the velocity change parallel to the surface.

They also discuss viscosity:
"This
variation of viscosity induces a decrease of air
velocity inside the boundary layer from its outer
frontier to the aerofoil surface."
This is completely wrong. For a Newtonian fluid (like air and water), viscosity only changes with temperature. This only becomes important for very high speed flows. The velocity is lower inside the boundary layer because of the no slip condition at the surface.

Their conclusion:
"In summary, lift occurs when flow is shifted
downwards."
This is the first time they use the word "shift", thereby making this statement very ambiguous. In actuality, lift is created by sending more streamlines above the body than below.

-David
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