# Airfoil doubt

Gold Member
Sorry that i am re-entering this zone after a long time.
Firstly, kutta condition is not a cause for air flowing at a greater speed over the airfoil. It is an effect of it.

No. Viscosity is what ensures the fluid cannot travel around the sharp trailing edge, enforcing the condition that the rear stagnation point must be on that trailing edge. That is what is termed the Kutta condition. To satisfy the conservation laws while still keeping that stagnation point, the upper air must move faster.

The cause comes from the fact that airfoil in air, behaves like half venturi tube, which can explain all the features of the airfoil.

No it isn't. That cannot be applied here given the fact that a Venturi tube is a control volume, this is not. The similarity is purely a coincidence, not physically meaningful.

It can explain why air flow is faster on the upper surface
This assumption can also help in explaining the airfoil behaviour in supersonic flow as well
I dont know how to add figures in this space, to explain.
This assumption of half venturi tube can explain everything of airfoil, without any trace of doubts.
I knw some people are not in agreement of the term HALF VENTURI TUBE, i am ready for criticism.

It can if invoked incorrectly. Magic could explain it too, but that doesn't make it correct.

Remember a super critical airfoil have flattend upper surface and curved aft surface. This is because, a supersonic flow accelerates in divergent zone and decelerates in convg zone of CONVG-DIVG nozzle.

Actually supersonic airfoils work by taking advantage of shocks on the bottom half to raise the pressure by decelerating the flow and Prandtl-Meyer expansion waves on the upper half to accelerate the flow, leading to the same sort of circulation as dictated by the Kutta condition on a subsonic nozzle.

The cause comes from the fact that airfoil in air, behaves like half venturi tube, which can explain all the features of the airfoil.

This is not an explanation. You're saying that the half-Venturi tube explains everything about the airfoil just because they behave the same way. You can't say that the Kutta condition doesn't explain it because it just happens as an effect AND that the half-Venturi tube explains it because some effects are the same. This is a contradiction in your requirements of proof.

According to this website (http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html" [Broken]), the half-Venturi tube explanation of lift is a wrong theory. You might not like boneh3ad's explanation, but it seems that a few people at NASA agree. It seems they happen also to do aerodynamics for a living, but I know that doesn't mean that much to some. As for me, I tend to respect people who get paid regularly for their knowledge of a certain topic.

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