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Because it's not an accurate representation of Bernoulli. When someones blows air, which was compressed by their lungs, the velocity of the air is due to the higher pressure air escaping ones mouth by flowing into an area of ambient pressure, and continuing along due to momentum. The air pressure does not decrease below ambient air pressure once its exhaled. It matches it. And ambient pressure is what is below and above the paper in the first place. So its not exactly Bernoulli. Which applies for air accelerating in a duct. Or a wing traveling through undisturbed air. Although, Bernoulli doesn't account for how a wing generates lift, as the upper surface is often only 1.01-1.05x longer than the lower surface and that minute speed difference makes almost zero change in air pressure. Then you have the fact that symmetrical airfoils fly just fine, as seen on the Extra, Edge, and MX aircraft. All have identical top and bottom surface curvatures. It can be seen that Bernoulli has nothing to do with anything related to generating lift by a wing. It would seem the Coandă effect is a better description of the blowing over paper trick, Which is due to air velocity supplied by blowing, and the viscosity of air "sticking" to the paper as it travels along its surface, the jet-flow pulling in ambient air, which pressure is lowered. But primarily its Newtonian physics, air has density and mass, and a wing impacts air at some angle of attack, and deflects the air downward which has an equal and opposite reaction which is lift. Or the Coandă effect pulls ambient air from above the paper and draws it downward into the blown air jet-stream. This downward acceleration of air mass has an equal and opposite reaction on the paper which causes it to lift.
Perfect fit. ;-)
Erm, whatever happened to blowing over the top a piece of paper... far simplier to show and do...
Because it's not an accurate representation of Bernoulli.
When someones blows air, which was compressed by their lungs, the velocity of the air is due to the higher pressure air escaping ones mouth by flowing into an area of ambient pressure, and continuing along due to momentum. The air pressure does not decrease below ambient air pressure once its exhaled. It matches it. And ambient pressure is what is below and above the paper in the first place. So its not exactly Bernoulli. Which applies for air accelerating in a duct. Or a wing traveling through undisturbed air. Although, Bernoulli doesn't account for how a wing generates lift, as the upper surface is often only 1.01-1.05x longer than the lower surface and that minute speed difference makes almost zero change in air pressure. Then you have the fact that symmetrical airfoils fly just fine, as seen on the Extra, Edge, and MX aircraft. All have identical top and bottom surface curvatures. It can be seen that Bernoulli has nothing to do with anything related to generating lift by a wing.
It would seem the Coandă effect is a better description of the blowing over paper trick, Which is due to air velocity supplied by blowing, and the viscosity of air "sticking" to the paper as it travels along its surface, the jet-flow pulling in ambient air, which pressure is lowered.
But primarily its Newtonian physics, air has density and mass, and a wing impacts air at some angle of attack, and deflects the air downward which has an equal and opposite reaction which is lift.
Or the Coandă effect pulls ambient air from above the paper and draws it downward into the blown air jet-stream. This downward acceleration of air mass has an equal and opposite reaction on the paper which causes it to lift.