ORIGINAL: LouW
I think you are confusing the mathematical model with reality. Circulation theory was developed to make the math conform more closely to the observed flow, not visa versa. Perfect fluid theory doesn’t predict lift (or drag) but if you add circulation to the rectilinear flow it quite closely conforms to experimental data.
We're not talking about perfect fluid theory. We're talking about a newtonian fluid... a fluid
with viscosity. You're
kinda referring to d'Alembert's Paradox, but not quite. That's not what we are talking about.
As far as the upwash ahead of the wing, it obviously exists, but when the downwash aft of the wing is considered, for a finite wing, the net motion of the air is downward.
[snide comments snipped]
There is a
small net motion of the air downward, but it is because real wings are finite and inefficient. The small downward component is
not the lift.... it is due to induced drag and the lift vector being tilted back. It is the inefficiency of the wing... not the cause of lift. The pressure field around the wing causes the lift... the pressure field is caused by the thrust from the engine. (in the case of a glider.... the thrust is replaced by an exchange of potential energy and kinetic energy as the glider flies through the air)
I would never dismiss theory as just a bunch of equations. The years I spent in industry as a working engineer after earning my degree more than fifty years ago has given me a respect for their usefulness in design and research. On the other hand, to deny the observed acceleration of the air as an integral part of understanding lift is to lose touch with reality.
I don't deny that the air is accelerated, i'm just saying that it is not in the direction you think it is. It is accelerated around it... not down and behind it. Accelerated over the top... decelerated over the bottom.... both of which contribute to upward force by causing lower pressure on the top and higher pressure on the bottom. This is circulation.
As acropilot_ty said, “both are true”. When designing wings ‘n things use the pressure stuff, but when flying, the momentum explanation makes the most sense.
It doesn't make the most sense... it's just the easiest to visualize.... "air deflected in this direction moves me in this direction". I don't deny that... it's a great explanation for a lay person.
But try this... ask someone unfamiliar with engineering to explain why, when you blow over the top of a tissue held in your hand... it moves up toward the flow, not away from all the air your'e blowing on it. When the angle between the "jet" coming out of your mouth and the surface of the tissue changes from blowing over it... to blowing on it... they get the expected effect.. the tissue moves away.
That difference is exactly what i am trying to explain.... pressure lift verses momentum lift.... they are two different things. Pressure lift is good and efficient.... momentum lift is bad and inefficient... as least as far as wing design is concerned. Downward motion of air does not explain lift... pressure does.... at least to this engineer. There are much easier ways of pushing air downward to cause a vertical force... such as standing a jet on end... but airplanes are not designed that way.