Lyman...
I have never understood the apparent need to make lift complex, and pay homage to a Swiss hydrologist.
Lift is complex! And Bernoulli wasn't a hydrologist, he was a physicist and mathematician.
Daniel Bernoulli versus a
hydrologist.
I think the quote that Owain posted needs to be emphasized more. Until you accept this, I see no point in continuing to try to understand the complexities of lift. A scientist or physicist or aerodynamicist all have one thing in common... they all are studying nature and discovering the way it is. If it turns out the way nature is is simple, then that's the way it is. If they find out that nature is complex, then that's the way it is! Lift happens to be one of those complex things.
"Aerodynamics is a difficult subject, and all attempts to simplify it for the average person leads to wrong interpretations." - Arvel
That is fundamentally the point I try to make, the wing compresses air...
When studying lift at low speeds, which is where you always start, it can be said that air is incompressible at speeds below Mach 0.3. That satisfies one of the conditions of Bernoulli. Outside of the boundary layer air can be considered inviscid, another condition for Bernoulli.
Henra...
Yes, expansion means increasing pressure. But increasing can mean increasing back to free stream static pressure. No discrepancy to the 'closed' system here.
Yup... I made an error with my description. What you say makes sense.
Owain Glyndwr...
I disagree. The energy added is used to overcome a side effect of lift generation – drag.
There is abundant evidence from wake survey experiments that total pressure is not constant behind a wing producing lift, so in broad terms they are right, although not because energy is added – rather it is subtracted. However, it is also true that this loss of total pressure is confined to a small area just behind the TE – in the wing wake in fact.
Would it be correct to say that the flow outside the boundary layer does not have energy added or subtracted (and it's incompressible and inviscid) so Bernoulli applies to it? Is the boundary layer then responsible for the induced and parasitic drag?
I'm trying to get an understanding of the energy of the system - how energy is added or subtracted to the air by an airfoil passing through. It makes sense that energy has to be added to overcome drag. Regarding the loss of total pressure behind the TE, where does that energy go?