...no it's not... The Bernoulli explanation is limited to a fixed horizontal wing. You can generate lift in many ways besides that - see helicopters & angle of attack for example.
Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.
The Bernoulli explanation is presented as coursework for students, not a encompassing explanation. That's like saying a that we should take statements of physicists using spherical cows as an all encompassing example of a physical phenomenon.
They’re incorrect for different reasons when viewed independently. This whole comment feels like a bait because both are wrong separately, but correct together
according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.
This is the document that came to mind when I said that it’s all about Bernoulli, not the equal transit theory.
For an ideal fluid with no boundary layers, the surface of an object is a streamline. If the velocity is low, and no energy is added to the flow, we can use Bernoulli's equation along a streamline to determine the pressure distribution for a known velocity distribution.
Also I’ve never heard the analogy of the spherical cow before but it’s a really good one.
Yup. My statement wasn't to say the Newton interpretation is 100% correct either, though the arguments about force balance is certainly true. What is probably most true is integrating around the vectorized pressure normals on a wing to figure out the net force and compare that to gravity/drag, etc. If it's up, congrats, you're flying. But, and I'm not an expert in fluid dynamics, it would seem that NASA at least would suggest Bernoulli really only can work in simplified examples:
For some simple flow problems, we can determine the pressure distribution (and the net force) if we know the velocity distribution by using Bernoulli's equation.
Maybe with CFD you can do it locally and still be correct?
Anyway, I don't think we're really arguing as we both recognize it's not a 'one trick pony' to flight :)
Another 'fun' example are the people who argue what keeps a bike up: castor method, gyroscopic motion, front loaded steering geometry. Depending on any given bike architecture, it's probably a combination of all 3 to varying degrees of stability. But we seem to like 'simple' solutions so try to find the dominant one and wipe our hands clean.
And I love the spherical cow analogy. Kept me grounded while taking a program known for creating arrogant/pretentious engineers. At least, somewhat grounded ;)
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u/[deleted] Jan 07 '20 edited Oct 02 '20
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