I see so many people trying to get this and still making some mistakes.
Mike Dunlavey and Paul Townsend have a lot correct, but still miss something very important that puts this all completely to rest.
Bernoulli’s Principle and Equation are two completely different things Please stop conflating them.
You need to understand that Bernoulli’s Principle/Theorem/Effect does not cause anything, nor explain the science.
Bernoulli’s Principle only points out the inverse relationship between pressure and speed -- in a special (and very limited) situation AND, it must be observed in a special way (I’ll explain that below ***). It says nothing about what causes what. In addition, the “increase in speed” mentioned in the classical wording is an acceleration and that is the most important take-away from this.
Euler, in the mid 1700s followed up on Bernoulli’s work and determined that Pressure Gradients Accelerate Fluid and derived Bernoulli’s Equation. <-- this is critical!
Bernoulli’s Equation can be used to calculate the lift value. So when Paul says: “100% can be explained by Bernoulli's equation.” This is correct, but incomplete and misleading. Integrating The Equation over the area of the wing does give the lift force, but you still don't know what causes the pressure in the first place.
Bernoulli’s Equation does not explain the science of the cause, it only gives the upward force value, but not why it happens in the first place.
When Paul also says: “100% of the lift can be explained by Newton's laws” same thing, you get the lift value from the down-wash momentum transfer, but it is also incomplete.
So what’s the real deal?
When a wing moves through the air, this relative motion directly causes pressure differences at the surfaces.
It is that simple.!.
A. The pressure increase under the wing is caused by the simple fact that the wing is advancing on the air, pushing on it. That makes an increase of pressure. This is Adak47‘s “RAM effect” put in better terms.
B. The pressure reduction above the wing is a bit more complex to describe, but is due to the simple fact that the air and wing are ‘trying’ to move away from each other (the opposite effect of “ram”).
Then, and most important:
These very same pressures do two things at the same time:
1. They are the pressure difference that pushes up on the area of the wing.
2. They also cause all of the accelerations of air that we see around the wing: Ahead, below, above, behind and around the tips.
In other words, the pressure difference and the downwash/deflected air are both TWO Parts of the Whole story, not two different ways to look at it.
They ARE, however, two ways to calculate the force.
that is what everyone is missing.
Stop focusing only on the wing shape. What is critical is the air-flow pattern around the wing and it is NOT the same as the wing’s shape. The fact is that ALL wings produce very similar flow patterns. This goes for symmetric, asymmetric, inverted and flat.
Then, one more thing to blow your minds before I provide a reference explaining this all in detail.
If you stand on a hill and watch a wing go by, the fastest moving (highest speed) air is under the wing!! Yep!, This is measured and actual data on a wing generating lift. This blows the doors off this “fast air” nonsense, because it shows that acceleration of air is what is key, as I mentioned above.
What is going on is that the lower air is pushed forward pretty fast in the direction of flight. However, the upper air is accelerated rearward, really yanked rearward, with high acceleration.! The reduced pressure is part of the Euler Pressure Gradient that causes this acceleration. It is actually traveling rearward at slightly lower speed than the lower air is traveling forward…
This is all called out more carefully here:
P.S. The bound vortex (circulation) also doesn’t cause anything, but it is something we can observe because it happens to be an effect of the way the air is accelerated near the wing.
In linear fluid motion.
*** When the pressure upstream is higher than downstream, the net force is forward, thus speeding up (positive acceleration) the air. This is called “proverse” pressure Gradient.
This is the ONLY case that the classical words called “Bernoulli’s Principle” describes.
The opposite is also true.
When the pressure downstream is higher than upstream, the net force is rearward, thus slowing (negative acceleration) the air. This is called an “adverse” pressure gradient.
When the Pressure Gradient is across the stream, the flow is curved. This is what curves (or “turns”) the flow both below and above a wing.
I’ve studied this and discussed this with two noted experts actually in the field and who have practiced aerodynamic design, Boeing’s Doug McLean and Embry-Riddle’s Charles Eastlake.