Ever since I started to be interested in science, the explanation that I’ve read about how planes stay in the air has been based on the Bernoulli principle: As the wing moves through the air, air flows around it, and the profile of the wing causes it to flow faster on the upper side than the lower side. This creates a pressure differential between the two sides, and that’s what pushes the wing up.
So far, so good. But lately I have repeatedly read an explanation (and in publications that I wouldn’t consider fringe) that relies on Newton’s third law: The shape of the wing, as it moves through the air, pushes the air that it hits downward, and this creates upward lift.
This explanation sounds plausible, but it is so radically different from the Bernoulli effect explanation summarised above that I almost feel I’m experiencing a Mandela effect here. Surely in the (more than a) century that humans have been flying with heavier-than-air machines, and all the research that is going into improving planes, the theory of flight is thoroughly understood. Can it really be that the view of what is happening in physical terms, or at least the way this is explained in popular science, has changed so drastically?
My understanding is that it is both the shape of the wing (and the Bernoulli effect) as well as the angle of attack of the wing, which pushes down on the surrounding air, and which by Newton’s third law pushes back to create the force referred to as lift.
It can’t solely be the shape of the wing, or no plane would be able fly inverted.
Actually, it’s a god question. Several years ago I saw a spate of articles going into how airplanes reallly fly, and professors being annoyed at the incorrect and lazy explanations for th processes, especially ones that misused the Bernoulli efect.
In private flight school in the 1980s the textbook presented the Bernoulli Effect. However, more recent discussions focused on the idea that the too-side air “doesn’t care” and has no incentive to flow faster. Further, some wings are symmetrical, so no difference in chord between top and bottom.
So my once rock-solid understanding is in crumbles.
Hold a sheet of paper in front of your mouth so that it droops down. Blow over the top of it, and you’ll see the paper rise. That is a simple demonstration of the Bernoulli principle.
I’ll post something more substantial later if I can. For now, this is the best book I’ve read which reconciles (or at least explains side by side) the Bernoulli and the Newtonian explanation of lift:
That’s true. It’s also about 95% irrelevant to how airplanes fly. As @Raza said, “Bernoulli effect” was the standard pedagogy until fairly recently.
Airplanes fly because the wing forces a mass of air downwards with sufficient F=ma that it offsets the gravitational force trying to make the airplane drop like a brick.
The vast majority of lift is the “barn door effect”. Hold the wing at an angle to the incoming air such that the incoming air “bounces off” the bottom side and is deflected downwards.
A secondary effect at lower speeds is Coanda effect. Air will tend to follow a convex curvature. With the result that a curved upper surface will impart a downward vector to the air departing the trailing edge. Trailing edge flaps get some of their effectiveness from this topside flow in addition to simply being big barn doors that force bottom-side air downwards.
Somewhere as a rounding error, Bernoulli gets a look in. But much more as a result of accelerating air upwards than any notion that topside and bottomside air have to meet back up exactly aligned at the trailing edge.
Consider that any upward acceleration of air is a direct subtraction from net lift. So the pressure drop is real and readily measurable even with primitive 1900s tech. But the contribution of the pressure drop to net lift is real small.
For reference, here’s a previous discussion about the Bernoulli effect and its relative irrelevance:
Also here, nominally about jet engines, but where I was educated about the fact that even the “barn door effect”, or a flap at the edge of a wing, is technically an airfoil – it doesn’t have to be the pedagogical Bernoulli-touting wing cross-section:
Indeed, it seems to me that early airplanes, with their thin flimsy fabric wings, exploited both of those effects, and not at all the Bernoulli effect because they didn’t have the cross-section profiles of modern wings.
It may be hard to explain to the layperson, but it’s well-understood by aerodynamicists. The lift equation sums it up fairly well, and includes all of the variables that contribute, along with the extent of their contribution:
L=Cl⋅(ρV^2)/2⋅A where:
CL = coefficient of lift - the complicated part. It is a function of flow conditions, wing geometry, and angle of attack, and can be mathematically determined for a range of geometries and simple flow conditions, but usually ends up being determined experimentally for a given lifting body.
ρ = air density
V = velocity
A = wing area.
density times the square of the velocity, and then divided in half, is the dynamic pressure and the Bernoulli equation part.
When I was arguing with someone deeply wedded to the Bernoulli explanation I asked what motive the top-side air had to accelerate so it wound up even with the bottom-side, shorter path air again. “Was it having an interrupted conversation?”