Eli5 what bernoulli’s principles is and how it works for aviation. Been watching videos about it and still cant wrap my head around it, thanks heaps for any help.
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Most of these replies are terrible and dying on the hill of one aspect of lift generation when the facts is there are many different principles at play, all intertwined, which contribute. No one principle alone makes airfoils work.
At the top level both newton and pressure differences add to the lift of a wing.
Addressing newton, yes, the obvious hand out the window effect is there. You can generate lift with a piece of plywood, it’s just INCREDIBLY inefficient.
The “magic” of airfoils occurs on the upper surface. Coanda effect makes the air follow the surface of the wing and turn downward. This turning both creates a force through newton and a pressure difference from a change in velocity. This “magic” is what allows airfoils to work at a reasonably usable amount of efficiency, it’s “free” lift.
BOTH things add to the lift generation if someone tells you it’s one or the other just laugh at them and walk away.
Wow. Lots of crazy stuff in here. Let me give a shot at ELI5.
First, “pressure” is made of “dynamic” pressure and “static” pressure. Total pressure remains the same. If you can *increase* dynamic pressure, static pressure will *decrease* to compensate, and vice versa. When standing still, dynamic pressure (created by moving) is zero. If you can increase it, you can decrease static pressure. And key here is, *if you can increase dynamic pressure more on the top of a wing than on the bottom, the static pressure on the bottom will be higher, creating lift*.
How to do that is to make air traveling over the top of the wing go faster than air going over the bottom of the wing – faster air has more dynamic pressure, and so less static pressure. If you compare two molecules of air right next to each other, one going over the top of the wing, and one going over the bottom of the wing, they will start and reach the end at the same time. To make one go faster, you need to travel a greater distance. So the shape of the wing makes the molecule of air going over the top of the wing travel a greater distance to get to the backside of the wing then the molecule of air going under the wing. In order to travel past the wing and end up at the backside at the same time, the top molecule has to go a farther distance, and will have had to have traveled faster. This gives it more dynamic pressure, creating lower static pressure on the top of the wing versus the bottom of the wing, creating lift.
If you want a simple example of Bernoulli‘s principle, in action, which also somewhat debunks this “deflection theory“, take a napkin, or a tissue, hold it tot right under your mouth and blow. The faster air going over the top of the napkin will create a lower pressure area and cause the napkin to be lifted by the higher static pressure beneath it.
Enjoy!
Air is a fluid. Fast air is thinner because there’s less of it, it’s all moving out of the way. Slow air is thicker because it’s sitting there chilling.
Thick air pushes harder than thin air so stuff gets pushed in direction with thinner air.
This is grossly over simplified and inaccurate but you said eli5 so
Simple example for Bernoulli’s principle is if you open your mouth while skydiving the air moving outside your mouth is moving fast while the air inside your mouth is not moving. The faster moving air is a low pressure area while the slow moving air inside your mouth is high pressure. Just like an inflated tire that has a hole in it, stuff will get pushed from the high pressure area inside your mouth to the low pressure area which means you will have saliva all over your face.
In the case of the wing, the wing is between the high and the low pressure areas. The high pressure area is underneath the wing pushing it up towards the low pressure area.
A few people are getting it wrong with the theories with lift, especially the ones who say air molecules has to go farther on the top. Please read on incorrect lift theory. NASA has 3 pages on wrong theories and 2 on right.
[https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html](https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/wrong1.html)
And for OP, to understand Bernoulli’s principle, think of a garden hose with a constant flow of water. If you cover the opening, leaving a small space for water to go, water has to go fast. It means the flow speeds up due to the reduction of static pressure or potential energy.
And if you look in one of the correct theories of lift,
[https://www.grc.nasa.gov/www/k-12/airplane/right1.html](https://www.grc.nasa.gov/www/k-12/airplane/right1.html)
You can see the page stating that Bernoulli’s principle is used for some simple flow problems to find how pressure is distributed across the wing if we know the local velocity across the wing then finding a net force helping us know if there is lift or drag.
Am aerodynamicist.
Many people are missing the real principle behind Bernoulli’s principle, which is the conservation of energy : ie. energy cannot be created or destroyed.
Imagine a balloon filled with air; when you push it inwards, the pressure increases. In other words, you have added energy into the air.
When air moves, it has kinetic energy, just like anything else.
Bernoulli found a way of relating “pressure energy” to “kinetic energy”. As aerodynamics developed, Bernoulli’s equation was eventually superseded by more accurate models, but the fundamental point remains.
How does it relate to flight? This is where stuff gets really messy and not well explained. Bernoulli’s is best thought of as a way to calculate the pressure and velocity in an airflow of interest. The rest is more philosophy than science. No equation can really ever truly explain “why”. We just know that this equation can describe what you see.
To clear up the many misconceptions on this thread :
– The pressure integrals and Newton’s third law are different faces of the same coin. Pressure forces are the -only- forces acting on a wing (apart from shear forces ie drag). So the net force on a plane can be completely determined by the pressure distribution.
-Every aerodynamic effect you can think of essentially alters the surface pressure distribution. So, as far as an engineer is concerned, pressure distribution easy to measure, so we do that.
– As far as the momentum change in the air column is equal and opposite to the wing, that’s true too. Its more that there’s no clean way of measuring how much a given amount of air was deflected. So it’s perfect science, but pretty useless engineering. However, for drag (where the air simply slows down), momentum rakes have been used to some success. (momentum in – momentum out = drag force).
– To marry the two you could look as pressure distribution as the kinematics way of looking at it (resolve all forces in detail and get the resulting lift), and the conserved quantities approach as the big picture approach (this momentum change must be equal and opposite to the other one).
the basic idea of bernoulli’s equation is that the amount of energy in a streamline is constant.
if you’ve ever seen a wind-tunnel where they had white smoke snaking across an object, that smoke is caught in a streamline.
energy can be in the form of velocity, pressure, or height. for air, the height difference is trivial, so the air flowing over a wing can be fast *or* high pressure.
how that relates to a wing is complicated. basically, the bottom of a wing pushes air down and the top *pulls* the air above it down. this is a function of geometry and the coanda effect (basically streamlines like to follow objects) but the pulling part on the top decreases the pressure. per bernoulli’s equation, that means the air above the wing moves faster.
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