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.
In: 847
[removed]
[removed]
>what bernoulli’s principles is
Air moving faster has lower pressure; air moving slower has higher pressure.
>and how it works for aviation
Airplanes are designed to create a faster flow of air on the top side of the wings, and a slower flow of air on the bottom side of the wings. This create higher pressure *under* the wing, and lower pressure *above* the wing…which ends up shoving the wings upwards.
The idea that lift comes entirely or mostly from pressure differences created by Bernoulli’s principle is a myth.
Most of the lift comes from deflection of airflow via the aerofoil shape and angle of attack, consistent with Newton’s third law. Air flowing past the wing is pushed down, so the wing is pushed up. Think of how it feels to hold your hand out the window of a car.
There are however many different forces at work, including air pressures, which would all need to be included in a detailed calculation of lift (which is well beyond my knowledge).
I suggest this article:
[No One Can Explain Why Planes Stay in the Air](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/)
Particularly these two graphics:
[The Flawed Classics](https://static.scientificamerican.com/sciam/assets/Image/2020/XXsaw0220Regi31_d.png)
[New Ideas of Lift](https://static.scientificamerican.com/sciam/assets/Image/2020/saw0220Regi32_d.png)
I’ll try to merge a lot of concepts here for one, all encompassing explanation.
Bernoulli’s principle says that flowing fluids exhibit less pressure in directions *other than the one they’re flowing in* (in that direction, things are more complicated). If you want an intuition for this, it’s roughly speaking that the fluid is still exhibiting the same pressure, but now more of that pressure is directed forward instead of equally in all directions.
There’s a common explanation that airplane wings are longer on the top, forcing air to move faster over them than under them because it needs to take a longer path in the same time, and thus generating lift. This is totally wrong. There’s nothing that requires air to take the same time to do this. In fact, air does tend to move faster over the top of wings, but way faster than this theory would suggest.
Flight is complicated because fluids are complicated, but it’s more straightforward and accurate to say planes fly because they push air down. You may have heard Newton’s Third Law: “Every action has an equal and opposite reaction”. That’s what’s happening here: airplane pushes air down, air pushes airplane up.
While Bernoulli’s principle is at play here, it’s more accurate to say that the wing pushing air down is creating the faster flow than to say that the faster flow is lifting the plane. But again, these things are complicated and everything gets an asterisk. But roughly speaking pushing air down requires pushing it forward too, so it bunches up under the wing and goes more slowly. At the same time, this creates a lower pressure region over the wing, since the wing has pushed down air that would have normally gone over the wing. Air rushes to fill this low pressure region and therefore moves faster.
Ok just to clarify one thing for you regarding a lot of comments here: when they say that faster air flow has less pressure, they actually mean static pressure. Bernoulli equation says (in general) that total pressure is constant and total pressure is sum of static (p/rho) and dynamic pressure (v^2/2). If you increase one component ( speed, for example) then the other component must become lower (static pressure p). There is also a gravitational component of this sum (g*h), but it is neglected in most of the calculations which include gases because of the low density. This method is used for calculation of air speed of the plane. You will often see a thin pipe at the front of an airplane – this instrument is using Bernoulli principle to determine air speed of the plane… Wow this is more like ELI25
A lot of partial answers here. Bernoulli’s law actually contributes very little to lift.
The purpose of a wing is to force a mass of air down while minimizing drag.
Most replies seem less clear about how wings accomplish this.
When an airfoil meets airflow, it splits the air in two, with some going under — and the rest going over.
If the airfoil meets airflow at a slightly positive angle (angle of attack) the bottom surface displaces air mass down, which exerts a force up.
However, air going over top of the wing follows a surface that begins to recede away from the airflow, which creates a partial vacuum. Ambient atmospheric pressure tries to fill this void from all directions. The wing inhibits air underneath from filling the void (but lifts the wing by trying) so air from above does it instead. Air held against the upper surface still moves down as it passes the wing’s trailing edge, so the net force of moving this air down implies an equal force that pushed the wing up.
The wing rides this constant pressure differential as it moves forward, which we call lift.
Now if the wing’s angle of attack is too steep, the airflow’s intertia overcomes atmospheric pressure holding it against the wing and it separates. Tendrils of atmospheric gas seep into the void from around the wing’s leading and trailing edges which create’s turbulence and drag.
So now, not only does does substantial portion of the passing air mass stop moving down, but drag substantially increases too, destroying the wing’s efficiency in a condition known as a “stall.”
A wing’s stall angle (the attack angle at which flow separates) is the same, regardless of speed. This occurs because even though slow moving air holds less inertia than fast moving air, this also means that less differential pressure is needed to draw air from around the leading and trailing edges to fill the pressure void above the wing and separate the boundary layer.
Think of air as a bunch of sand grains but invisible.
When you stick your hand out a moving vehicle, and prop ur hand in a slightly upward position, think about what happens to your hand?
You catch a lot of sand (air) on the palm side of ur hand to the point where it feels like it’s pushing your hand backwards.
While on the top side of your hand, you basically block all the sand (air).
In the palm of your hand where theres a lot of sand, we call this the high pressure zone.
(Abundance of air molecules that are slow moving as they are stopped)
While on the top side of your hand, where there is no sand, we call this the low pressure zone.
(Lower than normal air molecules creates a vacuum as it tries to find anything to fill the void being created – your are catching what should be there and pushing it to the high pressure zone.)
Because the world likes balance, the low pressure sand looks for a way to fill the void (vacuum) and since the high pressure zone wants to also achieve normal balance, it will push its way (Lift) to the low pressure zone and help fill that void.
It will push ur hand (wing) if it has to causing you to feel your hand pushing back.
Conservation of energy for a flow moving in a streamline, but every term is divided by volume to give a sort of conservation of different pressures.
Essentially the equation shows that faster flow has lower pressure.
However, it doesn’t really apply to lift that well, though it forms part of a common but incorrect explanation.
Lift essentially forms because if you have curved flow, there is a pressure gradient acting towards the “centre” of that circle (this is derived by just considering the pressure/force balance on a small element of fluid) so pressure closer to the centre is lower than further away, which results in lower pressure above the wing than below due to its shape, and since force = pressure * area that causes a net force upwards.
Latest Answers