Physical Science 3.5c - Bernoulli`s Principle and Airplanes

Bernoulli's Principle is stated and discussed. The lift produced by airplane wings is discussed, including the lift produced by Bernoulli's Principle and also the lift produced by deflecting the air downward, according to Newton's 3rd Law.

[Observer]

The lift force is the pressure difference around the wing (more under than above) caused by the relative (air-wing) motion.
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These same pressure changes are what also push the air down behind the wing (down-wash).... as well as forward and upward ahead of the wing (up-wash) and around the wing-tips into the tip vortex.
Bernoulli's Principle as interpreted here is an all too common *misconception*. Speed DOES NOT lower static pressure.
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Pressure gradients (differences in pressure between regions) *accelerate* (push) on the mass of air. That's simply Newton for fluids.
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Three years since I explain all of this and this Bad Science is still here!

[LawtonDigital]

What role does Bernoulli's principle play when a plane is flying upside down?

[snehika_connections]

Superb explanation. I have been through many explanations but this is the one which clarifies all my doubts about the lifting up of aircraft. Thank you so much!! Great job!!

[spacegazette]

That was surprisingly good. The bit about faster air not being less pressurized but by depressurizing the objects it passes by. And the direct air pressure hitting something which this is not.

[yakx]

Pressure is the result of molecular collisions with the surface of something. If there is no movement of the molecules (absolute 0 temperature) there is no pressure. As a stream of fluid passes faster along a surface the collisions become increasingly glancing blows at an angle to the surface and impart less pressure. It's like hitting a ball against a wall at increasingly more shallow angles. That explanation is from a 1950's aerodynamic book that I've misplaced, but I've never heard a better explanation.

[aidansvoboda]

Excellent video! One thing to keep in mind is that Bernoulli's equation only applies to a single streamline. Thus, Bernoulli's is not a great explanation when it comes to airfoils, as the many streamlines above and below the airfoil cannot be related using Bernoulli's. While velocity and pressure are related when it comes to airfoils, it is more due to pressure and velocity fields and how they change together.

[carmelpule]

It is not enough to just state Bernoulli's principle, and the next question to ask is what causes Bernoulli's principle?
So let us jump in at the deep end and describe what actually happens. Let us assume we are using a well designed airfoil section where the top and bottom of the leading edge were designed to accelerate the air particles out of the way, for the wing to go through the gap formed and generate lift on the whole wing. The shape of the accelerating zone at the leading edge to cause minimum acceleration to provide the right momentum to the air particles, without too much drag is a science in itself about which we shall not cover in this comment.
The air particles are forced to hit heavily the leading edge using the energy of the engine, hence they are accelerated and reflected upwards very forcefully solely due to engine power or dive in a glider, where their momentum would tend to carry them far higher than the surface of the wing causing a depleted zone hence low pressure bubble above the wing This is a sort of bubble of low pressure above the wing, which will try to collapse with the low pressure accelerating the air particles downwards while at the same time pulling the wing upwards. But with the less mass, viscous air particles forming the upper side of the low pressure bubble being lighter than the solid wing with a higher mass, these will accelerate down faster than the wing will try to accelerate upwards, with an equal reaction and so the wing must move forward fast enough not to let the air particles cover so much distance down as to hit too much of the upper trailing edge and preferably will come down as down wash behind the trailing edge. This is a continuous action with new particles being given energy by the engine or a dive, and shot up at the leading edge to keep the low pressure bubble above the top surface of the wing through them being accelerated down into an area of low pressure thus increasing their velocity before they go past the trailing edge, perhaps even hit it but the over all effect is still the fact that the wing trying to be sucked up by the low pressure area above it. Again it has to be noted that it is very important that the wing will move forward fast enough so that the air particle on top accelerating down will not hit too much of the upper surface of the wing to lose their momentum downwards on the wing itself. The fact that the upper surface of the wing recedes down after its higher point, it will give the air particles some more room, hence time before they hit the trailing edge and preferably go past it.
Meanwhile at the lower leading edge the air is also accelerated down, but due to the fact that the lower part of the wing forms an angle of attack there will now be a high pressure zone which will accelerate the air particles under the wing to change their momentum downwards and due to the fact that they are in a high pressure zone all the way to the trailing edge, they will slow down a little while those on the upper side on the wing accelerating in to a low pressure zone of their own making, they will increase velocity hoping that they will not hit too much of the upper surface near the trailing edge. In fact this is the main reason why the plane has a minimum safe speed for take off so that the upper downward accelerating particles on top of the wing will not hit the upper surface to lose their momentum on the wing surface which would push it down, where other zones on the upper surface are lifting it up!
The over all effect is a change of momentum downward of the air around the wing and so the force to do this reacts to lift the wing, be it a high pressure under the wing or a continuously collapsing and continuously repaired low pressure bubble on the top surface.
We should stick to the facts that are taking place and not mention, Bernoulli nor Newton. That would be like some one asking me what my wife looks like and I would tell them that she looks like her sister. If I had to use that type of description, No one would be any wiser if they do not know her sister, would they?

[Inveltz]

May I ask why would the air passing above the wing move faster than the air moving below it?

[samkavin]

The sum of total energy, PE, KE are constant is bernoullis principal

[david_porthouse]

An inclined hydrofoil moving slowly through liquid helium will not generate a transverse force, or lift in other words. The same hydrofoil moving through liquid sodium with exactly the same geometry or kinematics will generate a transverse force. An attempt to explain this in terms of Bernoulli or Newton will be a failure.

What happens in the case of liquid helium is that fluid doubles back round the trailing edge and nothing much else happens. In the case of liquid sodium viscosity frustrates the doubling back behaviour and a starting vortex is dumped in the flow. By conservation of vorticity, there must be bound vorticity associated with the hydrofoil and this generates lift by the Magnus effect.

If the hydrofoil stops, then it will dump a stopping vortex. If it moves intermittently in a start-stop action, then it will dump a vortex street. This can actually be visualised.

This is the basics of lift generation. I'll talk about the Kutta condition and the Kutta-Joukowski circulation theorem if you like, but the Magnus effect is a good simple alternative. Bernoulli's theorem is merely wisdom after the event of a starting vortex being dumped.

[Observer]

The wing's movement through the air is what causes the pressure changes.  Those pressure differences is the lift force.  Those same pressure differences then cause the accelerations of the air; the various velocity changes of the air around the wing.

[GayanAthapatthu]

because of the aero dynamic design 1:55

[vennessalorret]

It is not enough to just state Bernoulli's principle, and the next question to ask is what causes Bernoulli's principle?
So let us jump in at the deep end and describe what actually happens. Let us assume we are using a well designed airfoil section where the top and bottom of the leading edge were designed to accelerate the air particles out of the way, for the wing to go through the gap formed and generate lift on the whole wing. The shape of the accelerating zone at the leading edge to cause minimum acceleration to provide the right momentum to the air particles, without too much drag is a science in itself about which we shall not cover in this comment.
The air particles are forced to hit heavily the leading edge using the energy of the engine, hence they are accelerated and reflected upwards very forcefully solely due to engine power or dive in a glider, where their momentum would tend to carry them far higher than the surface of the wing causing a depleted zone hence low pressure bubble above the wing This is a sort of bubble of low pressure above the wing, which will try to collapse with the low pressure accelerating the air particles downwards while at the same time pulling the wing upwards. But with the less mass, viscous air particles forming the upper side of the low pressure bubble being lighter than the solid wing with a higher mass, these will accelerate down faster than the wing will try to accelerate upwards, with an equal reaction and so the wing must move forward fast enough not to let the air particles cover so much distance down as to hit too much of the upper trailing edge and preferably will come down as down wash behind the trailing edge. This is a continuous action with new particles being given energy by the engine or a dive, and shot up at the leading edge to keep the low pressure bubble above the top surface of the wing through them being accelerated down into an area of low pressure thus increasing their velocity before they go past the trailing edge, perhaps even hit it but the over all effect is still the fact that the wing trying to be sucked up by the low pressure area above it. Again it has to be noted that it is very important that the wing will move forward fast enough so that the air particle on top accelerating down will not hit too much of the upper surface of the wing to lose their momentum downwards on the wing itself. The fact that the upper surface of the wing recedes down after its higher point, it will give the air particles some more room, hence time before they hit the trailing edge and preferably go past it.
Meanwhile at the lower leading edge the air is also accelerated down, but due to the fact that the lower part of the wing forms an angle of attack there will now be a high pressure zone which will accelerate the air particles under the wing to change their momentum downwards and due to the fact that they are in a high pressure zone all the way to the trailing edge, they will slow down a little while those on the upper side on the wing accelerating in to a low pressure zone of their own making, they will increase velocity hoping that they will not hit too much of the upper surface near the trailing edge. In fact this is the main reason why the plane has a minimum safe speed for take off so that the upper downward accelerating particles on top of the wing will not hit the upper surface to lose their momentum on the wing surface which would push it down, where other zones on the upper surface are lifting it up!
The over all effect is a change of momentum downward of the air around the wing and so the force to do this reacts to lift the wing, be it a high pressure under the wing or a continuously collapsing and continuously repaired low pressure bubble on the top surface.
We should stick to the facts that are taking place and not mention, Bernoulli nor Newton. That would be like some one asking me what my wife looks like and I would tell them that she looks like her sister. If I had to use that type of description, No one would be any wiser if they do not know her sister, would they?