This video provides a good introduction into the general theories of lift with a solid explanation of the relevance of conservation of momentum and the physics which create boundary layers and their role in circulation and the magnus effect. One thing I do want to clarify is his examination of fluid velocity and pressure. Namely, he states that fluid velocity and pressure are causally linked, and that an increase in one directly results in drop in the other, this is not necessarily the case for freestream flow like we see in flight. Bernoulli's Equation is modeled for the scenario for control volumes and control masses which assumes a constant value for each parameter respectively. Because of this it is not safe to assume that an increase in fluid velocity over one section of the airfoil causes a decrease in pressure as there are a number of factors beyond fluid velocity which affect pressure over freestream flight.
A cylinder rotating clockwise is initially surrounded by a sheet of anti-clockwise vorticity. This thickens up due to viscosity, and is then convected away by the crossflow. This leaves a cylinder which is a naked clockwise vortex, which then generates a transverse force, this being the Magnus effect. The distribution of vorticity around the cylinder can be solved as a Fredholm integral equation, but the solution is easy because the coupling coefficient around a circle is a constant. Then Bernoulli’s principle can be applied to the solution to calculate the transverse force, or the Kutta-Joukowski circulation theorem may be applied.
Bernoulli's equation only is consistent only across a given airstream, not to two separate ones like you have in the case of a wing. So saying that the pressure is lower at the top of a wing because it's velocity is higher than a separate airstream below the wing has nothing to do with Bernoulli's equation.
hamboot The point regarding different flow velocities resulting in different static pressures above and below the airfoil (thus an upwards net force on the airfoil) still stands, because, as per Bernoulli, the static pressure decreases with higher velocity. Basically, you don't have to consider them as a single stream, they can be two separate streams each applying a different magnitude force on the airfoil, thus causing a motion (lift). As to why the the velocities of flow would be different is the key question, which you are right about in saying that Bernoulli's equation doesn't apply.
Bernoulli’s equation has nothing to do with lift. This is a common misconception. Reference NASA’s Glenn website and the RUclips video from Cambridge University.
+hamboot, Correction: it is consistent only WITHIN a given airstream. ... It actually turns out that because the air ahead of the wing has one pressure and velocity that you can use the equation to relate the upper and lower flow because they both have that common point. .. However, that has nothing to do with what CAUSES the pressure difference. Bernoulli's Equation only allows calculation of values and "explains" nothing. ... 99% of these explanation are simply repeating bad science they don't understand. Amateurs. ... The fact is that the air over the wing is ACCELERATED much more than the air below BY the lower pressure there. Pressure differences accelerate rthe air. Newton. The force from the pressure accelerates the mass of the air. Newton
+hamboot , This is only partly true. Bernoulli's Equation is calculated ALONG a streamline. HOWEVER, the air going above and below a wing has the SAME Bernoulli constant because they both start ahead of the wing at the same pressure (atmospheric) and velocity (zero). THEREFORE, Bernoulli's Equation CAN be used to CALCULATE the upper and lower pressures around a wing from the velocities.
@@curvebal , Bernoulli's Equation CAN be used around a wing. As i just explained in this reply thread. . It is Bernoulli's Principle which does not explain lift because it is misunderstood.
This explanation is a myth. Even physicists often believe it. Bernoulli's effect might be present but it is quite weak and I am not sure whether it can be applied on the explanation. The wing's lift is produced by: -deflection and pressurizing of the air on the bottom part of the wing ("angle of attack") -the coanda effect on the upper part of the wing creates the lower pressure which bends the air-stream downwards. This is caused more by air inertia than Bernoulli's principle. Finally the only force that has strength to keep the airplane in the air is caused by Newton's action-reaction principle. The air must bend downwards or there will be no lift.
While having some similarities, above the wing is technically not Coanda. ... Bernoulli explains and causes NOTHING. It is only a way to calculate various pressures when used correctly. . The total lift is from the difference in pressure bottom to top caused by themotion and surfaces. .... Also, it is the pressures created by the motion that ALSO produces the downward 'bend'. Air is accelerated by pressure differences. Newton.
I should add that pilots do not *_need_* to understand the physics of lift. We need to understand many more practical aspects of flight and aircraft control.
And few pilots know even that, because they are pretty much all told bad science... but it doesn't matter because the practical aspects of flight are more important such as knowing how to get and keep lift regardless of the physics..
This video provides a good introduction into the general theories of lift with a solid explanation of the relevance of conservation of momentum and the physics which create boundary layers and their role in circulation and the magnus effect. One thing I do want to clarify is his examination of fluid velocity and pressure. Namely, he states that fluid velocity and pressure are causally linked, and that an increase in one directly results in drop in the other, this is not necessarily the case for freestream flow like we see in flight. Bernoulli's Equation is modeled for the scenario for control volumes and control masses which assumes a constant value for each parameter respectively. Because of this it is not safe to assume that an increase in fluid velocity over one section of the airfoil causes a decrease in pressure as there are a number of factors beyond fluid velocity which affect pressure over freestream flight.
A cylinder rotating clockwise is initially surrounded by a sheet of anti-clockwise vorticity. This thickens up due to viscosity, and is then convected away by the crossflow. This leaves a cylinder which is a naked clockwise vortex, which then generates a transverse force, this being the Magnus effect. The distribution of vorticity around the cylinder can be solved as a Fredholm integral equation, but the solution is easy because the coupling coefficient around a circle is a constant. Then Bernoulli’s principle can be applied to the solution to calculate the transverse force, or the Kutta-Joukowski circulation theorem may be applied.
Bernoulli's equation only is consistent only across a given airstream, not to two separate ones like you have in the case of a wing. So saying that the pressure is lower at the top of a wing because it's velocity is higher than a separate airstream below the wing has nothing to do with Bernoulli's equation.
hamboot
The point regarding different flow velocities resulting in different static pressures above and below the airfoil (thus an upwards net force on the airfoil) still stands, because, as per Bernoulli, the static pressure decreases with higher velocity.
Basically, you don't have to consider them as a single stream, they can be two separate streams each applying a different magnitude force on the airfoil, thus causing a motion (lift).
As to why the the velocities of flow would be different is the key question, which you are right about in saying that Bernoulli's equation doesn't apply.
Bernoulli’s equation has nothing to do with lift. This is a common misconception. Reference NASA’s Glenn website and the RUclips video from Cambridge University.
+hamboot,
Correction: it is consistent only WITHIN a given airstream.
...
It actually turns out that because the air ahead of the wing has one pressure and velocity that you can use the equation to relate the upper and lower flow because they both have that common point.
..
However, that has nothing to do with what CAUSES the pressure difference. Bernoulli's Equation only allows calculation of values and "explains" nothing.
...
99% of these explanation are simply repeating bad science they don't understand. Amateurs.
...
The fact is that the air over the wing is ACCELERATED much more than the air below BY the lower pressure there.
Pressure differences accelerate rthe air. Newton. The force from the pressure accelerates the mass of the air. Newton
+hamboot
,
This is only partly true. Bernoulli's Equation is calculated ALONG a streamline. HOWEVER, the air going above and below a wing has the SAME Bernoulli constant because they both start ahead of the wing at the same pressure (atmospheric) and velocity (zero). THEREFORE, Bernoulli's Equation CAN be used to CALCULATE the upper and lower pressures around a wing from the velocities.
@@curvebal ,
Bernoulli's Equation CAN be used around a wing. As i just explained in this reply thread.
. It is Bernoulli's Principle which does not explain lift because it is misunderstood.
many thanks
I'm in the class
This explanation is a myth. Even physicists often believe it. Bernoulli's effect might be present but it is quite weak and I am not sure whether it can be applied on the explanation.
The wing's lift is produced by:
-deflection and pressurizing of the air on the bottom part of the wing ("angle of attack")
-the coanda effect on the upper part of the wing creates the lower pressure which bends the air-stream downwards. This is caused more by air inertia than Bernoulli's principle.
Finally the only force that has strength to keep the airplane in the air is caused by Newton's action-reaction principle. The air must bend downwards or there will be no lift.
While having some similarities, above the wing is technically not Coanda.
...
Bernoulli explains and causes NOTHING. It is only a way to calculate various pressures when used correctly.
.
The total lift is from the difference in pressure bottom to top caused by themotion and surfaces.
....
Also, it is the pressures created by the motion that ALSO produces the downward 'bend'. Air is accelerated by pressure differences. Newton.
*This video only repeats misconceptions and bad science. I've already explained it in this and His other wing design video.*
As a pilot, mechanical engineer, and scholar of fluid dynamics, I was screaming like a baboon at this video.
I should add that pilots do not *_need_* to understand the physics of lift. We need to understand many more practical aspects of flight and aircraft control.
Good pilots know what the aircraft does. Great pilots know why it does it.
And few pilots know even that, because they are pretty much all told bad science... but it doesn't matter because the practical aspects of flight are more important such as knowing how to get and keep lift regardless of the physics..