In special cases, like a Joukowski airfoil in an inviscid flow, you can get a pencil and paper analytical solution, but in general you need to go to CFD, even for 2D problems.
The question is why pressure differential increases as speed increases? As aircraft speed increases, velocity on bottom and top of wings increases too. Then, pressure on top and bottom of wings decreases too. Why does a pressure differential comes up and lift force increases?
No. Wings change the pressures around them - some are above ambient [positive gage pressure] and some are below ambient [negative gage pressure]. . Lift is the difference in pressure from top to bottom of a wing. . As speed increases, the effect of the wing increases. The wing changes the pressures around it FARTHER from the ambient free stream. This means that the pressure below the wing goes UP and the pressure above the wing goes DOWN, thus making the difference GREATER. . I can only assume you bought into the most common misconception around Bernoulli's Principle: FACT: "Fast air does not produce a lower pressure. . The TRUE Physics: Air is ACCELERATED toward a lower pressure. The higher pressure pushes more, thus providing a net force, on the mass in air, toward the lower pressure. It's Newton. When there are two opposing forces on an object, there is a NET FORCE toward the lower force.
Great video. It helped me to find lift coefficient for NACA0015 for different angles of attack. But now, I am wondering how to find drag coefficient. Someone could tell me how to do that?
Calculating drag coefficient for airfoil is not very useful as it would vastly underestimate the drag of wing (full 3d structure as opposed to 2d slice has flow component along the wing edge messing things up). That said you could just integrate the horizontal component of pressure over the circumference since drag is a sum of horizontal surface forces.
I am a little concerned that you show the pressure points as pointing up vertically. I have always been taught that the pressures at different parts of the wing point at angles away from the surface.
Hello! I’m in the process of planing the wings for a flying wing without any sweep, but all reflex airfoils I can find have a Cm of -0.05 or something like that, and since I have no tail I need a Cm close to 0, right? I don’t know, what to do now. (really sorry for my bad english)
Nice video. Why does the air has to increase speed (per Bernoulli's principle) after the stagnation point? The air flowing on top of the wing do not have to meet the air flowing on the bottom of the wing at the same speed when they meet at the trailing edge. I believe that using Bernoulli's principle to explain lift of an airfoil is not entirely correct. Most of the lift is produced by the down wash of air at the trailing edge of the wing (Newton's 3rd law of motion).
And what is the down wash reacting on??...The forces acting On the wing, pressure On the lower surface, plus the loss of pressure On the upper surface, give the up force, the Down flow off of the aerofoil accounts for the Total energy Loss of the system....
Edwin, It speeds up because the pressure ahead of the wing pushes more than the relatively lower pressure above the wing. It is Newton accelerating the mass in air.
@@regdor8187 Downwash does not "act" on anything. The downwash is the RESULT of the pressures around the wing. . In the mid 1700s, Euler figured out that it is a Pressure Gradient (a difference between two locations) that Accelerates fluid/air toward the lower pressure region. The higher pressure region pushes more, which is a net force on the air toward the lower pressure region. Simple Newton. .. Pressure at a point pushes outward in all directions. .. Pressure under the wing pushes up on the wing AND pushes down on air further below the surface -- contributing to the down wash. .. Atmospheric pressure far above the wing is pushing down toward the upper surface and the lower pressure there. THEREFORE, there is a vertical Pressure Gradient Accelerating air downward that continues after the wing passes, contributing to the down wash - thus making the total downwash. . . . For a full explanation see this: *Understanding Lift Correctly:* rxesywwbdscllwpn.quora.com/ - - Regards
You already have F_l = integral (P_bot-P_top) dA Integrate (P_bot-P_top)*x(A) dA instead and you will get moment of rotation around the x=0. x(A) being camber distance from leading edge to the dA. M_l = integral (P_bot-P_top)*x(A) dA Then notice M_l = x_l*F_l So x_l = M_l/F_l.
This is simplified for slim wing, so the surface normal is not in equation. You would get different pressure plots for different attack angles but it would not work worse as long as angle of attack was relatively small.
There is - you need to know the functions of the pressures, probably by curve fitting and measuring. I think knowing the camber line helps a lot with developing the distributions, but i am not profficient enough yet to tell you how :X
at 2:37 you say "negative pressure" this is not correct: the pressure coefficent is negative and it leads (bernoulli) to a positive pressure lower than the "undisturbed" pressure. negative pressure can not exist.
wikipiero you are correct that total absolute pressure is always positive. It is the dynamic component of the pressure that goes negative, taking the total pressure at high speed locations below the ambient pressure.
wikipiero Bernoulli includes pressure, velocity, and elevation components in the total. The velocity component is often referred to as “dynamic”. In air the elevation effects are usually negligible and the pressure coefficient Cp represents the dynamic effects.
I was looking for a clear and simple explanation for some friends of mine to the fact that no negative pressure exist on extrados and no suction exist for the airfoil I.e. the air which flows on the extrados pushes the airfoil down. Your video is quite clear except when you say there is a negative pressure without specifying “dynamic pressure” nor specifying how to take this into account. On the other hand, although I can understand your explanation, I’ve never heard about “dynamic pressure” and as far as I know Cp is just a coefficient to calculate how the (total) pressure changes: always above zero. At the end your video is useless to me. Anyway thank you for the time you spent answering me.
@@wikipiero , The term "negative pressure" is very commonly used to indicate pressure that is *less that the ambient atmospheric pressure.* .. We also use the term "gauge pressure" to indicate the pressure relative to the ambient atmospheric pressure. .. For example: Assume we have an ambient atmospheric pressure of, say 14 psi. Then we have some flow that meets a surface normal (perpendicular to) the flow and stops at the surface where we measure 15 psi. The "Total Pressure" is 15 psi. The "Dynamic Pressure" is 1.0 psi. The "Static Pressure is 14 psi. The "Gauge Pressure" is also 1.0 psi. ... IF ... We find a surface with 13 psi on it. This is a Gauge Pressure of -1.0 psi This is a negative pressure of 1.0 psi. ... Sometimes we can also use the term "Dynamic pressure" to mean any pressure that has formed as a result of the relative motion of the air and some surface. ... So, yes. The pressure on the top of a wing always pushes down and the pressure on the bottom of a wing always pushes up.
The authors have two wrong scientific approaches: researching the creation of Lift force and Low pressure at upper side of the wing, relative to the ground surface and Earth. I explain the aerodynamic cavitation and existence of Lee side aerocavern, and creation of Aerodynamic force.
These explanations don’t explain anything when they do not show why the airflow speeds up over the curved surface. Of course no such thing happens. What does happen is this : imagine a container with a gas ,all the particles are in random motion and the pressure on the walls is equal. If by some magic the motion of these particles could be arranged in one direction the pressure would not be equal in all directions etc. Another relevant fact is that gasses occupy only a small fraction of the space they inhabit due to the large voids between the particles,the reason why they are compressible. In this way two imaginary steams of gas with their particles neatly parallel could pass through the same space at perpendicular angles without interfering with each other. Here we find the solution to the problem of the explanation of “lift’. The motion of the front edge of the airfoil creates an air mass with special properties,ie alignment of motion of its particles,this air mass is added to the layer above without change to its pressure in the direction perpendicular to the airfoil . This special mass is also subtracted from the air layer near the airfoil. These conditions rapidly return to normal random motion as the airmass passes over the wing. Well, I hope this is of interest to you and goodbye!
Oscar, I see this every time when people try to analyze this at the molecule level. They always have some misconception of how velocities asdd, or some other misconception. . First, his purpose is not to explain anything but pressure distribution , not really explain the fundamental reasons. .. Now. . . . . RE: "If by some magic the motion of these particles could be arranged in one direction the pressure would not be equal in all directions etc. " Correct. You need to study what are called static and dynamic pressure. In short. . . Static pressure is the pressure that the molecules exert on each other. This is what you would measure if you traveled along with any overall directed, average flow in one direction. This is some average speed of all these vibrating molecules still bouncing around amongst each other. This internal pressure is not affected by that overall, directed motion at all. They still vibrate and bounce around as before the flow was caused (magically). BY THE WAY, that magic is called a Pressure Gradient - a difference on static pressure between two locations is called a "Pressure Gradient. . The higher pressure pushes more so there is acceleration toward the lower pressure region. This pressure is also the same as that exerted on a surface parallel to the flow. Because velocities add vectorially, any velocity added to a flow parallel to the overall average flow, does not change the velocity, and therefore the momentum and pressure of the molecules on each other, nor does it change that velocity and momentum toward the surface it is near and traveling PARALLEL to. . HOWEVER, as that flow approaches a surface, either directly or at an angle, we are now seeing the pressure caused by the entire mass of its inertia toward the surface. This is dynamic pressure. It is like you running into a wall or another person. The momentum of the molecules is indeed increased in the forward direction, but not changed in the sideways direction, nor of them against each other. If you bounce a ball on the ground out a car window, the bounce force on the ground is NOT changed when the car moves. The forward motion and vertical motion are fully orthogonal - - fully independent. . You contradict yourself as follows. A: "the particles are in random motion" then, B: "two imaginary steams of gas . . . could pass through the same space at perpendicular angles without interfering with each other." You ignored the fact that all that "empty space" is full of molecules wizzing rapidly through all that space bouncing off each other, and the two flows wound FOR SURE be colliding. Then you say "with their particles neatly parallel" which contradicts " at perpendicular angles " - makes no sense. . . . Finally, I am unable to make sense out of your last 4 sentences. . . . For an explanation see this: *Understanding Lift Correctly:* rxesywwbdscllwpn.quora.com/ - - Regards
Nope Oscar. BUT you've come up with a brand new MISCONCEPTION. . . . Every neophyte who tries to use molecules, as you have gets, it wrong. The supersonic vibrating molecules prevent two flows flowing through each other. . One undisputed fact is that [Static] Pressure at a point has no direction. It is, therefore, called a scalar, not a vector. The pressure of a moving point is also EQUAL in all directions. The large-scale fluid flow [in one direction] can not change the random motions within a flow. Learn what Static and Dynamic pressure is. The kinetic energy of the moving flow has already been added by a pressure up stream; such as inside the blower or your lungs. The directed flow DOWS NOT alter the randomized, supersonic molecular vibrations. . SPEED reduce the pressure in a flow. NOPE. That is an all-too common of Bernoulli's Principle. It is the Pressure Gradient that is the CAUSE of the Acceleration of fluid. It is Newton in fluids. Force causes Accelerations.!. . . . . . .This "flows passing through each other" is just another misconception and YOU have been the first I've seen to come up with it. So take pride in you lack of physics knowledge. . And, BTW this video gives absolutely no explanation of the physics that causes these pressures and lift. He only shows you what the pressures are.
![[Aero Fundamentals #12] Pressure Coefficient Explained](http://i.ytimg.com/vi/ttaZt41lBXw/mqdefault.jpg)
![[Aero Fundamentals #12] Pressure Coefficient Explained](/img/tr.png)
In special cases, like a Joukowski airfoil in an inviscid flow, you can get a pencil and paper analytical solution, but in general you need to go to CFD, even for 2D problems.
Absolutely made it clear to me I've been struggling to understand this topic , Thanks a million
Good stuff, helped me understand the principle very easily.
Thank you so much, easily explained and makes my life easier going through ATPL theory.
The question is why pressure differential increases as speed increases? As aircraft speed increases, velocity on bottom and top of wings increases too. Then, pressure on top and bottom of wings decreases too. Why does a pressure differential comes up and lift force increases?
No. Wings change the pressures around them - some are above ambient [positive gage pressure] and some are below ambient [negative gage pressure]. .
Lift is the difference in pressure from top to bottom of a wing.
.
As speed increases, the effect of the wing increases. The wing changes the pressures around it FARTHER from the ambient free stream.
This means that the pressure below the wing goes UP and the pressure above the wing goes DOWN, thus making the difference GREATER.
.
I can only assume you bought into the most common misconception around Bernoulli's Principle: FACT: "Fast air does not produce a lower pressure.
.
The TRUE Physics: Air is ACCELERATED toward a lower pressure. The higher pressure pushes more, thus providing a net force, on the mass in air, toward the lower pressure. It's Newton. When there are two opposing forces on an object, there is a NET FORCE toward the lower force.
Excellent explanation
Very good, congrats!! Could you explain about Reynold number? and if it is possible with examples?
Great video. It helped me to find lift coefficient for NACA0015 for different angles of attack. But now, I am wondering how to find drag coefficient. Someone could tell me how to do that?
Calculating drag coefficient for airfoil is not very useful as it would vastly underestimate the drag of wing (full 3d structure as opposed to 2d slice has flow component along the wing edge messing things up). That said you could just integrate the horizontal component of pressure over the circumference since drag is a sum of horizontal surface forces.
Thanks for this video, Can I find drag coefficient from that cp vs x/c graph
Thank you so much, were can i fined the graph for example?
I knew that I would regret not paying attention is school.
Ty, that was explained so that I could understand it.
Really great! thank you
Excellent video
I am a little concerned that you show the pressure points as pointing up vertically. I have always been taught that the pressures at different parts of the wing point at angles away from the surface.
Pressure acts perpendicular to the surface, but in this video I was focussed on the lift generated by that pressure, hence vertical.
The area isn't the lift. It is the lift coefficient
Hello? Georgia Tech admissions? Please rush my application so I can understand this video!!
Well-Done
Very good explanation! :)
What is formula for area under each curve for wing?
that helped a lot! Thanks!
Hello!
I’m in the process of planing the wings for a flying wing without any sweep, but all reflex airfoils I can find have a Cm of -0.05 or something like that, and since I have no tail I need a Cm close to 0, right? I don’t know, what to do now.
(really sorry for my bad english)
Total moment needs to be zero to hold level. I don't know what the best choices would be in that configuration.
Look at NACA 22112-jf....
Nice video. Why does the air has to increase speed (per Bernoulli's principle) after the stagnation point? The air flowing on top of the wing do not have to meet the air flowing on the bottom of the wing at the same speed when they meet at the trailing edge. I believe that using Bernoulli's principle to explain lift of an airfoil is not entirely correct. Most of the lift is produced by the down wash of air at the trailing edge of the wing (Newton's 3rd law of motion).
And what is the down wash reacting on??...The forces acting On the wing, pressure On the lower surface, plus the loss of pressure On the upper surface, give the up force, the Down flow off of the aerofoil accounts for the Total energy Loss of the system....
Edwin,
It speeds up because the pressure ahead of the wing pushes more than the relatively lower pressure above the wing. It is Newton accelerating the mass in air.
@@regdor8187 Downwash does not "act" on anything.
The downwash is the RESULT of the pressures around the wing.
.
In the mid 1700s, Euler figured out that it is a Pressure Gradient (a difference between two locations) that Accelerates fluid/air toward the lower pressure region. The higher pressure region pushes more, which is a net force on the air toward the lower pressure region. Simple Newton.
..
Pressure at a point pushes outward in all directions.
..
Pressure under the wing pushes up on the wing AND pushes down on air further below the surface -- contributing to the down wash.
..
Atmospheric pressure far above the wing is pushing down toward the upper surface and the lower pressure there. THEREFORE, there is a vertical Pressure Gradient Accelerating air downward that continues after the wing passes, contributing to the down wash - thus making the total downwash.
. . .
For a full explanation see this: *Understanding Lift Correctly:*
rxesywwbdscllwpn.quora.com/
- -
Regards
Muchas gracias
What is the formula to find the position of center of pressure ?
You already have F_l = integral (P_bot-P_top) dA
Integrate (P_bot-P_top)*x(A) dA instead and you will get moment of rotation around the x=0. x(A) being camber distance from leading edge to the dA.
M_l = integral (P_bot-P_top)*x(A) dA
Then notice M_l = x_l*F_l
So x_l = M_l/F_l.
QUESTION: Isn't that plot Cp-1?
en.wikipedia.org/wiki/Pressure_coefficient
Hmm...How does angle of attack influence the lift characteristics and how would this wing provide lift when inverted?
This is simplified for slim wing, so the surface normal is not in equation.
You would get different pressure plots for different attack angles but it would not work worse as long as angle of attack was relatively small.
большое спасибо)
thank you.. thank you... so much
There is no analytical solution to find the pressure distribution?
There is - you need to know the functions of the pressures, probably by curve fitting and measuring. I think knowing the camber line helps a lot with developing the distributions, but i am not profficient enough yet to tell you how :X
at 2:37 you say "negative pressure" this is not correct: the pressure coefficent is negative and it leads (bernoulli) to a positive pressure lower than the "undisturbed" pressure. negative pressure can not exist.
wikipiero you are correct that total absolute pressure is always positive. It is the dynamic component of the pressure that goes negative, taking the total pressure at high speed locations below the ambient pressure.
can't understand what the "dynamic component of the pressure" is. can u explain please?
wikipiero Bernoulli includes pressure, velocity, and elevation components in the total. The velocity component is often referred to as “dynamic”. In air the elevation effects are usually negligible and the pressure coefficient Cp represents the dynamic effects.
I was looking for a clear and simple explanation for some friends of mine to the fact that no negative pressure exist on extrados and no suction exist for the airfoil I.e. the air which flows on the extrados pushes the airfoil down. Your video is quite clear except when you say there is a negative pressure without specifying “dynamic pressure” nor specifying how to take this into account. On the other hand, although I can understand your explanation, I’ve never heard about “dynamic pressure” and as far as I know Cp is just a coefficient to calculate how the (total) pressure changes: always above zero. At the end your video is useless to me. Anyway thank you for the time you spent answering me.
@@wikipiero ,
The term "negative pressure" is very commonly used to indicate pressure that is *less that the ambient atmospheric pressure.*
..
We also use the term "gauge pressure" to indicate the pressure relative to the ambient atmospheric pressure.
..
For example:
Assume we have an ambient atmospheric pressure of, say 14 psi.
Then we have some flow that meets a surface normal (perpendicular to) the flow and stops at the surface where we measure 15 psi.
The "Total Pressure" is 15 psi.
The "Dynamic Pressure" is 1.0 psi.
The "Static Pressure is 14 psi.
The "Gauge Pressure" is also 1.0 psi.
... IF ...
We find a surface with 13 psi on it.
This is a Gauge Pressure of -1.0 psi
This is a negative pressure of 1.0 psi.
...
Sometimes we can also use the term "Dynamic pressure" to mean any pressure that has formed as a result of the relative motion of the air and some surface.
...
So, yes. The pressure on the top of a wing always pushes down and the pressure on the bottom of a wing always pushes up.
awesome!
4 cұрақтың жауабы
4:54 Aaaah seeing the center of pressure drawn at the center of chord line makes me instinctively cringe. Lol
It is obviously Cp-1, not Cp.
The authors have two wrong scientific approaches: researching the creation of Lift force and Low pressure at upper side of the wing, relative to the ground surface and Earth. I explain the aerodynamic cavitation and existence of Lee side aerocavern, and creation of Aerodynamic force.
This video author is not explaining the physics of lift.
NOTHING he says requires the Earth to be down in that drawing.
See :
These explanations don’t explain anything when they do not show why the airflow speeds up over the curved surface. Of course no such thing happens. What does happen is this : imagine a container with a gas ,all the particles are in random motion and the pressure on the walls is equal. If by some magic the motion of these particles could be arranged in one direction the pressure would not be equal in all directions etc. Another relevant fact is that gasses occupy only a small fraction of the space they inhabit due to the large voids between the particles,the reason why they are compressible. In this way two imaginary steams of gas with their particles neatly parallel could pass through the same space at perpendicular angles without interfering with each other. Here we find the solution to the problem of the explanation of “lift’. The motion of the front edge of the airfoil creates an air mass with special properties,ie alignment of motion of its particles,this air mass is added to the layer above without change to its pressure in the direction perpendicular to the airfoil . This special mass is also subtracted from the air layer near the airfoil. These conditions rapidly return to normal random motion as the airmass passes over the wing. Well, I hope this is of interest to you and goodbye!
Oscar,
I see this every time when people try to analyze this at the molecule level. They always have some misconception of how velocities asdd, or some other misconception.
.
First, his purpose is not to explain anything but pressure distribution , not really explain the fundamental reasons.
..
Now. . .
. .
RE: "If by some magic the motion of these particles could be arranged in one direction the pressure would not be equal in all directions etc. "
Correct. You need to study what are called static and dynamic pressure.
In short. . .
Static pressure is the pressure that the molecules exert on each other. This is what you would measure if you traveled along with any overall directed, average flow in one direction. This is some average speed of all these vibrating molecules still bouncing around amongst each other. This internal pressure is not affected by that overall, directed motion at all. They still vibrate and bounce around as before the flow was caused (magically).
BY THE WAY, that magic is called a Pressure Gradient - a difference on static pressure between two locations is called a "Pressure Gradient.
.
The higher pressure pushes more so there is acceleration toward the lower pressure region.
This pressure is also the same as that exerted on a surface parallel to the flow. Because velocities add vectorially, any velocity added to a flow parallel to the overall average flow, does not change the velocity, and therefore the momentum and pressure of the molecules on each other, nor does it change that velocity and momentum toward the surface it is near and traveling PARALLEL to.
.
HOWEVER, as that flow approaches a surface, either directly or at an angle, we are now seeing the pressure caused by the entire mass of its inertia toward the surface. This is dynamic pressure. It is like you running into a wall or another person. The momentum of the molecules is indeed increased in the forward direction, but not changed in the sideways direction, nor of them against each other.
If you bounce a ball on the ground out a car window, the bounce force on the ground is NOT changed when the car moves. The forward motion and vertical motion are fully orthogonal - - fully independent.
.
You contradict yourself as follows.
A: "the particles are in random motion" then,
B: "two imaginary steams of gas . . . could pass through the same space at perpendicular angles without interfering with each other."
You ignored the fact that all that "empty space" is full of molecules wizzing rapidly through all that space bouncing off each other, and the two flows wound FOR SURE be colliding.
Then you say "with their particles neatly parallel" which contradicts " at perpendicular angles " - makes no sense.
. . .
Finally, I am unable to make sense out of your last 4 sentences.
. . .
For an explanation see this: *Understanding Lift Correctly:*
rxesywwbdscllwpn.quora.com/
- -
Regards
Nope Oscar. BUT you've come up with a brand new MISCONCEPTION. . .
.
Every neophyte who tries to use molecules, as you have gets, it wrong.
The supersonic vibrating molecules prevent two flows flowing through each other.
.
One undisputed fact is that [Static] Pressure at a point has no direction. It is, therefore, called a scalar, not a vector.
The pressure of a moving point is also EQUAL in all directions. The large-scale fluid flow [in one direction] can not change the random motions within a flow. Learn what Static and Dynamic pressure is.
The kinetic energy of the moving flow has already been added by a pressure up stream; such as inside the blower or your lungs.
The directed flow DOWS NOT alter the randomized, supersonic molecular vibrations.
.
SPEED reduce the pressure in a flow. NOPE. That is an all-too common of Bernoulli's Principle. It is the Pressure Gradient that is the CAUSE of the Acceleration of fluid. It is Newton in fluids.
Force causes Accelerations.!.
. . . . .
.This "flows passing through each other" is just another misconception and YOU have been the first I've seen to come up with it. So take pride in you lack of physics knowledge.
.
And, BTW this video gives absolutely no explanation of the physics that causes these pressures and lift. He only shows you what the pressures are.
useless. an old man need to improve knowledge