So Confused about The Theories of Lift Generation
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So Confused about The Theories of Lift Generation
Hi all,
I am sorry to raise such basic question here, but I really hope that you can help me clarify my understanding. When it comes to lift generation, there are plenty of theories. When I dive in with this topic, I am so confused because some widely accepted theories are proven wrong by NASA. This is my current understanding:
Bernoulli’s Principle states that the higher the dynamic pressure (speed of airflow), the lower the static pressure. Since the air flows along the upper wing surface in a faster speed relative to the lower wing surface, the upper wing has a lower static pressure and while the lower wing has a higher static pressure. Such pressure difference creates an upward force which is lift.
I acknowledge that equal time assumption has been proven wrong a long time ago. However, I am so confused about why the air flowing along the upper wing surface is faster.
FAA’s PHAK explains this by a venturi tube, when the mass of air entering the tube needs to be the same as that exiting the tube, the airspeed must increase in the throat, therefore the pressure will decrease. First I am not sure how an airfoil relates to that, and second NASA has also disprove this.
The book also explains the lift generation by “air stream strikes”. Here is the extraction (FAA, 2016):
I don’t know why there are two air streams striking two different positions of the leading edge, one is the lower section and the other is the upper. I learned that the relative airflow is just divided into two path at the leading edge, but not having a force striking to the wing. In addition, it seems to be proven as an incorrect theory here by NASA.
Apart from the theories that explains lift generation by pressure difference, Coanda Effect and Newton’s Third Law seem to be the two that are widely accepted. The former explains why the air tends to stick to and flow along the wing surface, which should be the precondition of all lift theories; while the latter is used to prove that the air flowing along the upper wing surface is deflected downward (action), an upward force (reaction) is created as lift. Are these two still correct in explaining lift generation? Is my understanding correct?
Hope you guys can help!
I am sorry to raise such basic question here, but I really hope that you can help me clarify my understanding. When it comes to lift generation, there are plenty of theories. When I dive in with this topic, I am so confused because some widely accepted theories are proven wrong by NASA. This is my current understanding:
Bernoulli’s Principle states that the higher the dynamic pressure (speed of airflow), the lower the static pressure. Since the air flows along the upper wing surface in a faster speed relative to the lower wing surface, the upper wing has a lower static pressure and while the lower wing has a higher static pressure. Such pressure difference creates an upward force which is lift.
I acknowledge that equal time assumption has been proven wrong a long time ago. However, I am so confused about why the air flowing along the upper wing surface is faster.
FAA’s PHAK explains this by a venturi tube, when the mass of air entering the tube needs to be the same as that exiting the tube, the airspeed must increase in the throat, therefore the pressure will decrease. First I am not sure how an airfoil relates to that, and second NASA has also disprove this.
The book also explains the lift generation by “air stream strikes”. Here is the extraction (FAA, 2016):
As the air stream strikes the relatively flat lower surface of a wing or rotor blade when inclined at a small angle to its direction of motion, the air is forced to rebound downward, causing an upward reaction in positive lift. At the same time, the air stream striking the upper curved section of the leading edge is deflected upward. An airfoil is shaped to cause an action on the air, and forces air downward, which provides an equal reaction from the air, forcing the airfoil upward. If a wing is constructed in such form that it causes a lift force greater than the weight of the aircraft, the aircraft will fly.
Apart from the theories that explains lift generation by pressure difference, Coanda Effect and Newton’s Third Law seem to be the two that are widely accepted. The former explains why the air tends to stick to and flow along the wing surface, which should be the precondition of all lift theories; while the latter is used to prove that the air flowing along the upper wing surface is deflected downward (action), an upward force (reaction) is created as lift. Are these two still correct in explaining lift generation? Is my understanding correct?
Hope you guys can help!
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First things first : air pressure is lower on the upper side, and there is some downward movement of air behind the wing, these things are 100% certain.
However, the newton explanation seems wrong, you'll see why later in my post.
I think the problem is even more confusing if you consider the other side of the relative movement.
I mean, in order to explain lift, most explanations consider a wind tunnel, with the wing stationary.
However, if you consider a real airplane, then the airplane is moving and the air is stationary.
So the only way this air above the wing could be accelerated is if it moved backwards. Which is really a mind
Really, I've completed a master's degree in aerospace engineering and we never learned how to explain lift in layman's words.
We only learned that lift comes from a circular movement of air (which you already know about) : air moves backward above, forward under, upward in front of and downward behind the wing.
But the reason why this circular movement would appear is just because the fluid mechanics equation make us come to that conclusion.
Obviously I forgot about the involved integro-differential equations almost as soon as I saw them
However, the newton explanation seems wrong, you'll see why later in my post.
I think the problem is even more confusing if you consider the other side of the relative movement.
I mean, in order to explain lift, most explanations consider a wind tunnel, with the wing stationary.
However, if you consider a real airplane, then the airplane is moving and the air is stationary.
So the only way this air above the wing could be accelerated is if it moved backwards. Which is really a mind
Really, I've completed a master's degree in aerospace engineering and we never learned how to explain lift in layman's words.
We only learned that lift comes from a circular movement of air (which you already know about) : air moves backward above, forward under, upward in front of and downward behind the wing.
But the reason why this circular movement would appear is just because the fluid mechanics equation make us come to that conclusion.
Obviously I forgot about the involved integro-differential equations almost as soon as I saw them
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Thanks for your response! Would you mind explaining the "circular movement" further? I am not sure what you meant by "air moves backward above, forward under, upward in front of and downward behind the wing"...
I have also heard another theory that states that the air (boundary layer???) on the upper surface is expanded while that on the lower surface is compressed, due to angle of attack. May I know why the air gets expanded/compressed when it flows along the wing?
I have also heard another theory that states that the air (boundary layer???) on the upper surface is expanded while that on the lower surface is compressed, due to angle of attack. May I know why the air gets expanded/compressed when it flows along the wing?
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Hi,
very nice video to proof that the equal time theory is nonsenses:
https://youtu.be/e0l31p6RIaY
A long and detailed explanation of the latest theory:
https://youtu.be/XWdNEGr53Gw
Have fun.
very nice video to proof that the equal time theory is nonsenses:
https://youtu.be/e0l31p6RIaY
A long and detailed explanation of the latest theory:
https://youtu.be/XWdNEGr53Gw
Have fun.
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Thanks very much! I have found the presentation file for your second videos from the youtube comment section.
Basically it considers the curvature of the airfoil as a part of circular motion in which the air sticks to the wing because of centripetal force. Since the force needs to be larger when the air is farer from the centre, the pressure needs to be larger as well.
Do I understand that correctly?
Basically it considers the curvature of the airfoil as a part of circular motion in which the air sticks to the wing because of centripetal force. Since the force needs to be larger when the air is farer from the centre, the pressure needs to be larger as well.
Do I understand that correctly?
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First of all, Lift is a reaction, not a force in and of itself - you are not sucked up into the air, as implied by EASA questions - if that were the case, where does downwash come from? Indeed, if you relied totally on Bernoulli, a Cessna 172 would have to do 472 knots to fly, so it's not the whole story. There are some who say that Coanda comes into the mix, but this is outwith the scope of exams.
Starting with the flat plate in an airflow, there is a difference in static pressure between the front and the rear of the plate due to the turbulence, from the air speeding up as it goes round the plate and reduces its pressure - this difference is enough to suck the plate backwards, and is similar to what happens in mountains and why you get turbulence in the lee of them. If you could get a situation where there was no change in static pressure from front to back, you would have no form drag, and this is what they measure in wind tunnels.
Now incline the plate at 45 degrees, and you will get some sort of impulse reaction (and downwash) that makes the flat plate want to fly up. Add that to the difference in pressure and we have the first two reasons why anything flies.
But the trick with aerodynamics is not creating "Lift" - we can do that with a plank of wood, as mentioned above. The trick is reducing drag, hence the shaping of aerofoils.
So, on to reason no 3....
Take a square room at sea level, inside which air molecules are bouncing around at random, providing an equal pressure on all surfaces of 2116 lbs per square foot. On a 10-foot square ceiling, that would be enough to support a 737, but we can't magically take away the static pressure on top of its wings, which would make it leap up into the air like startled rabbit. Instead, we can mess with the dynamic pressure by moving forward which reduces the static pressure for us. It is atmospheric pressure from below that pushes the 737 up into the air. It is the same atmospheric pressure that pushes the accelerating airflow over the upper surface against that surface and keeps it there.
Finally (for this discussion), we have wingtip vortices that rotate from the outside to the inside of the wing, providing a additional downwash effect.
The "circular movement of air", BTW, is only an impression due to vector differences, not a real circulation.
The tip vortices make the air trailing behind the tips flow lower than that from the root, which is assisted by the upwash at the leading edge, and which is instrumental in creating induced drag (it is the backwards moment from the difference between the vectors).
Thus, the air doesn’t rotate about the wing, but we get an equivalent effect if you imagine that the airflow over the upper surface is the same as the average speed plus a bit, and that through the lower surface is similar to the average speed minus a bit - rather like the difference between a headwind or a tailwind when it comes to groundspeed.
Starting with the flat plate in an airflow, there is a difference in static pressure between the front and the rear of the plate due to the turbulence, from the air speeding up as it goes round the plate and reduces its pressure - this difference is enough to suck the plate backwards, and is similar to what happens in mountains and why you get turbulence in the lee of them. If you could get a situation where there was no change in static pressure from front to back, you would have no form drag, and this is what they measure in wind tunnels.
Now incline the plate at 45 degrees, and you will get some sort of impulse reaction (and downwash) that makes the flat plate want to fly up. Add that to the difference in pressure and we have the first two reasons why anything flies.
But the trick with aerodynamics is not creating "Lift" - we can do that with a plank of wood, as mentioned above. The trick is reducing drag, hence the shaping of aerofoils.
So, on to reason no 3....
Take a square room at sea level, inside which air molecules are bouncing around at random, providing an equal pressure on all surfaces of 2116 lbs per square foot. On a 10-foot square ceiling, that would be enough to support a 737, but we can't magically take away the static pressure on top of its wings, which would make it leap up into the air like startled rabbit. Instead, we can mess with the dynamic pressure by moving forward which reduces the static pressure for us. It is atmospheric pressure from below that pushes the 737 up into the air. It is the same atmospheric pressure that pushes the accelerating airflow over the upper surface against that surface and keeps it there.
Finally (for this discussion), we have wingtip vortices that rotate from the outside to the inside of the wing, providing a additional downwash effect.
The "circular movement of air", BTW, is only an impression due to vector differences, not a real circulation.
The tip vortices make the air trailing behind the tips flow lower than that from the root, which is assisted by the upwash at the leading edge, and which is instrumental in creating induced drag (it is the backwards moment from the difference between the vectors).
Thus, the air doesn’t rotate about the wing, but we get an equivalent effect if you imagine that the airflow over the upper surface is the same as the average speed plus a bit, and that through the lower surface is similar to the average speed minus a bit - rather like the difference between a headwind or a tailwind when it comes to groundspeed.
Last edited by paco; 15th Jun 2017 at 10:29.
We only learned that lift comes from a circular movement of air (which you already know about) : air moves backward above, forward under, upward in front of and downward behind the wing.
But the reason why this circular movement would appear is just because the fluid mechanics equation make us come to that conclusion.
But the reason why this circular movement would appear is just because the fluid mechanics equation make us come to that conclusion.
It's like metalurgists who find it convenient to consider atoms to be solid spheres of matter packed closely together so they form crystal latices. This model is extremely useful in predicting and explaining metal properties, byt the solid spheres themselves do not exist. Ditto electron orbits. Ditto neutrons etc.
It always fascinates me that people feel there is a "problem" if we can't come up with a layman's explanation of lift, but they are utterly unphased at the lack of a layman's explanation of why the element of an electric fire gets hot when it passes current, or why magnetism is a thing* at all!
* because it isn't - it's merely the relativistic consequence of electrostatics
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I like to think of it either one of two simple ways.
1 - Next time you are driving on the highway at 100km/hr or so, stick your hand out the window and orientate your palm facing down or at a very slight angle upwards. You'll see that it takes some force to keep your hand the way it is without it flying upwards. The faster you go, the more force you have to apply. Now, think about when you are going at 250km/hr+, and the size of your palm is almost a MILLION times greater (compared with the wing area of the A380).
2 - Throw a paper dart and you'll obviously notice it will fly a bit and then gradually descend until it glides down to the ground. Now imagine if there's a force continuously pushing it from behind, then it will keeping flying.
Not scientific but it helps, especially for nervous flyers.
1 - Next time you are driving on the highway at 100km/hr or so, stick your hand out the window and orientate your palm facing down or at a very slight angle upwards. You'll see that it takes some force to keep your hand the way it is without it flying upwards. The faster you go, the more force you have to apply. Now, think about when you are going at 250km/hr+, and the size of your palm is almost a MILLION times greater (compared with the wing area of the A380).
2 - Throw a paper dart and you'll obviously notice it will fly a bit and then gradually descend until it glides down to the ground. Now imagine if there's a force continuously pushing it from behind, then it will keeping flying.
Not scientific but it helps, especially for nervous flyers.
I've been reading through a very comprehensive online book written by a certain John S. Denker, who seems to have taken it on as a mission to try to explain all aspects of flight, including lift generation, in as layman-ish terms as possible:
See How It Flies
3 Airfoils and Airflow
I'm not claiming to understand all of it, especially the circulation part I didn't understand at all.
See How It Flies
3 Airfoils and Airflow
I'm not claiming to understand all of it, especially the circulation part I didn't understand at all.
The thing that has always fascinated me about lift is the fact that we can get lift coefficients greater than 1
In the lift equation we have Cl 1/2Rho Vsquared
Where
1/2 Rho Vsquared is the dynamic pressure.
S is the surface area generating the lift.
Cl is a measure of how efficiently we are generating lift by applying the dynamic pressure to the surface area.
Dynamic pressure acts downstream and lift acts upwards (assuming straight and level flight here) so we could say that Cl indicates the efficiency with which we have changed the direction of the dynamic pressure from dowstream to vertically upwards. But if we managed to deflect the entire dynamic pressure upwards, the lift would only be 1/2 Rho V squared S. Which means that the maximum possible Cl should be 1. But some configurations can achieve a Cl greater than 3.
The mind boggles!
In the lift equation we have Cl 1/2Rho Vsquared
Where
1/2 Rho Vsquared is the dynamic pressure.
S is the surface area generating the lift.
Cl is a measure of how efficiently we are generating lift by applying the dynamic pressure to the surface area.
Dynamic pressure acts downstream and lift acts upwards (assuming straight and level flight here) so we could say that Cl indicates the efficiency with which we have changed the direction of the dynamic pressure from dowstream to vertically upwards. But if we managed to deflect the entire dynamic pressure upwards, the lift would only be 1/2 Rho V squared S. Which means that the maximum possible Cl should be 1. But some configurations can achieve a Cl greater than 3.
The mind boggles!
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I'm not following you in that interpretation of the lift formula.
Firstly, the Cl coefficient depends (also) on the angle of attack and can be anything from 0 to a lot. When Cl = 1 we are not deflecting "all" air upward. Usually a wing stalls when Cl is around 1.5 to 2 for a "standard" wing, and in that scenario we are deflecting more air than with Cl=1, but not even close to "all air".
Secondly, the amount of air (i.e. dynamic pressure) deflected is not the same as lift.
Lift is a force measured in Newton, dynamic pressure is a pressure measured in pascal.
The amount of Lift force generated (with Cl=1) will depend on the wing surface area, and the larger the wing, the more lift force you will get from the exact same dynamic pressure.
In other words: With constant dynamic pressure and constant lift coefficient (Cl=1), you can change the amount of lift generated by changing the size of the wing, but we are no where near deflecting all air.
Firstly, the Cl coefficient depends (also) on the angle of attack and can be anything from 0 to a lot. When Cl = 1 we are not deflecting "all" air upward. Usually a wing stalls when Cl is around 1.5 to 2 for a "standard" wing, and in that scenario we are deflecting more air than with Cl=1, but not even close to "all air".
Secondly, the amount of air (i.e. dynamic pressure) deflected is not the same as lift.
Lift is a force measured in Newton, dynamic pressure is a pressure measured in pascal.
The amount of Lift force generated (with Cl=1) will depend on the wing surface area, and the larger the wing, the more lift force you will get from the exact same dynamic pressure.
In other words: With constant dynamic pressure and constant lift coefficient (Cl=1), you can change the amount of lift generated by changing the size of the wing, but we are no where near deflecting all air.
I see no reason why Cl can't be large - it's not a fundemental property, it's just a non-dimentional coefficient. We can get drag coefficients of 10 or more and all.
Also lift isn't a matter of "deflecting the moving air from horizontal to vertical". Lift is a manifestation of the kinetic energy in the gas - lift is the hydrostatic part.
Also lift isn't a matter of "deflecting the moving air from horizontal to vertical". Lift is a manifestation of the kinetic energy in the gas - lift is the hydrostatic part.
