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-   -   How is lift generated at airfoils ? (https://www.cfd-online.com/Forums/main/1043-how-lift-generated-airfoils.html)

 Victor Serov July 19, 1999 08:59

How is lift generated at airfoils ?

Hi there,

Can someone give a short and consistent explanation why there's lift at an airfoil ? If I asked some fluid mechanics they say, it's because the air has higher speed at the upper side because of the longer way the air has to go on the upper side. But what about symmetric airfoils ? The way on the downside is almost equal to the upperside. But I have lift at the airfoils as well ! So, perhaps I think in a wrong direction. I would be very glad if someone could give me enlightment in that matter !

Thanks,

Victor

 John C. Chien July 19, 1999 10:49

Re: How is lift generated at airfoils ?

(1). Flow over a body ( a ski jumper, an aircraft, or a roof of a house...) will produce pressure on the body surface. The pressure times the surface area is defined as the force. The resultant total force over a body is a vector, which has a magnitude and a direction. (2). The component of the total force in the verticle direction is called the lift. (the other component is called the drag). (3). Even a flat plate can produce lift if it is positioned at an angle to the flow direction. (4). An airfoil will be able to produce lift easier with less drag. (5). Inside a tornado, almost everything will fly because of the lift produced on the body. ( if the resultant force on the body has a negative lift force, then it will not fly.)

 Victor Serov July 19, 1999 11:18

Re: How is lift generated at airfoils ?

O.K., but what physical process lowers the pressure (or increases the velocity) on the upper side of an airfoil ? That's the point I have difficulties with to understand !

 John C. Chien July 19, 1999 12:44

Re: How is lift generated at airfoils ?

(1).That is the main goal of CFD. (2). CFD is trying to answer your question by solving the Navier-Stokes equations with given geometry and boundary conditions.(3). The flow over an airfoil depens on the geometry, the Reynolds number, the surface condition, the Mach number, the angle of attack,..etc. For example, at a negative angle of attack, the pressure on the upper surface will be higher than that on the lower surface. At transonic spped, when a shock is form, the pressure behind the shock on the upper surface will also be higher. At high angle of attack, the flow separation on the upper surface will create higher pressure and the airfoil will stall ( no lift). At the supersonic speed, the subsonic airfoil will create stronger shock on the upper surface, thus the pressure will be higher there. (4). So, there is no general answer. ( For very, very simple condition, at low subsonic speed, you can say that the higher displacement of the upper surface tends to reduce the flow area locally, thus increase the velocity and reduce the pressure. But this can be very misleading for your future understanding of the fluid dynamics of flow over an airfoil) (5). Don't over-simplify the problem. The consequence can be similar to flying an aircraft into a black hole at night over the ocean with thick haze or fog. Suddenly, for no particular reason, the lift disappears. And the aircraft also vanishes. If you still are not sure, don't fly an aircraft.

 Juhee Lee July 19, 1999 18:03

Re: How is lift generated at airfoils ?

in potential theory, if fluid flows over the airfoil which has different passages , upper and lower, this difference makes circulation. this circulation, like a magnet in the electric field, introduce a pressure deviation between upper and lower side. the magnitude of the deviation can be calculated by Bernoulli' eq.

thank you.

 Tom July 20, 1999 01:06

Re: How is lift generated at airfoils ?

the circulation is an interessting feature. you often calculate with it, you think about the integral, everybody in turbomachinery seems to know but you always only get the mathematical side of it. who understands the circulation really ? Can someone give a not a mathematical explanation but a really practical one ?

 MYM,Rameez July 20, 1999 03:11

Re: How is lift generated at airfoils ?

Yeah I thing like this An answer for who has no knowledge of fluid dynamics.

Normally every object has a force by atmospheric pressure from all its surface.When air or liquid pass through the object, the air or liquid pull some particles from its surroundings(You can imagine that you pull something).It makes a little decrease in amount of force that has applied on it.The relative decrease in force caused the lift.

 clifford bradford July 20, 1999 15:18

Re: How is lift generated at airfoils ?

the explanation you were given is essentially (without getting too technical) correct. the flow on top (of a non-symmetrical) airfoil is faster due to the higher curvature and thus by bernoulli's principle the pressure is less on the top than on the bottom and so you have lift. as for your problem of lift by symmetrical airfoils you're probably having some problems. if the airfoil is alligned with the flow, ie the angle of attack is zero, then there is no lift. there are thousands of experimental results to prove this. as for you other guys, don't think so hard

 Sergei Chernyshenko July 22, 1999 06:53

Re: How is lift generated at airfoils ?

Hi, Victor.

When a fluid dynamicist is asked such a question, he has a difficult choice. First, it is possible to give an intelligent answer in terms of fluid dynamics, see for example Clifford's, and leave the one who asked unsatisfied (because why, for example, higer curvature causes higher velocity etc.). Second, it is possible to teach a little of fluid dynamics first, and this is again unsatisfactory, since this was not what was asked for.

This is in marked contrast to what we see in, say, mechanics of rigid bodies. If one asks, for example, how does a clock work, you can make a picture and explain: see, this spring pushes that wheel, this pendulum swings being pushed by that wheel via that level etc. This is possible because everyone has intuitive understanding of the basic properties of rigid bodies. With fluids, this is different. Most people do not know even what vorticity is, and certainly do not know how it behaves. Physical mechanism of lift generation can be explained in simple and clear physical terms, but first you need to know those terms, notions etc. So far, the only way to study them is to study the corresponding mathematics. Being faced with this type of questions for many years, I eventually worked out kind of a system of notions which can be studied without much mathematics, and illustrated it with a collection of various flow mechanisms explained, lift included, but it is a lecture course which I read to students at Moscow Inst. of Physics and Technology, it is not published. So, there is no king's road in science yet (as you surely know :)), you have to go through mathematical fluid dynamics. However, you may still wish to be more close to the physical-mechanism-side, and in that case I would recommend books by L.Prandtl, they are old but good. In Russian the one most suitable is called "Theory of Wing" but I am afraid this title is misleading, and English translation can be called "Hydrodynamics", for example. You have to find a book by Prandtl which deals with entire fluid dynamics, not wings only. Good luck.

Also, there are various levels of understanding. For example, if a body is not symmetric, one can expect a side force. And if the force is directed downward, you just turn the body upside down and you have a lift :)

I was really pleased to see this question asked again :). Not entire fluid dynamics is CFD.

Sergei

 John C. Chien July 22, 1999 21:24

Re: How is lift generated at airfoils ?

(1). Human brain is designed to store easily a lot of data , pictures, and sound for a hundred years or so. (2). It is very poor in storing and creating logic information and rules. (3). A computer can do both easily because it sotres data and logic both in the memory. (4). A human brain store information like the above comment-3 through repetitive use of the statement so that it can link all the associated data,pictures to re-construct the rule. (5). Birds probably don't ask their mothers why bird can fly. They practise and practise and eventually they learned how to fly. (6). On the other hand, human brain constantly searches simple rules to link existing data,pictures which already are stored there permanently, in order to form a network or a link so that data can be easily recalled when needed. (7). If you say that the bird can fly because the bird has a pair of curved wings. This rule will be very useful because later on he can compare his observation and data to check whether the object has a curved wings or not to determine the ability to fly.

 Kim July 23, 1999 03:55

Re: How is lift generated at airfoils ?

I think lift force can be affected by pressure, angle of attack, and shear stress (I guess this is small). As you know, pressure is in normal direction to the surface, and shear stress is aloing the surface. My answer of your question is as follows. (i) If the profile is symmetrical with zero angle of attack, there is no lift force because the effect of the pressure and shear stress will be cancelled out for upper and lower sides. (ii) If the profile is symmetrical with non-zero angle of attack, there can be the lift force because the magnitude of the vertical components of the pressure from upper and lower surface can be different, while the magnitudes of the pressure on upper and lower sides are equal. (iii) If the profile is not only just symmetrical but also ideally flat, the magnitude of the pressure will be the same, and the vertical component of the pressure will be the same independently of the angle of attack. Accordingly, there is no lift force caused by pressure difference. However, in theory, there could be a lift force because of the shear stress (depending on the angle of attack, the force could be negative). Again, there is no lift with zero angle of attack.

If my answer is not clear, I apologize in advace. Simple drawing of any profile with pressure and shear stress may help.

Kim

 Pavel July 26, 1999 05:31

Re: How is lift generated at airfoils ?

I would also like to know !PHYSICAL! explanation of circulation.

 John C. Chien July 26, 1999 11:22

Re: How is lift generated at airfoils ?

(1). That is really easy. I guess you are not watching baseball games. (2). When a baseball suddenly sink before the catcher, it is because the ball was rotating in the horizontal axis and rolling forward. "circulation means rolling" . (3). When a baseball is rolling forward (which creates circulation, a different name for rotation) and also moving through air, on the surface of the ball, the bottom flow moves faster than the upper flow.(because of the viscous effect). The flow with faster speed gets lower pressure, hence the ball sinks. (4). If the ball is moving through the air and rolling backward, the upper surface speed will be faster, and the ball will fly upward. (5). So, an object moving through air having a lift created on it, we say that it has created this equivalent "rolling" or "circulation" effect. (naturally, the rolling effect of an airfoil is not created by the rotation, but by the geometry itself) (6). So, an airfoil with a lift force on it is equivalent to a baseball moving through air having rotation backward.

 Tom July 27, 1999 01:47

Re: How is lift generated at airfoils ?

But in the "ball-problem" you have in your explanation viscosity. But you can calculate circulation without viscosity and this I still do not understand.

 John C. Chien July 27, 1999 02:48

Re: How is lift generated at airfoils ?

(1). Without the viscous effect, the rotation of a baseball is not going to have any effect on the air. And thus no lift will be created. (2). So, the mathematician came up with a simple approach to explain the results of the lift created by a rotating baseball. (3). They model the flow over a non-rotating baseball by assuming that the flow has no viscosity ( inviscid flow). In this way, the equation to be solved is simplified. And one can find a mathematical solution for flow over a moving baseball now ( based on this inviscid model equation.) without having to use the computer. That was last century's classical approach. ( and the particular field is called the potential flow) (4). But when they applied the theory to the ball, they were unable to find the lift. When they applied it to the airfoil, the lift was not there. So, the potential flow theory failed. (5). But then, finally they realized that the problem was because they over-simplified the model. The real flow is viscous and their model was inviscid. (6). Well, they could wait till now and use the CFD to solve the viscous Navier-Stokes equations to come up with the lift ( this is the current approach), or they had to invent a short cut approach. (7). Their invention was to solve two problems for flow over a baseball, the first one is the potential flow over a ball moving at a constant speed, and the other one is the potential flow over a ball rotating at a constant rate. Well, these are two separate potential flow problem and each has its own solution. Each will not produce lift. Still no good! (8). But since the simplified potential flow equation is a linear equation ( linear means the solution is additive), they decided to add the solutions of the two problems together. ( sort of arbitrary. and you think you can do it also.) It turns out that the resultant solution has the effect of viscous flow, and it also produced lift. That is the most important part. (9). To apply this theory ( or model) to the airfoil to produce lift, you compute the potential flow over the airfoil with additional circulation on it by imposing the circulation boundary conditions. In other words, you combine two solutions together to produce the lift. (10). But we are modern man, those are classical models. We use CFD instead of solving inviscid potential flow equations. That is a long story of how the lift was created mathematically.

 Md. Ziaul Islam August 10, 1999 18:01

Re: How is lift generated at airfoils ?

For simplicity, If you look at an airfoil you will see that bottom surface is almost flat and the upper surface is partially convex shape. The distance between leading and trailing edge of the upper surface is higher than bottom surface. So when air flows over an airfoil, air has to travel longer distance over the upper surface. Therefore, the velocity is higher at the upper surface. If you look at the Bernoulli's equation you will see pressure and velocity terms. So as velocity increases then pressure has to decrease for energy balance. This physical process lowers the pressure or increases the velocity at the upper surface of the airfoil producing lift.

 Alex Kopylov August 13, 1999 23:01

Re: How is lift generated at airfoils ?

Hello !

http://www.allstar.fiu.edu/aero/airflylvl3.htm At this page I've found (second time in my life)the very best and cleverest explanation on how the wing lift force is really created. By David Anderson. First time a similar explanation I've heard from late Sergey Eger - one of the best russian aircraft designer.

Regards.

Alex.

 John C. Chien August 14, 1999 12:07

Re: How is lift generated at airfoils ?

(1). Thank you very much for the information about this site. (2). I have just visited the site briefly, and I think his theory of lift was based on two figires, a streamline plot for flow over an airfoil (Figure 4) and the velocity vector plot (Figure 6). (3). So, his whole concept was based on these two pictures. The nice streamline plot looks like a CFD streamline plot, and the velocity vector plot is rather crude and could be hand drawn. (4). It is not a good idea to go through this intermediate steps to provide explanation about the lift. I think the flow field and the lift can be modelled by the Navier-Stokes equations. And therefore, the CFD solution will provide the direct answer about the lift. (5). The basic problem about his theory is " Where are you going to find the Figures 4 and 6 ? Can one always find the same streamline plot and the vector plot? " It is really very important for this person to identify the source of Figure 4 and Figure 6 . ( or is he trying to say that those are computed results of CFD ? Not a good idea either, because CFD is still trying to come up with accurate prediction of lift and drag.)

 Alex Kopylov August 15, 1999 22:12

Re: How is lift generated at airfoils ?

Dear sir,

I've just read through your message and I want to say that I'm really perplexed with your reactions to "his theory". It's (the reaction) is so different of mine. First thing - it's not "his theory" at all. (By the way, "him" i.e. David Anderson, I suspect is the same well known author of many books on CFD. May be not? I'll check.) The theory of wing lift force was developed by such science pillars as Bernoulli, Prandtl, Zhukovski (sorry for any wrong spelling - English is not my native language) and others. This theory was very well developed theoretically and experimentally long before CFD came to scene. There are thousands of pictures of visualized air flow around different profiles in wind tunnels and results of calculating velocity distribution and stream lines by methods of traditional aerodynamics. And...there are thousands of real wings built based on those results. One don't need to invent new theories to explain how lift force is developing on wings. All the author does is explains very well and popularly what was well known long before him. Well known but not wide known. It's a very wide spread delusion to explain lift force formation referring to Bernoulli equation. I know about it not from the Internet article. I met it many times in the institute, during my postgraduate studies and years of working in aerospace industry. Even people professionally involved in aerospace very often cannot explain well how physically the force is created. The first simple book on aerodynamics, that I read when was a child, explained it the same wrong way! (By the way, my first concern when I decided to address myself to this news group, was to save a guy asking this question from being mislead. Hope it's not late. It's so important to get right direction at the start.) Dealing with intrinsically limited mathematical models it's easy to lose contact with the real world. That's what in my opinion explains why very often a person capable to manipulate very complicated mathematical formulas and perfectly communicating with a super computer, find himself quite helpless trying to understand what's actually happening to the air hitting his hand stretched out of the car window. Navier-Stokes and Bernoulli equations are just derivatives of a few quite simple fundamentals laws of physics. Referring to those root laws and some experimental data, that's what is necessary and enough to explain the phenomena so vividly discussed at this CFD forum.

Friendly and with best regards.

Alex. alexk@globalserve.net

 John C. Chien August 15, 1999 23:45

Re: How is lift generated at airfoils ?

(1). Thank you very much for the discussion. (2). I was surprised right from the begining about this question of lift. (3). I can only say that, at various time in history, people (including mathematicians) have tried to provide various simplified theories of lift. These were intended to give a quick link back to the physics world (various laws). (4). The Bernoulli's equation is all right, the use of the equation is also all right, because at the place where the velocity is high, the pressure is low. (5). One thing I have to say is that, for commercial aircrafts, the power required at cruise is mainly used to overcome the drag on the wings, the fuselage. It is not used to produce the lift. A soaring plane without the power can still produce lift and stay in the air for many hours. (The other thing is based on his explanation, a door panel or flat plate can also produce lift. I don't think it is appropriate, because no aircraft uses flat plate as wing, not even a bird or eagle.) (5). I think it is an interesting , and trivial question about the origin of the lift. But the question can only be answered when the complete wing geometry is known.

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