Is there just one correct way to calculate wing lift?

[Neon]

Wings generate lift because of the curved shape of the top wing air flow faster over the top and sucked up the plane. This would be bad for a fighter jet flying upside down. Wright brothers plane wings are flat so the wing must be deflected down. So curved wings are just aerodynamic. Soo which is correct?

 

[jbriggs444]

Wings generate lift because of the curved shape of the top wing air flow faster over the top and sucked up the plane. This would be bad for a fighter jet flying upside down. Wright brothers plane wings are flat so the wing must be deflected down. So curved wings are just aerodynamic. Soo which is correct?

Which of what is correct? What two contradictory statements do you see?

 

[Neon]

Curved wings produce lift or air deflection do everything as flaps the deflection of air.

 

[jbriggs444]

Curved wings produce lift or air deflection do everything as flaps the deflection of air.

How are those statements contradictory?

 

[Neon]

If curved wings produce lift, you can't fly upside down. If flat wings are better why so many curved wings?

 

[jbriggs444]

If curved wings produce lift, you can't fly upside down.

Curved wings do produce lift and you can fly upside down. What leads you to think differently?

If flat wings are better why so many curved wings?

Because "better" depends on your requirements. What is better for fighter jets may not be better for commercial airliners or for paper planes.

 

[russ_watters]

The airfoil of the Wright Flyers was most certainly not flat. Identifying the optimum curvature was one of their major innovations.

As others said, what is optimum depends on the requirements. But it is noteworthy that by increasing the angle of attack you can make a surface that curves up (the underside of a wing) direct air down.

 

[Nidum]

Some aircraft designed specifically for aerobatics have symmetrical wing profiles .

 

[CWatters]

If curved wings produce lift, you can't fly upside down.

That's not correct. The angle of attract and the shape of the wing both contribute to the production of lift. A flat plate can generate lift if given an angle of attack. Some curved wing sections can generate lift with zero angle of attack or even a negative angle of attack.

 

[cjl]

Some aircraft designed specifically for aerobatics have symmetrical wing profiles .

Yes, and even aircraft with asymmetrical airfoils can fly upside down - they just need to use a much larger angle of attack to do so than they need when flying normally.

 

[Neon]

Ok after some research, I got it. Thanks everyone

 

[Joe Pancottini]

Good job Neon. It is relaxing to hear just that. I have witnesses some horrific aftermath from aircraft crashes and they are almost always pilot error. Like John Kennedy and hundreds more. They were up and did not fully understand what can happen and how to get out of a lethal situation.
Again great job NEON. If you become an aviator, know every aspect of the aerodynamics and the physics behind it and you will have a great and wonderful and long life enjoying the freedom of the skies.

 

[BreakingBaDude]

The phenomenon of lift can be explained in its entirety using Isaac Newton's 2nd and 3rd law. The blades/wings have a huge forward momentum and exact force on the air. As a result, the air is deflected in a slightly downward direction. Bringing in the Newton's 3rd law, this deflection 'air stream' caused by the wing will provide a net upward force on the wing, which is termed as lift.
No matter what, the air stream HAS to be deflected downwards. You can induce this deflection through a non-symmetric airfoil with or without an angle of attack, or a symmetric one with an angle of attack.
There are other subsidiary effects which make the airflow to 'stick' to the profile of the wing. One should look up Coanda effect. It explains why a person standing next to a charging train is pushed into the train.
For a particular velocity, a higher angle of attack means higher lift only until a certain angle. After that, the effects like flow separation and turbulence come into picture.

 

[boneh3ad]

There are other subsidiary effects which make the airflow to 'stick' to the profile of the wing. One should look up Coanda effect. It explains why a person standing next to a charging train is pushed into the train.

The Coanda effect pertains most correctly to a fluid jet adhering to a surface, not the flow over an object such as a wing like you cite. The air "sticks" to a wing due to viscosity and the simpl fact that if it didn't stay near the surface, there would be a vacuum, which obviously can't happen.

This is also not why a person would tend to get drawn toward a passing train. In fact, in general that doesn't happen near the sides anyway. The real danger of that is right after the train passes and air tends to rush into the low-pressure wake.

For a particular velocity, a higher angle of attack means higher lift only until a certain angle. After that, the effects like flow separation and turbulence come into picture.

Flow separation and therefore stall causes the effect you cite. Turbulence can occur at any angle of attack, however. In fact, in most situations, the majority of the flow over a wing is turbulent.

 

[sophiecentaur]

No matter what, the air stream HAS to be deflected downwards

Yes, this is something that some Bernouli enthusiasts will argue against, vehemently, on the grounds that Bernouli is 'enough' of an explanation. But there absolutely has to be a vertical force and that can only be caused by a continuous process of changing momentum of some air, possibly quite distant from the wing. To see this downward flow of air, it's no good going into a wind tunnel (the argument commonly used) because a wind tunnel has that vertical force directed at its bottom half and such an experiment only shows the amount of lift force but does not 'explain' it. The wing will effect the region of air in the immediate vicinity and Bernouli works in the near region but you need to look outside this region to get the downdraft effect. I make that statement on the grounds of faith in Newton's laws and not as result of any fluid dynamics argument. But Newton is ultimately in charge of all these situations The downwards force reveals itself impressively in the wake of a large jet aircraft which is a region of lethal downdrafts. .

 

[DaveC426913]

The phenomenon of lift can be explained in its entirety using Isaac Newton's 2nd and 3rd law. The blades/wings have a huge forward momentum and exact force on the air. As a result, the air is deflected in a slightly downward direction. Bringing in the Newton's 3rd law, this deflection 'air stream' caused by the wing will provide a net upward force on the wing, which is termed as lift.
No matter what, the air stream HAS to be deflected downwards. You can induce this deflection through a non-symmetric airfoil with or without an angle of attack, or a symmetric one with an angle of attack.

By this logic then, as long as you provide an angle of attack, a flat slab of a wing with no curve and no asymmetry would get jut as much lift as a modern cambered wing.
Yet such is not the case.

 

[boneh3ad]

By this logic then, as long as you provide an angle of attack, a flat slab of a wing with no curve and no asymmetry would get jut as much lift as a modern cambered wing.
Yet such is not the case.

The logic you cite makes no such claims. It only claims that a flat slab will generate lift provided it has nonzero angle of attack. It makes no claims about the amount of lift relative to an actual airfoil. In fact, this is why we have airfoils, as they operate much more efficiently at subsonic speeds.

 

[DaveC426913]

The logic you cite makes no such claims.

The phenomenon of lift can be explained in its entirety using Isaac Newton's 2nd and 3rd law.

i.e., he is claiming there is no other component.

 

[sophiecentaur]

By this logic then, as long as you provide an angle of attack, a flat slab of a wing with no curve and no asymmetry would get jut as much lift as a modern cambered wing.
Yet such is not the case.

Is this not ultimately affected by Drag? If you pull a flat plate through the air on a bar, you can get deflection force at all angles. It would just be a lunatic design for a wing to have such a steep attack angle.

 

[rootone]

I think BreakingBaDude meant that you could have a working wing without the necessity of the aerofoil profile.
A completely flat winged plane COULD fly as long as the wing has an angle of attack.
It would be very inefficient and would waste a lot of energy as unnecessary drag, but it could actually fly.
In fact I know it can, A Iong time ago I made catapult launched balsawood models with flat wings (delta wings too!), and yes they worked.
They just didn't stay airborne very long, but they flew alright .. and they looked cool too.

 

[boneh3ad]

i.e., he is claiming there is no other component.

Well in a sense there isn't. 100% of lift can be accounted for by the downdraft generated by a lifting body. It just turns out it's not easy to measure that downdraft, and a cambered airfoil, for example, generates a whole lot more downdraft than does a flat plate at angle of attack relative to drag.

That said, some of these "Newton purists" seem to act like Bernoulli's equation is simply useless. It is not. It's a lot easier to measure pressure or calculate velocity (which can be converted into pressure trough Bernoulli's equation) near the surface than it is to measure or calculate the entirety of the downdraft created by a shape, so Bernoulli's equation is still very useful.

If you want to know how lift is generated, therefore, you can take several approaches. Newton's laws and the downdraft can provide a compact, easy to understand picture of where lift comes from but does not address the multiple ways to generate said downdraft. Lift can also be directly connected to the fluid pressure on all of the surfaces of a lifting body. This naturally lends itself to using Bernoulli's equation, and that's completely justified in many cases. It naturally leads to the question of why the air on the top and bottom travel at different velocities, and that usually leads to a whole host of misunderstandings.

The fun fact is that the nature of the downdraft from an airfoil and the reason the air moves faster over the upper surface are related concepts, and therefore, the two approaches honestly have some of the same "weaknesses". At the end of the day, the how portion of lift lift generated by an airfoil is because it generates a downdraft and because there is a pressure difference on the top and bottom. Those facts cannot be separated. Why an airfoil has a faster velocity and lower pressure on the top and why it generates a lot of downdraft can really only be addressed fluid dynamically, and that's a lot more complicated.

 

[boneh3ad]

Is this not ultimately affected by Drag? If you pull a flat plate through the air on a bar, you can get deflection force at all angles. It would just be a lunatic design for a wing to have such a steep attack angle.

Interestingly enough, this would be the most efficient possible design for a supersonic wing. It just isn't very practical for many reasons.

 

[boneh3ad]

To see this downward flow of air, it's no good going into a wind tunnel (the argument commonly used) because a wind tunnel has that vertical force directed at its bottom half and such an experiment only shows the amount of lift force but does not 'explain' it.

A wind tunnel is a perfectly valid way to look at lift, and s properly-designed experiment will show a large flow deflection as a result of the airfoil. The issue is that you have to have a tunnel test section that is substantially larger than the model. This is why, if you look at pictures of plane models being tested by someone like Boeing or Lockheed or NASA, the mode usually looks tiny relative to the tunnel. Even when the downwash impacts a tunnel wall, it is often possible to correct for that.

 

[Vanadium 50]

What is better for fighter jets may not be better for commercial airliners or for paper planes.

Agreed. As someone who has flown 2 million miles on commercial airliners, I am glad they don't regularly fly inverted.

100% of lift can be accounted for by the downdraft generated by a lifting body

That is the key insight that people forget in this argument: air goes down, plane goes up, The rest are details.

 

[boneh3ad]

That is the key insight that people forget in this argument: air goes down, plane goes up, The rest are details.

The rest really aren't just details though. Heavier-than-air flight would simply not be possible if that's all we understood about lift. Air goes down, plane goes up is true, but you can't ignore the importance of how much air and how much drag.

EDIT: That was kind of confusing. Let me clarify. You can explain lift pretty much entirely with downwash. The same cannot be said about airfoils (and why they are better than a flat plate) or heavier-than-air flight.

 

[CWatters]

Playing the devil for a moment...

Take a look at wing sections designed for unswept "flying wings" aka tailless aircraft. These curve up not down at the trailing edge (to change the normal negative nose down pitching moment to a positive pitch up moment.)

Scroll down here for example section.

http://www.scalesoaring.co.uk/VINTAGE/Documentation/AV36/AV36.html

 

[boneh3ad]

Playing the devil for a moment...

Take a look at wing sections designed for unswept "flying wings" aka tailless aircraft. These curve up not down at the trailing edge (to reduce or eliminate the normal nose down pitching moment.)

Scroll down here for example section.

http://www.scalesoaring.co.uk/VINTAGE/Documentation/AV36/AV36.html

They still generate downwash, though.

 

[CWatters]

Agreed.

 

[Neon]

I realized that the Bernoulli's equation BELIEVES the air flow over the top faster(true) BUT reach the tail at the same time as wind at the bottom. But the air over the airfoils flow over way faster then the air bottom.So Bernoulli's equation is useless since it got 1 major thing wrong.
#NewtonianFan

 

[boneh3ad]

I realized that the Bernoulli's equation BELIEVES the air flow over the top faster(true) BUT reach the tail at the same time as wind at the bottom. But the air over the airfoils flow over way faster then the air bottom.So Bernoulli's equation is useless since it got 1 major thing wrong.

The problem with this statement is that it's not true. Bernoulli's equation in no way requires the air moving over the top and bottom to reach the end at the same time. That's just a commonly repeated falsehood that sounds logical but isn't. Bernoulli's equation simply relates the pressure to the velocity. It says and cares nothing about why a given bit of air is faster or how much faster it is.

 

[A.T.]

The phenomenon of lift can be explained in its entirety using Isaac Newton's 2nd and 3rd law.

i.e., he is claiming there is no other component.

He just talks about the lift component, and makes no claims that other components don't exist.

 

[sophiecentaur]

He just talks about the lift component, and makes no claims that other components don't exist.

But, of course, exactly the same effect will be transferring forward momentum to the air as the wing passes through. (Where the air actually goes is a more complicated matter.)

 

[Joe Pancottini]

Without a doubt, the eureka moment was the warped wing theory. And the understanding of warp-age which ignited the true apex moment and the major innovation to control. Keen observation and the understanding of the applied physics spinning in the brothers minds were the great steps in aerodynamics and control.

 

[Mark44]

I realized that the Bernoulli's equation BELIEVES the air flow over the top faster(true) BUT reach the tail at the same time as wind at the bottom. But the air over the airfoils flow over way faster then the air bottom.So Bernoulli's equation is useless since it got 1 major thing wrong.

No, it's not useless. The length of the path the air takes over the top of the airfoil is longer (because the top is curved) than it is along the bottom. For the air to maintain laminar flow, the airstream taking the longer path (across the top) has to flow faster to be able to rejoin its counterpart on the shorter path. The stream with higher speed is lower pressure, which exerts a net force upward.

The same principle works in carburetors. The throat of the carburetor narrows down to a venturi, which causes the airstream to speed up, producing a region with lower pressure. This causes gasoline to be sucked out of the carb's jet, to mix with the incoming air.

 

[boneh3ad]

The length of the path the air takes over the top of the airfoil is longer (because the top is curved) than it is along the bottom. For the air to maintain laminar flow, the airstream taking the longer path (across the top) has to flow faster to be able to rejoin its counterpart on the shorter path.

No. This is absolutely 100% false. First of all, the situation has absolutely nothing to do with laminar flow, and most airfoils in practice have turbulent flow over the vast majority of their surface anyway. More importantly, as has been discussed at length here and elsewhere, there is nothing that says that a given parcel of air flowing over the wing has to meet back up with its counterpart from the bottom side of the wing. In fact, the air over the top moves so much faster, it typically leaves the trailing edge long before its counterpart on the bottom does. Shoot, the length of the path around the top isn't even required to be longer to generate lift.

See, for example,

The stream with higher speed is lower pressure, which exerts a net force upward.

Low pressure on the top of an airfoil will never exert an upward force. The force on an airfoil can be understood in terms of pressure only when considering the pressure on all sides. The pressure on the bottom is higher and pushes up with a large force. The pressure on the top is lower and pushes down with less force than the bottom air is pushing up. Their sum is a net upward force.

The same principle works in carburetors. The throat of the carburetor narrows down to a venturi, which causes the airstream to speed up, producing a region with lower pressure. This causes gasoline to be sucked out of the carb's jet, to mix with the incoming air.

This is not the same concept. The only similarity is the Bernoulli relationship between velocity and pressure, but they are otherwise entirely dissimilar processes.

 

[Mark44]

The length of the path the air takes over the top of the airfoil is longer (because the top is curved) than it is along the bottom. For the air to maintain laminar flow, the airstream taking the longer path (across the top) has to flow faster to be able to rejoin its counterpart on the shorter path.

No. This is absolutely 100% false. First of all, the situation has absolutely nothing to do with laminar flow, and most airfoils in practice have turbulent flow over the vast majority of their surface anyway. More importantly, as has been discussed at length here and elsewhere, there is nothing that says that a given parcel of air flowing over the wing has to meet back up with its counterpart from the bottom side of the wing. In fact, the air over the top moves so much faster, it typically leaves the trailing edge long before its counterpart on the bottom does. Shoot, the length of the path around the top isn't even required to be longer to generate lift.

My explanation comes from what I learned many years back in school. I'm not an aeronautics engineer, so my explanation is probably not 100% accurate, but I'm not sure that it is 100% wrong, as you say.

See, for example,

The stream with higher speed is lower pressure, which exerts a net force upward.

Low pressure on the top of an airfoil will never exert an upward force. The force on an airfoil can be understood in terms of pressure only when considering the pressure on all sides. The pressure on the bottom is higher and pushes up with a large force. The pressure on the top is lower and pushes down with less force than the bottom air is pushing up. Their sum is a net upward force.

I didn't say this very well. Overall, with a smaller force downward and a larger force upward (due to lower speed flow on the undersurface of the wing), there is a net force upward -- that's what I meant.

 

[boneh3ad]

My explanation comes from what I learned many years back in school. I'm not an aeronautics engineer, so my explanation is probably not 100% accurate, but I'm not sure that it is 100% wrong, as you say.

I mean, the part about a lower velocity air flow having a higher pressure is correct, but the part I highlighted is not at all correct. That portion of your response was 100% wrong. Neither the length of the path nor laminar flow have anything to do with the the reason the speed is so fast over the top, and the transit time while generating lift is not equal. The speed has everything to do with the shape of the trailing edge and the conservation laws that govern fluid flow.

 

[A.T.]

the airstream taking the longer path (across the top) has to flow faster to be able to rejoin its counterpart on the shorter path.

There is absolutely no reason to "to rejoin its counterpart".

 

[DaveC426913]

He just talks about the lift component, and makes no claims that other components don't exist.

No no. He is claiming the down wash of air is the only component needed to explain lift. He ignores the Bernoulli Principle.
If the Bernoulli Principle did not contribute to lift, then there would be no reason for a cambered wing, you would simply use a plank.

 

[boneh3ad]

No no. He is claiming the down wash of air is the only component needed to explain lift. He ignores the Bernoulli Principle.
If the Bernoulli Principle did not contribute to lift, then there would be no reason for a cambered wing, you would simply use a plank.

No, either one can fully account for lift. If you could account for every bit of downwash, you could back out the lift. Cambered airfoils simply create more downwash than, say, a flat plate at the same angle.

On the Bernoulli side, you can fully account for lift by integrating the pressure and a cambered airfoil will accelerate the upper air more relative to the underside than a flat plate.

 

[sophiecentaur]

No no. He is claiming the down wash of air is the only component needed to explain lift. He ignores the Bernoulli Principle.
If the Bernoulli Principle did not contribute to lift, then there would be no reason for a cambered wing, you would simply use a plank.

The Bernouli principle merely accounts for a difference in pressure and is, of course, very relevant to aeroplane design. But there is more to it than that. The pressure applies in all directions. The lift force, due to a difference in pressure, must also be applied (equally and oppositely) to the atmosphere around the plane. Why does no one seem to consider this in their narrow 'explanation' of lift?
If you try to explain why an object is suspended on a piece of string, would it be regarded as satisfactory to talk in terms of the molecules of the string that are actually in contact with the object? The string and the hook at the top would normally be included in the explanation. So why not include the rest of the atmosphere in the explanation of flight?

 

[A.T.]

He is claiming the down wash of air is the only component needed to explain lift.

The downwash (downwards momentum change of air) fully accounts for the opposite (upwards) force component on the wing.

If the Bernoulli Principle did not contribute to lift,

The Bernoulli Principle doesn't additionally contribute lift to the above, it just describes a different relationship.

 

[sophiecentaur]

It's big endians and little endians all over again. ( lookitup)

 

[boneh3ad]

I don't understand how this question always devolves into this debate between two sides of the same coin. Neither is wrong (so long as equal transit time is not invoked) and they both lead to the same, tougher questions. The endian analogy is apt.

 

[jartsa]

Wings generate lift because of the curved shape of the top wing air flow faster over the top and sucked up the plane. This would be bad for a fighter jet flying upside down. Wright brothers plane wings are flat so the wing must be deflected down. So curved wings are just aerodynamic. Soo which is correct?

Wing has an underside which functions like a water ski.

Wing has an upperside which functions like train: people on the platform standing too close to the tracks are sucked in.

 

[boneh3ad]

Wing has an underside which functions like a water ski.

Wing has an upperside which functions like train: people on the platform standing too close to the tracks are sucked in.

This has already been addressed. It's not true.

 

[jartsa]

This has already been addressed. It's not true.

What? Oh yes, you talked about passing trains, and I'm not disagreeing. Passing train pulls air along it. But there has no been any previous mention about water skis :)What do airfoils do? Do they somehow push air at distance? At greater distance compared to a flat board? I think airfoils are just boards with a shape that allows a larger angle of attac than flat boards.

 

[A.T.]

I think airfoils are just boards with a shape that allows a larger angle of attac than flat boards.

Air foils can have positive lift with a slightly negative AoA, where the water ski analogy for the underside breaks down.

 

[Neon]

Aerodynamics is freaking complicated.So both Newtonian and the other one are like two sides of a page-cannot have paper without either one.Correct?

 

[Nidum]

http://i.stack.imgur.com/fqmEI.gif

(Edited - more reliable link)