Is there just one correct way to calculate wing lift?

[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)

 

[sophiecentaur]

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?

I guess it would be smartarse to introduce the Mobius Strip.

 

[A.T.]

Aerodynamics is freaking complicated.

Yes, but that's not even the issue with those lift discussions. People want to know "the cause". But physics often just states relationships between quantities measured at the same time, so cause-effect ordering is arbitrary.

 

[boneh3ad]

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 :)

Passing trains (or passing anything) pulls air along. You will not get sucked into the side of a train. You might get sucked into the low-pressure wake behind a train, though, but that is a different concept. The water ski analogy is pretty rough. Both an airplane wing and a water ski deflect a fluid down in order to stay up, but that is about where the similarity stops.

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.

Airfoils have a special shape, particularly at their trailing edge. Most of them are "sharp", though it works with a flatback airfoil, too. The important thing is that the trailing edge of the airfoil fixes the separation point at a location that it otherwise would not have been. This combined with the conservation of mass and momentum results in the air over the top being greatly accelerated compared to that over the bottom. The results is a greater pressure differential and more downwash compared to a flat plate. A flat plate at angle of attack (or a ski) doesn't have this action, and so while it can generate lift, it doesn't generate nearly as much.

 

[rcgldr]

A flat plate at angle of attack (or a ski) doesn't have this action, and so while it can generate lift, it doesn't generate nearly as much.

There's a scale factor involved. For small balsa type gliders, a flat or nearly flat plate wing is reasonably efficient. For gliders with wingspans somewhat less than a meter, a thin symmetrical wing with some tapering at the trailing edge is good enough. Thin airfoils with some camber show up at around 1 to 1.5 meter. At 2 meters or greater, more conventional airfoils are used.

 

[boneh3ad]

There's a scale factor involved. For small balsa type gliders, a flat or nearly flat plate wing is reasonably efficient. For gliders with wingspans somewhat less than a meter, a thin symmetrical wing with some tapering at the trailing edge is good enough. Thin airfoils with some camber show up at around 1 to 1.5 meter. At 2 meters or greater, more conventional airfoils are used.

That's not a strictly accurate statement. It has less to do with scale and more to do with the "mission requirements" of each of the objects you mentioned. A small balsa glider can get away with simple, flat wings because it is made of extremely light materials and has no requirements other than gliding. If it has propulsion, like a wind-up propellor, they are generally very large and provide a large thrust to weight ratio. In other words, it doesn't take much lift to keep them aloft, they don't have to haul anything, and they have plenty of thrust, so having an inefficient wing is no big deal. As you get larger, the weight goes up due to materials requirements, internal structure, propulsion requirements, etc. Generally the thrust to weight ratio is likely to go down and therefore, the efficiency of lift generation is more important.

So yes, there is a scale factor involved. I just wanted to clarify that it is not a matter of the scale of the lifting body affecting its performance (e.g. a flat wing being more efficient than an airfoil at small scales) so much as the efficiency requirements of the system going up as the vehicle gets larger.

 

[rcgldr]

scale factor

I was making a reference to small gliders similar to this one.

http://www.4p8.com/eric.brasseur/glider2.html

The article mentions that a lighter model flying at slower speeds would do better with larger wing chord and/or "flatter" wings due to Reynolds issues (slow speed, small wing chord). I'm wondering if there is much to gain in lift to drag ratio by using a more cambered airfoil for this type of model glider.

In the case of 1.5 meter gliders, thin but otherwise conventional air foils are used as seen in the attachment. The wings have full length ailerons that can be moved in unison to increase / decrease camber to provide a wider speed range. There are issues related to scale, speed range, and wing span being limited to 1.5 meters by the rules. The wings use a relatively large wing chord to increase wing area, which decreases wing loading and sink rate.

 

[boneh3ad]

I was making a reference to small gliders similar to this one.

http://www.4p8.com/eric.brasseur/glider2.html

The article mentions that a lighter model flying at slower speeds would do better with larger wing chord and/or "flatter" wings due to Reynolds issues (slow speed, small wing chord). I'm wondering if there is much to gain in lift to drag ratio by using a more cambered airfoil for this type of model glider.

The problem with using this site as a source is that, while he seems to have made a pretty neat balsa glider, his grasp of aerodynamics is at best incomplete. Sure he uses the term Reynolds number a lot (in quotes!), but it is pretty clear to me that he doesn't know what it is.

There is one bit in his article that supports what I said though. He points out that for the same lift and span, a flat wing would have a larger chord and therefore will fly slower. This is an awkward way to phrase it, but it is true. Really what that means is that you need a larger surface area for a flat wing to generate the same lift as a cambered wing. This is because a flat wing is less efficient. Essentially, the lift to drag ratio is lower for the flat plate. That is what I have been saying.

In the case of 1.5 meter gliders, thin but otherwise conventional air foils are used as seen in the attachment. The wings have full length ailerons that can be moved in unison to increase / decrease camber to provide a wider speed range. There are issues related to scale, speed range, and wing span being limited to 1.5 meters by the rules. The wings use a relatively large wing chord to increase wing area, which decreases wing loading and sink rate.

All of which is in line with what I have been saying. Unless you go to an extremely tiny scale, an airfoil will outperform a flat plate at generating lift regardless of the size (assuming that neither is experiencing stall, of course). If you go small enough to change that, you are likely in the range where we are talking about insect wings, and I am not nearly as familiar with aerodynamics on such a small scale as that.

 

[rcgldr]

One issue related to Bernoulli are statements that imply faster moving air results in lower pressure, as opposed to air accelerates from higher pressure zones to lower pressure zones, and as it does, Bernoulli describes the relationship between the speed^2 of the affected air and it's pressure. What isn't explained is how wing produces low pressure zones above the wing. A simple explanation is that the upper surface of a wing curves and/or recedes away from the flow, which has to accelerate towards the surface of the wing to fill in what would otherwise be a void (or if stalled, vortices fill in what would otherwise be a void). A pressure gradient coexists with the acceleration towards the upper surface. The reduced pressure near the upper surface of the wing also causes the air to accelerate in the direction of the flow (the faster moving air from the wing's perspective).

 

[boneh3ad]

One issue related to Bernoulli are statements that imply faster moving air results in lower pressure, as opposed to air accelerates from higher pressure zones to lower pressure zones, and as it does, Bernoulli describes the relationship between the speed^2 of the affected air and it's pressure. What isn't explained is how wing produces low pressure zones above the wing. A simple explanation is that the upper surface of a wing curves and/or recedes away from the flow, which has to accelerate towards the surface of the wing to fill in what would otherwise be a void (or if stalled, vortices fill in what would otherwise be a void). A pressure gradient coexists with the acceleration towards the upper surface. The reduced pressure near the upper surface of the wing also causes the air to accelerate in the direction of the flow (the faster moving air from the wing's perspective).

That is not an issue with Bernoulli's equation itself, as Bernoulli says nothing about why air moves faster. It only provides a relationship. Further, there really isn't necessarily a cause and effect here between pressure and velocity. Really, the two are interrelated and it doesn't really matter which one you want to say comes first as far as I can see. As long as you don't fall victim to the equal transit time myth, then it doesn't really matter anyway.

That is the true weakness of the Bernoulli's principle approach to lift. It simply doesn't explain why the air moves faster over the top, so it encourages people to make things up or to fall victim to common fallacies like equal transit time or the Venturi analogy.

 

[256bits]

All of which is in line with what I have been saying. Unless you go to an extremely tiny scale, an airfoil will outperform a flat plate at generating lift regardless of the size (assuming that neither is experiencing stall, of course).

Outperform is a good description..

A blunt object ( as which the leading edge of a flat plat could be described, except for very thin ) separates the flow earlier than other shapes. The basic teardrop has a shape where the flow will follow the contour, leading to less drag and less power requirements. Wings are a somewhat modified teardrop. Taking a wing and a flat plate, the wing can experience a greater angle of attack than a flat plate before separation, and thus produce lift for a wider range of speeds and angle of attacks.

 

[mrspeedybob]

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 Bernoulli principle describes pressures. Pressure x Area = Force, which is the lift force.

If you consider the down wash of air then what you are mainly interested in is how much air is pushed down and how fast it is pushed down, aka, it's momentum. Except, that you are dealing with a continuous process, so it's really momentum imparted to the air per unit of time. mv/t. since v/t is acceleration mv/t=ma

So, Bernoulli gives you force, Newton gives you mass x acceleration...Drum roll please...

F=ma

 

[boneh3ad]

I don't want to take away from the fact that you are correct here, but I feel the need to point out that just because something is dimensionally correct doesn't make it the right equation.

 

[DaveC426913]

Well, I got to concede to the greater wisdom. I don't quite have my head around it, but I guess what I'm hearing is that there are multiple independent ways of envisioning lift, each of which explains it, fully, in a different way.

 

[russ_watters]

A great many things can be looked at multiple ways, depending on your personal preferences, provided input data and needed output. Indeed, Newton's third law tells us that all forces come in pairs, and that's kind of what we are looking at with the two methods here. I would say that though you can often get the correct answer two different ways, being "complete" means understanding both.

I like the pressure profile around the wing because it speaks to me and is visual. But for a helicopter, it tends to make more sense to view the issue from the momentum of the rotor downwash.

If someone wants to start from scratch though with "how much lift will this airfoil shape I just designed create?" that's a very difficult question to answer, going far beyond a simple/superficial pressure summing/momentum change.