Explain Lift creation [theories]

Koehn on Aug 17, 2011

The short version: lift is generated by the wing pushing air downward. The more air that’s pushed down, the more lift that’s generated. What could be simpler?
For a more complete explanation try the NASA GRC. It may not be simple, but it’s pretty complete.

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Nathan Parker on Aug 17, 2011

All aerodynamics books focus on the pressure distribution around the airfoil. When you sum up the negative or positive pressures, taking into account the direction of these forces, the net result is the lift vector.

The pressure at any point on a subsonic airfoil will bear a relationship to the local velocity of the air at this point; the relationship is provide by the Bernoulli equation. This version of the equation that we’ve learned is only valid below about Mach .3, but there are equivalent relationships at higher speeds, so the fundamental point remains that the pressure is governed by the local velocity.

As a result of the pressure distribution around the airfoil, the air around the wing will be deflected downwards, due to Newton’s third law, so if you’d rather describe lift this way, you have some justification for doing so.

As for references, pick up any college aerodynamics text book. (A real one, not one geared towards pilots. Say, “Introduction to Flight” by John D. Anderson.)

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Brian on Aug 17, 2011

To elaborate on what Nathan said, I would like to quote John D. Anderson from his book on Aircraft Performance and Design page 52:

“the only two sources of aerodynamic force felt by the body are the integral of the pressure over the surface and the integral of the shear stress over the surface”

John italisizes the word only, that is not added by me. In this reference John goes on to highlight shear stress and pressure being responsible for drag; that is friction/parasite drag and drag caused by lift respectively. Further pointing out that pressure is the sole cause of lift and discussing a multitude of methods for calculation.

Hope that helps.

PS Thank you Nathan for recommending this book. I’m half way through and finding it to be one incredible read.

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Brian on Aug 17, 2011

Khoen,

Downwash is drag, not lift. This downwash you speak of is created by induced drag; in other words, the wing tip vortices. These vorticies are the drag we suffer as a result of producing lift. For a reference see my post on this topic some months back: http://www.askacfi.com/3322/how-is-lift-really-created.htm

In that post I link to an article (also provided to be by Nathan many months ago) that disproves downwash (tip vorticies) being a function of lift.

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Jon Anderson on Aug 17, 2011

I personally like the explination in John Denker’s “See How It Flies” Chapter 3.
http://www.av8n.com/how/htm/airfoils.html
I apparently need to read Aircraft Performance and Design by my doppelgänger.

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Brian on Aug 17, 2011

Jon,

I appreciate the link. It appears I have more reading to do. At a cursory glance it looks to be a good read and agrees with everything this ‘doppelgänger’ has said. 🙂 Not surprisingly John mentions, in his Bibliography, the overt use of aerodynamic misconceptions published in many pilot related texts (particularly the FAA texts). Hence the recommendation to explore a topic such as this elsewhere.

Oh and I contend your use of ‘doppelgänger’!! I’m not evil. Nathan might be though 😐

Thank you again.

Brian

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Wes Beard on Aug 17, 2011

I recently tackled this question on my blog. Instead of reiterating everything here I will just provide the link to what I wrote. It’s easier as I can’t input pictures here at this site.

http://allaboutairplanes.wordpress.com/2011/08/06/how-the-lift-formula-was-created/

It turns out that the lift formula uses dynamic pressure, skin friction, aspect ratio, wing planform, surface area and of course velocity of the air and the mach ratio. On the blog, I list the references for my information.

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Kyler Dalton on Aug 18, 2011

Again, I’m asking for the simplest explanations. Powerful, easy to understand, statements of truth. For instance: Newton explained in his third law that for every action there is an equal and opposite reaction. An airplane wing deflects air downward. The airplane reacts by moving upward.
I understand there are many different aspects and reasons for how and why an airplane flies, but I’m not going to whip out formulas and things that pilots will forget the next day. There must be simple explanations for why anything flies. I can get a piece of plywood to fly if I put a jet engine on it–it has no airfoil. I can get a kite to fly with a little wind and a string. When I throw a football, what keeps it going? Is it the air that pushes it forward through the air? Is it the squeezing force of the air molecules that don’t like being separated like a bar of soap in wet hands? Or is it the inertial net force; the momentum and inertia that keeps it moving until it reacts with something with enough friction to stop it?
Does anyone agree on the simple basic concepts that create the lifting force?

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Nathan Parker on Aug 18, 2011

“Again, I’m asking for the simplest explanations.”

I don’t know how it can get any simplier than what I posted. And I gave you exactly what you requested at the end, a one-sentence explanation using Newton’s third law.

And a piece of plywood IS an airfoil, it’s just a bad one. It generates lift just the way a wing does; so does a kite. A football, however, is a projectile and doesn’t fly.

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Brian on Aug 18, 2011

“There must be simple explanations for why anything flies.”

Pressure difference across the airfoil is the sole reason an airfoil remains aloft, as quoted from the source earlier. A pilot can change/control this pressure in two ways: angle of attack and airspeed. Everything else; Newton, Bernoulli, Kutta condition, magnus and coanda effects, etc are just methods of calculating this pressure difference. In other words, they are in no way the reason the airfoil is flying, but instead are our way of quantifying (giving numerical meaning to) the aircrafts production of lift.

So your simple one sentance explanation is: An airfoil produces lift through a the sum of all pressures acting on that airfoil and this lift is controlled by the pilot through angle of attack and airspeed.

Is that what you’re looking for?

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Kyler Dalton on Aug 20, 2011

The following is more what I’m looking for.
An airfoil is something that redirects/moves/deflects air. The redirected air creates a force. To counteract that force, the airfoil reacts in an opposite direction (Newton’s 3rd). The amount of force can be controlled by 1. airspeed (quantity of air), and 2. angle of attack (severity of deflection).
Any refutations? Or is that concept widely agreed upon as the primary reason?

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Wes Beard on Aug 20, 2011

Kyler, this question is a lot like other questions where people can argue quite aggresively on their point of view. I am a pitch control altitude and power controls airspeed guy but I am certain there are those that will argue with me till their last breath the other way is better; and that is OK.

I personally believe Bernoulli has more influence than what it seems you are giving him credit in your previous post. I also know that Newton’s classical mechanics equation (kinetic energy) is used in the creation of the lift formula but not necessarily Newton’s third law. (In fact, my advanced aerodynamics book completely disregards Newton’s three laws in how lift is created.) Since the air pressure pushing down on the wing, defined as Force / area, decreases from the top; the result is a net upward force or lift.

I think that if you stay with the lift formula you can keep it as simple as possible. Lift is the product of air velocity, angle of attack, surface area and air density. There are other factors as well but these are the big ones. I hope this helps you.

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Steve Pomroy on Aug 20, 2011

Hi Kyler.

I’m a little late arriving, and this conversation is getting a little long in the tooth, but here’s my $0.02 worth anyway.

If you’re looking for straight simplicity, go with your most recent post as an explaination. What you’ve described is “momentum change” and is a perfectly valid description of the source of lift. In fact, if you were to study engineering (speaking from direct experience here, for what it’s worth), your first course in fluid dynamics would include calculating forces on pipes with bends in them and curved nozzles. The method used at this stage is exactly your description of lift, plus the added complexity of actual equations and number crunching.

Having said this, I have to vote with some other posters in expressing a preference for the continuity/Bernoulli explanation. Yes, it’s a touch more complicated, but not that much more. Plus, it gives you the added benefit of now being able to explain how a venturi works in a carbureted engine and how a pitot tube works at low speeds (below 250 knots). As a final point, if you’re teaching at the commercial or instructor level, and not to PPL’s, the continuity/Bernoulli approach gives some justification for the “adverse pressure gradient” that causes stalls (not that you can’t teach this to PPL’s, but we usually want to keep things a little simpler at that stage).
For some further thoughts, check out these blog entries:
   – http://www.flightwriter.com/2010/12/straw-man-of-equal-transit-time-lift.html
   -http://www.flightwriter.com/2010/12/wherefore-art-thou-lift-lift-part-2.html
   – http://www.flightwriter.com/2010/12/going-in-circles-lift-part-3.html

Wes:
I’m curious to know what advanced aeordynamics textbook makes no reference to Newton’s laws. That strikes me as very odd. Ommitting the first and third laws I can see. That’s quite common in engineering books since they are just corollories to the second law (despite the terminology of calling them “laws”). But the second law is fundamental to everything mechanical, which includes fluid mechanics. I think it’s more likely that you’re seeing Newton’s second law, but the author hasn’t bothered refering to Newton because readers are assumed to recognize the equations. If you’re seeing any equations with the term “Sigma F”, that’s Newtons’ second law, even if it isn’t labelled as such. Also, any reference to momentum or change of momentum is Newton’s second law. Some examples include the momentum equation(s), eulers equation(s), and the Navier-Stokes equations.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

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Andy Neumann on Aug 21, 2011

I think Newton’s 3rd Law is a perfectly sound way of explaining *in general* how the wing lifts the weight of the airplane and keeps it suspended. Bernoulli wouldn’t disagree. The Bernoulli effect actually causes a downward movement of air–downwash that can be seen behind the wing in its wake if you were to fly through a cloud. Think of the lower pressure on the top of the wing as “sucking” air downward… the amount of air that gets sucked downward by the top of the wing plus the amount of air that gets physically pushed downward by the bottom of the wing is directly proportional to the amount of lift created. If you were to fly a wing through perfectly still air, after the wing had passed you would see a net downward acceleration of that air (the equal and opposite reaction to lift) and a net forward acceleration of that air (the equal and opposite reaction to drag).

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Kyler Dalton on Aug 21, 2011

Andy, that is exactly what I’m talking about, well done. I think we all understand basically what is happening, but don’t all agree on the same method for explaining it. I agree with Steve that the pressure discussion is a good precursor to explaing air/fluid pressures in other systems like the before mentioned venturi, or a turbocharger, or even hydraulics. I agree that there are different levels of complication to explain the same outcome and the explanations should reflect the audience’s understanding.

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Wes Beard on Aug 21, 2011

Steve,

I appreciate your insight. I have looked through the most common book “Aerodynamics for Naval Aviators” and the author only briefly mentions Newton’s second law in helping to describe Bernoulli on page 6. It is Newton’s classical mechanics law on kinetic energy that is part of the lift equation not his other three laws.
The mathematically based book “Introduction to Flight” by Anderson published by McGraw Hill only mentions Newton when describing thrust required and available curves.
I like what H.C. “Skip” Smith says on page 48 of his “Illustrated Guide to Aerodynamics” book published by Ashgate.

“Occasionally one might hear, among the hangar crowd, that this [Newton’s Second Law with downwash creating a upward lifting motion] is the true explanation of lift and that Bernoulli’s principle and the lift theory associated with it are just myths. This is not correct. The two methods of explaining lift are not opposing theories. They are merely different ways of looking at the same actions. Remember that the momentum change comes from the downwash, which is caused by the wingtip vortices. The vortices, in turn exist because of the pressure differential between the upper and lower surfaces, and this differential is explained by Bernoulli’s theory. The two approaches mutually support each other, and simply address the same physical phenomenon in different ways.”

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Andy Neumann on Aug 21, 2011

“Remember that the momentum change comes from the downwash, which is caused by the wingtip vortices.” Whoa, is Smith implying wingtip vortices are the only cause of downwash? If so, I don’t understand why flaps increase lift since wingtip vortices supposedly decrease when flaps are extended. Similarly, is the blast of air you feel behind a spinning propeller just caused by the vortices coming off the propeller tips? And wouldn’t winglets dramatically decrease lift? I agree that there is downwash at the wingtip, but surely this can’t be the only place air is being accelerated in the downward direction. Maybe I’m just defining “downwash” differently than Smith et al.

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Steve Pomroy on Aug 21, 2011

Wes:

Skippy is dead-on about the two descriptions just being different perspectives on the same phenomenon. That is precisely why the discussion about which description is a better explanation for pilots (as opposed to for engineers) is a valid one. As noted, I prefer the continuity/Bernoulli description for a number of reasons, but that doesn’t make the Newton description any less valid.

I’m afraid I’ve only read parts of Aerodynamics for Naval Aviators, and it wasn’t recently, so I would be ill advised to comment on the content. Having said that, I’ve written another book, Applied Aerodynamics for Private and Commercial Pilots, which has been compared favourably to Naval Aviators by some who have read both.

Bernoulli’s principle does indeed follow from Newton’s Second Law, and can be derived mathematically by applying Newton’s law to a fluid element moving along a streamline. The derivation isn’t really appropriate for pilot groundschool, as it involves calculus and is well beyond the scope of pilot training.

I’m not sure what you mean by Newton’s classical mechanics law on kinetic energy. If you mean the conservation of energy, that, technically, is the First Law of Thermodynamics, although it’s usually just referred to as the conservation of energy. If you’re talking about the definition of kinetic energy itself (half the mass times the square of the velocity), that is indeed derived directly from Newton’s Second Law. It is sometimes referred to as “Newtons’ Second Law in scalar form”, whereas the version we are more familiar with (F=ma) is “Newton’s Second Law in vector form”.

I’m fairly certain that Introduction to Flight contains more references to Newton than you say. Although my copy is in storage in another province, so I can’t go confirm that. My guess is that, as I noted in my previous post, Anderson uses force sums but doesn’t explicitly refer to Newton, assuming the reader will see the equations and know their basis. In any case, he definitely used Bernoulli to describe lift production at low speeds, as he does again in his more advance Fundamentals of Aerodynamics (if you’ve got the math background, that’s one I would recommend – you really don’t need it as a pilot, but it’s interesting). Fundamentals is heavy on the calculations, and as such uses the concept of circulation. But it’s worth noting for the non-engineer pilots that circulation is rooted in potential flow theory, which in turn is rooted in continuity. We can use continuity directly to describe the airflow over the wing by representing the top of the wing as half a venturi tube (note that this is an analogy, not a theory, so don’t flame me when you read NASA’s criticism of the venturi “theory”! http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html).

Anyway, the bottom line here, I guess, goes back to your quote from Skippy. Both descriptions are valid, but one has to decide which one is more useful to the audience.

Cheers
Steve
http://www.flightwriter.com
http://www.skywriters.aero

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Steve Pomroy on Aug 21, 2011

Andy:

QT: Whoa, is Smith implying wingtip vortices are the only cause of downwash?

Yes. The mathematical theory of lift predicts that as the wingspan increases, the strength of both the vortices and the downwash will decrease. This is indeed what we see when we compare short-span aircraft to long-span aircraft. Further, when airfoils are tested in wind tunnels as wings that span the entire tunnel, they don’t produce vortices or downwash. The data obtained from these tests is refered to “2-D data” or “airfoil data”, and must be modified to be applied to a three dimensional wing (with wingtips!).

QT: If so, I don’t understand why flaps increase lift since wingtip vortices supposedly decrease when flaps are extended.

This is a common misconception, probably caused by the unfortunate turn of phrase “high lift device”. Flaps don’t increase your lift. If your aircraft weights 3,000 pounds, it needs 3,000 pounds of lift — with or without flaps. What flaps do is increase drag, enable you to fly at a lower angle of attack (improving visibility during critical phases of flight), and increase your angle/speed margin from the stall. These are all good things, but none of them include producing more lift.

More to the point with regard to your question, flaps have the effect of breaking up a single strong vortex into two weaker vortices. If you look at a wing with flaps extended, it effectively has two wingtips — one at the end of the flap, and one at the original wingtip. Each of these will produce an independent vortex, with each partial vortex being weaker than the full vortex coming off a clean wing under the same flight conditions.

QT: Similarly, is the blast of air you feel behind a spinning propeller just caused by the vortices coming off the propeller tips?

Propeller theory is a bit more complex than wing theory, mostly because of the tendency for the blades to interfere with one another. But the short answer here is yes. Imagine a much larger prop producing the same amount of thrust. It would have weaker tip vortices (longer span) and less backblast (larger area, same thrust).

QT: And wouldn’t winglets dramatically decrease lift?

I hate to admit it, but you got me there. I have to reflect on that one for a while.

QT: I agree that there is downwash at the wingtip, but surely this can’t be the only place air is being accelerated in the downward direction.

It isn’t. In fact, for most wing designs (it varies according to planform, twist, and airfoil variation), downwash is strongest at the root and weakest at the tip. This is because, although the vortex originates at the wingtip, the inflow above the wing and the outflow below the wing are present across the entire span.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

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Brian on Aug 22, 2011

“I agree that there is downwash at the wingtip, but surely this can’t be the only place air is being accelerated in the downward direction. ”

I would question Skip’s mention of downwash in this case, though I myself could certainly be misreading it. That said these vortices, as you state, are not the only reason for downwash; or to better term this, ‘downward momentum of air’. Vortices, no doubt, impose a downward momentum on the air. However, as stated earlier, they are not an addition to lift. (I suspect Skip knows this.) Instead, vortices are a result of lifts creation. In other words, they do not contribute to lift. To help prove this I’ve cut through the technical jargon in the source I mentioned earlier in this topic thread and this quote follows:

“It follows from this and from the consideration of the transfer of momentum that the vortices behind the wing do not create the lift. They are in their turn created by lift.”
(Emphasis added) Source: http://www.archive.org/stream/nasa_techdoc_19930080967/19930080967#page/n11/mode/2up 

The downwash I think Skip means to infer is that caused by pressure. Pressure is nothing more than a parcel of air impacting a surface and changing direction. During this change of direction it imparts a force on the object and the object imparts a force on the air parcel. For our considerations, this object would be an airplanes wing. It is this broken down description that gives Newtons second law merit. As all pressure change can be defined in this way, Newton and pressure are one symphony; not two separate entities.

To help understand this better, one might consider the following quote from Mechanics of Flight:
“When an airplane flies overhead, the downwash produced by the lifting wing could be measured on the ground if instruments of sufficient sensitivity were available. This downwash can be viewed as the result of the momentum imparted to the air by the force of the airplane’s weight.”

As one final note to this discussion, consider a wing in a wind tunnel. In such a case the wing butts up against the walls of the wind tunnel; hence there are no wing tip vortices. These wings produce a downward momentum on the air for reasons discussed above. It is through such wind tunnel experiments that a wings lifting coefficient is determined.

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Andy Neumann on Aug 22, 2011

Thanks for the lengthy and well thought out replies! I’m afraid this thread has strayed way beyond what Kyler was looking for, but this is how we all learn and it’s fun.
I finally took down my dusty (but pristine) copy of Aerodynamics for Naval Aviators from the top shelf of my towering aviation library. Pages 63-66 address the issues we are dealing with here. I figured out that the downwash behind a wing is dependent on how far away from the wing that you meausure it. The farther back you measure it, the more what Skip says is correct–the downwash is a function of wingtip vortices only. However, if you were to measure the downwash right on the trailing edge, you would have downwash as a function of wingtip vortices AND “bound vortices” which are created by the wing section independent of the wingtip vortices.

There is also a quote that struck me as quite succinct: “With the action of (wing)tip vortices and bound vortices coupled, a final vertical velocity … is imparted to the airstream by the wing producing lift. This result is an inevitable consequence of a finite wing producing lift. The wing producing lift applies the equal and opposite force to the airstream and deflects it downward.”

As I was reading, I also found it helpful to remember that a force is created only when air is accelerated. So downwash is kind of a red herring. What we need to measure when talking about lift is the rate at which the downwash is changing in the vicinity of othe wing. It’s the acceleration of the air downward that makes lift–it’s not enough to just look at a parcel of air moving downward at a constant velocity somewhere behind the wing and say “lift must have been created!” Maybe this is obvious to you all, but every once in a while I have to rememember F=ma.

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Brian on Aug 22, 2011

QT: And wouldn’t winglets dramatically decrease lift? I hate to admit it, but you got me there. I have to reflect on that one for a while.

Steve, I wonder if you too are assuming vortices off the tips to create lift? Winglets decrease the formation of vortices. This results in a decrese in drag and, in turn, an increase of two crutial design parameter; T/D and L/D ratios. See my previous post for proof of the statement on vortices.

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Kyler Dalton on Aug 22, 2011

Take away the trailing edge downwash foce and tip vortices: is there still lift? What if you took a wing that had an eternal [perfectly flat and rigid] trailing edge and an eternal wall of winglets. This in essence isolates pressure as being the only factor to create lift. Can lift be created this way? If not, then we know that pressure is just means to an end; the end being that the foil’s reaction to the downwash is actually what creates the lift. Thoughts?

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Steve Pomroy on Aug 22, 2011

Brain:

No, I’m clear on where lift comes from. Forces can only be transmited between the air and wing by pressure and shear. Pressure dominates in the production of lift and induced drag, and shear dominates in the production of parasite drag. Vortices and downwash are a consequence of the pressure imbalance between the upper and lower surfaces. As such, vortices are a consequence of lift, not a cause. Nothing in either of my prior posts contradicts this.

As for the winglets, they are usually explained as “extending the effective wingspan, and therefore reducing induced drag”. This is a crude but effective (and reasonably accurate) description. However, the same description can be applied to wing endplates, which are nowhere near as effective as winglets. And the wing still has to produce the same amount of downwash (in terms of total mass flow times velocity change) in order to produce the same amount of lift (or as a consequence of the same amount of lift, however you look at it). This is a consequence of the fact that, however long the wing is, it is still finite and has wingtips. The complexity of winglets is that they are wings themselves, and are used to alter the vortex formation and the downwash distribution in a manner that reduces drag while maintaining lift.

Kyler:

Read my above point about wind tunnels. Sure enough, lift can be (and is) produced without vortices and the associated downwash. Pressure is the ultimate source of lift. Downwash is a consequence of lift when it’s produced by a finite wing.

However, referring back to your original post, you wanted a simple and accurate description of lift, with emphasis on the simple. The simplest accurate description of lift is the action-reaction description. However, the most useful for pilots (in my view) is the continuity/Bernoulli. It involves a little more complexity, but provides much more utility.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

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Brian on Aug 22, 2011

Steve,

Apologies if I offended you, this was not my intent. I suspected you knew that, but read into what you said improperly. That said, I thank you for being the first here to directly confirm/agree upon the role downwash/vortices play on lift. 🙂

Kyler,

What you described is what every book on the topic of aerodynamics (not necessarily the pilot geared texts), that I’ve read, starts its discussion of how a wing produces lift; a 2d look at the flying barn door. Where 2d refers to a wing of infinite wingspan. Meaning no wing tips and thus no vortices.

Note: I caution saying no downwash as the action of changing pressure imparts a downward momentum on the air, which one could term downwash. Search this page for “Mechanics” and read the quote I provide from the book Mechanics of Flight to see what I’m referring to.

There is one caveat to your wording, and yes it is nit picking, but it is necessary. That is, this ‘barn door’ must be at some angle of attack relative to the wind. Otherwise, like a symmetrical wing, at zero angle of attack the barn door does not produce lift.

The short answer to your question is yes, this wing does produce lift. For which I also restated after Steve. It seems I’m always just behind the ball with this man!

I’m not sure I’ve answered this clearly for you yet, but like Steve I also stick with the Bernoulli version to explain lift for reasons he’s already stated. Though I open the discussion by first making the claim I made in my reply to your second post, repeated here to save you the search:

“An airfoil produces lift through a the sum of all pressures acting on that airfoil and this lift is controlled by the pilot through angle of attack and airspeed.”

Then…Bernouli – Venturi – Conservation of energey – Etc.

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Andy Neumann on Aug 22, 2011

Kyler,

To take a stab at your first question: If a wing had a perfectly flat and rigid trailing edge so that the air stream flowing off the back had no net vertical movement, I don’t think you get any net lift. The airstream is initially accelerated downward, but then has to be accelerated upward so there is zero net acceleration in the vertical direction. In my mind that means there is zero net force in in the upward direction. You’d have plenty of drag though.

I’m not sure I quite understand what you mean by “eternal wall of winglets” in your post. I’ll try to state my point of view anyway. I define downwash as the downward motion imparted by the wing on otherwise still air. The downwash is behind the wing and is a result of what was happening as the air passed over the wing. Vortices (wingtip and bound) make up the downwash directly behind a normal wing. However, I don’t see vortices as a requirement for lift–they are simply the result of a “non-eternal” wing (if I understand your word usage correctly). So, I believe that yes, you can create lift without vortices.

I’d like to put the vortex/downwash discussion to bed for the simple reason that it is (by my definition) what happens behind the wing. The lift is created AT THE WING, and in my view that means we should be talking about what happens AT THE WING, not behind it. And what happens at the wing? Air is accelerated in a downward direction. This imparts an upward force on the wing. Bernoulli explains why the shape of the top of the wing accelerates air downward in a beneficial and effective way. Newton says that the force imparted on the wing will be directly proportional to the mass of air being accelerated and also directly proportional to the amount of acceleration.

In this description, you could interchange pressure with force by just dividing both sides of the equation F=ma by AREA. (Force / area = Pressure = mass * acceleration / area) What has that changed? Nothing. We see that pressure, like force, is still directly proportional to the mass of air accelerated and to the amount of acceleration (if the area in question remains constant). So, don’t get caught up in pressure vs. force vs. lift etc. They are all ways of quantifying a “push” or a “pull” on something. Pressure is not the means to an end. It is the end. Net pressure differential measured around the entire wing and multiplied by the wing area IS the LIFT. Simply put–pressure IS lift, just spread out over the entire wing instead of visualized at a single point.

For those of you still awake after reading my thoughts above, here’s another stumper that has got me confused. Let’s look right at the wingtip and examine the vortex there. Isn’t there actually upwash right there? Doesn’t the high pressure air underneath the wing actually escape and move UPWARD towards the low pressure area on the top of the wing? Wouldn’t this indicate a downward force on the wing at that point? (This is why I say that downwash should be measured at some distance behind the wing and we are always talking about a NET (total) downward movement of air, even though there are local movements of air that actually go upward. This is purely my opinion based on what I’ve read and seen in drawings.) Maybe someone can explain the magic behind this apparent paradox.

Adios amigos.

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Wes Beard on Aug 23, 2011

Steve,
I didn’t realize that KE=.5mv^2 is the scalar form (for those that don’t know is the quantity but not the direction) of F=MA which is a vector (quantity and direction). A search of the internet confirms what you stated and looks like Newton’s second law is fundamental in creating the lift formula. Interesting.
Thanks.

I almost posted this in my previous response but didn’t want to seem argumentative. I believe the downwash created by the wing is a byproduct of the Bernoulli principle. Since the wing is being lifted, an equal and opposite reaction force is applied to the air as it leaves the trailing edge. This is induced drag. Thus Newton’s third law better explains how induced drag works than how lift is created.

Andy,
I think the theory and application of winglets prove the fact that Bernoulli’s principle is correct. Dr. Anderson in his book referenced above compares an infinite wing in a wind tunnel to a finite wing and concludes a finite wing has a correction factor on the angle of attack making it less effective. We know this as the wingtip vortices.

We know the higher pressure on the bottom of the wing finds it way over the wingtip to the lower pressure on the top of the wing. In effect, that portion of the wing where the two meet produces little to no lift. A winglet stops the ability of the air below to come into contact with the air above the wing. A winglet effectively extends the lifting surface of the wing. As a result of the wingtips, an aircraft can cruise at a lower angle of attack which translates into a lower induced drag penalty. The aircraft can either cruise faster or pull power and cruise at the same speed. In any instance, the fuel savings for winglets are well worth the modification… as Southwest Airlines figured out.

P.S. For those that are curious. Engineers are now designing blended wings where the higher pressure on the bottom will react with the winglet in such a way to actually produce a little thrust on impact. How? I don’t know.

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Steve Pomroy on Aug 23, 2011

Brian:

No offence taken. Sometimes it’s hard to tell in these forums, the lack of vocal inflection can lead to all kinds of misinterpretations. Smileys help a little though:).

Wes:

The ability of a properly designed winglet to produce a little bit of “thrust” is precisely what makes them so much better than simple end plates. In many cases, endplates aren’t used because their benefit in induced drag reduction is outweighed by their cost in parasite drag production. Winglets are more effective, and therefore often worth the investment — especially for aircraft that operate for long periods under high induced-drag conditions, which is typical at high altitudes.

If I could draw a picture the thrust would be a pretty simple thing to explain. Not sue how well I can do with words only, but here goes!

A winglet is a mini-wing, not just a plate that blocks airflow to reduce vortices. It has a specified planform, chord, taper ratio, camber, airfoil section, etc. Hence the name “wing”let. If you look at a winglet from above, you will usually note that it isn’t aligned perfectly with the flight path. Instead, it is alighed to take advantage of the relative airflow to produce lift. The aftward flow of the freestream combined with the inward flow of the vortex creates an angled spiralling flow. This is the relative airflow to the winglet. If the winglet is designed and positioned properly, it interacts with this relative airflow to produce “lift” in the inward and forward direction. The forward component is the thrust.

Andy:

QT: Let’s look right at the wingtip and examine the vortex there. Isn’t there actually upwash right there? Doesn’t the high pressure air underneath the wing actually escape and move UPWARD towards the low pressure area on the top of the wing? Wouldn’t this indicate a downward force on the wing at that point? Maybe someone can explain the magic behind this apparent paradox.
The lift produced by a wing is distribute over the span. More importantly, it isn’t constant over the span. The amount of lift-per-unit-span is maximum at the root and minimum at the tip. In fact, planar planforms (flat wings, no winglets) produce zero lift at the tips. This variable distribution doesn’t occur in wind tunnels. It is entirely an effect of the 3-dimensional wing shap, the wing tips, and the vortices.

It can be shown that the most efficient lift distribution (minimum induced drag production) is an elliptical one. This is why elliptical wings were popular during WWII. However the manufacturing costs have made other method of lift control preferable. Nowadays, manufacturers use a combination of taper, sweep, twist, and airfoil variation to get the lift distribution they want — or as close to it as they can.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

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