Wingtip Device Questions for aero folks..

seagull said:
What I don't like is that the bottom diagram states "large vortex, higher drag", "smaller vortex, less drag". That leads some to think that the drag reduction is a result of the vortex reduction, rather than the fact that the vortex reduction is a result of the drag reduction. In other words, it's putting it backwards.
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Actually, wrong, and here's the physics:

F=MA (force = mass x acceleration)

As we're flying, the air is for lack of a better description, not moving. As we pass through it, we make the air go above or below the wing, and the resultant vaccum force then pulls it back to where it was, kindof. All of this movement is a displacement, and that displacement took an acceleration to put it there... Up... then down for air moving above the wing. To accelerate it, it took a force, we sum that up as drag.

We call all of the forces that are taking out of our energy drag.

We call all of the force that are adding in energy as thrust.

Now one of the side effects of moving that air is we also create a pressure differential, high pressure air below the wing (relatively) and low pressure air above the wing (relatively). When the wing separates that, no problem. However, behind the wing and around the tips the air is accelerated High -> Low. This takes more force, we sum it up as drag. Induced drag to be precise as it is only created when we produce lift and is proportional to it (exponentially, but still related). As the air swings around the tips of the wings in creates a vortex, we created it. The air was not swirling before we got there, but it is afterwards. This is drag. A winglet makes a SMALLER vortex as less air mixes around the wing, but the remaining pressure differential now mixes behind the wing. That takes less energy as it just equalizes and doesn't swirl.

So if we have a smaller vortex we have less force that it took to move that air, or also know as less drag. The resultant force also just happens to be slightly forward or "thrust." Most books call it "negative drag" to simplify it.





So, for your students, just say it reduces drag. If they really really want to know the science, read them that off. It takes some decent physics and aerodynamics to understand it. Here is the vortex without winglets:

0239080.jpg


Vorticies on any lifting surface:
http://cdn-www.airliners.net/aviation-photos/photos/4/9/6/1131694.jpg

With winglets:
0882622.jpg


Wingtip vortex in action... that's a lot of air moving!
[video=youtube;uy0hgG2pkUs]http://www.youtube.com/watch?v=uy0hgG2pkUs&feature=related[/video]

Here's a 737 comparison I just found at: http://www.b737.org.uk/winglets.htm
737winglets.jpg
 
SteveC said:
Are you saying that the vortex causes the drag?

I'm thinking I'll go with seagull on this one.
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I think I'm going to stay with newton on this one.

However, I will try to counterpoint. You're saying that because we have drag, we have a vortex? So then why isn't there a vortex generated by the landing gear? (there actually are millions of tiny ones, but nothing stable to continue to rotational force).

Or the vortex was just there in the air and the wing happened to hit it. So if we get rid of it we get bonus energy? Kindof like getting the rings in sonic the hedgehog?
 
jwp_145 said:
ANd I understand that this force isn't just being created out of thin air... I know that it is just an already-existent force that is simply being redirected.

So the one bit that hans't been answered... if the 744 and 32X wintip devices don't do much, why are they there?
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Because the science wasn't there and early simulations thought that simply holding the air on it's proper side at the aft portion of the wingtip would cause the air to simply re-equalize after the wing passed it. However, the computations were wrong and it was certified as that. It makes some difference, but not enough. Air isn't very dense, and so it takes much more volume to dissipate the pressure. Hence forth you see vortexes that are 30' in diameter or bigger on many aircraft. A 12" winglet just doesn't do much. A full winglet creates lift and downwash (side wash) and dissipates that force better.

See the science posts above.
 
jwp_145 said:
So the one bit that hans't been answered... if the 744 and 32X wintip devices don't do much, why are they there?
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My understanding is that they are there as a "fence" to keep the high pressure from meeting the low pressure. They don't do any force re-directing like blended winglets.

I also recall something about them adding some stability, but I could be mistaken on that one.
 
Minuteman said:
I'll readily concede that my understanding of how wingtip devices offer an advantage is small. But I still have trouble with the statement that winglets produce thrust. Maybe it comes from how we're defining the boundaries of a system.

My understanding goes like this:
  1. In order to produce thrust you have to exchange momentum with the surrounding airmass.
  2. When the momentum exchange accelerates the freestream, that's thrust.
  3. Because the ideal "surrounding airmass" is homogeneous and equipotential, the energy to accelerate an airmass has to come from somewhere else; usually either by burning dinosaurs or by incurring an equal or greater momentum extraction from another part of the airmass (i.e., drag).
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You're correct in that we don't gain any new energy. We still have conservation of energy, and we get new energy by converting mass to energy (burning fuels). What the winglets are is a drag reduction. It does this by creating lift perpendicular to its relative wind, which is the vortex. This produces a lift that is aimed towards the fuselage and slightly ahead. That ahead vector we refer to as lift. There's a diagram a few posts up.

Alternatively we call it "negative drag."

I'm trying to answer everyone's post. I have a BS in Aeronautics, enough books on aeronautics to prop up a mouse to be eye-to-eye with an elephant, and started a degree in Astrophysics but never finished it. I don't know it all, but this stuff you just have to read into. I'm more than willing to explain it really in depth if anyone wants to in a PM message or on here if requested. I always prefer understanding knowledge over rote.

For any of your students, just remember...

They simply reduce drag therefore requiring less fuel to go fast, and offsetting their increased weight.
 
So the reference to sailboats got me thinking. Indeed they do need a keel to compliment the forces on the sail. For a winglet the keel is replaced by the rest of the wing, which is attached to the other wing, which has a symmetric winglet producing complementary forces, everything lateral is nominally balanced (n'est-ce pas?).

If I may commit a vulgar act of elementary thinking...:
I realized I was thinking about the momentum exchange backwards. The "price of admission" had already been paid by the creation of wingtip vortices, which is just an inescapable fact of having a wingspan shorter than infinity (unless it's one of those wings in a NASA artist's conception). Without a winglet, the air in the tip vortex would just keep spinning along, shedding its energy while the plane becomes a dot on the horizon (the spinning originated because the airplane transferred some momentum that couldn't be turned into lift because there's no wing past the wingtip).

So, it would be nice to recover some of that energy. If you can't increase the span and make more lift, why not make a like a sailboat and turn some of that lateral air movement into a longitudinal force with some magical complicated geometry (i.e., the "thrust" seems to be the result of recovering some energy from the wingtip vortex)? And that's what blows my mind about winglets. Unlike a sail that can constantly be trimmed and adjusted for the conditions, a winglet is a fixed structure at the whim of the airplane's speed and aeroelasticity. To design something that can provide a 4%-5% decrease in fuel burn at its optimum without being objectionable in other regimes of flight is impressive (and maybe that is part of why newer, floppier-winged transports don't have them).

I don't see that it is entirely correct to describe winglets as reducing drag so much as they reduce the thrust required from the engine. In the context of the four-forces-of-flight that seem confusing or trivial, and it is advantageous to describe the recovery of momentum from producing lift as 'thrust' (and that may also be the convention of legitimate aerodynamicists, among whom I do not congregate).

Bonus vulgarity:
I suspect that wingtip fences or plates may work on the hope that by disrupting the wingtip circulation, some of that energy will be entrained by air coming off the wing-proper and somehow result in more-efficient lift being produced(?).

Question:
Why was the oval planform of a WWII Spitfire sometimes described as "ideal?"
 
Minuteman said:
I don't see that it is entirely correct to describe winglets as reducing drag so much as they reduce the thrust required from the engine. In the context of the four-forces-of-flight that seem confusing or trivial, and it is advantageous to describe the recovery of momentum from producing lift as 'thrust' (and that may also be the convention of legitimate aerodynamicists, among whom I do not congregate).

Bonus vulgarity:
I suspect that wingtip fences or plates may work on the hope that by disrupting the wingtip circulation, some of that energy will be entrained by air coming off the wing-proper and somehow result in more-efficient lift being produced(?).

Question:
Why was the oval planform of a WWII Spitfire sometimes described as "ideal?"
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Yes and Yes, reducing drag IS reducing the thrust required. That is the right concept, but it's not either or, it's BOTH! We're reducing drag, hence requiring less thrust to maintain any given speed. Or allowing us increased thrust available to go faster or higher. Remember, performance is mainly based on thrust available over thrust required (thrust required being equivalent to the sum of all drags).

The elliptical wing reduces the lift generated near the tips which decreases overall producing of induced drag due to reduced vorticies. However, it does stall all at once which can be unstable and hard to fly. Along with that, constructing a strong elliptical wing was challenging at that time. The increased weight and increased wingspan (comparable to a rectangular wing) may have made it a wash in the end. With current technology it's a very good wing design that only really lacks once approaching the speed of sound due to the wingspan required interacting with the shockwave rather soon and propagating it along the leading edge.

Very good for WWII tech, and the ability to stall all at once could make for a nice tail slide maneuver if entered right, or a nice flat spin if entered wrong.
 
Part of the problem with winglets is that they create a large bending moment in the wing that has to be countered with additional structure, which is weight. The ideal winglet for an aircraft like a 747 is 16-18 feet tall, and would require significant structural modification. This is partially why they are not on a 747. Another problem is wing flex on that aircraft They (Aviation Partners) have tested blended winglets on a 747, the remaining sample is in front of the museum of flight in Seattle. There is also a very large "not invented here" mentality with Boeing, which keeps them off.

The winglets on the 767 cause a structural mod that is over 2200 pounds. Even with this extra weight you still get a significant drag reduction. Winglets also give a huge second segment climb improvement, which can really help how much you can get into the air, especially at high hot airports.

Airbus has seen the errors of their ways and are installing "sharklets" on the new A-320s. Expect to see a retrofit market for the older 320s. The current fences are good for somewhere in the order of 2% reduction in drag.

The stability addition of fences is almost not measurable. The increase from blended winglets is a small gain.

crazyjaydawg said:
My understanding is that they are there as a "fence" to keep the high pressure from meeting the low pressure. They don't do any force re-directing like blended winglets.

I also recall something about them adding some stability, but I could be mistaken on that one.
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Proud, you can post all the internet sites you want, but you are not arguing with me, but rather with the inventor of Winglets, Whitcomb, and also with Chris Carpenter, former head of aerodynamics and the RAF college, and also John Anderson and a host of others. Your posts are the popular view of the situation, but the bottom line is that the reduction in vorteces is a result of the action the winglets are doing, and not the reason they work. They do impart a forward lift vector. Go take a look at the link I posted earlier and take it up there and see how far you get!
 
seagull said:
Proud, you can post all the internet sites you want, but you are not arguing with me, but rather with the inventor of Winglets, Whitcomb, and also with Chris Carpenter, former head of aerodynamics and the RAF college, and also John Anderson and a host of others. Your posts are the popular view of the situation, but the bottom line is that the reduction in vorteces is a result of the action the winglets are doing, and not the reason they work. They do impart a forward lift vector. Go take a look at the link I posted earlier and take it up there and see how far you get!
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I went to the link, which was an ad to go buy a book, of which I have plenty that all agree on this. The links I found in the posts are:

http://www.nasa.gov/centers/dryden/pdf/89234main_TF-2004-15-DFRC.pdf?forumid=486771 which agree with the information I provided above and,

http://www.bedecorp.com/images/pricelists/bd4_blended_winglets.pdf?forumid=486771 which also agrees with the information I provided above.

Which is the same as from here: http://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-15-DFRC.html
and another NASA here: http://www.grc.nasa.gov/WWW/K-12/airplane/winglets.html

Ok, so now that I am arguing with you, as I only see your point is to sell a book, what is your practical expertise in this area? 20 years ago, the understanding of vortex generators, winglets, and wing fences were fairly different than today. I remember studying books in High School that were massively changed by the time I started aerodynamics in college. NASA does fairly good research, I would trust them. Would you prefer I go to a different source? Do you want me to start photocopying pages out of a book to help prove a point?

Simply posting that I'm arguing against Whitcomb (who brought them into mainstream, but didn't invent them... and didn't do the most current fluid dynamic simulations on them) or Chris Carpenter (who is the author of Flightwise, which is advertised on your website) doesn't make your right, or them. Science gets things wrong all the time, but I'm going to agree with my stack of current books, the naval aviators, NASA, Rutan, and Boeing. So far their practical applications have been spot on.

I don't get why you're not willing to look into the science, and instead you just argue with it. If I'm wrong, great! Educate me so I teach the next person correctly. I want to get it right, so I study, a lot. I've trusted NASA, but I'm willing to look into other proofs that are out there, but post some of them. I'm not going to go buy another book just because you said so. Post some diagrams, do some math, or show some research.




By the way, for the other posters and readers, the information that I posted, did that help you understand it? I'm used to teaching one on one, and sometimes I miss the crowd. This truly isn't rocket science, and there is a way for everyone to understand it. I'm sure this post will get pulled up on google for years to come. I'll be watching this thread for another week or two until work gets crazy again. So ask away!
 
I won't bother with my resume here, as that is not the point (if you want to go there, be careful what you wish for). The website I pointed to is NOT a book advertisement. The author of the book created the website, and he is Chris Carpenter, former head of Aerodynamics at the RAF college. There are forums there. I am sure you are able to click that link and read the thread there on this topic.

What are your credentials that makes you believe you are an "expert" in the face of some of the leading aerodynamicists of the past century?

The bottom line, though is simple. A wing tip vortex is, by definition, a RESULT of what is occurring. While you can use their reduction as a useful and simple way to measure the EFFECTIVENESS of winglets, the REASON that winglets work is not the same. The winglets are, in fact, producing a "lift force" in the forward direction. It is as simple as that. The more effective they do that, the less vortex there will be.

Your argument is like arguing that a rocket is going forward do to the reaction of the air rushing out the back, rather than the fact that the unequal forces are PUSHING it. Objects move because they are PUSHED, not just because some airflow is moving a different direction. The argument you make is also analogous to saying that a wing is moving up as a RESULT of the fact that air is deflected downward after the wing moves through it, rather than the fact that the air moves downwards as a RESULT of the lift.

By the way, did you even read the links you posted? From the Langley (where I have done some work in the past) post : "Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices and reduce their strength..."
 
seagull said:
I won't bother with my resume here, as that is not the point (if you want to go there, be careful what you wish for). The website I pointed to is NOT a book advertisement. The author of the book created the website, and he is Chris Carpenter, former head of Aerodynamics at the RAF college. There are forums there. I am sure you are able to click that link and read the thread there on this topic.

What are your credentials that makes you believe you are an "expert" in the face of some of the leading aerodynamicists of the past century?

The bottom line, though is simple. A wing tip vortex is, by definition, a RESULT of what is occurring. While you can use their reduction as a useful and simple way to measure the EFFECTIVENESS of winglets, the REASON that winglets work is not the same. The winglets are, in fact, producing a "lift force" in the forward direction. It is as simple as that. The more effective they do that, the less vortex there will be.

Your argument is like arguing that a rocket is going forward do to the reaction of the air rushing out the back, rather than the fact that the unequal forces are PUSHING it. Objects move because they are PUSHED, not just because some airflow is moving a different direction. The argument you make is also analogous to saying that a wing is moving up as a RESULT of the fact that air is deflected downward after the wing moves through it, rather than the fact that the air moves downwards as a RESULT of the lift.

By the way, did you even read the links you posted? From the Langley (where I have done some work in the past) post : "Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices and reduce their strength..."
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I tend to agree witho your descriptions and answers... I'm at the beginning of an engineering egree, so I wans't able to fully understand what you were saying, but from what I got of the basic concepts you are explaining, you are mirroring what the people like Whitcomb say, as opposed to the other guy who is spouting off regularly misunderstood answers.
 
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