winglets

[bLizZuE]

So I've got this ground school instructor who claims that winglets create forward lift (thrust) due to the way they're installed. Claiming they are airfoils, and thus are capable of creating lift.

I've always seen them just as ways of reducing wing-tip vortices.

Comments?

[meritflyer]

Just because they're attached to the wing doesnt mean they "reduce" wingtip vortices. They just use them to their advantage to reduce drag.

Usually the vortex increases drag by the downwash it produces. By placing the winglet on the tip, it combines with the free airstream which results in somewhat of an angled flow.

By doing this there is a lift vector provided which acts perpindicular to the local airstream therefore producing a vector that is pointed forward.

Just remember that the winglets still produce a parasite drag penalty and added weight to the wing.

So, I'd say your instructor was right!

(how'd I do tgrayson?)

[bLizZuE]

heh, I researched a little and found this

"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. Weaker vortices mean less drag at the wingtips and lift is restored. Improved wing efficiency translates to more payload, reduced fuel consumption, and a longer cruising range that can allow an air carrier to expand routes and destinations."

from: http://www.nasa.gov/centers/dryden/a...4-15-DFRC.html

[tgrayson]

Originally Posted by meritflyer

(how'd I do tgrayson?)

Flawless.

bLizZuE is probably thinking they function as end plates. An effective end plate would have to be much larger than a winglet and would produce a lot more weight and drag penalty than a winglet, rendering them useless.

The end plate effect is only useful when the "plate" must be there regardless. For instance, twin vertical stabilizers on many aircraft will function as end plates on the horizontal stabilizer.

[bLizZuE]

Nah see, I've never heard of end plates. Your description makes sense, but have never heard the term.

[seagull]

Exactly!

The reduction of vorticy does happen, but it is a result of the rest, and not the cause of the increased efficiency.

Incidentally, the MD11 winglets are 9' from the top of the upper to the bottom of the lower.

[tgrayson]

Originally Posted by bLizZuE

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. Weaker vortices mean less drag at the wingtips and lift is restored.

The description offered by MeritFlyer is accurate and in all the aerodynamic books that discuss the subject. The NASA page is concurring when it says

Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices

but creates what I consider to be an "attractive nuisance" when it follows with what appears to be a non sequitur

and reduce their strength

Are these two opposing views?

I don't think so. Take a look at what Richard T. Whitcomb, the inventor of the winglet said in NASA Technical Note D8264.

Induced drag is, of course, directly related to the total energy of the crossflow circulation. Thus, the induced drag reduction caused by the addition of the winglets must result from a diminishing of this energy. (See ref. 10.) This energy reduction is associated primarily with the marked reduction of the measured high crossflow velocities near the vortex center for the configuration with the basic tip. Analyses indicate that the primary mechanism by which the reduction of the drag force on the model is accomplished is the forward inclination of the side-force vectors on the winglets resulting from the local crossflows. The reduction is also conjectured to be caused by a small decrease in the downwash of the flow approaching the wing. Link: http://ntrs.nasa.gov/archive/nasa/ca...1976019076.pdf

So what appears to be the case is that the vortex velocities are reduced in proportion to the lift being generated by the winglet. What this says to me is that the two explanations are essentially the same; however, the forward pointing vector has the advantage that it demonstrates how the reduction occurs.

[Maximillian_Jenius]

So what about wingtip fences like on Airbus planes are they less effective then winglets? And are raked wingtip extension more effective then the classic winglet design?

Enquiring minds would like to know!

[ananoman]

Originally Posted by Maximillian_Jenius

So what about wingtip fences like on Airbus planes are they less effective then winglets?

Yes, they are less effective. Airbus wanted to add winglets to an updated A320, but has since claimed that they would have to strengthen the wings and this will add enough weight that the winglets would not add any efficiency. There is speculation that with the A380 and A400 eating Airbuses lunch, they probably don't have the engineering capacity to spare and just made the weight problem up as an excuse not to introduce winglets in the near future.

[seagull]

Originally Posted by ananoman

Yes, they are less effective. Airbus wanted to add winglets to an updated A320, but has since claimed that they would have to strengthen the wings and this will add enough weight that the winglets would not add any efficiency. There is speculation that with the A380 and A400 eating Airbuses lunch, they probably don't have the engineering capacity to spare and just made the weight problem up as an excuse not to introduce winglets in the near future.

Well, not sure, but winglets DO add a lot of wing bending moment. We have speed limitations with the tips tanks less tha full due to that issue. I would not be surprised at all that a retrofit might not add enough over the current tips to make up for what it would take to compensate for the bending moment.

[caliginousface]

Since the other one was lost in the 2006 server shutdown, I figured we could repost some of the info about them.

tgrayson had some good links that I didn't get to read through completely.

Here are the Boeing links:
http://www.boeing.com/commercial/aer...t_story.html#1

http://www.boeing.com/commercial/aer...p_devices.html

[meritflyer]

I like Boeing's thoughts on winglets however, I feel like they dont touch on a few key points about a winglets funtion.

[Champcar]

My dad always asks me what the winglets do and I just tell him that it reduces drag.

I don't understand it in depth yet but I see if at stoping the over flow of low pressure seeking high pressure.

[SpiraMirabilis]

Originally Posted by Champcar

My dad always asks me what the winglets do and I just tell him that it reduces drag.

I don't understand it in depth yet but I see if at stoping the over flow of low pressure seeking high pressure.

On a normal wing the wingtip vortices accumulate at the tip of the wing, causing induced drag on a portion of the wing causing that part of the wing not to produce any net lift.
Winglets move the vortices to the very tip of the winglet, allowing the entire surface of the wing to produce lift. It's like getting extra wing without any additional weight (except for what the winglet weighs) and without redesigning the entire wing. Winglets also change the patterns of the wingtip vorticies, spreading them out and reducing the kinetic energy the vortices have and that also reduces drag.

[caliginousface]

I'm a dumbass. I thought the original winglet thread was in CFI corner, but it's in Technical Talk. Can a mod merge this thread to the other one?

[meritflyer]

Originally Posted by SpiraMirabilis

On a normal wing the wingtip vortices accumulate at the tip of the wing, causing induced drag on a portion of the wing causing that part of the wing not to produce any net lift.
Winglets move the vortices to the very tip of the winglet, allowing the entire surface of the wing to produce lift. It's like getting extra wing without any additional weight (except for what the winglet weighs) and without redesigning the entire wing. Winglets also change the patterns of the wingtip vorticies, spreading them out and reducing the kinetic energy the vortices have and that also reduces drag.

???

I will respond to this once I have some more time.

I immediately see a few "misconceptions" with this idea.

Stay tuned.

[Doug Taylor]

Originally Posted by caliginousface

I'm a dumbass. I thought the original winglet thread was in CFI corner, but it's in Technical Talk. Can a mod merge this thread to the other one?

Soitenly.

[Maximillian_Jenius]

Doug. Remember at your b-day dinner when I mentioned that they created winglets for the MD-80/90 family here are the pics.

http://www.airliners.net/open.file/1088763/L/

http://www.airliners.net/open.file/1075745/L/

http://www.airliners.net/open.file/1075139/L/

Bwaaaa...you don't care you're goin to the ER!

[SeanD]

I saw some winglets on a NWA 757 here in LAS the other day. They look pretty effing sweet on the 75s .

[grnclvrs]

Originally Posted by Maximillian_Jenius

Doug. Remember at your b-day dinner when I mentioned that they created winglets for the MD-80/90 family here are the pics.

http://www.airliners.net/open.file/1088763/L/

http://www.airliners.net/open.file/1075745/L/

http://www.airliners.net/open.file/1075139/L/

Bwaaaa...you don't care you're goin to the ER!

They look more like the sweptback wingtips on the 767-400 than winglets.

[Maximillian_Jenius]

Originally Posted by SeanD

I saw some winglets on a NWA 757 here in LAS the other day. They look pretty effing sweet on the 75s .

Yea...see em here too on NWA 757-200/300's. At first I thought they were EMB-190's. I had to do a double take,was thinkin when NWA get 190's!

[caliginousface]

Originally Posted by meritflyer

Just because they're attached to the wing doesnt mean they "reduce" wingtip vortices. They just use them to their advantage to reduce drag.

Usually the vortex increases drag by the downwash it produces. By placing the winglet on the tip, it combines with the free airstream which results in somewhat of an angled flow.

By doing this there is a lift vector provided which acts perpindicular to the local airstream therefore producing a vector that is pointed forward.

Just remember that the winglets still produce a parasite drag penalty and added weight to the wing.

So, I'd say your instructor was right!

(how'd I do tgrayson?)

According to the attached photo, do I have the vectors drawn correctly? The red being hte local airstream and the yellow being lift produced by winglets. How does the lift produced by the winglet, perpendicular to the local airstream become a forward pointed vector?

Originally Posted by Boeing

"The winglets increase the spread of the vortices along the TE, creating more lift at the wingtips (figs. 2 and 3). The result is a reduction in induced drag (fig. 4). The maximum benefit of the induced drag reduction depends on the spanwise lift distribution on the wing."

So the spreading of vortices towards the end of the wing lead to a reduction of localized vortice strength allowing more of the wing to produce more lift?

Originally Posted by boeing

"The 747-400 commercial airplane needed a significant span increase to meet the range requirement. However, structural constraints prevented the total span increase, so a combination of winglet and span increase was used."

I'm slightly confused by explanations given, and have tried reading the wind tunnel test by NASA, but have still not been able to make total sense. Any help would be appreciated.

[tgrayson]

Originally Posted by caliginousface

According to the attached photo, do I have the vectors drawn correctly?

No, the yellow arrows need to point much more forward.

Originally Posted by caliginousface

How does the lift produced by the winglet, perpendicular to the local airstream become a forward pointed vector?

Because the lift generated depends on where the relative wind is coming from. In this case, it's coming from outside the wingtip. Draw a lift vector perpendicular to it.

Originally Posted by caliginousface

I'm slightly confused by explanations given, and have tried reading the wind tunnel test by NASA, but have still not been able to make total sense. Any help would be appreciated.

That's because they switch back and forth between two different ways of saying the same thing. Although the winglets do develop lift in the forward direction, when you place these airfoils on top of another wing which is also developing aerodynamic forces, the resulting forces are going to be a combination of the two. Consider that as an airfoil, the winglet will have high pressure on the bottom, low on the top. The high pressure air on the bottom of the main will not experience as much of a pressure difference with the top of the wing, because the high pressure of the winglet separates them. This tends to reduce the intensity of the vortices.

So the idea of the winglets developing lift or the winglets diminishing the vortices is pretty much the same thing.

[IcarusGR]

Winglets do 2 things

Reduce wing tip vorticies and induced drag, thats it!!