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Total Drag Reduction Of Winglet?  
User currently offlineairbusb0y From Sri Lanka, joined Dec 2010, 9 posts, RR: 0
Posted (3 years 4 months 3 weeks 5 days 11 hours ago) and read 4899 times:

If an aircraft was tested in a wind tunnel with and without winglets(for a range of AoA), for the winglet configuration would the total drag against angle of attack be reduced as opposed to the no winglet configuration?

I am confused about the fact that winglets do reduce induced drag, however don't they increase the profile drag slightly therefore increase the total drag?

If some one could show this on a Cd vs AoA graph I would appreciate it!

14 replies: All unread, jump to last
 
User currently offlineKPWMSpotter From United States of America, joined Dec 2006, 442 posts, RR: 2
Reply 1, posted (3 years 4 months 3 weeks 5 days 10 hours ago) and read 4867 times:

Certainly any addition to the structure of an aircraft will increase the profile/parasite drag, but think about the size of a winglet compared to the total size of the aircraft (or even just the wing itself) - it's tiny!

The largest single component of drag on most aircraft is the induced drag - upwards of 50% of the total drag on a typical subsonic transport. Most winglets boast a 5-15% reduction in induced drag, equating to a 2-7% total drag reduction across a "typical" airframe.
Now, if we make the simplification that aircraft area is directly corrolated to parasite/profile drag (which isn't too far from the truth), you would be hard pressed to find a winglet that increases the total aircraft area by even 2%. Generally, the reduction in induced drag far outweighs any penalties of having the additional structure out in the breeze.

As for a winglet's effectiveness vs. AoA, that's a trickier question. To answer your first question, yes, a wingletted wing will generally show a lower Cd vs Alpha curve when plotted against the same airfoil sans winglets. I don't have any specific examples to show how much the drop would be, or if there's a given range at which winglets are most effective, but any reduction in the induced drag will show a reduction in the Cd curve. Everything else really depends on the airfoil and the winglet design.
In general, winglet effectiveness increases as Cl incrases. Higher Cl equates to stronger trailing vorticies and higher drag, which is where winglets really start to show their benefits. Since Cl increases with AoA, you could draw the correlation that winglet effectiveness also increases with AoA.

After a little browsing I found this Cl vs Cd graph which compares simulations a given airfoil with and without winglets. While it's not exactly the Cd vs Alpha graph you wanted, I think you can draw the same conclusions:


Source: http://www.mh-aerotools.de/airfoils/winglets.htm


Regardless of the type of winglet, you can see an increasing reduction in drag as Cl increases. Hope this answers your question!

[Edited 2011-03-30 07:19:49]


I reject your reality and substitute my own...
User currently offlineairbusb0y From Sri Lanka, joined Dec 2010, 9 posts, RR: 0
Reply 2, posted (3 years 4 months 3 weeks 5 days 9 hours ago) and read 4850 times:

Thank you sir! That makes more sense to me now.

User currently onlineprebennorholm From Denmark, joined Mar 2000, 6424 posts, RR: 54
Reply 3, posted (3 years 4 months 3 weeks 4 days 23 hours ago) and read 4737 times:

Quoting airbusb0y (Thread starter):
...however don't they increase the profile drag slightly therefore increase the total drag?

Strictly speaking they do increase profile drag.

But winglets are angled slightly. They are pointing a little outwards to both sides seen in the flight direction. That way, when they intercept the tip vortex, then they produce sort of forward "thrust", much like a sail on a ship.

Forward thrust for free is the same thing as reduced drag.

For optimal performance their angle related to flight direction should be variable. But such an extra complexity is maybe not worth the effort.

But it makes it very hard to tell an exact value, how much winglets improve performance. Too slow / too fast, too low / too high, too heavy / too lightweight - or any combination - and they won't work to their full potential.

On planes, which are cruising at well over Mach 0.8, there is another consideration, transonic drag on the winglets. The winglets intercept the air at a higher speed than cruising speed since the vortex adds to the local airspeed. Therefore you see on faster planes like B747, MD11, A330/340 that the winglets are sharply swept back, while on slightly slower planes like B737 and B757 (and soon A320) they have a very much different shape.

On other pretty fast planes like B777 and A380 we see raked wingtips and wingtip fences respectively. They are both vortex reducing devices rather than thrust generating devices. Part of the reason for these choices are that it is hard to make a winglet really work at Mach 0.85. And when Airbus chose fences instead of raked wingtips, then it is likely because they would put the wingspan of the A380 well above the 80m limit agreed for airport compatibility.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (3 years 4 months 3 weeks 4 days 19 hours ago) and read 4669 times:

Quoting prebennorholm (Reply 3):
But winglets are angled slightly. They are pointing a little outwards to both sides seen in the flight direction. That way, when they intercept the tip vortex, then they produce sort of forward "thrust", much like a sail on a ship.

Although this may be true for certain installations, it is *not* where the vast majority of the drag reduction comes from. The tip vortex is an extremely tiny part of the overall vorticity (tty fast planes like B777 and A380 we see raked wingtips and wingtip fences respectively. They are both vortex reducing devices rather than thrust generating devices.[/quote]

They're all vortex reducing devices. Induced drag is caused by shed vorticity. If you reduce induced drag, you reduce vortex strength. It's physically impossible to do it any other way.

Tom.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19515 posts, RR: 58
Reply 5, posted (3 years 4 months 3 weeks 4 days 18 hours ago) and read 4666 times:

Quoting prebennorholm (Reply 3):
They are both vortex reducing devices rather than thrust generating devices.

In the case of the A380 wingtip fences, how does eliminating the vortex without converting it back into thrust reduce drag? The energy from the vortex has been bled away from the wing, so simply sticking up a fence to stop it isn't going to return that energy to the wing, is it?


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 6, posted (3 years 4 months 3 weeks 4 days 18 hours ago) and read 4659 times:

Quoting DocLightning (Reply 5):
In the case of the A380 wingtip fences, how does eliminating the vortex without converting it back into thrust reduce drag?

It doesn't eliminate the vortex. You can't eliminate the vortex for non-infinite-span wings. It weakens the vortex by reducing the end effect of the wing tips...it reduces the "leakage" from the high pressure side to the low pressure side (the cause of shed vorticity).

A perfect wingtip treatment would make the wing shed no vorticity, which means it flies like a wing of infinite span, which means zero induced drag. The only place you actually see this is wind tunnels with full span wings, where there is no flow path around the end of the wing.

Quoting DocLightning (Reply 5):
The energy from the vortex has been bled away from the wing, so simply sticking up a fence to stop it isn't going to return that energy to the wing, is it?

It doesn't return it to the wing (this is the erroneous idea that winglets work by generating thrust), it prevents the wing from shedding the energy in the first place.

A winglet/fence/raked tip works by altering the flowfield across the entire wing (this is why effective wing tip treatments are large), not just right at the tip. There are some very good articles for those interested in more detail:
http://www.boeing.com/commercial/aeromagazine/aero_17/winglets.pdf
http://www.boeing.com/commercial/aer...03_09/pdfs/AERO_Q309_article03.pdf

Tom.


User currently offlineKPWMSpotter From United States of America, joined Dec 2006, 442 posts, RR: 2
Reply 7, posted (3 years 4 months 3 weeks 4 days 9 hours ago) and read 4578 times:

Quoting tdscanuck (Reply 6):
It doesn't return it to the wing (this is the erroneous idea that winglets work by generating thrust), it prevents the wing from shedding the energy in the first place.

  

I hadn't heard this "winglets generate thrust" argument before now, but it appears to be a very common misconception in forums all over the internet (find me a reference that isn't a link to another A.Net or PPRUNE thread and I'll give it some thought...)

Winglets don't add energy to any system, they simply prevent loss from occuring. I think that a lot of the misconceptions go back to a misunderstanding of induced drag.

Induced drag is a component of the total drag which is caused solely by the generation of lift. When a wing is generating lift, the local air pressure on the bottom surface of the wing is greater than that of the top. Where the airfoil ends, there is a tendancy for the higher pressure fluid to curl around the wing tip from bottom to top. This movement of high pressure to low pressure causes spanwise flow (air flow from root to tip rather than leading edge to trailing edge) along the airfoil, and results in the formation of a trailing vortex. The spanwise flow itself reduces the effective area of the airfoil (bad), but the trailing vortex creates some problems as well. The rotation of the vortex induces a downward flow in the fluid behind the wing. The downward flow ("downwash") further reduces the aerodynamic efficiency of the wing. Both of these phenomenons combine to reduce the lift-producing ability of the wing, requiring the wing to be flown at a higher angle of attack to compensate for the lost lift.
When a wing is flown at a higher angle of attack the resultant vector of lift will be tilted aft, proportional to the angle of attack. The rearward component of the total lift is what we call Induced Drag.

Winglets basically allow a wing to generate lift more efficiently, reducing the formation of wing tip vorticies. Think of winglets as a sort of barrier to keep the fluid flowing in the direction it's supposed to go. When a wing can efficiently generate lift it can fly at a lower angle of attack, reducing the amount of lift which opposes the forward motion of the aircraft. No thrust generation or violations of the 1st Law of Thermodynamics necessary...



I reject your reality and substitute my own...
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17017 posts, RR: 67
Reply 8, posted (3 years 4 months 3 weeks 4 days 9 hours ago) and read 4574 times:

Quoting tdscanuck (Reply 6):
You can't eliminate the vortex for non-infinite-span wings

Yeah life would be much simpler if we didn't have those pesky gate size restrictions preventing infinite span wings. 



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlineprebennorholm From Denmark, joined Mar 2000, 6424 posts, RR: 54
Reply 9, posted (3 years 4 months 3 weeks 3 days 22 hours ago) and read 4482 times:

Quoting KPWMSpotter (Reply 7):
I hadn't heard this "winglets generate thrust" argument before now, but it appears to be a very common misconception in forums all over the internet

Some of you guys, who do not believe in the "forward thrust theory", do not agree with NASA and Dr. Richard T. Withcomb (inventor of winglets). I think you should read http://www.nasa.gov/centers/dryden/a...hnology/Facts/TF-2004-15-DFRC.html

A short quote:
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.

To produce as much forward thrust as possible, the winglet's airfoil is designed with the same attention as the airfoil of the wings themselves. Performance improvements generated by winglets, however, depend on factors such as the basic design of the aircraft, engine efficiency, and even the weather in which an aircraft is operating.

The shapes and sizes of winglets, and the angles at which they are mounted with respect to the main wings, differ between the many types and sizes of aircraft produced but they all represent improved efficiency. Throughout the aviation industry, winglets are responsible for increased mileage rates of as much as 7%.

Endof quote

Those who don't believe in "winglets generating forward thrust" should argue with NASA Dryden Flight Research Center. Unfortunately it is too late to argue with Dr. Withcomb himself since he died 18 months ago at age 88 after having worked for NACA/NASA for 40+ years and invented not only winglets, but also "area ruling" and "supercritical airfoils".



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (3 years 4 months 3 weeks 3 days 19 hours ago) and read 4460 times:

Quoting prebennorholm (Reply 9):
Some of you guys, who do not believe in the "forward thrust theory", do not agree with NASA and Dr. Richard T. Withcomb (inventor of winglets).

No, we don't. Or, at least I don't.

You're misinterpreting the key phrase (emphasis added):

Quoting prebennorholm (Reply 9):
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.

The thrust reference frame is the vortex circulation field, not the aircraft. The point of the winglet is to reduce the vortex strength, which reduces induced drag.

Tom.


User currently offlinejetmech From Australia, joined Mar 2006, 2687 posts, RR: 53
Reply 11, posted (3 years 4 months 3 weeks 3 days 19 hours ago) and read 4458 times:

Quoting tdscanuck (Reply 4):
The tip vortex is an extremely tiny part of the overall vorticity

I always thought the contribution from the tip was significant. Nothing like 40-50% mind you, but I always pictured something in the order of 10-15%?

Quoting KPWMSpotter (Reply 7):
When a wing is flown at a higher angle of attack the resultant vector of lift will be tilted aft, proportional to the angle of attack. The rearward component of the total lift is what we call Induced Drag.

I suspect that the rearward component of the lift vector may also be present at low angles of attack?

Quoting tdscanuck (Reply 10):
The thrust reference frame is the vortex circulation field, not the aircraft.

Out of curiosity, and with respect to Dr. Whitcomb, how could one have a "forward thrust" component to a vortex? Even if one tilted the axis of rotation of a vortex, how would this induce a "forward thrust" component?

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineKPWMSpotter From United States of America, joined Dec 2006, 442 posts, RR: 2
Reply 12, posted (3 years 4 months 3 weeks 3 days 11 hours ago) and read 4415 times:

Quoting prebennorholm (Reply 9):
Some of you guys, who do not believe in the "forward thrust theory", do not agree with NASA and Dr. Richard T. Withcomb (inventor of winglets). I think you should read

I'm certainly not trying to argue with Dr. Withcomb, but the quotes you've selected are quite...misleading. I suspect that the document you reference was written by a non-technical author, and they've simply convoluted the meaning of "thrust." Yes, the winglet counters the rotation and formation of a tip vortex, but again, that doesn't result in any forward thrust in the traditional "lift, weight, thrust drag" sense.

Quoting jetmech (Reply 11):
I suspect that the rearward component of the lift vector may also be present at low angles of attack?

Of course, a wing will always create some induced drag. The aerodynamic forces on the wing will result in a vector that points up and slightly to the aft, basically perpendicular to the wing's chord line. The component of the vector perpendicular to the relative wind ("up") can be termed as lift, while the rearward vector component becomes "drag." The lower the angle of attack, the smaller the drag component becomes.



I reject your reality and substitute my own...
User currently onlineprebennorholm From Denmark, joined Mar 2000, 6424 posts, RR: 54
Reply 13, posted (3 years 4 months 3 weeks 2 days 22 hours ago) and read 4338 times:

Quoting jetmech (Reply 11):
Out of curiosity, and with respect to Dr. Whitcomb, how could one have a "forward thrust" component to a vortex? Even if one tilted the axis of rotation of a vortex, how would this induce a "forward thrust" component?

It is quite simple. The winglets are not oriented exactly in the flight direction, but are mounted with a "toe-out angle". On for instance the B737 the toe-out angle is two degrees. In addition they have a "lifting" airfoil section like a wing, where the upper surface is pointing inwards against the fuselage.

Above the wing the vortex blows inwards, hits the winglet, and due to the toe-in angle and lifting airfoil creates forward thrust, like ordinary wind makes a wind turbine rotate.

Dr. Withcomb originally imagined double winglets. It hasn't become very popular since it is only seen on one airliner, the MD-11.

View Large View Medium
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Photo © Eerebout Stefaan


The smaller downwards pointing winglet works exactly the same way, just opposite. Below the wing the vortex flow it outwards, therefore the low winglet it mounted with a toe-in angle instead of toe-out. And the airfoil section has the "upper surface" pointing outwards.

It's a shame that it is impractical to have variable toe-out (toe-in) angle on winglets - like variable pitch on propellers. That would make them able to work perfectely well during all flight conditions.

When mounted with a small toe-out angle, then with an extremely high alpha on the wing, maybe combined with a sideslip, a winglet could actually be stalled and produce enormous drag and no thrust. The good thing is, that should it happen, then it will happen on the low wing in the sideslip, and the asymmetric drag will tend to reduce the sideslip angle - not increase it.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (3 years 4 months 3 weeks 2 days 21 hours ago) and read 4330 times:

Quoting jetmech (Reply 11):
I always thought the contribution from the tip was significant. Nothing like 40-50% mind you, but I always pictured something in the order of 10-15%?

It would depend on the exact lift distribution for a particular wing, but large jets have appreciable taper 10-15% sounds really high too me...most of the lift (and hence vorticity) is coming from well inboard.

Quoting jetmech (Reply 11):
I suspect that the rearward component of the lift vector may also be present at low angles of attack

Absolutely....it's there any time you have a non-zero angle of attack. But the induced drag varies with the square of the angle of attack, so it gets big fast as you increase angle of attack.

Quoting jetmech (Reply 11):
Out of curiosity, and with respect to Dr. Whitcomb, how could one have a "forward thrust" component to a vortex? Even if one tilted the axis of rotation of a vortex, how would this induce a "forward thrust" component?

Thrust and drag are defined relative to the airflow direction. For a vortex you've got rotating air, so thrust relative to a vortex flowfield is opposite the rotation.

Tom.


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