Hi freely190 !!

Many commercial planes use winglets but the most modern
commercial plane in the world the big Boeing 777 does not use those winglets.
Why ??? May be its are not to much effective ??

Try to find (at http://techreports.larc.nasa.gov) works of Mr Richard Whitcomb about winglets.

Regards

freely190

See also other threads on winglets in keyword search.

Freefly190:
Winglets aide in the reduction of wingtip vorticies.  As I'm sure you know, aircraft are able to fly due to the physics of Bernouli's principle and airfoil shape....high pressure below the wing; low pressure above it.  On a "standard" wing, of say a Cessna, the pressure gradient on the wingtip causes high pressure air below the wing to flow to the lower pressure above the wing, thus essentially causing a horizontal tornado.  These vorticies are not only drag inducing, but they can also be very dangerous in the vacinity of airports.  (Vorticies from a large jet can easily flip a gen. av. aircraft!)  The winglets are there to disrupt this flow due to the pressure gradients.  I'll try to explain in a picture:

no winglets:  ______X______

winglets:     |_____X______|

For example, if you draw the flow pattern on the left wing, it will be clockwise flow. You can see how the winglets disrupt the flow.

So in short, winglets are used to increase efficiency as well as for safety concerns.  They DO NOT ELIMINATE the wingtip vorticies, but they can help greatly reduce the effects.  I hope this helps.

Winglets do not actually decrease the strength of the vorticies as such, and as a result do not cause a decresase in aircraft wake turbulence, as wake spacing for aircraft is purely determined by A/C AUW. THey utilise the secondary flow from the lower wing surface to the upper to create lift, as they are designed in such a way tyo generate an incriment in lift in the flight direction. they themselves however also generate drag, and it takes much testing to ensure that winglets are truly beneficial. They generally will create a reduction in induced drag of around 1-2%, which does not sound much, however, when compared to the operating cost of a heavyweight over a year, can be significant.

Relying on a book I read, the induced drag can not be suppressed because it generates lift.  So winglets decrease cL and make airplanes fall!

Hi Rollerblades

Allow me to say, don't be so dramatic!

Winglets take care about the extreme local way how you finish aerodynamic the main wing behauviour, performance.

It helps to reduces the local phenomena concerning the particular local induced drags.

However, improving globag aerodynamic efficiency has its local costs, I mean, it must be careful studied and after assembling, to go for a very delicated monitorization/ validation  process.

Please take a look to the other winglets thread in this Forum.
zzzo   

Please correct me if I'm wrong, but isn't it the other way round? That induced drag is created by lift and can be - more or less - reduced by using winglets?

Hi Rollerblades

Here, it goes my second turn:

- Take the Bernouli theorem, applied to the flow over an unity span wing, on the top and bottom surfaces.

-Consider the integration of the pressure along airfoil chord, on the top and bottom surfaces respectively thus having V1 > V2 or P1<P2. (1 stands for the top surface and 2 stands for the bottom surface.)
 
-It results an up-driven force, the Lift. Or in terms of the local vortice generation, being  V1-V2>O and the corresponding vortex sheet, integrating that velocity difference along the chord, the intensity of the vortex sheet (or the vortex strength) along the airfoil chord is defined by the variable “circulation”, expressed by Circulation = Integral along the chord of V1-V2.
 
- The Kutta-Joukowski theorem explains the resulting Lift in terms of the circulation variable: Lift= specific air density x Mean flow velocity x circulation.  

- Notice, this analysis is for one length unity span wing. So, you have to integrate this effect along the wing span dimension, y axis. If the wing is clearly of 3D nature, you will have different circulation values, Thus Circulation = f (y axis).

- Along the airfoil chord and along the span wing, you have generation of circulation, thus the generation of Litf happens all over the wing. That means, all the wing  produces Lift, not only in its extremes!   

- Now, we arrive finally to the wing extremes. The way we treat locally these singular wing portions, a better (or worse) aerodynamic effects/efficiency we will get.

- The wing extremes have the trend generate big vortex, that means local Lift and, at the same time as well, local strong induced drags. So what you get is the positive effect, Lift, againsts a negative effect you have to pay the local extremes wing induced drag.

- Therefore, one (and not so expensive) way we have to decrease such unconvenient balance in the already existing aircrafts is to adapt winglets, in order to reduce the vortex strength, thus the respective induced drag.

- I must say that all we said before stands for it, in those expressed terms, we did.  
 
May I hope to have been clear this time.
zzzo

Hi Rollerblades

Here, it goes my third turn, I should add:
 
- You can have a high induced drag wing or/and you can have a low induce drag wing. Just give exemples! It's almost intuitive!

- The main task is to reduced the induced drag effects at the wing extremes (be ware that in the wing aerodynamics includes other types of drag) behind of the scope of this post.

zzzo  

Hi Rollerblades

I'm sorry, but I still have to add the following:

- How has invented the winglets? The Nature did!

- By the way: Watch certain birds! How they fly. You may get clearly the answer.
zzzo    

"For the third time, you guys cannot suppress tip vortex which create induced drag because it generates lift.  A 2D airfoil is a profile with AR=infinite because the surface tends to zero.  A 3D wing has thickness. length and width as dimensions."

As a matter of interest, if you join the wing tips together to form a circular wing, in front view, do you still get tip vortices?

I would guess not, thereby demonstrating that the tip vortex is NOT necessary for developing lift.

Cheers

Greg Locock

Hi!

Or, following GregLocock: -The last but the least, going to the absurd, if such effect were so important for the the Lift you need, why the airplanes have wings?

They would need to have only the extreme of those wings, "the only source of the Lift", looking as fish, or imagine having only the winglets itselves !?...

zzzo

I just want to say that I can see several errors in the above discussion.  No one has mentioned the fact that an airfoil, in order to produce lift, must pump air in the opposite direction.  The energy necessary to lift the aircraft is deposited in the air as downward momentum.  So, no matter what, at some point vortices will be generated behind the aircraft.  Think of it more along these lines:  You have a column of downward moving air behind any aircraft, the "undisturbed" air on either side is basically still.  The vortex is then unavoidable since air shearing by itself will inevitably cause rotation.  An experiment was conducted where a lear jet was flown just above some clouds, in order to see what effect was left behind.  It was evident that there was quite a bit of air moving downward just behind the jet.

Wether winglets help or not is not really the question, since they exist in nature in many forms, they obviously help some.  But they do not "stop" wing tip vortices.  A circular wing will still cause there to be a column of downward moving air...but there will be no sharp point for vortex attachment.

Imagine if you will, a wing which has the majority of its lift in the middle, tapering off as one approaches the wingtips.  If one reduces the lift at the wing tip to zero, leaving the pressure differential near the middle of the wing....will vortices form?  Or how about a wing which actually reverses the lift at the very tip?  Not too much, but just enough to stop the localized pressure gradient.

Also, a description of how a wing functions can be derived from F=MA.(This I have done myself)  The pure Bernoulli description leaves too much out.

Anyway, sorry to be long winded "typed"...

Hi there

Winglets are in fact a way of 'artificially' increasing the span of the a/c (or reducing the aspect ration) and therefore help reduce induced drag.

One side effects of the winglets is that they also increase the wing root bending moment (due to the side force they generate.

Hence, if you are not span limited, then a simple (true) span extension is preferable...