winglets
 

hi gent's
what is the function of winglets???
thanks

The function of the winglet is to reduce drag created as a consequence of the creation of lift.

They limit the formation of wingtip vortices and thus the amount of drag created.

Borrowed from the Boeing website, this picture shows how effective they are at reducing the wingtip vortices!

http://i35.photobucket.com/albums/d1...37winglets.jpg

Wing tip vortices are created by span wise air flow from the lower wing surface (area of higher pressure) to the upper wing surface (area of lower pressure). Generally, the airflow on the upper surface is forced towards the fuselage, whilst the airflow on the lower surface is being forced toward the tip. This results in the airflow from the lower side trying to climb over the wing tip and move to the area of lower pressure, the upper surface.

The winglets act like a solid fence, stopping this movement of air from the lower to upper surface. It doesn’t stop these vortices completely, as the winglet is essentially just an extension to the wing, but does reduce them significantly, as shown in the diagrams above.

Just why is a winglet better than extending the wingtip in the same direction?

As then, without the winglet (above Boeing) or wing fences (Airbus) you'd still get the span wise flow meeting.

Basically you have to try and stop the meeting of the top and bottom air flows. Which creates the 'swirls' you see on a damp morning when acft are landing.

If you mean extending the wing away from the tip and then bending it toward the vertical, then that is what the blended winglet is.

If you mean extending the wing without the winglet, I would suggest there would still be the issue of greater air movement from High to Lower pressure areas. Remember, you are trying to stop this movement.

Ah - just noticed the above... I agree.

Aren't Airbus now looking at winglets on their A320? Although, I've always though their wing fence would function better given it is stopping this movement at both lower and upper surfaces?

Aye - thought I could remember something about the A320 and winglets.

The following link takes you to a news letter from Airbus – you want page 2.

Winglets - Page 2

Cheers!

Raked wingtips do work as well - by extending the span. Winglets are not the be-all and end-all (see 777-300ER or -200LR and 767-400). The main problem is that you can't go on extending the wing ad infinitum as then you hit gate compatibility problems. For a 737 or A320, the ramp space allows about 36 metres for the wingspan. Winglets allow you to effectively extend the wingspan - even though it isn't in line with the wing - without hitting the ramp space limitations.

A wing with infinite length has ZERO spanwise flow as there is no pressure differential along its span. No vortices are then created and therefore no drag as a result of the vorticies. So you are right, a longer wingspan, or ASPECT RATIO, is good.

However, the longer the wingspan, the more bending (fatigue) it creates at the wing root where it connect so the fueselage (longer moment arm). With more bending comes the requirement of more strength and it's associated weight penalty with a beefed up airframe, which cancels out the original bennefit.

Also, a bigger wingspan has issues when getting into the passenger terminal area for parking.

So winglets do increase the SPAN of the wing but just in a vertical direction, the result is the same. Less spanwise flow, less wingroot bending and the weaker vorticies are actually moved up and away from the main airflow over the wing.

Most books/texts don't tend to actually explain HOW the vorticies increase drag - which is nice to know (INDUCED drag from lift).

Imagine a pen stuck straight out the top of a wing. This is called the TOTAL REACTION FORCE. As the angle of attack increases (creating more lift), the pen tilts back. The TR force is broken down to a vertical component, LIFT, and a horizontal component pointing backwards called INDUCED DRAG. It's this horizontal force which opposes thrust that winglets try to reduce.

How?............

Every wing has an associated UPWASH and DOWNWASH. With low pressure on top and high pressure underneath, air approching the leading edge of the wing wants to move up to the the lower pressure. This increases the "angle of attack" (AoA) of the air as it hits the wing (increases lift). As the air passes behind the trailing edge an opposite downwash is created. This reduces the EFFECTIVE angle of attack. So the pilot needs to select a higher pitch attitude to produce the same lift.....and unfortunately, out "pen" gets tilted further back and with it, an increase in the horizontal component of INDUCED DRAG.

Wingtip vorticies exacerbate the DOWNWASH, so reducing the size of the vorticies, reduces the downwash. This means the pilot doesn't have to increase the AoA as much for a given amout of lift. Also, having the winglet tips high above the main wing also aids in stopping the vortices impacting the airflow over the wing.

Hope this schpeeeeel helps you :ok:

Does it mean that instead of adding winglets to a wing, you could retrofit an existing wing by sawing off some of the wing and installing a winglet instead? The wing, having shortened wingspan and less wing root bending moment, would still provide as much or more lift?

No, because you've shortened the horizontal span of the wing (the lift producing area), although it would have less drag than a wing of the same span without winglets.

But you could fly faster to produce the same amount of lift from the shorter wing with winglets. But..err...then you produce more drag:}

So now we have to introduce the word "compromise" which is always the end result in aircraft aerodynamics, and I guess that Mssrs Boeing and Airbus have got it just about right in their own individual ways.

Gee, and all this time I just thought they provided a nice blank canvass to paint your airline Logo / advertising !!

The sailplane folks often refer to "span loading" (gross wt./unit span) as a measure related to spanwise flow. Bigger span => lower span loading => less spanwise flow.

Let´s have a look of what the wingtips and winglets do, then:

A wingtip extension adds lift and improves L/D ratio. But the extra lift, with long leverage, requires extra wing strength. Even if the plane is slowed so that the total lift stays unchanged at the gross weight, the extra leverage requires mor wing strength.

Whereas a winglet generates no lift, and improves L/D ratio by decreasing drag.

Correct?

That's correct.

If, in your original question, you added the winglet on to the existing span of the wing you would have a wing producing the same amount of lift but with a increased L/D ratio.

True or Not?

Don't winglets also produce a certain amount of lift themselves? Depending on the toe or cant.

winglets
 

hello ithernet

true, they produce some "horizontal" lift where the resultant vector has a forward angled component.

So, winglets (unlike wing extensions) produce no wing bending moment because they produce no lift because they are at a right angle to the wing.

There are two directions at right angle to the wing. Why do winglets extend up from wingtip, not down?

I’m not sure they produce no bending moment. They add to the mass of the wing so I would think the bending moment is increased to some degree. The bending moment does not occur at the wing tip, but at the wing root.

Why do they point up? Not so sure, but I would guess for obstacle clearance and aesthetics.