Winglets and wake vortex
 

Could you learned contributors please explain the effects possitive and/or negative of winglets on the vortex wake generated by aircraft in the approach phase of flight?
Many thanks in anticipation
David

This is only an assumption.

As winglets are generally used to reduce induced drag, of which wake vortex is a by product. I would guess that possibly, and only possibly, that wake vortex and the effects thereof are also reduced. This is a good thing!

Hope this helps:ok:

...standing aside to allow those with more knowledge than I to correct me... or is it...more knowledge than me to correct I...?:confused:

i don't think i'll be touching down at the same point as a 737-with winglets or not, in a Cherokee i don't think it will matter the funeral director never heard of winglets:} ....enough nonsense from me Chesty's post sounds very reasonable:)

A vortex is generated because when a wing produces lift, you have high pressure below the wing and low pressure above. They will try to equalize at the wingtip, combine this with relative wind and you get a swirling motion. At low speeds, because of higher AoA and higher camber (from high lift devices), this pressure difference will increase and create more powerful vortexes. A winglet stops this motion by effectively being a barrier/wall so that the pressure differences cannot be equalized.
On a 737, winglets generate a 2% fuelsaving. For a big company with small margines, this can be the difference between earning money or losing money.

You can have induced drag with no wake vortex whatever - in the theoretical limit of infinite airfoil, there is no wake vortex (of course), but there is a plenty of drag, incl. induced drag.

But just how does the effect of a winglet on a wake vortex compare with effect of simple wingtip extension?

JOCKEY, The 737 winglets give a total average of about 4,5% fuel savings on a 737

http://www.b737.org.uk/737winglets.jpg

M

I think it's essentially the same thing, an increase in span, but without increasing the overall dimensions of the a/c excessively. Like I say, I'm no expert, merely trying to drag up long forgotten P of F lessons.

Regarding the infinite aerofoil. Again I'm only guessing. Wake vortex is essentially caused by the span wise flow of air along the wing, due to the high and low pressure areas. If you take the infinite aerofoil, theoretically there would be no span wise flow and therefore no induced drag and wake vortex. Lots of profile drag though! Is that correct or am I completely rudderless now?!

Look forward to hearing the answer!:ok:

Some thoughts

(a) typical pilot PoF courses (like most non-specialist courses anywhere) leave some of the detail out so that the main points are driven home without too much distraction by the nice to know things which are important to the specialist (in this case the aerodynamicists)

(b) the flow is a vortex sheet coming off the trailing edge as well as the wiptip vortex. Within several spans behind the aircraft, the sheet wraps up into the tip vortex .. the pix linked to in the sticky (at Airliners.net) gives a reasonably good flow visualisation of this consideration. (Apart from that, it's a ripper shot)

(c) the wing tip sails (whatever term you prefer) are used for a range of things such as

(i) getting a bit more effective span without upsetting the airport gate infrastructure or having too great effect on overall wing bending loads reaction design problems. Notional figures are that the appendage is worth around a 70% simple span increase ..

(ii) marketing hype .. if it is on the big bird, it looks even better on the pocket rocket. The bending considerations still come into play

(d) you don't get a free lunch .. so, after market STC fits come with fatigue life penalties on the wing structure

(e) there are plenty of references on the net, including several in the sticky, if you want to get a more quantitative feel for what is going on.

I doubt whether an infinite airfoil wouldn´t have induced drag. After all, it can either be unloaded - just profile drag, pressures equal on both sides, no lift, no vortex sheet, just profile drag - or loaded at a nonzero angle of attack, in which case it would generate lift all along the airfoil, and have some extra, induced drag in addition to the profile drag.

An infinite airfoil in a slipping or oblique airflow ought to have spanwise flow, but no wake vortex, as the high pressure should be uniform over the airfoil length...

I suspect I may be showing my ignorance here, but aren't we talking about 2 differnet vortices?
If I dredge up what's left in my head from the wing design course, the trailing vortex is a function of the wing section, giving us the circulation term in 'Lift = rho * U * gamma" , and forms a line vortex roughly parallel with the trailling edge - and thus would exist for our infinite wing.
The tip vortex can be thought of as the result of the higher pressure air under the wing effectively spilling over the tip.
The interation between the two can get interesting, and is the bit that winglets can play around with.

Having touched on this during my degree I have to agree with the above posts, the winglets reduce 'form' drag and bring about bean counter pleasing savings - this is done by easing the split of the airflow off the wing. The tip vortex's are smaller and tighter than normal although are probably just as strong. as they are now tighter, you definately want to be avoiding these in light aircraft!! :eek: