Winglet thrust
 

Winglets do the fallowing:

1. Reduce Drag
2. Increase lift
3. Increase thrust?

I dont dont understand #3 fully, I get that it increases thrust like a sail going into the wind but I dont know how.

HELP:\ :confused:

Did my thesis on winglets at university. Never heard about the thrust bit! Ready to be proved wrong but sounds like bollocks to me......

The reason you get "thrust" is because of the way the vortex coming off the bottom of the wing affects the winglet. It causes a small element of lift in the direction of flight thus, it adds to the thrust vector.

Probably not the best explanation, I hope someone can elaborate. :O

Here is how I understand winglets. Winglet thrust, in itself, is quite small. In fact, the "forward" thrust contemporary winglets generate by means of vortex interaction barely offset the induced and parasite drag they create themselves. So no increse in "overall" thrust like an engine will provide, so much as a net reduction of self induced drag (pretty clever though).

However, taking thrust aside, the lift and drag they do create as a supplement to the wing is clearly positive and beneficial :)

Aerodynamically, wings generate "lift" normal to the local flow and "drag" parallel to the local flow. (In fact, of course, "lift" and "drag" are just convenient ways to break down the total forces....)
Now, because of the tip vortex the flow which is hitting the winglet isn't parallel to the general airflow - it has a significant spanwise component, as the vortex "rolls inwards over the top of the wing".
That means that the local flow is such that when the winglet acts on it you get a "lift" compoenent from the winglet which is partially inwards, partially upwards and partially forwards.
Something like this:
http://img178.imageshack.us/img178/1...gletlm8.th.jpg
DC-M is correct to say its minor, and its very much dependent on the conditions (which is why winglet optimisation is an art, and why winglets are much better on aircraft with a powerful single design point, rather than a range of design points)

Also discussed here