How raked wingtips reduce induced drag?

[Change in pressure]

Hi all,

I understand that both winglets and raked wingtips are for the same function (from what I have read) - to reduced the amount of induced drag.

I understand the principles behind winglets, however I am uncertain how raked wingtips actually achieve this? Could anyone please explain the aerodynamics behind it.

Thanks

 

[anorlunda]

The term raked wingtips was unfamiliar to me, so I looked it up. These images may help others provide an answer.

Raked wingtips

Winglets

 

[Change in pressure]

Raked wingtips have more sweep compared to the rest of the wing.
But how they reduce induce drag?
I can not find explanantion...boeing dreamliner

albatros

 

[anorlunda]

http://www.aerospaceweb.org/question/aerodynamics/q0148.shtml
Says that they create an aerodynamic washout.

http://www.aerospaceweb.org/question/dynamics/q0055.shtml

First we will cover wing twist, of which there are two kinds, geometric twist and aerodynamic twist. A technical definition of geometric twist would go something like this: "an actual change in the airfoil angle of incidence, usually measured with respect to the root airfoil." (Raymer, Aircraft Design) In other words, if you were to look at a wing from the side, the airfoil (cross section) of the wing would rotate as you moved away from the aircraft fuselage. In most aircraft, the airfoil twists down as we move along the wing further from the fuselage. This is referred to as "washout."

 

[jack action]

[...] they increase the effective wing aspect ratio and diminish wingtip vortices, decreasing lift-induced drag. [...] While an increase in span would be more effective than a same-length winglet, its bending moment is greater. A 3 ft (91 cm) winglet gives the performance gain of a 2 ft (61 cm) span increase but has the bending force of a 1 ft (30 cm) span increase.

I would say that they basically reduce the effect of a sharp parallel edge at the tip where the air can 'escape' and create vortices.

 

[LURCH]

Just by comparing the second illustration in Post #2 with the one in Post #4, I would say that wingtip vortices start with an upward motion of the air escaping from beneath the wing, and the downward angle at the tip of the wing opposes this motion, impeding the formation of the vortex.