spanwise vortices and induced drag

[lupidus]

What are spanwise vortices (I've seen the term used in a few places but never defined) and how do they relate to induced drag?

In this explanation of induced drag:

http://selair.selkirk.bc.ca/aerodynamics1/Drag/Page6.html

what causes the 'flow circulation' in the 6th picture?

Thanks

 

[MikeHalloran]

In a coordinate system that moves with the airplane, the air is flowing back under the wing at some velocity, and the air is flowing back over the wing at a higher velocity. Subtract the two, and you have a net velocity, _as_if_ the same air was actually circulating around the wing.



Mike Halloran
Pembroke Pines, FL, USA

 

[TheRaptor]

Wing tip vortices are created due to difference in air velocities or pressure on the upper and lower surfaces of the wing. Any fluid always flows from a region of high potential to a region of lower potential. Hence the air on the lower surface flows around the wing tips to the upper surface. These vortices can be minimised and almost eliminated by using wings of large aspect ratios. This is why an aerofoil (2D wing of infinite aspect ratio) in a wind tunnel would never experience induced drag.

Induced drag is significant only at low flight speeds. These wing tip vortices can also be clearly seen on the rear wings of F1 cars while they are zooming down the straights.

Cheers.

 

[GregLocock]

Well, I've aked this before and got no answer. What happens when you have a wing that is a circular ring, in front view?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[TheRaptor]

Greg

I am assuming that the view you are referring to, is also the front view of the aircraft. In that case, I dont see how there will be any pressure difference as the cross-section of the wing is circular.

 

[scotty7]

One ofthe reasons you get the vortex is that the low-pressure air above the wing tends to flow towards the fuselage, and the high-pressue air moves away from the fuselage (in both cases, high pressure region flows towards low pressure). At the trailing edge, tehrefore, there is a spanwise component to the flow, which is in opposite directions. This generates a vortex which runs down the trailing edge and spills off the wing tip. The winglets that many modern aircraft have sprouted are designed to unwind the vortex to some extent, hence reducing the induced drag.

TheRaptor, The wing that Greg is talking about is an annular ring, with a normal arerofoil cross-section wrapped round - sort of like bending both wings so far that the tips touch tangentally above the fuselage.
Don't know quite how the aero works on this, but apparently a paper aeroplane made in this style is very effective, sugesting that it is not traditional cambered-wing aerodynamics that is at work.

 

[TheRaptor]

scotty7,

Thanks for the info mate.

Greg,

Sorry pal. But never heard of such a design before.

 

[scotty7]

Try googling Annular Wing and go for images. There are a few that made it into metal there, as well as concept designs, with annular wings in both vertical and horizontal planes ('scuse the weak pun...)
Link to one below, can't get at the best one 'cos of the internet useage policy here...

http://www.unrealaircraft.com/gravity/snecmaC450-01.php