What prevents span wise flow on a swept back wing?

[south coast]

hi there people...

a quick question, what prevents span wise flow over a swept back wing?

would i be right in saying that; wing fences prevent span wise flow, and that winglets are used to help reduce the strength of the wing tip vortices therefore reducing induced drag.

(wing tip vortices being a product of span wise flow and therefore winglets also helping to prevent span wise flow)

comments please...

[BOAC]

Also the leading edge 'notch' which generates a vortex, helping to resist spanwise flow.

[boardnick]

In reality you don't always want to prevent spanwise flow, but if you do then the wing fence is an answer (invented by Wolfgang Liebe in 1938).
As you said, spanwise flow results in trailing edge vortices which are responsible foe induced drag. Winglets work by reducing the strength of the trailing edge vortices and displacing them further away from the wing and fuselage, thus reducing induced drag.

Boardnick

[Rainboe]

The main ingredient of wingtip vortices appears to be air from under the wingtip area flowing outwards to roll into a swirl, and being sucked around the wingtip inwards towards the top of the wing- is that correct? If that is so, why isn't it right to fit a fence under the wing near the tip angled forward end outwards to blow the outward flow back under the wing? I'd have thought you'd ideally want Airbus type wingtip plates angled to do opposite things above and below the wing, so they would look quite noticeably different above and below the wing.

[Astra driver]

Winglets "fly" with the airflow coming both from the side from the vortice and from the front from the normal airflow; the resultant net airflow is from ahead and to one side, since winglets are airfoils they produce "lift" that is perpendicular to this net airflow. If this lift vector is broken down a portion of it is in a forwards direction, and can be considered as thrust. It is by converting some of the induced drag that would otherwise be produced by the wingtip vortice into a small amount of thrust that winglets improve fuel efficiency.

[Milt]

south coast

I suspect you have a concept that the swept back wing is just a straight wing that has been installed at a sweep angle and that your span wise flow is then considered as the sweep angle.

Consider a narrow slice of a swept wing directly along the line of flight. The wing design will be such that that slice will be exactly, or very close, to the aerofoil shape desired. Each airflow streamline around this aerofoil will then have similar spanwise influence to that of a straight wing from the spanwise pressure gradient.