Spanwise air flow

[GirlInTraining]

In regards to induced drag/wingtip vortices, why does air flow spanwise along the wing towards the wingtips? I can think of a couple different reasons, but they fall apart when looking at, say, the X-29 with a forward-swept wing (which the Jepp private pilot book (on which I'm basing part of my lesson plans) says has spanwise flow to the inboard of the wing).

Just getting ready for my CFI checkride, and trying to tie up loose ends (and I hate aerodynamics)

 

[C650CPT]

This is primarily a problem on swept wing airplanes. Think of the air as if were water, as it hits the leading edge of the (swept) wing it trails aft or along side the sweep of the wing towards the end. Manufactures install flow fences to "stop" the flow of air along the leading edge and redirect it rewards over the wing itself and the control surfaces. With the X-29 the sweep of the wing is reversed so the "flow" would follow the leading edge towards the fuselage.
Hope it helps.

Jim