Yea 1515Blue, you got it, thickening of the boundary layer is the transition from laminar to turbulent flow, or for the DC-9 from semi-laminar, mostly turbulent flow to really turbulent flow with no cohesion.
At high alpha the boundary layer cannot negotiate the adverse pressure gradients formed at the wing leading edge, as a result the flow separates at the leading edge of the wing. The separated flow rolls into a set of vortices which reattach themselves to the wing. This reattachment line then encounters another pressure jump and again separates but is in opposite direction to the primary separation. The vortilon provides a mechanical blockage and creates a stable vortex that is at acute angles to the span-wise vortex, usually destroying it from that point on. The vortilon, though below the wing, creates a vortex above the wing, remember we are at high alpha.
Last edited by '%MAC'; 7th July 2002 at 18:00.