Crossunder
30th Apr 2003, 19:14
Why do wake vortices move outwards (viewed from behind) as they "touch down" behind a landing aircraft? This seems strange as they rotate inwards, towards each other?
PlaneTruth
30th Apr 2003, 20:21
Crossunder,
Votices are created by the interaction of a high pressure area below the wing and a low pressure area above the wing. In a swept wing aircraft, the airflow across the wing surface is not exactly inline with the slipstream. Next time you fly, look out at the vortilons (if you have them) or any other small turbulence generator on the upper wing surface (a fastener or panel line). You'll notice the dirt and grime marks left behind actually reveal a spanwise flow of air slightly towards the wingtip.
Air has mass and moist air has even more mass. As the air strikes the wing and begins this spanwise flow, it changes the vector of the static air into a direction away from the fuselage. The upper and lower pressure areas spill off the wingtip and continue away from the aircraft for a short distance due to this vector change. As you may imagine, pushing all this air is a great source of drag on the aircraft and a prime reason why many long haul aircraft are now being fitted with 'tip sails' to try to minimize this drag. Some aircraft have "fences" installed to stem spanwise flow prior to the wingtip to enhance effectiveness of an aileron or a flap. We most often see the tip vortices on an aircraft in the flare as they whip up dust adjacent to the runway or in rain or humid conditions.
Fairly recent research by NASA has revealed that vortices react violently after they depart the wing. These mini-tornados snake quickly in all axes. They may stack vertically or entwine with one another. The research has also revealed that they frequently strike the ground and bounce back up into calm air.
Hope that helps.
PT:ok:
Crossunder
1st May 2003, 00:52
Aha! Why didn't I think of the spanwise flow... Thanks!