Quote:
Originally Posted by Piotrsko
All these years I have believed the main purpose of VG's were to keep the airflow attached, typically at sub critical Reynolds numbers because when your flow becomes detached you fall out of the sky stalled, at any speed. I also understand that at some distance from the surface, the airflow returns to being laminar so I suspect dimples or divots are just keeping the surface from stalling (detached airflow) which for wings in flight is the highest drag point.
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* Once flow reaches the first minimum pressure on a wing it automatically transitions to a turbulent boundary layer ( TBL ).
* A TBL can withstand some degree of adverse pressure gradient ( pressure rise ), whereas, a laminar boundary cannot tolerate any of it!
* The angle of attack determines whether or not the flow will remain attached or not.
* On a Clark-Y for instance, the drag of the wing varies by 2,659%, depending on A-O-A.
* Zero lift occurs at -5.6-degrees cord.
* Trimmed cruise is at 12-degrees AOA.
* Max lift occurs at 15.6-degrees AOA.
* The wing begins to get mushy at 17-degrees AOA.
* At 19-degrees AOA the top of the wing is at 30-degrees and full stall.
* Max wing drag occurs at 22.8-degrees AOA. You're already falling. If you have enough altitude you just take your hands and feet off all controls, and the plane will recover in three spins.
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If you're forced into an emergency short field landing and must slip the plane to burn speed @ full flaps, VGs can help maintain aileron, rudder, and elevator authority, especially in crosswind or gusts.
At that point it's not about Reynolds number ( you've got that covered ), but tricking the flow into remaining attached when it would otherwise be separating along a contour of too radical a rising pressure gradient.
freebeard's Coanda-nozzles would be a big help just about then.
