Originally Posted by
DesertRat
Gurney flaps stick UP from the trailing edge of the wing. Gives downforce with only a tiny amount of drag added. They're not sure why it works so well.
Actually, they're quite sure why...on a symmetrical airfoil (the first application), the trailing edge vortices alternate; these turbulate the airflow at any angle of attack, but they're MUCH more pronounced at higher alpha. This turbulation causes laminar separation as far as 45-50% of chord length ahead of the trailing edge. This separation is seen as lost lift and increased drag.
The Gurney Flap (or Wickerbill or Zaparka Micro Flap, as patented in 1935) simply pulls the turbulation to one side of the airfoil (high pressure side) and stabilizes the airflow ahead on the opposite side (low pressure side).
There are many flap systems in use in the aviation world; almost all have complex linkages and tracks. The beauty of the Gurney flap is in it's simplicity.
There is a small penalty in drag at low speed; it is more than outweighed by the advantages at high speed.
Byrdman