What angle creates MAXIMUM air deflection?

I'm not a physicist, so please forgive me if my vocabulary is faulty
here. I really need this information, though; any feedback would be
greatly appreciated.

A flat surface, AB, is moving through the air, with the surface
exactly perpendicular to the direction of movement. In the graphic
below, the surface is moving downward. Point X is behind and slightly
to the inside of B.

X

A___________________________________B
|
|
V

As the surface moves through the air, the air "curls" around B and
hits X. To help eliminate this, we need to change the angle of
surface AB so that it deflects the oncoming air outwards and away from
X. In other words we need to move B "upwards" in the graphic above.

The question is, how many degrees does AB need to move to produce the
MAXIMUM amount of air deflection?

What happens at the A end is irrelevant. B really just pivots around
it.

For what it's worth, this is a fairly thin surface, about 1/4" thick.
The speed is typical highway driving speed, say 60-70 mph. Don't
worry about air temp, density, etc. It doesn't have to be that
precise.

A whole lotta thanks in advance. Please reply to the newsgroup.

Bill S.