Hi Tom,

One way to avoid significant drag penalties with vortex generators is to
align the long axis of the raised tab with the local airflow during cruise
flight. As the lift surface angle of attack is increased, the spanwise
airflow increases, thus increasing the angle of incidence of the vortex
generator to the local airflow, resulting in vortex formation. This
arrangement, however, requires the use of aligned vortex generators that
produce corotating vortices (and is commonly used on commercial aircraft),
and thus might be subject to the sort of adverse behavior mentioned in
Scotts post.

Another method to limit the cruise drag contribution of vortex generators is
to reduce their height to the minimum necessary to achieve the desired
effect. In some cases, it is only necessary to extend the tab just halfway
through the boundary layer. Remember, however, that at the lower speeds
commonly associated with higher angles of attack, the boundary layer will be
thicker than at the higher speeds associated with cruise flight.

It's all about the compromises you are willing to accept and the shape of
the performance envelope you are comfortable living within. If you want
improved low speed performance and are willing to sacrifice some high speed
performance, then vortex generators may be the right modification for you.
Maybe slightly larger flaps or wing extensions would accomplish the same
goals with fewer negative effects. Maybe the same results could be achieved
with the center of gravity moved farther aft.

Best Regards,
Mike Hutchins