[Apologies if this is a double-post - Google seemed to have lost the
first one. This version is shorter and more to the point anyway.]

Earlier, Ian Strachan wrote:

... If only one hook is fitted then it
will be somewhat forward of the pure
"C of G" position because its location
is a compromise for both air tow
and winch and it will be tested for
both before the initial C of A is
given for the type.

From the perspective of an amateur sailplane developer:

That might have been true some time ago. However, increased demand for
better performance have made such compromises less tenable in the last
generation or two of sailplane.

The way I understand it, there are only two good locations for a tow
hitch: At the stagnation point on the nose of the glider, and aft of
the point of maximum thickness of the fuselage.

With the tow hitch buried in the nose vent at the stagnation point,
there is no particular disruption to the airflow, and you can
reasonably expect to get laminar flow over most of the forward
fuselage. At least, until the air encounters a disruption such as a
canopy separation line or passes the point of maximum thickness and
encounters an adverse pressure gradient.

With the to hitch located aft of the point of maximum thickness, the
airfow will already have tripped over into turbulent flow, and the
extra drag of that flow encountering the tow hitch will be minimal.

However, with the hitch located in the "compromise" area as Ian
suggests, it will almost certainly disrupt the laminar flow there, and
trip it over into draggier turbulent flow. Furthermore, the area of
turbulence will spread laterally at about a 7-degree angle aft of the
disruption. So you end up with a triangular patch of turbulent flow on
the belly with an included angle of about 14 degrees. That means extra
drag and poorer performance.

My own next glider will have a nose hook in the air vent duct at the
stagnation point on the nose of the glider, and a mounting location
for an optional CG hook that will be covered by the landing gear
doors.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24