Rolf,

With a glider trailer there are two centers you need to worry about,
the CG is your weight distribution. In general the minimum weight on
the tongue should be 10%, more is better. The second is the
aerodynamic center of gravity as related to your wheels. If the
aerodynamic center of gravity is behind the wheels your trailer will
tend to be unstable as well. Think about this as surface area in front
and behind the wheels. You want more area in front than behind if
possible. Rather than just adding weight to the tongue by adding
ballast it is better to move the wheels back. This will lengthen the
amount of trailer in front of the wheels and have less behind, i.e.
shifting the aerodynamic center forward as well. Most modern glider
trailers have the wheels on a sliding frame that allows you to move the
wheels back, if not it may be worth it in the long run to have someone
move them back for you.

Tim