I am having difficulty trimming my patten model, and can not seem to find any help
from the trim charts I found on various web sites.

When I do a vertical dive with low power, for example on the downward leg of a stall-turn, the model
yaws to the left way too much. An excessive amount of right rudder trim helps but this should not be necessary.
I have tested in calm air and this is not a wind effect. The model construction seems straight and everything else
is fine with the way the model flies. The upleg on stall turns is straight with no yaw, and there is no rolling on the
down-leg or up-leg of stall turns.

Any suggestions would be appreciated.

Brent

Does this happen when you stall turn either direction? If it yaws in the direction of the stall turn, then the rudder may not be centering after the full deflection.
Also, where is the rudder trimmed for normal flight? You might be carrying left rudder trim to counteract some excess of right thrust.

Try here:

http://www.nsrca.org/trimA.htm