Dihedral is a stabilizing design......its not intented specially to make the airplane turn witih rudder only. A straight wing with no dihedral is still subject to the same principle of banking using rudder that a wing with dihedral.

Hi Gents
One of the best places to look for how to stuff is a club called Blue Max RC. They have the best article on trimming I have seen on the net. Check it out you will like it. You may have to look a little but its there. http://www.bluemaxrc.com Jim

goplaneman,
Your's is definitely a lack of downthrust. Loosen the bottom bolts on the engine mount, then remove the top ones. Put at least two washers on the top bolts, BUT behind the engine mount, and re-install them. Then tighten the bottom bolts. (You'll have to remove the engine to do this.)

The right amount of downthrust will prevent the airplane from climbing at almost any motor speed, because the faster the prop turns the more "down" is created, so it offsets the wing (or horizontal stab) incidence.
Dennis-