It seems you are getting a bunch of complicated answeres to what seems to me to be a simple question. The reason the nose is dropping is because as you apply rudder, lets say right rudder, then you are putting the left wing into the reletive wind. Most sport planes and all trainers have some dihedral in the wings so as one wing in this case the left goes in to the releative wind that wind is getting under the wing and pushing it up. causing a roll. and as is always the case when you roll the plane you must apply up elevator to maintain altitude. If you want to slip (which is what you are trying to do by applying rudder in straight level flight) then giv some opposite aileron to hold the leading wing down.


A quick comment on the above post.

A plane turns, whether model or full scale, due to the horizontal component of lift. So no, rolling the wings does not cause a turn, but trying to hold the nose on the horizon while the wings are something other than horizontal does. Think about it, there is no flight control that controls "turns" ailerons are all about roll, elevator is about pitch and rudder is the benevolent dictator of yaw. turns happen when things are used properly.