Some aircraft are designed to accommodate rudder-only turns. My first RC model airplane had this, and my first hang glider did too (Quicksilver B). Generous amounts of dihedral seems to be the key ingredient in making reasonably successful rudder-only turns.. But I can't find much in the way of design guidelines for this type of aircraft. A friend of mine is getting close to flying a prototype hang glider that is strikingly similar to the Quicksilver, and I'd like to have some design assurance that he is using the right amount of dihedral, rudder area, rudder lever arm, etc. Happy landings!