A torque roll is produced by the amount of torque placed on the engine, right? You need a prop that will produce more thrust, to keep the plane hovering,with out producing much torque, load on the engine. If your 18 X 8 is producing to much torque on the engine, then try a 19 X 6 or, if ground clearance is a problem, try an 18 X 6 or even an 18 X 4. Both will require more RPM, but will put less load, torque, on the engine.

Good Luck,
Richard