How does the propeller position affect the effectiveness of control surfaces?
A propeller has more effect on a control surface behind it, than on one in front of it. That's because the propeller pulls in air from a large area in front, and ejects it in a relatively narrow stream behind. Since the mass flow rate is the same on both sides, the air in front moves fairly slowly, and the air behind moves faster. Forces on a control surface in an airflow depend on the square of the airspeed, so a control surface in the fast-moving air behind the propeller will deliver larger forces for a given deflection than one put in the slower-moving air in front. Look up Axial Fan Design on Wikipedia, for a cross-sectional view of the airflow through a propeller.