You guys are a bit off base. The rigid pusher does not simply deliver a thrust, it responds to flow changes with surprising changes in its thrust and moment. If the nose goes up, the prop will see a strong "P factor" that causes a yaw (remember the right rudder on climbout in an airplane?) This means the prop is always destabilizing, and often cross coupling as well (where yaw makes it cause a pitch probelm and vice versa).