Trajectory design of an Aircraft for Circular Motion

In this paper we propose an autopilot control strategy for a small radio controlled aircraft aiming to follow a circular path with a desired radius. In contrast to the usual way to compute the error as the distance between the plane current space position and the desired flight trajectory path (for example a circle of radius r(0)), here we propose an alternative way to compute the scalar quantity E(t)) such that the aircraft moves in a spiral path of a decreasing/increasing with time radius to approach the circle with desired radius r(0.) Our particular choice of error term is intended to cause only gradual changes in the aircraft's direction so as to prevent unstable flight due to over-control.