On Flight Path Control of an Unmanned Helicopter in Forward Flight using an Optimal Inner Loop with Body Frame Velocities

This paper presents the design of an inner-loop state-space control system for an unmanned helicopter in forward flight. It is assumed that the outer loop of the flight control system forms the desired velocities and turn rate based on flight path parameters such as altitude error, speed error, and cross track error. These desired velocities and turn rate are in the heading frame which rotates about the down (z) axis so that the x axis follows the helicopter heading and the x-y plane remains tangent to the Earth's surface. The helicopter model that is used to design the inner-loop control law is represented in the body-fixed coordinate frame, so the heading frame commands are transformed into the body frame using the current aircraft Euler roll and pitch angles. This transformation adds feedback to the system and can reduce stability margin even to the point of instability. This paper quantifies the additional dynamics from the transformation of commands, and develops a method to adjust the inner-loop control gains to account for the additional feedback, thereby returning the closed-loop eigenvalues and eigenvectors to the values intended by the inner-loop design.