Optimal Control Of The Helicopter Autorotation Landing

Gliding descent in autorotation is a maneuver used by helicopter pilots in case of engine failure. It requires considerable skill, and since it is seldom practiced, it is considered quite dangerous. In fact, during certification, a region of low altitude and low velocity (the H-V restriction zone) is established where it is considered impossible to make a safe descent.The landing of a helicopter in autorotation is formulated as a nonlinear optimal control problem. A three-dimensional rigid body model is used to study the landing of a helicopter in autorotation. The model assumes that a helicopter is rigid with the following six degrees of freedom: three translations in the x-y-z axes, and three rotations around three axes. To be convenient, we only study the motion of a helicopter in the critical plane, and a simplified point-mass model is used. The model considers as its states the helicopter vertical and horizontal velocities, vertical and horizontal displacements, and the rotor angular speed. The cost function of the optimal control problem is a weighted sum of the squared horizontal and vertical components of the helicopter velocity at touchdown. The control (horizontal and vertical components of the thrust coefficient) required to minimize the cost function is obtained using nonlinear optimal control theory. As an example, an OH-58A helicopter is introduced in the study, and the possible reduction in height-velocity restrictions is discussed using optimal control techniques.