Development of an integrated/multidisciplinay methodology for the analysis of rotorcraft flight mechanics with external loads

Helicopter is an invaluable tool in many civil applications and a fundamental asset in military operations. Two of its most appreciated capabilities, are the ability to transport external loads suspended from a cable and the possibility to carry stores rigidly mounted to the fuselage or to stub wings. Unfortunately, a suspended load adds its aerodynamics, rigid body and elastic suspen- sion dynamics to the helicopter bare airframe dynamics and less than satisfactory handling can ensue. Moreover, rigidly mounted stores must be dropped before a forced landing in order to get rid of dangerous items (e.g. fuel or explosives) and decrease weight as much as possible. In this conditions a safe store separation is not guaranteed and the store may collide with the helicopter. It is, then, clear that carrying external stores can pose safety issues. The first objective of this dissertation is the evaluation of helicopter handling qualities in presence of an external suspended load. A simple helicopter model is coupled with two different load models (pendulum and 6-DoFs). The system is, then, linearised, the modal and the frequency domain responses are evaluated and the handling qualities are assessed according to the current regulations. This approach requires very little knowledge of the actual helicopter an can be useful as a tool for preliminary handling qualities assessment during the design phase. The coherence of external load modal dynamics and the helicopter response to control input obtained with the present approach is compared with available reference data. Finally, the impact of the slung load parameters on helicopter handling qualities is assessed. The second objective of this dissertation is the study of a comprehensive approach to the problem of external load jettison from a helicopter and the development of an associated software tool. First a specialised tool for the simulation of the release of an external store is devel- oped. An important aspect of the simulation is the collision detection. The minimum distance between the helicopter and the store is evaluated at each simulation step. Multiple simulations are performed in order to determine the safe drop envelope. The following section describes a methodology to reconstruct the trajectory and the attitude of the jettisoned load from video frames. A manual procedure is developed to identify the same markers in two video frames taken at the same time by video cameras mounted on the helicopter. A simple camera calibration technique, capable of computing camera orientation and aperture angle, is also discussed. Finally the possibility to identify the aerodynamics properties of the released body from flight test data is investigated. The objective is to build, from the reconstructed trajectory, an aerodynamic model functional to the refining of the store drop simulation. The problem is a minimization one. An Evolutionary Algorithm is used with good results on different test cases, also including measurement noise.