Study of a Nonlinear Dynamic Model of Vortex Ring State for Bell 412 Helicopter.

The main objective of this thesis is to develop a Level 1 simulator specifically for the Bell 412 helicopter to study the Vortex Ring State (VRS). This is an unsteady aerodynamic condition that occurs at certain descent rates and low forward speeds. Entering the VRS causes a sudden loss of thrust, an increase in descent rate and a loss of controllability. It is therefore necessary that the flight envelope avoids this possibility and, if this state occurs, pilots are trained in recovery manoeuvres to get out. This is the context in which the project was developed to provide a satisfactory model for possible pilot training. It was divided into the following phases: modelling, trim evaluation, validation, development and implementation of VRS models. First, a non-linear model of the helicopter was built in the Matlab-Simulink environment. The main elements of an helicopter were modelled, while the engine was considered ideal. Next, the trim was studied. This is in fact the necessary starting condition for any simulation and the assessment of a good equilibrium point is therefore crucial for the progress of the project. The model was then validated by time scale comparison with results from FlightLab, an high fidelity simulator developed and validated as part of a joint project between the University of Liverpool and the National Research Council of Canada. Different flight conditions were analysed, to cover as wide a range as possible to confirm the validity of the model. The simulations were carried out with a doublet input of collective, longitudinal cyclic, lateral cyclic and pedal. Once the model was validated, the next step was to develop algorithms that would describe the variation of the inflow during the descent and thus allow the development of VRS to be observed or not. Two models were therefore studied: Young and Jhonson. A validation was also carried out by comparing them on a time scale with the results obtained by FlightLab, focusing in particular on the variation of the climb rate and the activation of the VRS. The model described by Jhonson gives satisfactory results and is suitable for the purpose of the project.