Some of the more challenging aspects of modelling the mechanical dynamics of rotorcraft systems are addressed in this report. The generic model here presented contains an articulated rigid-bladed rotor and fuselage. Special attention is given to the building of the main rotor model containing flap, lag and feather blade degrees of freedom for each of the equi-spaced blades. A full descriptive AutoSim model is built, simulations results are generated and they are processed for comparison with theoretical findings fi:om the literature and experimental data obtained using VTM code. Such comparisons show that the results are accurate. It is concluded that contemporary multibody analysis tools are applicable with advantage to the modelling of complex mechanical-dynamic systems like helicopters.

Helicopter ground resonance is a highly non-linear dynamic problem involving complex interactions between the various parts of a helicopter and constituting a major design consideration. Linearised models of the problem are often utilised as the conventional tool of analysis and design. These models, however, portray only the local dynamics of the problem. Therefore, a more useful capability would be to obtain a global picture of the behaviour when system non-linearities are included and for that purpose, novel tools of analysis and design are sought in this research. Dynamic systems theory provides the basis for the continuation and bifurcation methods of analysing non-linear systems of ordinary differential equations. A significant strength of these methods is the gained insight into the phenomena that affect the global behaviour of a non-linear model. The combination of these methods with conventional simulation, provides an unmatched tool for studying non-linear dynamic problems and is therefore an ideal choice for this research. This thesis demonstrates the application of non-linear continuation and bifurcation theory to helicopter ground resonance. The classical model of the problem is reduced into the non-rotating frame and the methods are implemented to obtain the boundaries of instability and to study their variation following helicopter parameter changes. The resulting process is found to be far more efficient than current methods, thus demonstrating the ability of the continuation and bifurcation method to become a powerful and practical design tool. Using these methods, the effects of fuselage stiffness and damping non-linearities on ground resonance are investigated by computing the type, amplitude and frequency of the limit cycles after the onset of instability. Some of these non- linearities are found to be beneficial by producing a boundary of stable limit cycles which surrounds the unstable range of critical rotor speeds. In other instances, however, the potential rise of catastrophic scenarios was noted when unstable limit cycles exist in regions where linear theory predicts a stable behaviour. The determination of the stability boundaries in the rotating frame is also achieved for the first time. This allowed the study of the effects of a non-linear lag damper on the response and valuable insight into how the damper parameters affect this response are gained. Multi-body dynamics formulations are then used to model the ground resonance problem and the procedure required to couple the continuation and bifurcation tools used throughout this research to this model is outlined.

This thesis is devoted to the study of rotor flows. A literature survey suggested that possible improvements can be achieved via better simulation of turbulence and prediction of the elastic blade de- formation. While the limits of the currently used URANS turbulence models have been shown, accurate predictions can only be achieved if the blade elasticity is taken account due to the large deformations a rotor blade can undergo in flight. An aeroelastic coupling strategy consists in three steps: computing the flow field, computing the structural deformation and transferring data between the CFD and CSD solvers. The transfer method also needs to deal with the different sizes for the structural and aerodynamic models. Most challenges come from the need to modify the CFD mesh following the blade deformation, and are linked to the higher refinement of the CFD grid. To tackle the problem, a new hybrid mesh deformation technique, adapted to rotor in-flight deformations was developed for the Helicopter Multi-Block solver of Liver- pool. Demonstration of the method was presented for multiple test cases: hovering HART-II rotors and forward flying ONERA 7A and HART-II rotors. The method proved quick at deforming the mesh and able to deal with large rotor deformations without downgrading the mesh quality. Another point of interest in this work was turbulence modelling. Aeroelastic calculations must capture the influence of the flow on the blade structure. Rotorcraft flows are complex, and due to the lim- its of the URANS models in predicting the frequency content in the flow, discrepancies in the structural forcing and the blade deformation might appear. Vibration levels might also see an improvement from a higher frequency content. The potential of DES models for rotorcraft flow was demonstrated using the stalled flow around a NACA002l wing as a test case. The frequency content obtained through DES was much wider and also allowed for better predictions of the mean flow field properties along with integrated loads. DES was applied to the HART-II rotor in order to assess the possible improvements coming from the use of DES. However, the difference between the URANS and the DES predictions of the flow field were limited, highlighting a grid or time step refinement need. A strong aeroelastic coupling strategy was also demonstrated, using the UH-60A rotor in high- speed forward flight. The key structural deformation was captured by the coupling strategy, and the dependency of the predictions to many flight and simulation parameters was highlighted.

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The elastic coupling properties of anisotropic composite materials offer the potential for aeroelastic tailoring and other structural couplings that are not fully exploited in current helicopter rotor blade designs. The full 3-dimensional analysis of slender prismatic structures (such as helicopter rotor blades) is routinely reduced to analysis of a 1- dimensional beam with associated cross-sectional stiffness and mass properties. It is therefore desirable to design the cross-section of such prismatic structures to given values of these cross-sectional properties. Although use of anisotropic composite materials offers additional degrees of freedom with which to obtain the desired values of cross-sectional properties, this introduces non-intuitive structural couplings and interactions between design variables, which increases the complexity of the design process. Rigorous optimisation techniques are therefore required to reliably and efficiently obtain an optimum design. This thesis addresses the main issues relating to the static optimisation of prismatic structures and their application to composite helicopter rotor blade design. Existing literature in composite materials, optimisation, and helicopter blade design is surveyed. A 4-ply laminated cylindrical shell is examined from analytical and computational perspectives as a simplified case study, which is used to develop understanding of how the choice of design variables affects the nature of the design space, and hence the solution methods which can be used. Flap-torsion coupling is an important variable in aeroelastic tailoring, and is therefore examined in some detail. A new analytical model is derived which is validated using finite element analysis, and compares favourably against existing models in the literature. Flap-torsion behaviour of laminated composite beams is studied experimentally, and compared with finite element results. Finally, the validity of the method has been demonstrated through the application of this work to the design of a generic helicopter rotor blade section, which meets given target values of cross-sectional stiffness.

A dynamic inflow model is a powerful tool for predicting the induced velocity distribution over a rotor disc. On account of its closed form and simplicity, the model is highly practical especially for studying flight mechanics and designing control systems for helicopters. However, scant attention has been so far paid to applying this model to analyse autorotative rotors (i.e. rotors in the windmill-brake state), which differ from powered helicopter rotors (i.e. rotors in the normal working state) in that the geometric relation between the inflow and the rotor disc. The principal aim of this research is to theoretically investigate the applicability of existing dynamic inflow models for autorotative rotors, and if necessary, to provide a new dynamic inflow model for autorotative rotors. The contemporary dynamic inflow modelling is reviewed in detail from first principles in this thesis, and this identifies a modification to the mass-flow parameter for autorotative rotors. A qualitative assessment of this change indicates that it is likely to have a negligible impact on the trim state of rotorcraft in autorotation, but a significant effect on the dynamic inflow models in certain flight conditions. In addition, this thesis includes a discussion about the small wake skew angle assumption, which is invariably used in the derivation of Peters and He model. The mathematical validity of the assumption is cast doubt, despite the resultant model has experimentally been fully validated. The author discusses on a theoretical ground the possible reason why the Peters and He model works well in spite of its inconsistent derivation