Static optimisation of prismatic structures as applied to helicopter rotor blades
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.