  This study deals with the optimisation of hybrid composite drive shafts
operating at subcritical or supercritical speeds, using a genetic algorithm. A
formulation for the flexural vibrations of a composite drive shaft mounted on
viscoelastic supports including shear effects is developed. In particular, an
analytic stability criterion is developed to ensure the integrity of the system
in the supercritical regime. Then it is shown that the torsional strength can
be computed with the maximum stress criterion. A shell method is developed for
computing drive shaft torsional buckling. The optimisation of a helicopter tail
rotor driveline is then performed. In particular, original hybrid shafts
consisting of high-modulus and high-strength carbon fibre reinforced epoxy
plies were studied. The solutions obtained using the method presented here made
it possible to greatly decrease the number of shafts and the weight of the
driveline under subcritical conditions, and even more under supercritical
conditions. This study yielded some general rules for designing an optimum
composite shaft without any need for optimisation algorithms.
