In time of helicopter flight, rotor blades produce significant vibration and noise as a result of variations in rotor blade aerodynamic loads with blade azimuth angle. For this reason future helicopters need to be improved with respect to environmental and public acceptance. Significant vibration and noise reduction can be achieved without the need for complex mechanisms in the rotating system using active twist control of helicopter rotor blades by an application of the macro-fibre composite (MFC) actuators. In this case MFC actuators are implemented in the form of active plies within the composite skin of the rotor blade with orientation at 450 to the blade axis to maximize the shear deformations in the laminated skin producing a distributed twisting moment along the blade. The present investigations are devoted to the comparative study of optimal active twist solutions obtained for the helicopter rotor blades with C and D-spars. The baseline helicopter rotor blades consist of C or D-spars made of uni-directional GFRP, skin made of +450/-450 GFRP, foam core, MFC actuators embedded into the skin and balance weight. To investigate an active twist of the helicopter rotor blades, the steady-state thermal analysis using 3D finite element models has been developed. In this case thermal strain analogy between piezoelectric strains and thermally induced strains is used to model piezoelectric effects. 3D finite element models of the rotor blades have been built by ANSYS, where rotor blade skin and spar thin parts are modelled by the linear layered structural shell elements SHELL99, and spar and foam by 3D 20-node structural solid elements SOLID186. The node-offset option is applied for the joint skin-thin spar structure with location of finite element nodes at the top surface to preserve the rotor blade profiles. Optimisation problems for the optimum placement of actuators in different helicopter rotor blades have been formulated on the results of parametric study using the finite element method. Due to large dimension of the numerical problems to be solved, an optimisation methodology is developed employing the method of experimental design and response surface technique. As the design parameters, the values characterising blades spar geometry, skin lay-up, position and size of actuators are chosen. The non-linear optimisation problems are solved by the random search method taking into account the producers requirements on location of centre of gravity and elastic axis, cross-section mass, first torsion eigenfrequency and admissible strains. The optimisation results have been obtained for two helicopter rotor blades and two possible applications of active materials. Comparative study of optimal design solutions for helicopter rotor blades with C and D-spars is presented for a designer convenience.