Keywords: macro-fibre composite, helicopter rotor blade, active twist, finite element method, optimal design.

Helicopter 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 a 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 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 an orientation at 45^o to the blade axis to maximise the shear deformations in the laminated skin producing a distributed twisting moment along the blade.

A number of theoretical and experimental studies have been performed to estimate an active twist of helicopter rotor blades required to effect noise and vibration reduction benefits, as well as to improve the overall performances of helicopters [1,2,3,4]. However, most of them do not include optimisation due to complexity and large dimensions of the corresponding numerical problems to be solved. For this reason the present investigations are devoted to the methodology development for the optimum placements of actuators in helicopter rotor blades.

An investigated helicopter rotor blade is equipped with a NACA23012 airfoil and has a rectangular shape with active part radius 1.56 m and chord length 0.121 m. This rotor blade consists of C-spar made of unidirectional GFRP, skin made of GFRP, foam core, MFC actuators embedded into the skin and balance weight. To investigate an active twist of the helicopter rotor blade, the steady-state thermal analysis has been developed. In this case thermal strain analogy between piezoelectric strains and thermally induced strains is used to model piezoelectric effects. The 3D finite element model of the rotor blade has been built by ANSYS, where the rotor blade skin and spar "moustaches" are modelled using the linear layered structural shell elements SHELL99, and the spar and foam using the 3D 20-node structural solid elements SOLID186. The node-offset option is applied for the joint skin-spar "moustaches" structure with a location of the finite element nodes at the top surface to preserve the rotor blade profile. Two design solutions for an application of active materials (Figure 1) have been studied to estimate their effectiveness.

The optimisation problem for an active twist of helicopter rotor blades has been formulated for the results of a parametric study using the finite element method. Due to large dimensions of the numerical problem 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 the blade spar geometry, skin lay-up, position and size of actuators are chosen. D-optimal experimental design is formulated for 4 design parameters and 30 experiments. Approximations of the original functions for behaviour constrains and objective function are obtained using low order polynomials with some eliminated points. Minimisation problems are solved by the method of random search employing the approximating functions instead of original functions. The optimisation results have been obtained for two design solutions and checked by the finite element calculations.

From optimisation results it is seen that the Approach 2 is more effective in comparison with the Approach 1 for the maximal torsion angle can be reached and voltage applied. Unfortunately it has more dispersed centre of gravity and elastic axis, and greater rotor blade strains. A designer can find a compromise between the necessary solutions using optimal results obtained.

1

J.P. Rodgers, N.W. Hagood, "Design, manufacture and testing of an integral twist-actuated rotor blade", in "Proceedings of the 8th International Conference on Adaptive Structures and Technology", 1997.

2

A. Büter, E. Breitbach, "Adaptive blade twist - calculations and experimental results", Aerospace Science Technology, 4, 309-319, 2000. doi:10.1016/S1270-9638(00)00134-6

3

V. Giurgiutiu, "Active-materials induced-strain actuation for aeroelastic vibration control", The Shock and Vibration Digest, 32/5, 355-368, 2000. doi:10.1177/058310240003200501

4

P. Masarati, M. Morandini, J. Riemenschneider, P. Wierach, S. Gluhih, E. Barkanov, "Optimal design of an active twist 1:2.5 scale rotor blade", in "Proceedings of the 31st European Rotorcraft Forum", 37.1-37.14, 2005.

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