Keywords: composites, optimisation, linear programming, finite element analysis.

The concept of the uninhabited combat air vehicle (UCAV) exploits the operational possibilities presented by removing the pilot from the vehicle. The cost and weight associated with pilot support systems are eliminated as are the physiological limitations imposed by human tolerance to acceleration loading. The removal of these design constraints promotes the development of small, highly manoeuvrable airframes that may be optimised for the envisaged mission profiles. The high strength, stiffness and low weight requirements of the UCAV application encourages extensive use of composite materials. The high specific strength and stiffness properties coupled with the reduced safety margin enables significant weight reduction to be obtained whilst still satisfying performance requirements. The use of composite materials in the manufacture of primary components in high performance structural systems has demonstrated the potential of composites; the design flexibility provides a material that may, in principle, be tailored to deal with complex operating specifications very efficiently [1].

The UCAV-TD is a prototype research vehicle used to investigate the design possibilities associated with the UCAV concept. The vehicle has a tailless configuration and attitude control is effected by the forward canards and all-moving wings. The UCAV-TD is designed to operate at a maximum loading of 14G and at a maximum Mach number of 1.6.

The paper presents the results of a study aimed at investigating the conceptual design for the composite wings of the prototype UCAV-TD aircraft. The work was focussed on identifying an appropriate structural configuration as well as addressing the optimisation of the structural design of the wing when subjected to the typical aerodynamic pressure loading encountered at the upper edge of the operational envelope. The optimisation methodology used is based on the use of simplified material property data, approximation methods and the integration of sequential linear programming with finite element analysis [2]. Two structural design concepts were considered for the wings of the aircraft; the first being a conventional approach using a system of longitudinal ribs and transverse spars as shown in figure 63.1a, and the second being a more unconventional configuration using a radial system of spars as seen in figure 63.1b.

The results of the optimisation studies indicate that the multi-spar arrangement of the radial configuration is more efficient in limiting the deflection of the wing, thereby allowing for a reduction in wing skin thickness. The weights of the conventional and radial configurations as a function the location of the centre of pressure is shown in figure 63.2. As can be seen the optimal weight of the wing with conventional spar and rib construction is consistently heavier than for the radial configuration.

1

Strarnes, J. R. and Haftka, R. T., "Preliminary Design of Composite Wings for Buckling, Stress and Displacement Constraints", Journal of Aircraft, 16, 564-570, 1979. doi:10.2514/3.58565

2

Reklaitis, G. V., Ravindran, A. and Ragsdel, K. M., "Engineering Optimisation Methods and Applications", John Wiley and Sons, London, 1983.

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