Journal of Aeronautics, Astronautics and Aviation, Vol. 51, No. 2, pp. 201 – 212 (2019) 201 DOI: 10.6125/JoAAA.201906_51(2).05 Effect of Skin and Spar Laminate Orientations on Flutter of Composite UAV Wing * Nurul-Zubaidah Zaki ** , Fareed A. Mazaha, Ainullotfi Abdul-Latif, Shuhaimi Mansor, Mastura Ab Wahid, Md. Nizam Dahalan, Norazila Othman, Shabudin B. Mat, and Mohd Nazri Mohd Nasir Aeronautics Laboratory, School of Mechanical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia. ABSTRACT This paper presents an analytical study on the optimisation of composite laminate orientation lay-up to achieve the maximum flutter with minimum weight penalty. The study is carried out by using UTM CAMAR UAV swept back wing as a case study where the wing consists of two spars located at 35% and 55% of the wing chord length. The laminate lay-up of the spar and the wing skin are optimised by considering different variations of laminate ply orientation while the number of plies for each part are set to be constant throughout the work. The finite element analysis software, Abaqus, was used to obtain the structural natural frequencies for bending and torsion modes. The bending stiffness, torsion stiffness and the eigenvalues of the aeroelasticity equation of motion are computed using the mathematical software, MATLAB. The maximum flutter speed obtained from the study is 238.93 m/s with laminate configuration at [45°/-45°/45°/-45°] for wing skin, [45°/-45°/45°] for forward spar, [45°/-45°]s for aft spar and [0°/0°/0°/0°] for the wing tip. Keywords: Flutter speed, Composite wing, Dynamic aeroelasticity, Composite laminates, Fibre orientation I. INTRODUCTION Flutter is the product of interaction between elastic, aerodynamic and inertial forces and is associated with two or more modes of motion such as bending and torsion, and can cause catastrophic failure [1]. This phenomenon usually occur on structures with high aerodynamic loadings such as wings, tails and control surfaces. The critical flutter speed is one of the important parameters that need to be considered in aircraft designing process as the oscillations of the undamped vibrations may be amplified beyond the critical speed. One of the factors influencing flutter speed is structural stiffness which is influenced by the structural shape, design and materials [2]. For the past 40 years, researchers have been trying to investigate and study composite materials application in the aircraft structure due to its high specific strength to weight ratio. The material also offers a great advantage to * Manuscript received, February 1, 2018, final revision, April 15, 2019. ** To whom correspondence should be addressed, E-mail: nzubaidah2@live.utm.my tackle dynamic and aeroelastic problems such as flutter and divergence [3-7]. Composite material stiffness and strength can be tailored by altering the fibre orientations and number of laminate plies [8-10]. Also, properties such as laminate fibre orientations and elastic modulus ratio have significant effects on the limit cycle oscillation and flutter speed [1]. In the case of real aircraft wings, the structures are usually composed of internal components such as ribs, spars, stringers and frames which act as stiffeners for the wing skin. Wing spars can be altered in order to control aeroelastic behaviours such as bending and torsion deflections [11]. According to Zaki [1], the location of spar in the wing gives a significant effect on the flutter speed and bending stiffness. In the study, the maximum flutter speed is achieved when the front and the rear spars are located at 0.35 and 0.55 of chord length respectively for the swept-back composite wing.