Recent decades have seen the range of applications for aircraft expand to niches like weather monitoring, reconnaissance and satellite launch (Air Launch to Orbit, ALO). The requirements for such roles necessitate unconventional features like twin fuselages (in case of ALO mother ships) and very high aspect ratio wings (for long endurance missions). For conventional aircraft, aside from improving safety, the driving factors for new designs have always been increased efficiency and performance. The prevalent approach has been to reduce weight and induced drag by using higher aspect ratio wings. Consequently, newer designs are lighter, more flexible and closer the failure limit. Therefore, the interaction between flight loads and the airframe, i.e.\ aeroelasticity has become important for a safe design. High aspect ratio wings undergo large magnitude, low-strain deformations. An accurate understanding of their aeroelastic behaviour requires nonlinear analysis methods. This thesis implements a reliable nonlinear aeroelastic analysis method in NASTRAN. The aeroelastic analysis modules in commercial finite-element analysis software are aerodynamically and structurally linear. At the same time, such software often has advanced nonlinear structural analysis capabilities. This thesis modifies and combines two approaches mentioned in literature. NASTRAN's aeroelastic module is used to obtain rigid aerodynamic loads and its nonlinear structural module to obtain structural deformations. For a given angle of attack, altitude and airspeed a wing is analysed iteratively until the rigid air loads and structural deformations for successive iterations converge. The nonlinear structural module is used to obtain pre-stressed structural modes for this converged condition. These modes are then used in flutter analysis. An idealised HARW with arbitrary properties is analysed using this method and using TU Delft's in-house aeroelastic optimisation software: Proteus. Static aeroelastic deformations and flutter analysis results are compared for validation. Following this, the results of the linear and nonlinear aeroelastic analysis are compared. It is shown that linear analysis over-predicts the deformation in flexible wings and that the inclusion of pre-stress changes the damping of critical flutter modes; thereby changing the flutter point for the wing. For the test case analysed in this thesis, the inclusion of geometric nonlinearity and follower force effects resulted in large changes in the flutter speed with change in the angle of attack. In this way, the thesis successfully implements a nonlinear aeroelastic analysis method which can be used to improve aircraft designs during the preliminary stage.