Composite Structural Analysis of a High Altitude, Solar Powered Unmanned Aerial Vehicle

Abstract

In this paper, a development of a low order composite structure module has been introduced. This module can design the wing structure for the given aerodynamic load. The wing structure is broken down into non-spar elements and spars. The weight of non-spar elements is estimated by using empirical equations that were used by NASA for solar powered high altitude UAVs. The Spar is sized by using a numerical approach, which is developed in this paper. The spar is modelled as a composite rectangular wing-box and assumed to withstand the entire load with no contribution from the secondary wing components. The required numbers of laminate on each side of the spar are found iteratively until no failure or buckling is detected. The orientation of laminate of each side of spar was inspired by the existing high altitude aircraft structure. A linear finite beam element is used to evaluate the wing-box deflection under the internal and the aerodynamic loads while only a quasi-static equilibrium is considered during the sizing process. The module has been written in MATLAB. This tool can be used either in the conceptual design stage or in an optimisation process because it facilitates rapid computation. This module has been validated with a high order commercial package (ANSYS). The deflection calculation shows excellent agreement with less than 0.25 % error. The stress calculations show a reasonable agreement with ANSYS with maximum error margin of about 4% at the maximum shear stress level. However, this amount of error could be unimportant as a high factor of safety is usually taken in the design of composite structures. The weight prediction function also has been validated using reference to a NASA Pathfinder aircraft. The predicted weight seems reasonable with a 1.6 % difference from the expected weight of the case study.