The Impact of Gust Load Design Criteria on Vehicle Structural Weight for a Persistent Surveillance Platform

This paper introduces a methodology for structural mass optimization of High-Altitude Long Endurance (HALE) aircraft across a complete mission profile, tailored for use in preliminary design. A conceptual HALE vehicle and its mission profile are assumed for this study, which also evaluates the impact of risk-based design decisions on optimized mass. The research incorporates a coupled aeroelastic solver and a mass optimization algorithm based on classical laminate theory to construct a geometrically accurate spar model. A novel approach is proposed to minimize the spar mass of the aircraft throughout the mission profile. This algorithm is applied to a representative T-Tail HALE model to compare optimized mass between two mission profiles differing in turbulence exceedance levels during the ascent and descent mission stages, while maintaining the same design robustness for on-station operation. Sample numerical results reveal a 10.9% reduction in structural mass for the mission profile with lower turbulence robustness design criteria applied for ascent and descent mission phases. The significant mass savings revealed in the optimization framework allow for a trade-off analysis between robustness to turbulence impacts and critical HALE platform parameters such as empty weight. The reduced empty vehicle weight, while beneficial to vehicle performance metrics, may be realized but comes with the added safety of flight risk unless turbulent conditions can be avoided during ascent and descent through risk mitigation strategies employed by operators. The optimization framework developed can be incorporated into system engineering tools that evaluate mission effectiveness, vehicle performance, vehicle risk of loss, and system availability over a desired operating area subject to environmental conditions.