Evaluation of Sizing Strategies for eVTOL UAV Configurations

Abstract

eVTOL sizing is tackled with a Multi-Disciplinary Optimization problem with nonlinear constraints in this work, focusing on UAVs with distributed vertical lift. Several optimization schemes are investigated for including airframe sizing with finite element analysis, vehicle trim, and blade aerodynamic shape design. The iterative weight convergence loop is replaced by a slack variable and equality constraint for the sizing optimizer. Airframe sizing and weight minimization (with stress inequality constraints) may be driven either by the sizing optimizer, or by a separate optimizer within the various constraint functions in a nested structure. It is preferable to drive the trim variables using the sizing optimizer; if a particular design cannot be trimmed, this information is propagated to the sizing optimizer through the corresponding equality constraints. The all-at once optimization strategy yields results in the shortest time compared to the other methods. Using modified momentum theory for rotor performance, all gradient-based optimizations from different starting points converged to the same minimum, indicating that the design space is convex for the chosen bounds and objective function. Blade shape design with BEMT is also included in the sizing, either directly with blade twist and taper as additional design variables, or indirectly through a response surface. The methodology is demonstrated on the sizing of two package delivery vehicle configurations (a quadrotor and a lift-augment quadrotor biplane tailsitter) for a mission with 10 km radius of action. The cruise airspeeds for the two configurations are also identified as part of the sizing/optimization. The quadrotor is more suited for this point mission owing to its lower empty weight compared to the quad-biplane, which has better cruise efficiency but higher empty weight.