Variable-Twist Propellers for Small Unmanned Aerial Systems

Small unmanned aerial systems are now often utilised in a wide range of military and civilian applications. However, propulsion technology development for these systems has lagged significantly. While most variable-pitch mechanisms are too heavy for small UAS, variable-twist propellers offer potentially significant performance benefits. One way to achieve twist variation is the use of composite tailoring to impart structural coupling in the propeller blades. In this thesis, a multi-disciplinary propeller analysis tool is developed, encompassing aerodynamic, structural and acoustic analyses, and optimisation of structurally-coupled variable-twist propellers conducted for small UAS. Aerodynamic modelling is provided by an unsteady free-wake panel method, and aeroelastic deformation predicted using a geometrically-nonlinear beam structural model. An implementation of Farassat 1A provides acoustic modelling. The propeller sectional shape, planform and internal structure are each parametrised, and a multi-objective optimisation framework implemented using the NSGA-II algorithm. Three case studies demonstrate the performance gains possible with extension-twist coupled propeller design. The first examines propeller optimisation for a small tactical UAS across minimum power and maximum climb-rate; and across minimum noise and maximum climb-rate. In both cases, extension-twist coupled propellers produce significantly higher climb-rates than non-coupled propellers, for equivalent power and noise levels. The second case study considers a small VTOL-capable tactical UAS. Propeller optimisation is conducted for power minimisation for both maximum endurance and at hover. For a given endurance power level, extension-twist coupled propellers exhibited lower power use at hover than non-structurally coupled designs. The same behaviour was observed in the third case study, with propeller optimisation for minimum power at both maximum range and hover for an urban-mobility UAS