Mission Based Design Optimization of Fixed Pitch Coaxial Propeller System for VTOL UAV

This paper investigates the mission based propeller design optimization methodology for coaxial rotors for vertical take-off and landing (VTOL) vehicles. The modified blade element momentum theory is used for blade performance prediction, which is first validated using the experimental data available in literature. Each segment has to be bound by some lower and upper bounds and it has to satisfy the design constraints specified according to the requirements for the mission. The mission considered in this paper involves a standard package delivery mission involving, vertical take-off, cruise in forward flight, drop-off in hover mode and return to origin in cruise mode. The optimization is carried out by a genetic algorithm, which is a variant of Nondominated Sorting Genetic Algorithm (NSGA -II). The performance of the optimal blade designed for a specific mission is evaluated for a range of different missions to evaluate whether the design generalizes well over a range of missions. The optimized blade profile is curve fitted and smoothened out using splines and polynomial curves. Then the performance of optimal blade is evaluated against available off-the-shelf propellers. The results indicate that the power required by using off the shelf propellers for a mission involving 100 s of hover and 500 s of cruise to be as high as 41% greater than the power required by the optimized coaxial rotor system. The optimal coaxial propeller system is observed to have different geometries for both upper and lower rotors.