Understanding Upward Scalability of Cycloidal Rotors for Large-Scale UAS Applications

Abstract

This paper investigates the upward scalability of a cycloidal rotor (also known as a cyclorotor) from an aeromechanics standpoint while utilizing a two-dimensional computational fluid dynamics (CFD) solver and a lower order aeroelastic model. The CFD results show that the nondimensional thrust remains almost unchanged with increasing Reynolds number, while the nondimensional torque and power decrease significantly from Re=104 to 105, which clearly shows that the cycloidal rotor scales up favorably from thrust production and aerodynamic efficiency standpoints. The structural scalability study shows that as the cyclorotor size is increased, the blade weight per unit thrust remains constant; however, the blade stress increases monotonically if the rotor geometry is kept similar. This monotonic increase in the blade stress is found to be independent of the blade structural design. To bound the blade stress with increasing size, the diameter of the cyclorotor needs to be increased at a faster rate compared to the blade span, which reduces the rotor aspect ratio (blade-span/rotor-diameter). Proper scaling laws necessary to bound the blade stress are formulated. Utilizing these insights, an optimization framework based on a genetic algorithm is developed to determine optimal cyclorotor configurations for a thrust range from 1 to 1000 lb.