Contributions to the Aerodynamic Optimization of a Coaxial Rotor System
Leishman, J. Gordon
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The present work analyses the aerodynamic complexities involved in the design of a coaxial rotor system in an attempt to maximize its performance in hover and forward flight. The aerodynamic methodologies of the simple momentum theory (SMT), the blade element momentum theory (BEMT), and a free vortex wake method (FVM) are used to help study this problem. It is shown that because of the inter-rotor aerodynamic interference effects, as well as the requirement of a torque balanced operating condition, the two rotors of the coaxial system generally operate at different thrust and different aerodynamic loadings. Therefore, for an optimally performing coaxial system, the geometric designs of the two rotors can expected to be different. To this end, parametric studies were performed to understand the effects of changes in inter-rotor spacings, blade twist, and blade planforms on both the upper and lower rotors. A more formal optimization was attempted by coupling FVM with an optimizer to find the best rotor geometry (if any) to maximize the figure of merit in hover or to minimize the total power required in forward flight. It was shown that the performance of the coaxial rotor system can, indeed, be improved significantly by having different blade geometries on the upper and lower rotors. However, it was also shown that the blade twist distribution has more significant effects on the rotor performance as compared to the blade planform shapes. The baseline geometry for all the optimization analyses had untwisted blades on both rotors. It was shown that a higher inter-rotor spacing is desired to reduce the interference effects between the two rotors in hovering flight. However, the spacing distance can be limited by the increased rotor weight and increased parasitic drag in forward flight. The results also show that a high blade twist is desired on the upper rotor to reduce the induced losses of the coaxial system, whereas a high blade twist on the bottom rotor increases the induced losses of the coaxial system. In forward flight, the results showed that at high advance ratios the aerodynamic interactions between the two rotors become smaller, and both rotors behave almost as isolated rotors. Parametric studies were also performed to study the effects of changing linear twist rates on both the rotors of the coaxial system in forward flight. The results showed that the total power required at an advance ratio of 0.25 is insensitive to the changes in the blade twist on upper and lower rotors. This outcome also showed that the optimum blade shapes obtained for hovering flight also offered better performance in forward flight.