PERFORMANCE OF ELECTRIC MEDIUM-SIZED VARIABLE-RPM ROTOR AND SHROUDED ROTOR

Abstract

Electric variable RPM rotors are increasingly being used for propulsion and control of unmanned air vehicles. As these vehicles scale to carry heavier payloads of 50 to 400 lbs (20 to 180 kgs) in the group 2 and 3 UAS category, there are concerns about their aerodynamic performance and handling quality degradation. Therefore, there is a need to develop a systematic experimental testing procedure to measure loads on these systems to evaluate performance and augment Computational Fluid Dynamic (CFD) validation tools. In this work, a universal electric powered test rig is designed and fabricated for hover and wind tunnel tests of open and shrouded rotors. Steady hover results are validated using blade element momentum theory. These predictions incorporate an empirical correction approach in conjunction with an interpolation scheme to capture Reynolds number variation along the span of the blade and variation with RPM. Results show good agreement with the interpolation method for the low Reynolds number rotor tested (Re_tip<500,000). For the variable RPM rotor, transient step and chirp inputs are also presented. System identification showed linear frequency responses between thrust and torque with RPM and RPM-square in hover. Therefore, when modeling this rotor, steady inflow appears adequate in the frequency range of interest (0.4 to 60 rad/sec). In addition to an open rotor, the electric motor-rotor test stand was used to test a shrouded rotor in hover and forward flight to systematically compare performance results. Test data showed the shrouded rotor gained 15% thrust for the same power in hover with the best configuration. For low speed forward flight, lift-to-drag ratio was found to increase by 8 to 10% for the shrouded rotor system over the isolated rotor.