The aerodynamic performance of a reduced-scale coaxial rigid rotor system in hover and steady forward flights was experimentally investigated to gain insights into the effect of interference between upper and lower rotors and the influences of the advance ratio, shaft tilt angle and lift offset. The rotor system featured by 2 m-diameter, four-bladed upper and lower hingeless rotors and was installed in a coaxial rotor test rig. Experiments were conducted in the Φ3.2 m wind tunnel at China Aerodynamics Research and Development Center (CARDC). The rotor system was tested in hover states at collective pitches ranging from 0° to 13° and it was also tested in forward flights at advance ratios up to 0.6, with specific focus on the shaft tilt angle and lift offset sweeps. To ensure that the coaxial rotor was operating in a similar manner to that of the real flight, the torque difference was trimmed to zero in hover flight, whilst the constant lift coefficient was maintained in forward flight. An isolated single-rotor configuration test was also conducted with the same pitch angle setting in the coaxial rotor. The hover test results demonstrate that the figure of merit (FM) value of the lower rotor is lower than that of the upper rotor, and both are lower than that of the isolated single rotor. Moreover, the coaxial rotor configuration can contribute to better hover efficiency under the same blade loading coefficient (CT/σ). In forward flight, the effective lift-to-drag (L/De) ratio of the coaxial rigid rotor does not monotonously change as the advance ratio increases. Increases in the required power and drag in the case with a high advance ratio of 0.6 leads to the decreasing L/De ratio of the rotor. Meanwhile, the L/De ratio of the rotor is relatively high when the rotor shaft is tilted backward. The increasing lift offset tends to result in reduced required rotor power and an increase in the rotor drag. When the effect of the reduced rotor power is greater than that of the increased rotor drag, the L/De ratio increases as the lift offset increases. The L/De ratio can benefit significantly from lift offset at a high advance ratio, but it is much less influenced by lift offset at a low advance ratio. The forward performance efficiency of the upper rotor is poorer than that of the lower rotor, which is significantly different from the case in the hover flight.
Hub system of coaxial rigid rotor has a significant contribution to the total vehicle parasite drag. Prior efforts have explored various faring designs to reduce flow separation and interference drag between each components of hub system. The aerodynamic interference physics of rotor wake on the hub fairing system components is still need to be resolved to examine its practicability. A numerical investigation was carried out to predict the influence of rotor wake on hub drag for a coaxial rigid rotor. The investigation was based on the solution of RANS equations in three dimensions using unstructured grids and main rotor modelled by an actuator disc. Results are presented for baseline configuration and optimized lower drag configuration under various forward flight velocities. The analysis shows that the presence of rotor wake has remarkable interference on hub drag, the swirl and downwash component of rotor wake have significant interference effect on each parts of hub fairing system. With the increase of wind speed, interference effects have been altered by the movement of rotor wake. The additional rotor wake remarkably increases the hub system drag in low speed range, and makes the hub system drag slightly reduction at higher speed. The drag of optimized model is still lower than base model in the interference of rotor wake, reveals that the optimized technique which is to reshape the shaft fairing and pylon is effective in the presence of rotor wake.
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