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In order to solve the trim problem of computational fluid dynamics (CFD) simulation for rigid coaxial rotor, a hybrid trim model coupling CFD method and high-efficiency model (blade element theory) is established. In the trim process, the Jacobian matrix is solved by the high-efficiency model, while the CFD solver is only called for rotor performances modifying after each trimming step. The influences of pseudo time step, number of CFD revolution, and inflow model are investigated. Validation cases of AH-1G and Harrington-1 rotors are carried out, and good agreements are obtained. Results show that the trim efficiency can be significantly improved by saving the calculation of CFD for the Jacobian matrix. The trim accuracy is guaranteed by the correction operation with the CFD solver at each step. Moreover, as only time-averaged rotor performances are useful for trim, the efficiency can be further improved by adopting appropriately small pseudo time step and CFD revolution. The hybrid trim model has high robustness. The accuracy of inflow model for the coaxial rotor affects the convergence speed, but the final convergence can be achieved generally. [ABSTRACT FROM AUTHOR]

A numerical method based on Reynolds Averaged Navier–Stokes (RANS) equations and a high-efficiency trim model is developed to simulate the aerodynamics of a coaxial rotor. Farassat 1A equations are used for the prediction of thickness and loading noise. Hover cases of different thrust coefficients with torque balance are conducted. The sound pressure history of different observation points positioned below the rotor disk plane is analyzed. Results indicate that the special noise characteristics of the coaxial rotor are mainly caused by the noise superposition of the twin rotors and the unsteady loads of aerodynamic interaction. A new kind of impulsive loading noise is induced by the blade-meeting interaction. In contract to the single rotor, the loading noise of a coaxial rotor has a much larger sound pressure level in the high-frequency band. The loading noise is obviously enhanced around the blade-meeting azimuths. The maximum noise of the coaxial rotor is located immediately below the rotor disk center, while for the single rotor, it is the minimum location. [ABSTRACT FROM AUTHOR]