  A numerical simulation method of computational fluid dynamics (CFD) coupling
with a trim analysis for coaxial rotor systems is described in this paper. The
trim analysis is implemented using a rotorcraft flow solver, rFlow3D. Six
target forces and moments, which are the thrust of the coaxial rotor system,
the rolling and pitching moments for each upper and lower rotor, and the torque
balance for yaw control, are considered as the trim conditions. The blade pitch
angles of both upper and lower rotors are adjusted to satisfy the target trim
conditions through the trim analysis by being loosely coupled with the CFD
solver. Verification of the trim analysis method and validation of the
prediction accuracy of aerodynamic performance are performed based on previous
experimental and numerical studies in hover and forward flight using the
lift-offset conditions. It is shown that the predicted hover performances of
the torque-balanced coaxial rotors are in excellent agreement with the
experimental data. It is also verified that the lift-offset conditions in
forward flight are simulated using this established trim analysis. Furthermore,
reasonable agreements with other computational results are indicated.
