C OAXIAL rotor configuration is one of the technological solutions for increasing helicopter forward speed, maneuverability, and load-carrying ability. Since two rotors produce the net thrust instead of a single rotor in the conventional design, the diameter of the rotors can be reduced to carry the same amount of weight. Themost attractive feature of a coaxial design is the resulting compactness and safety of the vehicle. Second, the antitorsional moment generated by the two rotors would be canceled due to the opposite rotational directions, and the tail rotor and tail boom can be eliminated, resulting in a smaller and lighter vehicle. Compared to the individual-rotor helicopter, the aerodynamics and flow physics of coaxial rotor helicopter are relatively less studied and understood. The method of slipstream theory was first used to study the coaxial rotor, and then the predicted wake vortex model [1,2], free wake vortex model [3], and vorticity transport model [4,5] were applied to the coaxial rotor analysis. All the methods above carry out some approximations to the rotor, and the detailed flow feature near the blades and blade vortex in the wake cannot be simulated. Recently, the unsteady Navier–Stokes equations were used to simulate the coaxial rotor flows [6–8] to provide a detailed understanding of the flow physics of coaxial rotor. In the present Note, the three-dimensional unsteady Euler equations were solved to simulate the unsteady flows around coaxial rotor helicopter and individual-rotor helicopter in hover based on the unstructured dynamic overset grids. The aerodynamic interaction property of the coaxial rotor helicopter in hover and the effect of fuselage on the unsteady performance were studied.