Numerical simulation of unsteady flows around a complete helicopter was conducted, and the effect of rotor downwash on the behavior of free-flight rockets air-launched from the helicopter and their plume was investigated. For this purpose, a three-dimensional inviscid flow solver based on unstructured meshes was developed, and an overset mesh technique was adopted to handle the relative motion of the main rotor, tail rotor, fuselage, and traveling rockets. The flow solver was coupled with six-degrees-of-freedom equations of motion to describe the trajectory of the free-flight rockets. To validate the flow solver for simulating the plume flow from the rocket nozzle, calculations were made for a jet flow impinging on flat plates. To demonstrate the accuracy of the flow solver for predicting rotor downwash, a rotor-fuselage aerodynamic interaction flow was calculated, and the results were compared with available experimental data. The trajectory simulation of an external store released from a fixed wing was also performed to validate the present flow solver coupled with the six-degrees-of-freedom equations of motion. Then the present method was applied to the simulation of free-flight rockets air-launched from a complete helicopter configuration. It was found that rotor downwash has nonnegligible effects on the trajectory of the air-launched rockets and their plume development, which may potentially affect the safety and the reliability of other equipments installed on the mother helicopter.