In the context of the development of high speed compound helicopters, the main rotor may not be an efficient propulsive device at high speeds and a propeller has to be added in order to reach high advancing velocities. On such configuration, at low speed, the propellers are in strong interactions with the main rotor wake which affects their performance and aircraft maneuverability. The present work numerically investigates the aerodynamics of the rotor / propeller interaction on rotorcraft similar to the Racer from Airbus Helicopters. Through the comparison of two different levels of modeling it is shown that at high advance ratio, a simple free wake model is suitable to give most of the interaction effects, while in hover, a full CFD unsteady computation is necessary to precisely capture all the unsteadiness of the interaction. By comparing two different CFD solvers it also demonstrates that the results are highly sensitive to the choosen numerical setup. This paper also outlines the different behaviors of the propeller when it is fully inside the rotor wake or out of it, and therefore the need for a precise control of the rotorcraft in the transition between hover to fast forward flight.