Aerodynamic investigation of rotor / propeller interactions on a fast rotorcraft

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Boisard, R.
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During the past few years, different concepts of fast rotorcraft have appeared. Most of them rely on additional propellers (usually one or two of them) to ensure the propulsive force at high speed in order to be able to slow down the main rotor rotating speed. On such configuration, the propellers are in strong interaction with the main rotor wake which affects their performances and the aircraft maneuverability. The present work numerically investigates the aerodynamic of the rotor / propeller interaction on rotorcraft similar to the Racer from Airbus Helicopters. By using two different levels of modeling it is shown that at high advance ratio, a simple free wake model is perfectly able to give most of the interaction effects, while in hover, a full CFD unsteady computation may be necessary to capture all the unsteadiness of the interaction. This paper also outlines the different behavior of the propeller while it is fully inside the rotor wake or out of if, and therefore the need for a precise control of the rotorcraft in the transition between hover to fast forward flight.