On the optimum separation distance for minimum noise of contra-rotating rotors

Overlapping rotor systems are currently used on some urban flight vehicles and will become increasingly common in future electric propulsion, since they provide one of the most compact platform volumes per thrust. This paper presents the results of a detailed experimental investigation of co-axial contra-rotating rotor configuration at hover condition. A range of rotor diameters considered in the study are 14-18 inch and range of Tip Mach numbers investigated in the present study are 0.2. A detailed parametric study is performed to investigate the sensitivity of radiated noise to axial separation distance. An optimum separation distance z to rotor diameter D of 0.25 has been identified, whereby minimum radiated noise is obtained. The reason for this optimum has been explored and the balance between various interaction noise sources in co-axial configuration is discussed in the paper. It is also shown that the minimum radiated noise was roughly found where the aerodynamic efficiency is maximum.