Experimental investigations of the performance of an electric contra-rotating fan as a function of speed ratio

Contra-rotating ducted fans are a promising concept for expanding the design space of electric propulsion systems towards high subsonic flight speeds. However, experimental data for small-scale, contra-rotating stages remain limited, particularly under off-design conditions. This study addresses this gap by experimentally investigating the influence of rotor speed ratio adjustments on the off-design performance in terms of the stability behavior of such a small-scale stage, designed using an extended iterative and CFD-based tool chain. The experimental data are provided in the form of performance maps and radial flow measurements under design and off-design conditions. The results confirm that the design operating conditions, including a high stage total pressure ratio of 1.21, are achieved. This demonstrates that the iterative design approach is suitable for small-scale propulsion systems with pronounced three-dimensional effects, particularly secondary flows. Local flow-field measurements and CFD evaluations provide initial insights into mechanisms related to the stability limit and rotor-rotor interaction. Hub corner separation appears to increase with throttling and may contribute to extensive flow separation in the rear rotor. Selecting an appropriate rotor speed ratio seems to enable aerodynamic unloading of the stability-critical front rotor hub, thereby shifting the stage stability limit towards higher throttling levels. An increase in rear rotor speed by 20 % relative to the design speed ratio shifts the stability limit by approximately 5 % in normalized nozzle area, thereby extending the operational envelope of the propulsion system.