This paper presents a experimental study on the design issues of long shrouded (L/D>1) coaxial propulsion system in hovering. It objects to explore the effects of different design variables, which include: shroud inlet radius and nozzle expansion ratio, rotor locations, rotational speed, rotor number and so on. Through applying the test bench with multiple possibilities of testing parameters and different combinations of shroud inlets and nozzles, the preliminary results show that the diverging nozzle limited by the flow separation and relative greater inlet radius are desired. Based on it, a shroud with a suitable combination of inlet and nozzle is chosen. The experiments on it reveal that: The system with double rotor at the upstream side of cylindrical section and with the second rotational speed higher than the first one performs best; compared to the shroud contribution, increasing rotor number doesn't improve the efficiency greatly
