Development of A Cowling With Cooling Ports For Co-Axial Propeller Operation At Low-Reynolds Number Conditions

This paper presents the development of a cowling with cooling ports for electric motors to evaluate performance of a co-axial propeller as tested on a dynamometer at low-Reynolds number conditions in a wind tunnel. Co-axial propellers produce more thrust than a single propeller with the same diameter. A cowling shields the complex motor hardware required for co-axial operation, preventing viscous flow interactions losses. Although drag associated with the cooling ports is expected to increase, the overall drag of the assembly is expected to be lower with the cowling installed. The cooling ports are intended to reduce electrical component temperature to operate at a higher efficiency. Cowling internal cooling effectiveness is expected to be reduced at low-Reynolds number operation because the external flow at this scale is less likely to pass through the cooling ports due to viscous effects overcoming flow momentum. A numerical study was conducted to help visualize the flow around the cowling and the cooling ports. Due to its high complexity, the internal hardware of the co-axial rig was not modelled. Instead, the focus of the numerical study is on the external flow interactions and inflow at the cooling ports, not the complex flow pattern within the cowling. For the experimental study, two co-axial 18-in propellers are driven by two electric motors attached to a dynamometer in a 3-ft by 3-ft wind tunnel test section. The experiment includes the measurement of propeller thrust, torque, and rotational speed, as well as cowling exit air temperature and motor voltage and current. These measurements will determine the mechanical and electrical power, cooling effectiveness and thrust loss to the co-axial motor assembly drag. Tests were conducted at a range of wind tunnel speeds from 0-ft/s to 50-ft/s, and 0 RPM to 7000 RPM and with different cooling port geometries. Results show cooling ports decrease the temperature of the air coming out of the cowling and decrease drag of the test rig. Observations from this study indicate cooling port B is the most effective at cooling the internal hardware with a limited increase to total drag. This study will help unmanned aircraft designers balance cooling effectiveness with cowling drag at low-Reynolds number.