Aerodynamic Performance Evaluation of Coaxial Propellers Using a Custom-Built Hover Test Stand

Coaxial propellers, with their innovative design comprising two superimposed propellers on top of each other, offer significant advantages in terms of high speed capability, maneuverability, and load-carrying capa- bility. These systems hold particular promise in the field of unmanned aerial vehicles (UAVs), where space and performance efficiency are critical. Despite the growing interest in coaxial propeller systems, the precise aerodynamic behavior of these configurations remains under-researched, particularly in terms of the complex interactions between the front and rear propellers. Coaxial propellers offer increased thrust while keeping a relatively low size of the propulsion system on the aircraft [1].Also, when the two propellers rotate in opposite directions, their torques cancel out, eliminating the need for a vertical stabilizer on a fixed-wing style aircraft, helping reduce overall aircraft drag, weight and power consumption. Previous studies have noted significant variations in thrust and power due to the aerodynamic interaction between the front and rear propellers, but few experimental setups have been able to systematically measure these effects. Recently, experiments have been performed for a high advance-ratio coaxial model rotor system in hover and also in a wind tunnel, and comprehensive analysis has been done for the same rotor system [2]. This research aims to bridge that gap by offering a highly precise and adaptable testing platform. One of the key features of the developed test rig is the independent control of each propeller in a coaxial system, as well as the independent measurement of thrust, torque, and power for each propeller, which provides valuable in- sights into aerodynamic interactions, efficiency optimization, and performance characterization under various operating conditions. The hover test stand developed in this work consists of three primary components: the rotating assembly, instrumentation for sensors, and a data acquisition system. The rotating assembly includes a coaxial rotor hub where the upper and lower propellers are independently driven to enable precise control. The propulsion sys- tem utilizes KDE Direct brushless motors with a power rating of 1.6 kW, supplied by a 2 kW power source, ensuring consistent power output and precise speed control. Customized parts are also designed and fabricated to attach the motor and load cell with the propeller and the test frame, respectively, ensuring secure and pre- cise mounting for accurate measurements. The motor control system includes an Electronic Speed Controller (ESC), which operates at a voltage range of 6-50V DC and supports a maximum continuous current of 35A. It provides precise motor control by regulating pulse width modulation (PWM) signals, ensuring stable operation under varying loads. A servo tester has also been used for manual ESC signal calibration and motor speed reg- ulation. It sends PWM signals to the motor, where the pulse width varies from 1000µs - 2000µs, representing 0% - 100% throttle. For example, a 1500µs pulse width sends 50% of the battery voltage to the motor, ensuring precise control of motor speed during testing. The instrumentation system includes two six-axis load cells, independently measuring axial thrust and torque for each propeller. The load limits are 2.4 kN for thrust and 20 Nm for torque. The rotational speed is recorded using two remote optical sensors and two separate tachometers. The tachometer can measure rota- tional speed up to 250,000 RPM. The data acquisition system consists of a USB Data Acquisition (DAQ) device with screw terminals, a DAQ interface power supply, and a TW transducer. The DAQ system records real-time aerodynamic force and power consumption measurements, allowing synchronized data collection of thrust, torque, and motor performance. The LabView 23.0 software runs on a lab computer to process and store acquired data efficiently. The power system includes a 12V, 2A, 24W wall-mount power supply to ensure stable operation. The test stand frame is built from aluminum 8020, measuring 7.75 ft in length and 4.75 ft in width. Given the vibration-induced noise from propeller imbalances, the frame is designed to be heavy and structurally robust, minimizing oscillations and ensuring precise force measurements. The effects of various design parameters on the performance of the front and rear propellers, along with their mutual aerodynamic interference, will be investigated. Propellers with different configurations and blade geometries will be tested at varying axial distances. More specifically, propellers with various blade counts, solidity, diameter, pitch, blade twist and taper, while operating over a diverse range of rotational speed will be tested. These experiments will reveal the nature of coaxial interference between front and rear propellers in terms of thrust production and power consumption and the effect of the above mentioned design parameters on these coaxial interference. The proposed study will be critical to understand the nature of aerodynamic interference in coaxial propeller system, which could lead to more efficient propulsion designs, reducing energy consumption and enhancing the flight performance of Unmanned Aerial Systems (UAS). Future work will involve development of a physics-informed machine learning (PIML) algorithm that can utilize these data to accurately predict performance of a coaxial propeller. References [1] Holzsager, Jonathan Elliot. The effects of coaxial propellers for the propulsion of multirotor systems. Rutgers The State University of New Jersey, School of Graduate Studies, 2017. [2] Cameron, Christopher, and Jayant Sirohi. ”Performance and loads of a model coaxial rotor part I: wind tunnel testing.” American Helicopter Society 72nd Annual Forum, West Palm Beach, FL. 2016.