Performance Evaluation of a Coaxial Propulsion System Using Test Bench Validation

This study presents the design, development, and experimental validation of a custom test bench for evaluating the aerodynamic performance of a counter-rotating coaxial propeller system as a function of inter-propeller axial spacing. The experimental campaign was conducted using 254 mm (10 in) GEMFAN 1050 three-blade propellers, representative of small-UAV-scale electric propulsion systems. While prior research has examined coaxial rotor interference effects, systematic experimental studies combining adjustable axial spacing, isolated-rotor baseline measurements, and performance characterization over a broad rotational-speed range remain comparatively limited. The test bench incorporates two BLDC motors instrumented with four strain-gauge load cells, a servo-actuated threaded rod mechanism enabling automated adjustment of rotor spacing, and an RP2040-based dual-core control system for synchronized data acquisition and motor control via DSHOT signaling. The structural frame combines extruded 6063 aluminum profiles, pultruded carbon fiber rods, and 3D-printed polymer components, validated through static and harmonic response finite element simulations. An analytical model based on blade element theory was developed to predict circulation, thrust, power, and induced velocity for each propeller. When evaluated over the experimental rotational-speed range, the analytical thrust prediction showed close agreement with the measured coaxial thrust in the medium-to-high-speed regime, with mean absolute percentage differences of approximately 1.9–2.5% above 5000 rpm across the five tested spacings. Experimental tests were conducted at five axial spacings (70, 90, 110, 130, and 150 mm) across a rotational speed range of approximately 1000–9000 RPM, comparing the coaxial configuration against the summed performance of the two motors operating isolated. Results show that thrust and power curves for the coaxial and isolated configurations remain nearly coincident up to 5000–6000 RPM, beyond which the coaxial system exhibits a measurable thrust deficit driven primarily by aerodynamic interference rather than increased power draw. Coaxial thrust ratios converged toward similar values (~0.83–0.85) at higher rotational speeds regardless of spacing, whereas the largest spacing-dependent differences were observed in the low-to-moderate rotational-speed regime. Because the relative uncertainty of the electrical current measurement increases substantially at low current, the low-throttle thrust-to-power measurements are interpreted as indicative trends rather than as a basis for quantitative ranking of the configurations. These findings provide experimental insight into spacing-dependent interference behavior and establish a validated, low-cost platform for future coaxial rotor characterization studies.