# Validation of a Wind Tunnel Propeller Dynamometer for Group 2 Unmanned Aircraft
###### Abstract  
This paper presents an approach to validate a wind tunnel propeller dynamometer applicable to Group 2 unmanned aircraft. The intended use of such a dynamometer is to characterize propellers over a relevant range of sizes and operating conditions, under which such propellers are susceptible to low-Reynolds-number effects that can be challenging to experimentally detect in a wind tunnel. Even though uncertainty analysis may inspire confidence in dynamometer data, it is possible that a dynamometer design or experimental arrangement (e.g., configuration and instrumentation) is not able to detect significant propeller characteristics and may even impart artifacts in the results. The validation method proposed here compares analytical results from Blade Element Momentum Theory (BEMT) to experimental data to verify that a dynamometer captures basic propeller physics, as well as self-similar experimental results to verify that a dynamometer is able to resolve differences in propeller diameter and pitch. Two studies were conducted to verify that dynamometer experimental data match the performance predicted by BEMT. The first study considered three propellers with the same 18-inch (0.457 m) diameter and varied pitch from 10 to 14 inches (0.254 to 0.356 m). The second study held pitch constant and varied diameter from 14 to 18 inches (0.356 to 0.457 m). During testing, wind tunnel speeds ranged from 25 ft/s to 50 ft/s (7.62 to 15.24 m/s), and propeller rotational speeds varied from 1500 to 5500 revolutions per minute (RPM). Analytical results from a BEMT code were compared to available experimental data from previous work to show proper application of the code to predict performance. Dynamometer experimental results for thrust coefficient and propeller efficiency were then compared to BEMT results. Experimental results were consistent with the expected effect of varying pitch and diameter and were in close agreement with BEMT predictions, lending confidence that the dynamometer performed as expected and is dependable for future data collection efforts. The method used in this study is recommended for validating wind tunnel propeller dynamometers, especially for Group 2 unmanned aircraft, to ensure reliable performance data. propeller; propulsion; UAS; dynamometer; thrust; pitch; torque; wind tunnel +
Footnote †: journal: Applied Sciences  
flights based on the risk on a risk-management framework proposed under the upcoming Modernization of Special Airworthiness Certificates (MOSAIC) rule-set. As UAS operations expand and businesses are growing to meet customer demand, the need to optimize mission performance becomes paramount to efficiently and profitably provide services. Propulsion system optimization is among the many areas a designer considers. Group 2 UAS are based on medium-sized unmanned aerial vehicles (UAVs), with 21-55 lbs maximum take-off weight and flying lower than a 3500-foot operational ceiling and under 250 knots cruise airspeed, according to the US Department of Defense. Among Group 2 UAS, common power plants include low-cost electric motors and internal combustion engines. These devices convert stored energy to propulsion with a simple fixed-pitch propeller, as weight and cost constraints most often preclude the use of variable-pitch propeller options. The use of fixed-pitch propellers requires a compromise between climb and cruise performance. Thus, accurately understanding propeller performance is an important factor contributing to the operating envelope and mission capability of the vehicle. Early in the design cycle for a new platform, performance estimates are developed using low-order models to take advantage of the ability to perform rapid design iterations and mission performance evaluations. As the design matures, an increase in fidelity of the estimates is desired. Commonly, models such as Blade Element Momentum Theory (BEMT) are applied in early design stages to predict propeller performance.