# Validation of a Wind Tunnel Propeller Dynamometer for Group 2 Unmanned Aircraft
## 5 Conclusions  
Group 2 unmanned aircraft represent a large and continually growing segment of aerospace operations and businesses that demand optimal mission performance enabled by propulsion systems. It is critical that reliable experimental propeller performance data are available to UAS designers and mission planners, especially when progressing from low-order models to validated, higher-fidelity estimates. Wind tunnel propeller dynamometer designs have been well-documented for a range of propeller sizes, and the principles have been applied to the dynamometer design in this study. However, it is important to have a proper method to validate dynamometer performance, which is particularly challenging for propellers at the low-Reynolds-number operating conditions often associated with Group 2 UAS. Because there is a lack of validated wind tunnel performance data for this particular scale, an approach to validating such wind tunnel propeller dynamometers is presented here. The method includes using BEMT code and experimental results to authenticate a dynamometer. The proper application of the BEMT code was shown by comparing results to existing propeller data of a smaller scale (14 \(\times\) 12E), revealing less than 10% difference between BEMT and experimental results over a range of advance ratios from 0.3 to 0.6. The BEMT code was then applied to larger-scale propellers to predict performance with a wind tunnel dynamometer at airspeeds relevant to Group 2 UAS. BEMT and experimental results were in good agreement, particularly for thin electric propellers, up to advance ratios of about 0.6, above which Reynolds number effects become problematic such that BEMT predicted propeller efficiency increases as thrust coefficient approaches zero. The validation method proposed here also involved experimentally demonstrating expected effects of propeller diameter and pitch. Results showed that a reliable dynamometer should resolve that peak efficiency occurs at higher advance ratios with increasing pitch, showing peak efficiency at about a 25% higher advance ratio when increasing pitch from an 18 \(\times\) 10E to 18 \(\times\) 12E propeller. This effect was particularly noticeable for thin electric propellers. Peak efficiency also shifts to lower advance ratios as propeller diameter decreases. Peak efficiency occured at a 10% lower advance ratio from an 18 \(\times\) 12E to 16 \(\times\) 12E propeller. Furthermore, a dynamometer should be able to resolve differences in propeller type, as shown by results for thin electric and sport propellers, particularly apparent when comparing 18 \(\times\) 12E thin electric propeller results to those of an 18 \(\times\) 14 sport propeller. Use of the method presented here is recommended for validating wind tunnel propeller dynamometers for Group 2 UAS. It is important to apply it to advance ratios between about 0.3 and 0.6 to ensure reliable propeller performance data. A validated dynamometer should  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Speed (ft/s)** & \(\mathbf{C_{T}}\) & \(\boldsymbol{\eta_{P}}\) & \(\mathbf{C_{T}}\) Error & \(\boldsymbol{\eta_{P}}\) Error & \% \(\mathbf{C_{T}}\) Error & \% \(\boldsymbol{\eta_{P}}\) Error \\ \hline
25 & 0.0744 & 0.5523 & 0.0006 & 0.0029 & 0.7812 & 0.5255 \\ \hline
30 & 0.0678 & 0.6147 & 0.0009 &produce thrust coefficient and propeller efficiency results within 10% of the results from BEMT analysis. Furthermore, a validated dynamometer should be able to resolve performance effects associated with varying propeller diameter and pitch, as well as propeller type. Future work related to this study is recommended to show the effects of novel flow control methods to mitigate degraded propeller performance due to low-Reynolds-number operating conditions. Results from such a study will be enabled with a validated propeller dynamometer. M.J.--Primary author, developer of BEMT code, experimental data post-processing. K.R.--Developed propeller dyno, writing and editing. R.P.--Writing and editing, assisting with BEMT code.