# Validation of a Wind Tunnel Propeller Dynamometer for Group 2 Unmanned Aircraft
### Previous Dynamometer Work  
There are many existing wind tunnel propeller dynamometers, which can generally be categorized by scale and configuration. Small-scale dynamometers are typically used to evaluate propellers with up to about 10-inch diameter, and include those at the Universityof Illinois at Urbana-Champaign (UIUC) and Ohio State University ([9; 10; 11; 12; 13; 14]). [PERSON] and [PERSON] ([9; 10]) and [PERSON] et al. ([11; 12]) noted the effect of low-Reynolds-number operation on such propellers from a wind tunnel propeller dynamometer, and [PERSON] et al. ([13]) reported the performance of small folding propellers. [PERSON] and [PERSON] ([14]) compared blade element momentum (BEM) modeling results with wind tunnel experimental data for small propellers operating at low Reynolds numbers. [PERSON] et al. ([15]) evaluated small UAS propellers designed for minimum induced drag using a wind tunnel propeller dynamometer at the United States Air Force Academy. [PERSON] and [PERSON] ([16]) described automatic dynamic propeller testing at low Reynolds numbers and designed a dynamometer. [PERSON] and [PERSON] ([17]) described the design of a wind tunnel propeller dynamometer at Oklahoma State University (OSU) for evaluating a jet-blowing flow controller on small propellers to suppress boundary layer separation. Figure 1 shows the previous OSU wind tunnel propeller dynamometer design described by [PERSON] and [PERSON] to evaluate a modified 10-inch diameter electric propeller. [PERSON] ([18]) presented a method for validating a mobile propeller dynamometer for UAS applications; however, there has not otherwise been previous work on a method to validate a wind tunnel propeller dynamometer for Group 2 UAS applications.  
Examples of large wind tunnel propeller dynamometers are typically found in government and industry. [PERSON] et al. ([19]) described a dynamometer used in the United Technologies Technology Research Center: a 10 ft by 15 ft large subsonic wind tunnel used to evaluate an advanced ducted propeller. National Aeronautics and Space Administration (NASA) facilities have been previous described, including a 2000 hp dynamometer at NASA Langley used in a 16 ft, high-speed wind tunnel ([20]), shown in Figure 2, and a 1000 hp dynamometer at NASA Ames used in a 12 ft wind tunnel ([21]), shown in Figure 3. The propeller diameters used in these NASA facilities range from 4 ft to 10 ft and are roughly one half to one third of the test section size. Further, the propellers are located between one half to two diameters ahead of the vertical strut. In order to collect credible propeller performance data, it is important for wind tunnel dynamometers to be designed to reduce fluid-structure interaction between the propeller flow-field and the wind tunnel test section and dynamometer vertical support.  
Figure 1: Previous OSU dynamometer for propeller flow control.  
Dynamometer configurations generally can be classified by the means by which they measure thrust and torque. Thrust is typically measured with a linear load cell that is inline or offset from the propeller shaft, or in a moment arm arrangement. The aforementioned OSU dynamometer includes a linear, offset load cell for measuring thrust, which requires accounting for moment created by the offset distance. Alternatively, the NASA dynamometer in Figure 2 includes inline thrust measurement with a pneumatic thrust capsule. Torque is also typically measured inline or by using a moment arm arrangement. Figure 1 shows an example of an inline torque meter integrated into the previous OSU dynamometer, whereas the NASA Langley dynamometer includes torque arms for taking measurements with a moment. The advantages and disadvantages of these measurement approaches are discussed later in the design rationale for our proposed dynamometer.