# Validation of a Wind Tunnel Propeller Dynamometer for Group 2 Unmanned Aircraft
## 2 Materials and Methods  
### Propeller Dynamometer Design  
The scale of the dynamometer components is dictated by size of the wind tunnel test section (3 ft by 3 ft) and max propeller diameter (18 inch) such that the propeller diameter is half that of the wind tunnel (consistent with dynamometer designs noted in the previous work in Section 1.1). A typical highly loaded APC 18 in propeller is expected to draw about 4 kW of power at 6000 RPM and low airspeeds. Therefore, a 4 kW Magna-Power direct current (DC) power supply is selected.  
To avoid overloading the dynamometer motor, a 5 kW Great Planes Rimfire 50 cc electric motor is selected to drive the propeller. The dynamometer drive motor has a max voltage of 55 V, which is higher than the DC power supply's 32 V range, avoiding the potential for the supply to over-volt the drive system. The motor has a 230 kV rating, which limits max shaft speed to 7360 RPM at 32 V, well within the dyno motor limit of about 12,500 RPM.  
A Castle Creations Phoenix Edge 160 HV electronic speed controller (ESC) is selected, as its 50 V and 160 A range is greater than the DC power supply output. The ESC is placed outside the dynamometer cowling such that freestream air and propeller wake provide adequate cooling flow. The ESC receives a pulse-width modulation (PWM) throttle signal from a GT Power Professional Digital Servo Tester that is powered by a 7.4 V to 12 V DC input and provides a 4.8 V output. Table 1 includes a summary of the dynamometer electrical and instrumentation components.  
A Honeywell SS460S Hall-effect sensor is epoxied inside the motor to detect shaft speed. A Futek MBA500 torque and thrust bi-axial load cell is mounted between and inline with the drive motor and dynamometer horizontal support, using custom-designed and 3D-printed cowling components, as shown in Figures 7 and 8. The inline arrangement is an improvement over the previous OSU dynamometer design, minimizing the effect of vibrations that can be experienced with an offset, moment-arm arrangement. The load cell has a 50 lb thrust limit and 50 in-lb torque limit with an error of 0.25% of read-out.  
The dynamometer support structure is fabricated from 2 in by 2 in quad-rail, t-slot aluminum extrusion. The horizontal support is shrouded in a 3 in diameter polyvinyl chloride (PVC) pipe, as shown in Figure 9. The space between the rail and pipe is filled with sand to damp vibrations induced by fluid-structure interactions. The length of the vertical support is such that the horizontal support is in the center-line of the wind tunnel when mounted to a 2.5 in thick, 6 ft long, 2 ft wide optical breadboard that rests on the bottom of the wind tunnel test section.  
The vertical support includes symmetric airfoil fairing pieces that were 3D printed from polylactic acid (PLA) filament and inserted into the quad rail slots. The airfoil leading edge extends 1.5 inches ahead of the quad rail, and the trailing edge extends 6.5 inches behind, such that the total chord length of the vertical support is 10 inches. The distance between the propeller plane of rotation and the leading edge of the vertical support fairing is 36 inches, equal to twice the distance of the maximum 18 in propeller diameter, consistent with that of NASA designs noted in previous work, and also an improvement over the previous OSU design.