# Study of Airfoil Deflections for Unsteady Aerodynamics Optimization in Pitching Airfoils +
## 2 Materials and Methods
### Experimental Pitching Design and Wind Tunnel Assembly  
To replicate the pitching experimentally, an apparatus capable of actuating an airfoil with sinusoidal motion was constructed. A stepper motor controlled by a driver and Arduino Due was utilized to pitch the airfoil at the desired angles and frequencies. The control system electronics layout with main signal and components is shown in Figure 7a, along with a long-exposure picture during wing pitching in Figure 7b, a setup in a wind tunnel (in the more general tandem configuration) in Figure 7c, and a long-exposure picture during tandem pitching testing with the laser sheet illumination in Figure 7d. Figure 7: Pitching design: Arduino control system (**a**), pitching wing long-exposure picture (**b**), wind tunnel with tandem wing setup (**c**), and tandem pitching wing long-exposure sample with laser sheet illumination (**d**). The stepper motors utilized were NEMA 34, model 34 HS61-6004S1, available from STEPPERONLINE. The digital stepper motor driver selected was model DM860T and is available from the same vendor. This driver allows for micro stepping, and 3200 steps per revolution was selected for testing, since it provided sufficient angular accuracy at 0.1125 degrees per step. When powered at full voltage and current, this combination provides a holding torque of 13 N.m and was sufficient to pitch the airfoil at frequencies up to 30 Hz. Two motors are shown in Figure 7c in linear tandem configuration for the wing/wing interaction studies. Pitching parameters were achieved by measuring the airfoil's angle of attack with a 9-degree-of-freedom (DOF) Inertial Measurement Unit (IMU). A Bosch BMX160 9 DOF was selected for its size, robustness, and relative ease of implementation. Preliminary tests indicated that a similar sensor, the Bosch BNO055, could not provide enough accurate angle measurements while the airfoil is pitching. A Teensy 4.1 microcontroller running Arduino IDE was utilized to read the 9 DOF IMU. A Mahony filtering algorithm was applied to the individual accelerometer, gyroscope, and magnetometer sensor readings, used to determine the sensor angle. As shown in Figure 7c, the test setup consists of VR-12 airfoils on keyed shafts. Each wing is supported by a roller bearing pressed into the optically clear front viewing panel and attached to a stepper motor with an ER32 shaft coupling. The wind tunnel test section is 12 inches square by 36 inches long. Here, the primary wing (or 'downstream' wing) is positioned at both the vertical and horizontal midline of the test section, while the secondary airfoil (or 'upstream' wing) is located at the horizontal midline, 13 inches (from pivot points) upstream of the primary wing (corresponding to 3.25 chords); with the current settings, this distance represent interactions when the spacing between wings (from front-wing TE to rear-wing LE) is approximately two chords. Future designs will allow for sliding the wing position to study other distance interactions. To control the stepper motor driver, a code was written in Arduino IDE and ran with an Arduino Due, as shown in Figure 7a,b. Each driver/stepper motor/airfoil combination required its own Arduino Due to run. The code utilizes the predefined Arduino library \"AccelStepper\" and allows the operator to set the maximum angle of attack, minimum angle of attack, speed of rotation, acceleration of rotation, as well as various other parameters. Knowing these parameters, the code works by moving the airfoil to the maximum angle of attack, where it outputs a signal, then moves to the minimum angle of attack, and repeats this cycle until stopped. The signal output at the maximum angle of attack acts as a synchronization trigger for the PIV circuitry, allowing data to be taken at any intended angle of attack or set to complete a full interrogation of the wing performance. The code was set up to allow the operator to switch between four different states. Each state enables specific buttons, which then allows the operator to perform various actions with the airfoil.