Figure 8 - Point of Calibration load application

Hanging weight rods were pushed through the hole so that with the application of weights onto the hanging weight support, the load would be applied directly to the area around the hole.

With the hanging weight rod and support in place, the readings from the strain gauges produced in Labview were observed and recorded. These values would represent ‘zero’ loading in the calibration process. In essence the bar was loaded, but the aim was to find the relationship between voltage difference and the application of load.

With these zero values recorded, the application of weights could begin. This process was performed through applying load in increments of 200 grams from 0 to 2kg. At each increment the voltage outputs from both ‘mean 1’ and ‘mean 2’ were observed and recorded in a spreadsheet. This process was repeated four times so that the mean averages of voltage output at each loading stage could be determined, and thus a more accurate relationship between load and voltage difference could be reached.

With all the desired readings obtained and recorded, the process of determining the relationship could be carried out. Through examining the results obtained through both outputs of ‘mean 1’ and ‘mean 2’ it was clear to see that although both sets of results displayed a similar linear trend for each of the four loading runs, it was the values from ‘mean 1’ that gave the most consistent results. For this reason it was decided that the data obtained from ‘mean 1’ would be used as the basis for determining the relationship between load and voltage difference.

Through manipulation of the spreadsheet used for recording the values, a trend line of average voltage difference against load was produced. The software was used to produce an equation to represent this trend line, and the relationship was found to be:

However, this equation represents loading normal to the bar, whereas in reality the bar would be facing the airflow at an angle (about its length) in the test section due to the connection between the bar and wing mirror not being normal to the flow. For this reason some trigonometry would have to be applied to the force values obtained from this relationship. Inspection of the wing mirror base indicated that the strain gauge bar would be mounted at an angle of 14˚ incidence to the flow. From this, it was then determined that the relationship between the force on the strain gauge bar and the overall drag force was:

Where ‘D’ represents the drag force and ‘F_s’ represents the component of drag force acting normal to the strain gauge bar.

With these equations obtained it was then possible to place the strain gauge bar (with wing mirror attached) into the wind tunnel for testing.

2. 
   TESTING SET UP
   

To support the ground board in place in the test section, two rectangular wooden beams were slotted into supports at either side of the test section in the flow-wise direction. On top of these two wooden beams, two rectangular metal beams were placed running across the test section and clamped in place. This four beam lattice formed the support for the ground board which was placed on top and clamped tightly.