The procedure for this project was rather long, and involved many changes from beginning to end. By the end, several new ideas were considered, and experimented. Below is the summary of all the steps taken before experimentation began.
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As this project dealt with aerodynamics and wind, the very first thing that had to be built was a device which could produce airflow. A simple wind tunnel was designed and built. It consisted of a 3 bladed fan, which was enclosed in a round tunnel. In front a testing area was added, with supports for the airfoils. The wind tunnel had some requirements before testing could begin. It had to blow laminar airflow, so that the results would stay consistent. This was accomplished by placing a grid in front of the fan. This straightened the overall turbulent nature of airflow produced by the fan. Another requirement for the fan was to blow air at variable velocities. This was accomplished by adding a switch for adjusting the voltage supplied to the fan. A voltmeter was also connected to keep the voltage consistent. The end result met all the criteria, so testing could begin.
To find the velocity of airflow produced by the wind tunnel, a measuring device was built. It worked on the same principle as an anemometer, only compacted for measuring airflow velocity in smaller spaces. It consisted of a small three bladed propeller that span freely even at low airflow velocity. A fiber optic sensor then measured the number of times the blades rotated and sent the signal to a counter which displayed the number. The counter took readings once every ten seconds, for a one second period (These times intervals could be adjusted though). This way, the readings stayed consistent. The anemometer was secured on a thin metal rod, which was connected to the fan box. It could be rotated, and shifted in all 3 dimensions. This made it convenient to take the measurements, and also eliminated any shaking if it were to be hand held.
Using the anemometer, the wind tunnel was adjusted to make it blow laminar airflow. The anemometer was placed in different locations near the tunnel, and airflow velocity measurements were taken. The design of the wind tunnel was adjusted until the airflow velocity stayed consistent at every point in the tunnel.
For testing, there had to be an airfoil in which different interchangeable holes could be inserted. An ideal profile for the airfoil was the found to be a NACA 6324 airfoil design. NACA was the company which evolved into the NASA we know today. It specialized in aerodynamics, and airfoil design. They had a specialized classifying system for all the possible airfoil profiles. 6324 was chosen because the profile was compact and had a large camber (% chord). This meant the airfoil created a lot of lift, as there was a lot of pressure difference between the top and bottom side. The effects the holes created on the airfoil would be clearly evident on this airfoil design.
The airfoil was fitted with endplates and was secured in front of the wind tunnel. The endplates eliminated the wing tip vertices effect. The airfoil could also rotate to change the angle of attack, which was another important factor in finding the ideal position for the most possible lift.
At the maximum pressure difference location, the airfoil had a large opening into which the boxes with the enclosed holes could be inserted (this saved the time of building an airfoil for every hole). In total there were 6 different holes. Each hole had a special characteristic. For example, there were different shapes for the holes, which included squared, rectanglular and circular openings. The variety of different holes ensured that an ideal design for a hole would be found.
HPC - Hole Properties Coefficient
When building the boxes with the holes, an idea came up for a classifying the holes. Only six holes were built, but there could be an endless amount of different holes, so a classification system would give any hole its own classification number. It also meant that from the six boxes experimented on, different classification properties for a hole in an airfoil would be found. The system was called the HPC classifying system, which stands for Hole Properties Coefficient. It consists of 6 coefficients, each telling information about the proportions and co-ordinates of the hole in relation to the airfoil.
The HPC of a hole can be calculated using the following measurements:
After the measurments have been taken, the following formulas would be used to find each of the 6 coefficients.
The six holes made earlier were classified according to the system. They were:
1. HPC 33 30 39 – 33 32 38
2. HPC 24 6 51 – 24 6 29
3. HPC 13 7 32 – 13 1 6 39
4. HPC 7 15 39 – 7 15 39
5. HPC 12 19 42 – 12 6 44
6. HPC 13 20 35 – 13 20 43
To find out what each coefficent shows, and an example of how to calculate the HPC of a hole,Click here.
At this point, there was a wind tunnel that produced laminar airflow, an anemometer for determining airflow velocity, an airfoil and different holes. All the necessary equipment for testing was built, so the experimentation began. A grid was drawn on the airfoil, with points where airflow velocity measurements would be taken. There were 5 vertical x 10 horizontal points on each side of the airfoil; a total of 100 points. The objective of this experiment was to find the effects a hole in an airfoil had on the surrounding airflow. A chart was printed for writing down the airflow velocity at each of the 100 points. About 6-9 measurements were taken at each point to ensure an accurate measurement.
In total there were 9 experiments done. In the first experiment, the airfoil was at 0 degrees angle of attack, with no hole. This would provide the control for the next experiments, and something to compare the results with. For the second and third experiment there was an angle of attack of 7.5 and 15 degrees, with no hole. From these experiments the angle of attack at which the airfoil created the greatest lift was determined. It was found that the airfoil created the most lift at about 7.5 degrees angle of attack. For the rest of the experiments, an angle of attack of 7.5 degrees was used, and for each experiment, one of the 6 different holes was inserted into the airfoil. The data collected from these experiments was collected and graphed.
The results can be found in the "Experiments" page.