IV.  EVALUATION
Following is an evaluation form that could be given to students who participate in labs and projects that use the wind tunnel.  This evaluation could be given as a pre-evaluation and as a post-evaluation in order to assess whether or not any students had a change in attitude towards math, science and computing technology after participation in a wind tunnel lab.

V.  BUILDING THE TUNNEL

A.  DESIGN
A design team of fifteen students was assembled at General B. O. Davis Jr. Aviation High School in May of 1995.  These students went on a field trip to NASA Lewis on June 5, 1995, for a wind tunnel project kickoff meeting.  The students received a tour of the various wind tunnels at NASA Lewis in the morning.  In the afternoon, the students were briefed by a NASA engineer, about basic aeronautical theory and about the parts of a wind tunnel.

The students began the design of their tunnel once back at school. Two NASA Engineers participated in the project by going to the school to review the students’ design and to give suggestions.  Once the design was completed, the students began building the various parts of the tunnel. The construction of the tunnel was completed during the summer of 1995.  The students volunteered to work on the construction of the tunnel during the summer.

The wind tunnel contains three main parts, the contraction cone at the front of the tunnel, the test section in the middle of the tunnel, and the diffuser at the back of the tunnel. The project team wanted to create the largest tunnel that their budget would allow. They, therefore, iterated between the size of the tunnel and the cost associated with it, to determine how big they could make the tunnel.

The size of test section and the size of the contraction cone are related through a contraction ratio. The contraction ratio used, was 12 to 1. This is the ratio between the area of the contraction cone and the area of the test section. It was determined that a 5’ x 5’ contraction cone was the maximum size that could be built. This then dictated that the test section would be 1.5’ x 1.5’.  The shape of the contraction cone was designed by the NASA Lewis aeronautical engineers working on the project. They developed a bell curve shape that would cause laminar flow and eliminate flow separation.

It was determined that an optimal maximum speed for the tunnel would be around 90 mph. The next step was to find the volumetric flow rate of a tunnel with a 1.5’ x 1.5’ test section and a velocity of 90 mph.  Ninety miles an hour was converted to 132 ft/sec.   Next, the area of the test section was found to be 2.25 square feet.  The volumetric flow rate equation is Q = v x A.  So, Q = 132 ft/sec x 2.25 ft squared = 297 cubic ft/sec.  This converted into cubic ft/min, is 17,820 CFM. The size of the motor was based on the desired CFM output of the fan. With a 5 hp motor, a 36” diameter fan can move 20,650 cubic feet of air per minute, this was greater than the desired CFM output, so these two pieces of equipment were selected.

The length of the diffuser section was dictated by the size of the fan, which was selected to be 36” in diameter.  A square to round conversion piece was designed and the square portion turned out to be 34” x 34”. This became the dimensions of the end of the diffuser section.  The diffusion angle was kept below 6 degrees.  The diffusion angle ended up being 2.9 degrees on each side for a total of a 5.8 degree diffusion angle.  This made the diffuser section 13’ long. Figure 1. provides further explanation about the diffusion angle and the diffuser section.

Figure 1.

One of the NASA engineers on the project provided calculations to ensure that the wind tunnel structure that was designed would be able to withstand the pressures that would be created by the tunnel.  He calculated the pressure per square foot that would be theoretically created by a tunnel of this size and specification. This was compared to the loading standards of the materials used, Plexiglas and plywood and it was determined that the wind tunnel would be able to withstand the pressures. The wind tunnel was to be housed next to a roll up door that led to the outside and the plan was to open the door when running the wind tunnel, to vent the exhaust.  Therefore, the air pressure forces created when the tunnel was running, was not a concern within the building.

B.  CONTRACTION CONE
The contraction cone, also referred to as the inlet, was the only piece that was not fabricated by the students, due to its the size and complexity.  The fabrication of this piece was contracted to Avon Sheet Metal Co.  The students specified the size and shape of the cone for the fabricators. Figure 2. shows the specifications developed. 

Figure 2.

C.  TEST SECTION
The test section is 1.5’ x 1.5’. The test section is made out of plywood with Plexiglas viewing windows.  One of the students became concerned that the Plexiglas protruding from the plywood frame would create places for error once the tunnel was used for testing.  Therefore the Plexiglas was inset into the plywood.  Figure 3. shows the test section.

Figure 3.

D.  DIFFUSER SECTION
Once the dimensions of the diffuser were calculated, how to build it had to be determined.  The main problem was that an eighteen inch square had to be centered in a thirty-four inch square, thirteen feet apart.  The solution was to build a jig for the diffuser section, that was later modified and used as the support for the whole tunnel.  Figure 4. shows the dimensions of the diffuser.

Figure 4.

E.  FAN SECTION
A  36 inch fan was purchased to create the airflow of the wind tunnel.  This was attached to the back of the diffuser section.

F.  MOTOR
The 5hp motor and a drive unit were donated by Eurotherm Corporation.  The motor was mounted on top of the fan. The drive unit was used to convert AC power to DC power and to control the speed of the motor. 

G. FLOW STRAIGHTENER
A honeycomb material, donated by Hexal Corporation, was mounted to the front of the contraction cone and used as a flow straightener,

 
H.  INSTRUMENTATION
Twelve pressure transducers are used to collect pressure readings. Tycon tubing is used to connect the pressure transducers the test points inside the test section.  Inside the test section, probes are attached to the tubing. The location of the probes vary within the test section depending on the test article.  The pressure transducers were mounted into an electrical box to protect them from the environment.  Students from the school’s electronics class worked on this part of the project.  From the electrical box, the transducers were then connected to a 16 channel A/D board which was installed inside a 486 IBM compatible computer.  The software package, Control, was used to display the data on the screen. 

VI.  EXPERIMENTS

 A. VENTURI
The first test object created was a venturi. A venturi is defined as a short tube with a constricted throat that is used to determine fluid pressures and velocities by measurement of the differential pressures generated at the throat, as fluid traverses the tube. The venturi was chosen as the first test object because it could be used to verify that accurate data could be obtained from the tunnel.  If the readings obtained from the tunnel closely matched theoretical calculations, verification, that the wind tunnel and instrumentation were functioning properly, would be obtained. 

Additionally, the venturi was selected as a test object in which to demonstrate a basic aeronautic phenomenon, Bernoulli’s Principle. When air flows through a venturi, the physical properties of the venturi cause the velocity of the air at the beginning of the venturi to be slower than at the middle of the venturi. The air velocity is also slower in the back of the venturi than in the middle.  Bernoulli’s principle states that there is an inverse relationship between air velocity and air pressure, which will be explored in more detail later in this report. But having said that, the air pressure within the venturi will be greatest in the middle and lower at the front and back.

Two attempts to create a venturi with which meaningful readings could be gathered failed.  These two attempts are explained. The first venturi had an hourglass shape and was made out of two pieces of Plexiglas. This venturi was put into the wind tunnel for testing, with probes located throughout the venturi with which to collect pressure readings. Pressure readings were obtained from the inlet of the venturi, but the pressure transducers read zero, for the middle and back sections of the venturi. These pressure transducers and probes were switched with the probes used in the front of the venturi, to make sure they were working correctly, and they were. So, it was decided that inlet area of the venturi was too restrictive and was not allowing airflow through the rest of the venturi.  It was determined that a venturi with a bigger inlet should be made.

The second venturi was made from fiberglass into an hour glass shape. This time pressure readings were obtained from the front and middle sections of the venturi, but the data from the back portion of the venturi was the same as in the middle section. One of the NASA engineers helping with the technical aspects of the project, suggested that the diffusion angle of the venturi (the angle between the middle section and the back section of the venturi) rose at too sharp of an incline and was causing air separation in the back portion of the venturi.  The problem was researched and it was found that the diffusion angle should be no greater than 6 degrees, in order for the venturi to function correctly.

The third attempt was successful.  The entire test section was made into a venturi.  Sheet metal was formed into the shape shown in Figure 5. The location of the probes, which were used to collect pressure readings at six different points, are illustrated in Figure 5.   The test data that was obtained is shown in Figure 6. The graphs in Figure 6. can be used to illustrate Bernoulli’s Principle.   As the velocity increases, the pressure decreases and as the velocity decreases, the pressure increases.  The students transferred the data from the data collection program into a ClarisWorks spread sheet and completed the graphs using the data. The outcome of the comparison of experimental velocities to theoretical velocities is shown in Figure 7.

Figure 5.

Collection of Experimental Data

                                                            Figure 6.

Comparison of Experimental and Theoretical Velocities
from the Wind Tunnel

Figure 7.

B. 2415 NACA AIRFOIL
The second test object created was a 2415 NACA airfoil. The correct shape for an 2415 NACA airfoil with an eight inch chord was obtained from references.  The airfoil itself was made out of wood and sheet metal.  Five ribs where made from 3/4 inch pine wood. Dowel rods were used to connect the ribs together, this completed the form.  Sheet metal was then bent over and glued to the top of the form.  Next, instrumentation tubes were installed. The airfoil was then mounted on a steel plate and secured to the wall of the test section with three bolts, see Figure 8.  In the wall of the test section three slots were cut which allowed the airfoil to rotate from -8 degrees to +25 degrees.  A protractor was used to determine the angle. The 0 degrees mark was lined up with where the wing was horizontal. Lastly, the sheet metal was glued to the bottom of the form.

The airfoil was used to demonstrate how lift is created.  Additionally, the airfoil was used to explain angle of attack and stall.  Tufts of yarn were added to the model to visualize the airflow.  The students were able to see when the airfoil first began to stall; the pressure readings on the computer dropped and the tufts began to swirl. 

Students collected the pressure readings, shown in Figure 9., from the airfoil as it was rotated in two degree increments.  They started at a negative angle of attack and took readings until the airfoil stalled.  Students created a graph from the data they collected.  Refer to Figure 10. to see how the graphs illustrate that at a negative angle the lower surface creates a lower pressure, at -2 degrees there is no lift and at 11 degrees the airfoil begins to stall. (Had the angle marking system been more accurate, 0 lift would have occurred at 0 degrees.) 

The data collected, by the students, on the 2415 NACA airfoil was used to calculate the pressure coefficient and the lift coefficient of the airfoil.  It was then compared to a computer simulation of the same type of airfoil.  The readings where satisfactorily accurate. See Figure 9. for the spread sheet created to calculate the coefficients and Figure 11 though Figure 16 for a graph of the results.  Refer to the Lesson Plans to see how to calculate the coefficients.

Figure 8.

Figure 9.

Figure 10.

 
Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

C.  WHEEL PANT PROJECT
The wind tunnel team was contacted by researchers in the NASA Lewis Structural Systems Branch.  These researchers were interested in using the Aviation High School wind tunnel to perform initial tests on a NASA Lewis research project.  The purpose of project was to design a wheel pant for small private aircraft. A wheel pant is an aerodynamic cover for the wheel of aircraft that do not have retracting landing gear.

The students built a strut and attached a 3/8” diameter rod and the 4.5” diameter wheel and the 1/3 scale fiberglass wheel pant to it that were supplied by the NASA engineers. This structure was mounted in the wind tunnel and testing occurred to determine if drag could be reduced by using the wheel pant. Drag is a force and this force was measured with gram weights on a balance/pulley system, see Figure 17.  The middle of the strut was mounted to the test section floor, but it was able to pivot back and forth.   When the tunnel was turned on, the top of the strut started to move backwards due to drag and the bottom of the strut moved forward, since it was pivoting in the middle.  A string was attached to the bottom of the strut and run through a pulley.  The other end of the string was attached to pan which held gram weights.  Gram weights were placed on the pan until the strut was straight again.  The weight, in grams, corresponded to the drag of the strut system.

Three tests were performed. The first test was on the strut and wheel alone. The second test was on the wheel and the wheel pant, with the tire protruding from the bottom of the wheel pant. The third test was on the closed wheel pant; no tire was protruding.   Measured forces were 200g for the strut and wheel, 150g for the strut, wheel and pant, and 120g for the strut and closed wheel pant.  These initial readings showed that the wheel pant does appear to reduce drag. 

Figure 17.

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