ntrod uction A wind tunnel is a device that pushes or pulls air through a tube and into a test chamber. Wind tunnels are useful in giving designers and engineers data needed when designing all sorts of objects; from measuring the lift and drag forces on airplane wings, to the wind loads on a suspension bridge. By observing the airflow around the test object, wind tunnels allow engineers to design more fuel-efficient cars and trucks, faster race cars and motorcycles. They allow a bicycle coach to analyze a rider's aerodynamic position that will allow the cyclist to shave seconds or minutes off a time-trial. The Wright brothers built their own wind tunnel and used it to create more accurate data tables on various wing designs. With the wind tunnel data collected during the fall and winterof 1901, and by using the scientific method they ultimately succeeded where others failed. Below is an excerpt from a paper written by Wilber Wright several years later stressing the importance of 'their wind tunnel testing 1. t took us about a month ofexperimenting with the wind tunnel we had built to learn how to use it effectively Eventually we learned how to operate it so that it gave us results that varied less than one-tenth ofa degree. Occasionally had to yell at my brother to keep him from moving even just a little in the room because it would disturb the air flow and destroy the accuracy of the test. Over a two month period we tested more than two hundred models of different types of wings. All of the models were three to nine inches long. Altogether we measured monoplane wing designs (airplanes with one wing), biplanes, triplanes and even an aircraft design with one wing behind the other like Professor Langley proposed. Professor Langley was the director of the Smithsonian Museum at the time and also trying to invent the first airplane. On each little aircraft wing design we tested, we located the center of pressure and made measurements for lift and drift. We also measured the lift produced by wings of different "aspect ratios." An aspect ratio is the ratio or comparison of how long a wing is left to right (the wing span) compared to the length from the front to the back of the wing (the wing chord). Sometimes we got results that were just hard to believe, especially when compared to the earlier aerodynamic lift numbers supplied by the German Lillienthal. His numbers were being used by most of the early aviation inventors and they proved to be full oferrors. Lillienthal didn't use a wind tunnellike Orville and did to obtain and test our data. We finally stopped our wind tunnel experiments just before Christmas, 1901. We really concluded them rather reluctantly because we had a bicycle business to run and a lot of work to do for that as well. t is difficult to underestimate the value ofthat very laborious work we did over that homemade wind tunnel. t was, in fact, the first wind tunnel in which small models ofwings were tested and their lifting properties accurately noted. From all the data that Orville and accumulated into tables, an accurate and reliable wing could finally be built. Even modern wind tunnel data with the most sophisticated equipment varies comparatively little from what we first discovered. n fact, the accurate wind tunnel data we developed was so important, it is doubtful if anyone would have ever developed a flyable wing without first developing this data. Sometimes the non-glamorous lab work is absolutely crucial to the success of a project. n any case, as famous as we became for our "Flyer" and its system of control, it all would never have happened if we had not developed our own wind tunnel and derived our own correct aerodynamic data. - Wilbur Wright The small-scale classroom wind tunnel can be used for many investigations. This guide will help get you started in how to use the wind tunnel and includes several activities. Hopefully this manual will help you to guide your students and encourage to them go beyond these activities and design wind tunnel experiments on their own. 1 http://www.wrightfiyer.orglwindtunnel/testing1.html 1
Fan Unit Wind Tunnel ntake Grille Power Transformer Teacher's Guide Balance Base Wing Holder (2) Decals Clear Tubing 30 cm Clear Tubing 20 cm Washer Weight Hanger Punk Stick (10) Wood Splint (500) Velum Square (50)
Activities For the Classroom Wind Tunnel 1. Measuring force (converting grams to newtons) 6 2. Measuring lift of a wing 7 3. Analyzing lift vs. drag (AOA) 9 4. Comparing the lift of different wing shapes; testing aspect ratios 11 5. Measuring lift and observing airflow of a paper airplane(s) 13 6. Observing the airflow around a wing at various angles of attack 14 7. Observing the airflow around various objects 16 8. Observing the effects of restricted airflow 18 9. An alcohol air pressure sensor; observing Bernoulli's Principle 20 10. An alcohol pitot tube 22 CD Always use eye protection when operating the wind tunnel CD
Assembly nstructions for the Wind Tunnel 1. Unfold the cardboard tunnel and place it on a table with the rectangular mid-section opening upwards. 2. n end opening on the box nearest to the small windows, place the grating inside so that it is flush with the opening. 3. On the opposite end of the box, place the fan unit inside as shown. 4. Separate from the box assembly, find the scale base and lay it down flat. 5. Put the cylinder shaped end of the balance scale on to the fulcrum. The washer rod can be place through either set of holes. The balance scale should rock back and forth freely. 6. Position the angular stickers on to the vertical arms. These stickers should be place on the base of the arms themselves and not the pivoting piece to show the angle of attack. 7. Using the remaining triangular bar, fit the vertical arms on both ends of the bar as shown. The pivoting hinged pieces of the arms should be facing the outside. 8. nsert the vertical arms assembly through the slit holes in the scale base and sit the triangular bar on to the white pivoting holder on the balance scale. You will have to elevate the base to do this. 9. Place any test wing into the slits of the vertical arms. You may need someone to help you hold the assembly while you do this. 10. nsert this entire wing/scale balance assembly into the opening on the top of the cardboard box. 11. Apply the air flow sticker as shown on to the box and plug in the power unit for the fan. Place test wing upside-down in-between the vertical supports Attach the test wing by inserting it into the slots - -........ - - -, &=8 /\ ;...?,.'Y'...: 0\ ~ 1.~.~.~. 1.8 9 ~ Washer Rod can be placed ~ in either hole.89.349.0259 @@ Place a combination of 3 different washer sizes on the balance rod to level the scale Place power output sticker around twist knob Air Flow sticker / / Grill Cardboard Tunnel Fan Unit (plug adapter into nearest outlet) 4
Activity 2: Calculating Torque and Lift Force Torque (r) is a result of Newton's Second Law in a rotational system. over a distance (r) on a lever at an angle (8). t is force (F) applied r =F r sin(8) Because measurements with the wind tunnel will be made when the arm is level, the angle will always be 90 and the sine term wih be unitary, leaving r =F r. The bar on which the masses will be supported can be placed at 8 or 14 mm. This is the r value for the counter weight. The mass of the washers will determine the force of the counterweight. Because the wing must provide an equal torque, the angle is also 90, and the distance is known (4.5 em), it is possible to determine the force on the wing. After setting up the wind tunnel properly use the following steps to measure the lift force of the wing. Procedure: Step 1: Put the mass bar into one of the holes and add masses to it until the lever is level. Step 2: To calculate the static torque of the wing, tstatic' inside the wind tunnel, use the following formula. t 1 =L 2 =F 1 r 1 =F 2 r 2 where 1 is the wing side and 2 is the counterweight side. Step 3: Turn wind tunnel on. The air flowing over the wing should push it downward. Step 4: Add more mass until the lever is level and record as in step 1. Step 5: Repeat the force calculations for tdynamic from step 2 to get the dynamic force, in Newtons. Step 6: Calculate the net (lift) torque by subtracting the dynamic force from the static force (t lift =tdynamic - Lstatic) Step 7: Calculate the lift force from the torque equation: t =F r 7