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Introduction

A wind tunnel is a device that pushes or pulls air through a tube and into a test chamber. Wind tunnelsare useful in giving designers and engineers data needed when designing all sorts of objects; frommeasuring the lift and drag forces on airplane wings, to the wind loads on a suspension bridge. Byobserving the airflow around the test object, wind tunnels allow engineers to design more fuel-efficientcars and trucks, faster race cars and motorcycles. They allow a bicycle coach to analyze a rider'saerodynamic 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 onvarious wing designs. With the wind tunnel data collected during the fall and winter of 1901, and by usingthe scientific method they ultimately succeeded where others failed. Below is an excerpt from a paperwritten by Wilber Wright several years later stressing the importance of 'their wind tunnel testing 1.

It took us about a month of experimenting with the wind tunnel we had built to learn how to use iteffectively Eventually we learned how to operate it so that it gave us results that varied less thanone-tenth of a degree. Occasionally I had to yell at my brother to keep him from moving even just alittle 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 behindthe other like Professor Langley proposed. Professor Langley was the director of the SmithsonianMuseum at the time and also trying to invent the first airplane. On each little aircraft wing designwe tested, we located the center of pressure and made measurements for lift and drift. We alsomeasured the lift produced by wings of different "aspect ratios." An aspect ratio is the ratio orcomparison of how long a wing is left to right (the wing span) compared to the length from thefront to the back of the wing (the wing chord). Sometimes we got results that were just hard tobelieve, especially when compared to the earlier aerodynamic lift numbers supplied by the GermanLillienthal. His numbers were being used by most of the early aviation inventors and they proved tobe full of errors. Lillienthal didn't use a wind tunnellike Orville and I did to obtain and test our data.

We finally stopped our wind tunnel experiments just before Christmas, 1901. We really concludedthem rather reluctantly because we had a bicycle business to run and a lot of work to do for that aswell.

It is difficult to underestimate the value of that very laborious work we did over that homemade windtunnel. It was, in fact, the first wind tunnel in which small models of wings were tested and their liftingproperties accurately noted. From all the data that Orville and I accumulated into tables, an accurateand reliable wing could finally be built. Even modern wind tunnel data with the most sophisticatedequipment varies comparatively little from what we first discovered. In fact, the accurate wind tunneldata we developed was so important, it is doubtful if anyone would have ever developed a flyablewing without first developing this data. Sometimes the non-glamorous lab work is absolutely crucialto the success of a project.

In any case, as famous as we became for our "Flyer" and its system of control, it all wouldnever have happened if we had not developed our own wind tunnel and derived our own correctaerodynamic data.

- Wilbur Wright

The small-scale classroom wind tunnel can be used for many investigations. This guide will help getyou started in how to use the wind tunnel and includes several activities. Hopefully this manual will helpyou to guide your students and encourage to them go beyond these activities and design wind tunnelexperiments on their own.

1 http://www.wrightfiyer.orglWindTunnel/testing1.html

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Fan Unit

Wind Tunnel

Intake 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 Instructions for the Wind Tunnel

1. Unfold the cardboard tunnel and place it on a table with the rectangular mid-section openingupwards.2. In end opening on the box nearest to the small windows, place the grating inside so that it is flushwith 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 placethrough 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 ofthe 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. Thepivoting hinged pieces of the arms should be facing the outside.8. Insert the vertical arms assembly through the slit holes in the scale base and sit the triangular bar onto 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 theassembly while you do this.10. Insert 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.

Attach the test wing byinserting it into the slots

Place power outputsticker around twist knob

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Place a combination of 3different washer sizes onthe balance rod to levelthe scale

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~ _ _ _ _ Washer Rod can be placed~ in either hole

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Place test wing upside-downin-between the verticalsupports

Air Flow sticker

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Grill Cardboard TunnelFan Unit(plug adapterinto nearest outlet)

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Activity 2: Calculating Torque and Lift Force

Torque (r) is a result of Newton's Second Law in a rotational system. It is force (F) appliedover a distance (r) on a lever at an angle (8).

r =F r sin(8)

Because measurements with the wind tunnel will be made when the arm is level, the anglewill always be 90° and the sine term wiH be unitary, leaving r =F r. The bar on which themasses will be supported can be placed at 8 or 14 mm. This is the r value for the counterweight. 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 isknown (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 ofthe wing.

Procedure:

Step 1: Put the mass bar into one of the holes and add masses to it until the lever islevel.

Step 2: To calculate the static torque of the wing, tstatic' inside the wind tunnel, use thefollowing formula.

t 1 =L2 =F1 r1 =F2 r2where 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, inNewtons.

Step 6: Calculate the net (lift) torque by subtracting the dynamic force from the static

force (tlift =tdynamic - Lstatic)

Step 7: Calculate the lift force from the torque equation: t =F r

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