Wind Tunnel Simulator Lessons

Glenn Research Center

Background

Following the kite and glider flights of 1900 and 1901, the Wright brothers began to doubt the accuracy of the aerodynamic data on which they based their designs. Comparing their measured flight glide angle to the drag to lift ratio predicted by the available data, the brothers correctly determined that some errors were present in the data, and in the use of the data. The brothers decided to develop their own tables of aerodynamic data through wind tunnel testing. They built nearly a hundred small wing models and performed preliminary tests in a wind tunnel which they constructed. They then chose about thirty models for more detailed testing in the fall of 1901.

By 1900, scientists knew that the lift and drag of an object depends linearly on the surface area of the object, varies as the square of the velocity between the object and the air, depends on the atmospheric pressure, and depends on the shape and inclination of the object to the air. The dependence on atmospheric pressure was expressed as a pressure coefficient, called the Smeaton coefficient after the scientist who first measured it. Equations had been developed to determine the lift and drag of an object in terms of the area, velocity, Smeaton coefficient, and shape and inclination. The dependence on shape and inclination were expressed by a single number in each equation which were called the lift coefficient and the drag coefficient. The coefficients represented the ratio of the forces generated by a surface to the drag of a flat plate with the same area. Numerical values for the coefficients were determined by measuring the forces on a model during a wind tunnel test. The objective of the Wright 1901 wind tunnel tests was to determine accurate values of the lift and drag coefficients for a variety of wing shapes. The coefficients could then be used in the lift and drag equations to predict the lift and drag of the full size aircraft design.

The wind tunnel testing technique used by the brothers is exactly the same method used today to design modern aircraft. Forces are measured in a wind tunnel by using an instrument called a balance which works by comparing (balancing) the magnitude of forces present on the model. The output from the balance is normally expressed as the ratio of the forces on the model. The brothers displayed the output by the deflection of a needle on a dial. They recorded the deflections in a log book, then performed some math to reduce the data to the ratio of forces. They graphed the results of many tests to determine trends and to pick an optimum design. Modern wind tunnels use computers to perform all of these functions.

The brothers constructed two balances for their tests. One balance compared the amount of lift generated by the model to the drag on a set of reference plates. The brothers had determined the drag on the set of plates from their previous flight experiments. The other balance compared the drag of the model to the lift of the model. Each wing model was tested over a range of flight conditions on both balances.

Wind Tunnel Simulator

Engineers at the NASA Glenn Research Center have produced a computer simulation of the Wright Brothers 1901 wind tunnel which duplicates their test techniques and results. The program is written in Java and can be run on-line over the internet, or downloaded and run off-line on the user's PC or Mac. There are two versions of the program. Version 1 exactly duplicates the procedure used by the Wright brothers and requires the student to record data, reduce data, and graph the results. Version 2 records the data, performs the data reduction, and the graphs the results for the student.

The computer program simulates the operation of a wind tunnel and a student must test wing "models" in the tunnel to determine an optimum design. There are 31 models available for testing which duplicate the models used by the Wright brothers in 1901. Each model can be tested on both the lift balance and the drag balance. Unlike modern tunnels, the Wright tunnel ran at only one speed (about 25 mph). Because their flight speed was also very low (about 35 mph) this was not a problem for the brothers. The student must test each model over a range of angle of attack of the wing. The angle of attack is the angle that the chord of the wing makes with the incoming air. To record the data, the student should print out the appropriate data form when using Version 1. To begin testing, the student sets the model on the balance at a selected angle of attack and turns the air on. As the air flows past the model, the balance moves because of the forces on the model. The motion is noted by the pointer on the output dial. The student records the data on the data form. This process is repeated as many times as necessary. The student then selects a different model for comparison and repeats the entire process. The raw data (dial angle) must be reduced to a usable form (lift or drag coefficient) using the math given on the data form. After graphing the results of several tests, the student can determine which model performs better by studying the graphs. When comparing models, high lift and low drag are good.

There are several geometric factors that affect the amount of lift and drag produced by a wing. The wing models tested by the brothers were all produced from thin sheets of steel. The planform of the model is the shape of the model when viewed perpendicular to the lifting surface (looking down onto the wing). The distance from wing tip to wing tip is called the span, the distance from leading edge to trailing edge is called the chord. The ratio of the span to the chord is called the aspect ratio and is one of the most important performance parameters of a wing design. The brothers tested rectangular planforms with a variety of aspect ratios. The brothers also tested several other planforms; elliptical wings, wings with curved trailing edges, and wings with curved wing tips. If we cut the wing from leading edge to trailing edge, we obtain a side view of the airfoil. The brothers tested a variety of airfoil shapes. Some were circular arcs (high point in the middle), some were parabolic (high point near the leading edge), and some were highly curved. The amount of curvature is called the camber. A camber of 1/12 is more curved than a camber of 1/20. The Wright brothers used two wings on their aircraft with one wing mounted over the top of the other. They tested one wing, two wing, and three wing configurations, and also varied the distance between the wings.

Possible Uses for Simulator

There are many different ways that this simulation package can be used in the classroom. Here are some ideas regarding tests and test techniques:

Following Instructions

To operate the tunnel simulator, the student must carefully follow a set of instructions which are described on the web page below the simulator. The instructions are not particularly difficult, but the procedure must be executed in the specified order to get meaningful results. To obtain a single data point, there are five steps. The last four steps must be repeated several times to generate a single plot for one model.

Parametric Studies

The lift and drag of a wing depends on several parameters. To determine these effects, the student must conduct a series of experiments. Between experiments, the student should change only one variable. If we change two or more variables, we cannot easily determine how the result depends on each variable. The shapes of the 31 models were selected by the Wright brothers to perform a variety of parametric studies of the factors that affect lift and drag. The student should study the shapes of the models and determine which models to test and compare. A web page at the Wright Way site describes how the brothers conducted their tests.

The student must determine how many data points to gather for each test. A student needs to learn how to conduct a test; to take enough data to determine a trend, and to take additional data in those regions where results change rapidly. There will surely be some students who will pick only two points and end up with a straight line, when the trend is actually much more complex.

Reading a Dial - Interpolation

If the student uses Version 1 of the program, the only output from the program is the angle on the output dial. The student has to record this reading to the data form. The dial is very crude and is only delineated at 5 degree intervals. The student must learn how to read the dial and to interpolate the value of the raw data.

Data Reduction

The raw output from an experiment is seldomly presented in a useful form; a scientist usually must perform some mathematical data reduction to obtain meaningful data. In the simulator, the output is always an angle from the dial, but we are interested in lift or drag coefficient so some additional math must be performed.

The math used for data reduction involves the trigonometric functions sine and tangent. Young students have probably seen these on their calculators but do not know what they are or why they are there. A rather simple explanation, in terms of the ratios of the sides of triangles, is given on another web page. This may be a good opportunity to introduce the ideas of functions and trig functions to the student and to demonstrate how they are used by scientists. We have provided tables of the sine and tangent (to further practice table reading and interpolation) but the students could just as easily use calculators to reduce the data.

If the data reduction is a problem for young students, you can use Version 2 of the program which automatically performs the data reduction.

Graphing Data

For Version 1, a stencil for a piece of graph paper is provided along with the data forms. Students must determine how to put on the axes, scales, and record the data from several tests. Drawing a line through the data is always a problem. There is an interesting letter from Wilbur Wright to Octave Chanute in which Wilbur describes that it is "difficult to let the lines run where they will instead of running them where I think they ought to go."

Reading a Graph

For Version 1 or 2, the student must learn how to interpret the results of a graph. For the lift balance, the higher the line the better the performance. For the drag balance, the lower the line the better the performance. But both graphs have some additional surprises. There is a sharp break that occurs on most lift graphs at high angle, when the wing goes into stall. On the drag graph there is a bucket, a condition which produces a minimum drag that you do not see on the lift graph. These fine points provide additional information to a scientist about the performance of the model, and are a good topic for discussion in any report.

Drawing Conclusions - Producing a Report

The student can use the simulator to produce data and graphs which can be included in a technical report. Report writing is very important for any scientist, since that is the mechanism for sharing results. There is a definite form to report writing which students need to learn before they get into high school or college.

An interesting exercise would be to use the simulator to demonstrate a scientific study. Have the student postulate which of three models produces the highest lift. Conduct the tests. Then produce a technical report presenting the data verifying (or contradicting) the postulate. Propose another test.

Use the Data

The student can use the data developed by the simulator in the lift and drag equations to determine the wing area required to lift a given weight, or to determine the thrust required to overcome the drag of a given design. This exercise would introduce the students to some simple algebraic equations for lift and drag, and demonstrate how you can use math to design an object.

Some Aerodynamic Results

In the following discussions, the word performance indicates a combination of the effects of lift and drag, normally expressed as the drag to lift ratio. A low value is better than a high value of this parameter.

• Curved surfaces produce more lift than flat surface. The greater the curvature the greater the lift. (This information was available in the Lilienthal data).
• Curved surfaces also produce more drag than flat surface. The greater the curvature the greater the drag. So the most desirable cross-section is a curved surface with a small camber. The brothers settled on a 1/20 camber for their designs.
• Amongst curved surfaces, parabolic curves (those with the highest camber nearer to the leading edge) have better performance than circular arcs (where the highest camber lies in the middle of the foil).
• Long thin wings (high aspect ratio) have better performance than short wide wings (low aspect ratio). This helped to explain the problems of the 1901 glider and directly affected the design of the 1902 glider. The 1902 had roughly the same wing area, but twice the aspect ratio of the 1901.
• For many of the wings tested, the highest lift does not occur at the greatest angle of attack; the lift peaks at a low angle of attack and then decreases. The brothers were surprised by this result and did additional tests with a weather vane balance to verify it. A modern aerodynamicist would recognize this pattern as indication of a wing stall which occurs at high angles of attack due to boundary layer separation.
• Curved wing tips produce lower drag than rectangular wing tips. The detailed shape of the wing tip has a large effect on wing performance. The next time you visit an airport, or go to an air show, notice the many different shapes of the wing tips on various aircraft. Designers still try to optimize the performance of their aircraft.
• There is a slight performance penalty associated with bi- and tri- wing configurations; putting one wing on top of the other does not give you exactly twice the performance of the single wing alone. The brothers attributed this difference to the increased number of wing tips. While there is a performance penalty, the structure can be made very strong and light with a bi-wing design. Modern aircraft typically have a single wing made of light, strong aluminum. But this material wasn't available in large quantities for the Wright brothers.
• The brothers tested Otto Lilienthal's wing geometry (Model #31) in their wind tunnel and compared the results with Lilienthal's published data. Wilbur wrote to Octave Chanute that there were no errors in Lilienthal's data within the accuracy of his test techniques. But Wilbur noted the importance of total wing geometry (airfoil shape and wing planform) on wing performance. In 1900 and 1901, the brothers had closely approximated Lilienthal's airfoil shape, but had a very different wing planform which generated very different wing performance from Lilienthal's published data.

We at NASA Glenn would be glad to include any other ideas for the use of the simulator in the classroom. We invite teachers to submit activities to any of the e:mails listed below and these activities will be included on this web page.

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