
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
online over the internet, or downloaded and run offline 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 crosssection
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 biwing 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.
Navigation..
 ReLiving the Wright Way
 Beginner's Guide to Aeronautics
 NASA Home Page
 http://www.nasa.gov
