Michael Lord and Joshua Mann
Tualatin High School
January 18, 1999
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Introduction (Table of Contents)
For centuries, human beings have felt the desire to fly. Within the last couple hundred years, this dream has been realized in many different forms, which include hot-air balloons and manned rocket ships, not to mention the airplane and jet-engined aircraft, which are commonplace today. However, airplanes and jets are the most widely used flying devices. These two flying machines use apparatus, known as wings, to stay airborne. The wings are useful in increasing an aircraft’s vertical drag, or its tendency to impede upward motion (Coyle 159). For that reason, they are not generally used on helicopters, as they would impair the function of the rotor, which is specifically to create vertical lift. More importantly, the wings are specially shaped to deflect the passing air and create what is known as a dynamic lift (Giancoli 253). Since the shape of the wings is vital to the performance of the aircraft, a great number of tests are employed on them to ensure effectiveness.
The chief instrument by which the wings are tested for aerodynamics is the wind tunnel, which is literally a tube in which an object is placed while a fan blows air at it. Various devices are used to measure many forces, such as wind resistance, lift, and vertical drag. In fact, the Navy, in cooperation with Boeing, have recently developed a new "variable porous wing fairing" to eliminate a fighter’s wing problem (Wall 55). Additionally, the United States Air Force has refined the aerodynamic performance of a nose-mounted laser turret (Proctor 61). Both of these new developments were achieved through the use of a wind tunnel. Important and useful as the wind tunnel may be, its results could only be exact if the tunnel model was exposed "to a strictly steady and uniform stream of air," and the airplane was flying through air having no velocity (Von Mises 168). While these conditions cannot be achieved, only minimal accuracy is lost, and can be regained by the use of what is known as the degree of turbulence of the wing.
In this experiment, we intend to use a wind tunnel and measure the forces
imparted upon various objects of differing diameter and length. We will
also compare these to a couple of objects similar in length and diameter,
but with a pointed end. We expect that the force of the wind upon the object
will increase as the surface area at the front of it increases. Additionally,
we
believe that wind resistance will decrease proportionally with the decrease
in the length of the object.
Procedure (Table of Contents)
The first step toward testing this hypothesis involved the acquisition of two wooden dowels of differing diameters (three-quarter inch and one inch). We then cut them into different lengths ranging from twelve inches to four inches, as well as sharpening the first inch of selected dowels into conical shapes (points). Then, we drilled suitable sized holes on the rear of each dowel so as to conform to the mounting specification of the drag-measuring instrument. Once this was accomplished, and we finished reading the instruction manual on the wind tunnel, we were prepared to begin our testing stage.
For each dowel length and diameter, we mounted the dowel on the drag
measurement device, balanced the small weights on the device for the specific
dowel, and turned on the fan. While the fan was blowing air through the
tunnel at approximately 40 miles per hour, we re-balanced the weights to
give the drag measurement of the dowel. The data taken from these trials
is given on the subsequent page.
Collected Data (Table of Contents)
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Conclusions (Table of Contents)
Upon careful consideration and analysis of the above data, we have drawn several conclusions. First, it seems that, in general, a linear relationship exists between the length of the dowel and its wind resistance. The following graph is of the logarithmic best-fit type and shows this linear correlation.
However, we did see a spike in our data at seven inches, which is evident on the connected dot graph below, which may suggest otherwise. We think this spike is an anomaly of some kind, despite it occurring twice. It could have been the result of a power fluctuation, which would have caused the fan to increase in velocity and blow faster air. The two particular dowels that have the spike were tested one right after the other. Despite this aberration, we have found proportionality between length and wind resistance.
Additionally, the diameter of the dowel seems to have a greater effect on the wind resistance. The graph below shows that the difference in drag between the three-quarter inch dowels and the one-inch dowels stays fairly constant. Excepting possible bad data points, the difference in drag remained approximately .5 units. We think the relationship between dowels of differing diameters but equal length is a constant; however, we tested only two different diameters, and more extensive experimentation is necessary to support this conclusion.
One other factor that affects wind resistance is the shape of the surface facing into the wind. We compared the drag on the flat-nosed dowels to the drag on pointed-nosed dowels of the same length and diameter. According to our third data table (see page 3), the drag was significantly less on the dowels with pointed ends than on those with flat ends.
Throughout the experiment, many different factors existed that could have caused errors in the data. One factor that may have played a significant role in undermining our experiment was whether or not the wind was hitting the front of the flat-nosed dowel at a perpendicular angle, or possibly whether or not the flat nose was perpendicular to the axis of the dowel. Also, the measuring device and wind tunnel that we used were intended for larger objects, so the units of measurement were not as precise as were required.
Finally, length, diameter, and nose shape were all found to have an
effect on the wind resistance of dowels; however, length seems to have
the least significant effect. We were originally hoping that these conclusions
we give us more insight on airplane wings; however, we found that the data
explains more about the fuselage of various flying and non-flying vehicles
than it does about the wings. Using the example of a rocket booster (i.e.
a large fuselage), we can now see why rockets have conically-shaped noses
and have a sacrifice a short length for a short diameter.
Bibliography (Table of Contents)
April 6, 1998, P.55
"Boeing Completes Key ABL Wind Tunnel Tests" Aviation Week & Space Technology
March 2, 1998, P.61
Von Mises, Richard Theory of Flight. New York: Dover Publications,
Inc., 1995
Coyle, Shawn The Art and Science of Flying Helicopters. Ames: Iowa State University
Press, 1996
Giancoli, Douglas C. Physics Englewood Cliffs: Prentice Hall,
1991
Links (Table of Contents)
http://www.letsfindout.com/subjects/aviation/rfiwitun.html
-The history of wind tunnels
-The Arizona State University wind tunnel page.
http://aocentral.arc.nasa.gov/
-Neat pictures of wind tunnels provided by N.A.S.A..
http://www.fi.edu/flights/first/tunnelparts/index.html
-Diagram and information about wind tunnels.
http://firstflight.open.ac.uk/experiments.html
-Wind tunnel experiments used by the Wright brothers.
http://www.lerc.nasa.gov/Other_Groups/K-12/windtunnel.html
- Tells about wind tunnels. It even says how to make your own.