During the fall of 1901, the brothers began to question the aerodynamic
data on which they had based the design of their 1900 and 1901 gliders.
They decided to measure their own values of
with a series of
wind tunnel tests.
To measure the lift and drag, the brothers built small
of their wing designs and placed them on a measuring device called a
balance. There were two balances, one for
and the other for
They would test each model on both balances to determine the
effect of design variables on lift and drag. Most of the models were
designed to perform a
the value of only one design variable between models.
Then by comparing the performance
of the two models they could determine the effect of the single
changed variable on lift and drag.
To obtain data, one of the brothers would look through the view
window on the top of the
and record the angles on the
balance output dial in the test section.
On the left side of the figure, we see the actual data as recorded by the
Wright brothers into their data book.
These photos are provided by the Franklin Institute
where the actual data books are kept.
After recording the raw data, the brothers had to do some additional math
to produce usable lift and drag coefficients from the output dial measurements.
Engineers call this step
Finally, the reduced data was plotted on graph paper, as shown at the right
of the figure. This allows a quick comparison between the results of several
tests. Engineers still use graphs to present data.
You can duplicate the wind tunnel tests of the Wright brothers
by using our interactive
wind tunnel simulation.
There are 30 different
from which to choose and you can test each model on both
balances and record and reduce your data the same way that the brothers did it
At the end of their 1901 wind tunnel tests, the Wright brothers had the most
detailed data in the world for the design of aircraft wings.
What did the data tell them?
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
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
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
which occurs at high angles of attack due to
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
(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.
- Beginner's Guide Home Page