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The 1901
Wright Wind Tunnel
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fter the 1901 flying season at Kitty Hawk, the Wrights
were seriously discouraged. Neither of their gliders from 1900 or
1901 had worked as anticipated or had produced sufficient lift. They
had begun to suspect that the data they were working with to design
their aircraft was incorrect. This data was gleaned from the
experiments of Otto Lilienthal who, before his death in 1896, had
designed 16 gliders and made thousands of glider flights, many more
than any pilot/scientist before or since. His unquestioned success
gave credence to his data.
Whatever the accuracy of Lilienthal's data, the Wrights had come
to a dead end. They simply did not know what to do to improve the
performance of their gliders. If they built a third glider at this
point, its design would be nothing more than guesswork. And at their
present pace, building and testing one glider per year, it could be
several lifetimes before they discovered a workable design. Even
then, they wouldn't know if it was the best possible design.
They decided to discard Lilienthal's data and generate their own.
They built a wind tunnel, the second in America. (The first was
built by Alfred Zahm.) Over the winter of 1901-1902, they
tested over 200 wing shapes to find the most promising, then
thoroughly investigated about 45 shapes to determine the very best.
To do this, they built two instruments for their wind tunnel, one to
measure lift and another to measure the ratio of lift to drag.
When they compiled all their data, they were surprised to find
that Lilienthal had been correct. They, in fact, had been at naive
in the way they applied his data. Lilienthal investigated a single
wing shape that he used for all his gliders and his data was correct
for that shape only. The Wrights had presumed that even
though they used different shapes, Lilienthal's data for lift and
drag would be close enough. They were astonished to find just how
much difference in performance there was between wing shapes. Armed
with this new knowledge, they felt ready to get back in the air.
We built our tunnel and the balance instruments from the same
materials that the Wrights used, down to to the used hack saw blades
and spoke wire. But despite their crude construction, we found them
to be amazingly sensitive. |
The Wrights' wind tunnel is little more
than a wooden box with a tin scoop on one side to direct and
compress the air stream.
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The air enters the tunnel through a
two-stage "straightener" designed to remove the roiling vortices
that spin off the fan blade.
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The air then travels down the length of
the tunnel, allowing it to settle into a uniform stream before it
reaches the instrument.
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There is a glass window directly over
the instrument. Wilbur and Orville stood on a box and looked down
through this windows to read the scales that measured lift and
drag.
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The fan blade was mounted on a grinding arbor, a
tool that was normally used to spin abrasive grinding
wheels.
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The arbor was spun by a leather belt
that was turned by a series of line shafts and pulleys.
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The line shafts were turned by a
one-cylinder engine that ran on natural gas. This one engine, in
fact, powered all the tools in the Wrights' bicycle shop.
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The first instrument the Wrights used
they called the "lift balance." It balances the lift generated by a
wing shape against a constant pressure on four metal plates or
"fingers."
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The moving parts of the balance pivot with almost
no resistance. The vertical parts – spoke wires – are ground
to a point. The points rest in small indentations in the horizontal
parts – used hacksaw blades.
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Each balance has a scale or "quadrant" marked off
in degrees. By reading the scale and applying a little trigonometry, the Wrights could predict how much
lift or drag a wing shape would produce.
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On the lift balance the top set of
"arms" are friction-fit to their vertical shafts. This allow you to
adjust the position of the top arms in relation to those just
below.
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The lift balance also has an indicator
to help calibrate and adjust the instrument.
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When the pointer is in the center of
the "V," the balance is properly adjusted.
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When it's off-center, adjustment is
needed.
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To use the lift balance place it in the
wind tunnel so the long horizontal arms are perpendicular to the air
stream. With the air blowing, make sure the indicator is centered
and the scale pointer in on "0."
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Mount the wing shape on the tab that
protrudes from the long upper arm. Adjust the shape so it's the
proper "angle" of attack" to the air stream.
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Turn on the wind for a few moments and
let the pointer come to a rest over the scale. Turn off the wind.
Note that the indicator is no longer aligned with the
"V."
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Holding the pointer steady on the
scale, move the top arms so the indicator is centered. Turn on the
wind and check that it remains so. You may have to make several
small adjustments to the top arms until it does. When it stays
centered, read the scale. The sine of the angle indicated is the coefficient of
lift for that particular wing shape.
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The second instrument the Wrights
called the "drift balance." Drift was their word for what we
now call drag. The scale
doesn't measure drag, however. It measures the ratio of lift to
drag. This is an extremely important number in aircraft design. The
higher the ratio – the more lift and the less drag – the more
efficient the wing shape.
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Notice that each of these balances has
two pointers, one of which points to nothing. The Wrights added the
second pointer so the pressure of the wind on the sides of the
pointers would cancel each other out and not interfere with the
measurement.
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To use the drift balance, place it in
the wind tunnel with the long arms parallel to the air stream. Turn
on the wind and make sure the pointer remains on "0."
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Mount a wing shape on one of the long
arms so the chord is parallel to the arm. Turn the balance so the
wing is at the desired angle of attack to the air stream.
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Turn on the wind, let the pointer come
to rest, and read the angle on the scale. Add this angle to the
angle of attack. The tangent of the resulting angle is the ratio of
lift to drag.
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To mount the airfoils on the balances,
the Wrights soldered two small brackets to the top surface of each
foil, so close together that the brackets were almost
touching.
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These brackets slip over the crossbars
of the balances. The fit is snug, but not so tight that the foils
cannot be easily mounted and dismounted.
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The Wrights made preliminary tests on
over 200 wing shapes, then thoroughly investigated 57 of the most
promising. The foils are made of sheet steel; thicker portions are
built up with wax.
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