The Wind Tunnel
The experiments of 1901 convinced the Wrights that
Lilienthal's data for curved airfoils were wrong. More careful measurements
had confirmed the result found in 1900 that the predicted lift was too
large. Beginning in the fall of 1901 the brothers carried out a lengthy
series of tests in a wind tunnel.
Theirs was not the first wind tunnel or even the
first in the U.S. They probably got the idea from articles in Aeronautical
Annual, which was published by James Means in Boston in the years 1895-97.
The contribution of the Wrights was characteristically to the point. They
recorded accurate data for many airfoils and wing shapes. They did not
make absolute measurements but rather compared the forces acting on an
airfoil with those acting on a flat plate oriented perpendicular to the
airstream. As a result of their tests they had by midwinter all the data
they would need to design their aircraft for the next decade.
The Wrights also decided on the basis of their own
data that Lilienthal's measurements were not as inaccurate as they had
initially believed. For their predictions of lift on their gliders they
had needed a constant (related to the drag on a flat plate), which had
been inferred from the data published by Smeaton. By comparing the wind-tunnel
findings with the measurements of lift on their gliders the Wrights determined
that the value of the constant was wrong. The error had caused them to
overestimate the lift by 40 percent.
On August 28, 1902, the Wrights arrived again at
Kill Devil Hills with another glider. This model had a span of 32 feet
and a chord of five feet, so that its total area and wing loading were
about the same as those of the 1901 glider. From the wind-tunnel tests
the brothers had learned that a long, narrow wing, which has a higher
aspect ratio, is more efficient than one with the same area but a shorter
span. Their choice of cross section for the airfoil was also based on
the wind-tunnel tests.
The 1902 machine was the first to include the hip
cradle the brothers devised to enable the pilot in a prone position to
operate the warping wires by shifting his hips laterally. The deflection
of the horizontal surface was changed by operating a lever with the left
The most obvious and important change of configuration
was the addition of a double vertical tail. It had a total area of 12
square feet. This surface was rigidly mounted in the 1902 glider and was
never intended for steering. The Wrights had figured out correctly that
the vertical tail was needed to counteract the adverse yaw.
Tests of the glider as a kite showed the brothers
they had a better airplane. They found that with the new wing they had
a much-improved lift-drag ratio and that the difficulties with longitudinal
control were reduced. Moreover, the fixed vertical tail did act to reduce
the yawing tendency in a turn.
Initial glide tests showed an excessive response
to side gusts, and so the wings were set with negative dihedral. Further
testing showed that the 1902 glider gave the pilot even more trouble with
lateral control than the 1901 machine had. When the glider was hit by
a side gust, it had an uncontrollable tendency to oscillate in yaw.
GLIDER OF 1901 was by far the largest glider anyone
had tried to fly. It had a wingspan of 22 feet and a chord of seven feet;
the weight without pilot was 98 pounds. In tests at Kitty Hawk, with Wilbur
as the pilot, the glider at first followed a strongly undulating flight
path. The Wrights modified the aircraft to reduce the camber they had
added to the wings since 1900 and then found that it flew much better.
GLIDER OF 1902 provided the Wright brothers with
most of their flying experience. It was the first of their aircraft to
have the double vertical tail, which was added to compensate for the tendency
of the craft to slew in a turn, and to include the hip cradle. The craft
had a wingspan of 32 feet and a chord of five feet. The Wrights had learned
from wind-tunnel tests that a long, narrow wing is more efficient than
a shorter one with the same area. Another feature of the 1902 glider is
that the wings are set in negative dihedral, that is, they droop.
Much more serious problems arose in several of the
turns. Both brothers experienced the beginning of a spin in the direction
of a turn. The stall-spin sequence in a turn has been estimated to be
the cause of loss of control in 70 percent of all flying accidents. The
Wright brothers were the first to discover it. They began a turn, say
to the right, by warping the trailing edges down on the left wings and
up on the right. The force acting on the vertical tail also tended to
turn the airplane to the right, counteracting the adverse yaw. If the
turn became too steep, the warping had to be reversed. This maneuver gave
rise to adverse yaw acting in the same direction as the turning effect
of the fixed vertical tail. For a short time the aircraft was therefore
turned even more to the right: the beginning of a spin. It could also
happen that excessive downward warping of the wings inside the turn would
cause them to stall and drop. Several times the brothers found that this
series of events ended when the wing that was inside on the turn dug into
the sand and the airplane pivoted to a stop.
It was Orville's idea to correct the behavior by
replacing the double fixed tail with a single movable one. Wilbur suggested
that its operation should be tied to the warping wires; it therefore could
not be operated alone. Nevertheless, a reversal of the warping also reversed
the yawing tendency produced by the movable tail, and the net result was
a significant reduction in the tendency to spin.
These various adjustments completed the configuration
and the control system the brothers employed in the Flyer of 1903. Not
until the end of 1904 did they disconnect the vertical tail and finally
achieve independent control of pitch, roll and yaw. Virtually all modern
aircraft depend on the same system of controls.
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