Dihedral |
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| When
comparing a low wing airplane to a high wing airplane, a number of
differences are seen. One of the most obvious, and most important to the
flight characteristics, is the angle at which the wings are attached to
the plane. If a line is drawn across the front of the plane through the center
of lift, as represented by the blue
dot, an angle is formed between the line and the wingtip.
This angle is called dihedral.
On the high wing plane, the line runs through the wing forming very
little dihedral. On the low wing,
there is a very noticeable dihedral
angle. Dihedral helps in the lateral
or roll stability of the
airplane. Not only is there a center of lift on the airplane, there is
also a center of weight as
represented by the yellow dot.
In a coordinated, undisturbed turn, the
center of lift and the center of weight remain in line thru the vertical
axis of the airplane. This is due to the action of
centrifugal force. But if the turn is disrupted as by a gust of wind,
the center of weight and center
of lift are no longer in line. The center
of weight will try to
shift in the same direction as the roll of the turn. The high
wing airplane has a natural action to correct for the disruption. Since
the center of weight
is located below the center of lift,
the weight of the airplane will try to come to rest directly under the center
of lift, just like a pendulum.
However, in a low wing plane, since the center
of weight is above the center
of lift, the weight will continue in the same direction of the roll,
just like a tall mast on a ship.
When the low wing plane is in a bank,
the lower wing to the inside of the turn has less of a bank angle than
the higher wing on the outside of the turn. This allows the lower wing
to develop more vertical lift than
the higher wing. It is the developed vertical
lift that will raise the lowered wing, counteracting the force of
the weight to continue into the roll.
The pilot is able to make simple control adjustments without over
correcting.
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High Wing
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Low Wing
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Center of Lift - the horizontal line runs
through the wing.
Center of Weight
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Center of Weight
Center of Lift - the wings are angled up
from the horizontal line through this point.
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| If a gust of wind
disrupts the coordinated turn. . .
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Center of weight rolls opposite to the
direction of bank and will put increasing force to upright the plane.
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Center of weight rolls in the same
direction of bank pulling the plane further away from vertical.
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The distance of each wing from the
horizontal is equal so the upward lift vector is the same, with the
center of weight pulling the airplane to the vertical position and
stabilizing the bank.
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The upper wing is further from the
horizontal so the upward lift component is smaller than the lower
wing. The lower wing then has greater lift pushing the airplane
upright.
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