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In the last section a mention was made about not having
one wing heavier than the other. If one wing were noticeably
heavier it stands to reason that the plane would tend to roll
out
of level flight toward the heavy wing. This would be more
pronounced in a high g maneuver such as a loop. The first
step toward roll stability is a balanced wing.
Dihedral is the
most common way to introduce roll stability after
wing balance.
Figure 1 depicts wing dihedral in which both wing
tips are equally
higher than the wing root.
In figure 2 while viewing all the
wings from the rear, the top
wing is in level flight. The middle
wing is banked to the left.
Since gravity is pulling straight down
the left panel of the
middle wing is offering more area to produce
lift against gravity
than the right panel which is at a high angle
to the pull of
gravity. Since there is more lifting area on the
left wing panel
the wing will tend to roll itself back to the right
into level
flight. The bottom wing will tend to roll left back into
level
flight.
A high wing design also adds some pendulum
effect to roll stability
because the weight of the fuselage and its
components are below
the wing.
Good roll stability is
important to the training pilot because
he can concentrate more on
other things during his flight training.
As with pitch
stability, roll stability can be adjusted to suit
a pilots
individual taste or skill by using more or less dihedral
and by
wing placement of high wing, shoulder wing, mid wing,
or low
wing.
An aerobatic pilot finds dihedral a hindrance and usually
does
not use it. When an airplane is intentionally rolled inverted
any dihedral becomes anhedral (droopy wings) and the plane
becomes very unstable during inverted flight. The exception is
a
low wing design which when inverted effectively puts the wing
on
top and permits the fuselage to become a pendulum. In this
case a
small amount of dihedral is desirable to counter the pendulum
effect.
There have been a few designs such as the A7 Corsair
which have
used anhedral during normal flight to counteract
fuselage pendulum
effect.
Now is a good time to sum
up some of the things that have
been discussed so far. We have
learned how the weather effects
flight performance, how to create
lift, the reasons they fly
a little strange at times, how to set up
for proper ground handling,
how to adjust pitch sensitivity, and
how to control roll. In
the next section we will learn how to
control yaw to suit our
individual preference.
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