From the
Pilot's Handbook of Aeronautical Knowledge (Chapter 3)
[This handbook is available on the FAA Web site at:
http://afs600.faa.gov/srchFolder.asp?Category=traininghandbook&cmdScroll=2]
LATERAL STABILITY (ROLLING)
Stability about the airplane’s longitudinal axis,
which extends from nose to tail, is called lateral
stability. This helps to stabilize the lateral or
rolling effect when one wing gets lower than the
wing on the opposite side of the airplane. There are
four main design factors that make an airplane stable
laterally: dihedral, keel effect, sweepback, and
weight distribution.
The most common procedure for producing lateral
stability is to build the wings with a dihedral angle
varying from one to three degrees. In other words, the
wings on either side of the airplane join the fuselage
to form a slight V or angle called “dihedral,” and this
is measured by the angle made by each wing above a
line parallel to the lateral axis.
The basis of rolling stability is, of course, the lateral
balance of forces produced by the airplane’s
wings. Any imbalance in lift results in a tendency
for the airplane to roll about its longitudinal axis.
Stated another way, dihedral involves a balance of
lift created by the wings’ angle of attack on each
side of the airplane’s longitudinal axis.
If a momentary gust of wind forces one wing of the
airplane to rise and the other to lower, the airplane
will bank. When the airplane is banked without turning,
it tends to sideslip or slide downward toward the
lowered wing. [Figure 3-17] Since the wings have
dihedral, the air strikes the low wing at much greater
angle of attack than the high wing. This increases the
lift on the low wing and decreases lift on the high
wing, and tends to restore the airplane to its original
lateral attitude (wings level)—that is, the angle of
attack and lift on the two wings are again equal.
The effect of dihedral, then, is to produce a rolling
moment tending to return the airplane to a laterally
balanced flight condition when a sideslip occurs.
The restoring force may move the low wing up too
far, so that the opposite wing now goes down. If so,
the process will be repeated, decreasing with each
lateral oscillation until a balance for wings-level
flight is finally reached.
Conversely, excessive dihedral has an adverse effect
on lateral maneuvering qualities. The airplane may be
so stable laterally that it resists any intentional rolling
motion. For this reason, airplanes that require fast roll
or banking characteristics usually have less dihedral
than those designed for less maneuverability.
