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Dihedral<br />
Some aircraft are designed so that the outer tips of the wings<br />
are higher than the wing roots. The upward angle thus formed<br />
by the wings is called dihedral. [Figure 5-28] When a gust<br />
causes a roll, a sideslip will result. This sideslip causes the<br />
relative wind affecting the entire airplane to be from the<br />
direction of the slip. When the relative wind comes from the<br />
side, the wing slipping into the wind is subject to an increase<br />
in AOA and develops an increase in lift. The wing away<br />
from the wind is subject to a decrease in angle of attack, and<br />
develops a decrease in lift. The changes in lift effect a rolling<br />
moment tending to raise the windward wing, hence dihedral<br />
contributes to a stable roll due to sideslip. [Figure 5-29]<br />
Sweepback and Wing Location<br />
Many aspects of an aircraft's configuration can affect its<br />
effective dihedral, but two major components are wing<br />
sweepback and the wing location with respect to the fuselage<br />
(such as a low wing or high wing). As a rough estimation,<br />
10° of sweepback on a wing provides about 1° of effective<br />
dihedral, while a high wing configuration can provide about<br />
5° of effective dihedral over a low wing configuration.<br />
Wing has decreased<br />
AOA, hence reduced lift<br />
due to sideslip.<br />
Restoring lift<br />
Wing has increased AOA, hence<br />
increased lift due to sideslip.<br />
Sideslip<br />
Figure 5-29. Sideslip causing different AOA on each blade.<br />
A sweptback wing is one in which the leading edge slopes<br />
backward. [Figure 5-30] When a disturbance causes an<br />
aircraft with sweepback to slip or drop a wing, the low<br />
wing presents its leading edge at an angle that is more<br />
perpendicular to the relative airflow. As a result, the low<br />
wing acquires more lift, rises, and the aircraft is restored to<br />
its original flight attitude.<br />
Keel Effect and Weight Distribution<br />
A high wing aircraft always has the tendency to turn the<br />
longitudinal axis of the aircraft into the relative wind, which<br />
is often referred to as the keel effect. These aircraft are<br />
laterally stable simply because the wings are attached in a<br />
high position on the fuselage, making the fuselage behave like<br />
a keel exerting a steadying influence on the aircraft laterally<br />
about the longitudinal axis. When a high-winged aircraft is<br />
Figure 5-30. Sweepback wings.<br />
disturbed and one wing dips, the fuselage weight acts like a<br />
pendulum returning the aircraft to the horizontal level.<br />
Laterally stable aircraft are constructed so that the greater<br />
portion of the keel area is above the CG. [Figure 5-31] Thus,<br />
when the aircraft slips to one side, the combination of the<br />
Dihedral<br />
Dihedral<br />
Figure 5-28. Dihedral is the upward angle of the wings from a horizontal (front/rear view) axis of the plane as shown in the graphic<br />
depiction and the rear view of a Ryanair Boeing 737.<br />
5-18