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extension of the wing. Low temperatures, lesser amounts of
liquid water, low velocities, and small droplets are conducive
to the formation of rime ice.
Mixed Ice
Mixed ice is a combination of clear and rime ice formed on
the same surface. It is the shape and roughness of the ice that
is most important from an aerodynamic point of view.
General Effects of Icing on Airfoils
The most hazardous aspect of structural icing is its aerodynamic
effects. [Figure 2-19] Ice alters the shape of an airfoil, reducing
the maximum coefficient of lift and angle of attack at which
the aircraft stalls. Note that at very low angles of attack, there
may be little or no effect of the ice on the coefficient of lift.
Therefore, when cruising at a low angle of attack, ice on the
wing may have little effect on the lift. However, note that the
ice significantly reduces the CL-max , and the angle of attack
at which it occurs (the stall angle) is much lower. Thus, when
slowing down and increasing the angle of attack for approach,
the pilot may find that ice on the wing, which had little effect
on lift in cruise now, causes stall to occur at a lower angle of
attack and higher speed. Even a thin layer of ice at the leading
edge of a wing, especially if it is rough, can have a significant
effect in increasing stall speed. For large ice shapes, especially
those with horns, the lift may also be reduced at a lower angle
of attack. The accumulation of ice affects the coefficient
of drag of the airfoil. [Figure 2-19] Note that the effect is
significant even at very small angles of attack.
A significant reduction in CL-max and a reduction in the angle
of attack where stall occurs can result from a relatively small
ice accretion. A reduction of CL-max by 30 percent is not
unusual, and a large horn ice accretion can result in reductions
of 40 percent to 50 percent. Drag tends to increase steadily
as ice accretes. An airfoil drag increase of 100 percent is not
unusual, and for large horn ice accretions, the increase can
be 200 percent or even higher.
Ice on an airfoil can have other effects not depicted in these
curves. Even before airfoil stall, there can be changes in the
pressure over the airfoil that may affect a control surface at
the trailing edge. Furthermore, on takeoff, approach, and
landing, the wings of many aircraft are multi-element airfoils
with three or more elements. Ice may affect the different
2-15
Figure 2-20. Effect of Ice and Frost on Lift.
Figure 2-21. Downward Force on the Tailplane.
Figure 2-22. Ice on the Tailplane.
elements in different ways. Ice may also affect the way in
which the air streams interact over the elements.
Ice can partially block or limit control surfaces, which
limits or makes control movements ineffective. Also, if the
extra weight caused by ice accumulation is too great, the
aircraft may not be able to become airborne and, if in flight,
the aircraft may not be able to maintain altitude. Therefore
any accumulation of ice or frost should be removed before
attempting flight.
Another hazard of structural icing is the possible uncommanded
and uncontrolled roll phenomenon, referred to as roll upset,
associated with severe in-flight icing. Pilots flying aircraft
certificated for flight in known icing conditions should be
aware that severe icing is a condition outside of the aircraft’s
certification icing envelope. Roll upset may be caused by
airflow separation (aerodynamic stall), which induces selfdeflection
of the ailerons and loss of or degraded roll handling
characteristics [Figure 2-20]. These phenomena can result
from severe icing conditions without the usual symptoms of
ice accumulation or a perceived aerodynamic stall.
Most aircraft have a nose-down pitching moment from the
wings because the CG is ahead of the CP. It is the role of the
tailplane to counteract this moment by providing a downward
force. [Figure 2-21] The result of this configuration is
that actions which move the wing away from stall, such
as deployment of flaps or increasing speed, may increase
the negative angle of attack of the tail. With ice on the
tailplane, it may stall after full or partial deployment of flaps.
[Figure 2-22]
Since the tailplane is ordinarily thinner than the wing, it is a
more efficient collector of ice. On most aircraft the tailplane
is not visible to the pilot, who therefore cannot observe how
well it has been cleared of ice by any deicing system. Thus, it
is important that the pilot be alert to the possibility of tailplane
stall, particularly on approach and landing.
Piper PA-34-200T (Des Moines, Iowa)
　
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