Pilot and student pilot community. Share your pilot lessons or aviation stories.



General Effects of Icing on Airfoils

in Aerodynamic Factors

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.

Figure 2-19. Aerodynamic Effects of Icing.

Figure 2-19. Aerodynamic Effects of Icing.

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 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.

Figure 2-20. Effect of Ice and Frost on Lift.

Figure 2-20. Effect of Ice and Frost on Lift.

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]

Figure 2-21. Downward Force on the Tailplane.

Figure 2-21. Downward Force on the Tailplane.

Figure 2-22. Ice on the Tailplane.

Figure 2-22. Ice on the Tailplane.

 

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.

 

Comments on this entry are closed.

Previous post:

Next post: