IFH 4-14

Rime Ice CL (coefficient of lift) Angle of Attack Clean airfoil Airfoil with ice CD (coefficient of drag) Angle of Attack Clean airfoil Airfoil with ice Figure 4-20. Aerodynamic effects of icing. Figure 4-19. Rime ice. Clear Ice Buildup with HornsFigure 4-18. Clear ice buildup with horns. for essentially the same type of ice accretion. This type of ice is denser, harder, and sometimes more transparent than rime ice. With larger accretions, clear ice may form “horns.” [Figure 4-18] Temperatures close to the freezing point, large amounts of liquid water, high aircraft velocities, and large droplets are conducive to the formation of clear ice. Rime Ice A rough, milky, opaque ice formed by the instantaneous or very rapid freezing of super cooled droplets as they strike the aircraft is known as rime ice. [Figure 4-19] The rapid freezing results in the formation of air pockets in the ice, giving it an opaque appearance and making it porous and brittle. For larger accretions, rime ice may form a streamlined 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 4-20] Ice alters the shape of an airfoil, reducing the maximum coefficient of lift and AOA at which the aircraft stalls. Note that at very low AOAs, there may be little or no effect of the ice on the coefficient of lift. Therefore, when cruising at a low AOA, ice on the wing may have little effect on the lift. However, note that the ice significantly reduces the CL-MAX, and the AOA at which it occurs (the stall angle) is much lower. Thus, when slowing down and increasing the AOA for approach, the pilot may find that ice on the wing, which had little effect on lift in cruise now, causes stall to