Page 95 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 82

82 CHAPTER THREE

ICE ON A WING (Continued)

The buildup of ice does several negative things to the wing.
The first is that it changes the shape of the wing and thus
changes its stall characteristics. In general, a wing with ice will
stall abruptly at a lower angle of attack than without ice. Ice
also adds weight, which is an additional load on the wing.
Therefore, to compensate, the pilot must increase the angle of
attack. Eventually, the angle of attack required to maintain flight
reaches the new stall angle of attack and the airplane can no
longer fly.An additional impact of ice is increased drag, both
induced and parasitic.The result is that the power
requirement increases.

Boundary-Layer Turbulence

Normally, as passengers or pilots, we associate turbulence with
atmospheric conditions. This is known as clear-air turbulence. Clear-
air turbulence is important to structural designers to design for gust
loading. But there is another type of turbulence that is critical to the
aerodynamic design of the wing. This is boundary-layer turbulence,
which occurs only in the very thin boundary layer.
Most pictures of wings with air flowing over them show
For a week every summer the smooth patterns of air. This is laminar flow, which is
busiest airport in the world is nonturbulent flow. Figure 3.22 illustrates both laminar and
Whitman Field in Oshkosh, WI. turbulent flows. In the picture smoke in laminar-flow air
This occurs during the passes through a special screen. Shortly after the screen the
Experimental Aircraft Association laminar flow becomes turbulent. The basic difference is that
(EAA) annual fly-in. the laminar flow is smooth while the turbulent flow is chaotic.
Skin friction, and thus skin drag, increases dramatically with
boundary-layer turbulence. A wing with laminar flow will
have much less skin drag than a wing with turbulent flow.
It would be a great asset to design a wing that was laminar over its
entire surface. But this has proved extremely difficult. Boundary-layer
turbulence is a natural phenomenon and it becomes more prevalent as
speed and the size of the airplane become greater. Besides this natural