Environmental Engineering Reference
In-Depth Information
The challenge for engineers was to devise a windmill whose blades will turn whatever
the wind's direction. They solved this problem by exploiting two forces produced by air
movement: drag and lift. Drag is the wind resistance that I experienced as a young cyclist.
Lift is the 'pull effect' created when the air pressure on one side of an object is greater
than on the other. There are several ways to explain how lift works. In the end, all of these
explanations boil down to the fact that the flow of air around an airfoil (or wing) generates
an increase in air pressure under the wing and a decrease in pressure above it, causing the
wing to lift (see Figure 4.9 ). Thanks to this effect, birds and, by imitation, airplanes are
able to fly. The faster the airflow, the stronger are the forces acting on the airfoil. When the
speed drops below a certain level, the lift is not sufficient to sustain the wing and the bird
or plane will descend towards the ground.
Figure 4.8. We are familiar with what happens when one solid object hits another. This
effect is particularly evident in ball sports where players use one object (such as a bat) to
move another (the ball). However, when fluid matter (a liquid or a gas) passes around a
solid object, the reaction is more complex. Sources: Grunkcommons at Wikimedia (left);
Robert Swier at Flickr.
Figure 4.9. Forces that act on an airfoil, causing lift.
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