Why Are Blades on Fans Tilted?

The Simple Machines

• Though ordinary people often think of a machine as a mechanical device like a car or a fan, physicists define a machine as any device, no matter how complex or simple, that helps with work. Physicists define six types of simple machines. They are the inclined plane, the wedge, the screw, the pulley, the lever, the wheel and the axle.

The Screw

• Tilted fan blades are an example of the "screw" simple machine; however, they are not the most intuitive example, so to learn the concept, start with an ordinary screw. It is easier to drive a screw into a block of wood because the threads serve to increase the distance over which the work is done.

Screw Versus Nail

• This can be seen by comparing a screw to a nail. A nail, going into a block of wood, moves a total of only 1 to 2 inches, which means all the force needed to drive the nail in must be applied over that distance. By comparison, a screw must be twisted, and this allows it to translate a large amount of circular motion into a small amount of forward motion. For example, if it takes 100 lbs. of force to drive a 1-inch nail into a block of wood, then you must apply 100 lbs. of force per inch. However, a 1-inch screw may require a total of 100 inches of circular motion to drive into the wood. Since the motion is spread out over a 100 inches, rather than a single inch, the force required per inch is much smaller: 1/100 lb. of force per inch.

Fan Blades

• Now, you can imagine a fan as a sort of reverse screw -- instead of screwing into the air, as a ordinary screw goes into a wood, the fan pushes the air away. By tilting the blade and using a screwing motion, the total force required of the fan is reduced, and that means the fan can use a weaker, cheaper motor and less electricity to achieve the same effect.