'Round and 'Round

As mentioned in the previous post, airplanes usually use either jets or propellers to make them move. In this experiment, you'll try out some different propeller designs.

What you'll need:
Several 3" x 5" index cards
Scissors
Straight pin
Pencil with eraser

Cut the index cards into 1" x 5" strips (they don't have to be exact). Push the pin through the center of one of the strips and move the pin around to make the hole slightly larger. The strip should be able to spin around on the pin. Insert the pointy end of the pin into the pencil eraser. Hold the pencil straight up, place the propeller close to your mouth, and blow. What happens? Do the propeller blades turn? Try making some other propeller designs. Fold one or both corners of the strip. Does that make a difference? Does it matter whether both corners are up (or down), or whether one is up and one down? You can cut the other strips into different shapes, or fold and bend them in different ways, to see which design makes the best propeller. You just need to try to blow the same in all cases.

What you'll find is that flat blades don't turn very well. The air just rushes over the blade. But when you put some sort of curve or fold in, the air pushes against it, turning the propeller. The more area there is to push, the easier the blades turn. The air pressure is higher in front of the blade and lower behind. Airplane propellers work on the same principle. They are not flat, but curved and twisted.