Spinning Symmetry with Pinwheels

Why do wind turbines and airplane propellers look the way they do? The secret is in the symmetry. Learn why engineers design them this way with this fun, hands-on activity. Give it a whirl! 

Key conceptsFrictionWind power

Introduction

Background

One thing all of these blades have in common is that they are installed in a symmetric pattern. This means they are equally spaced around the center, or "balanced." There might be two blades directly across from one another or four blades that form an "X" and meet at right angles in the middle. There could even be an odd number of blades (for example, three blades evenly spaced would be at 120-degree angles to one another). Why is it important for the blades to be symmetric? Why don't you see windmills with just one blade or with two blades on the same side? This project will show you the best way to get the blades spinning!

Materials

• At least one pinwheel (that can be cut apart)
• Hair dryer
• Scissors
• Adult supervision

Preparation

• Gather all of your materials in an area where you can plug in and use the hair dryer. Have an adult operate the hair dryer about one meter away from where you will be standing with the pinwheel.

Procedure

• Turn on the hair dryer.
• Hold the pinwheel about a meter away from the hair dryer. Gradually move the pinwheel closer to the dryer until it begins spinning.
• Now, have an adult help you use scissors to cut off one blade from the pinwheel as close to the base as you can. This makes the pinwheel asymmetric because the remaining blades are no longer evenly spaced around the center.
• Again, hold the pinwheel about a meter in front of the hair dryer and gradually move it closer until it starts spinning.
• Use scissors to cut another blade off the pinwheel. Depending on how many blades your pinwheel had initially, you will have to make a decision about which blade to cut and whether the pinwheel remains asymmetric or becomes symmetric again. For example, suppose your pinwheel had four blades to start. When you cut off one blade, it has three unevenly spaced blades, so it is asymmetric. Now you can cut off a second blade next to the one you just cut off, so the pinwheel will still be asymmetric, with two unevenly spaced blades. Or you could cut off a second blade across from the one you just cut off. This will make the pinwheel symmetric again, with two blades directly across from one another. If you have more than one pinwheel, you can try different combinations.
• Hold in the pinwheel a meter in front of the hair dryer again and slowly move it closer.
• Repeat this process until your pinwheel only has one blade left.
• Extra: Use a ruler or measuring tape to record how far the pinwheel needs to be from the hair dryer to start spinning in each trial. Then you can make a graph of your results, with the distance from the dryer versus the number of blades.

Observations and results

More to exploreSpinning Your Wheels: Pinwheel Sensitivity, from Science BuddiesWind Turbine Design for Optimum Energy Output, from Strong Wind Science: The Power of a Pinwheel, from Wild Winds: Turbulent Flow around Structures, from Science Activities for All Ages!, from Science Buddies

Science Buddies

see also: