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Flying cups

By David Shaw, 9 August 2018

Do you like paper planes? Here’s a different glider to try, made from cups. You might be surprised at the path it takes when it flies!

You will need

• 2 disposable cups*
• Sticky tape
• About 10 rubber bands

* Note: Foam or plastic cups work best, but paper cups can still be used

Making the glider

1. Put one cup upside down and the other the right way up on top, so the bottoms touch.
2. Wrap a piece of tape around the join to stick the two cups together. This is your glider.

Making the launcher

3. Hold one rubber band horizontal.
4. Hold a second rubber band vertical inside the horizontal band.
5. Push one end of the horizontal band through the vertical band. Pull that end with one hand, and the top of the vertical band with the other hand, in opposite directions.
6. Continue this pattern with the remaining rubber bands to form a chain, keeping the existing chain at the top of the vertical band. This is your launcher.

Flying the cups

7. Find a clear area where you won’t hit anyone or anything. In the following steps, be careful to keep the rubber chain away from your face.
8. Stretch the rubber chain launcher and wrap the end three times around the join between the cups on the glider.
9. Keeping everything tight, hold the glider with one hand near your body. With your other hand, hold the launcher away from your body. The launcher should be coming out from under the bottom of the glider. If it’s coming off the top, turn the glider over.
10. Make sure the launcher is taut, then let go. It might take a few goes, but when you get it right, the glider will fly and spin up into the air!

Trouble-shooting: If the glider keeps bumping into the hand holding the launcher, make sure the launcher runs across the top of the thumb of the hand that’s holding it.

What’s happening?

As the cups spin, air curves around one side (blue arrow) and the cups are pushed the other way (red arrow).
Image: Wikimedia commons/MatSouffNC858s CC BY-SA 4.0

This cup glider’s flight might seem surprising, but scientists have been studying the cause for hundreds of years. It’s known as the Magnus effect, and it’s named after Heinrich Gustav Magnus, a scientist who studied the phenomenon almost 170 years ago. He wasn’t the first to look into it though – Isaac Newton noticed its effect on tennis balls 180 years earlier.

The Magnus effect is most commonly seen in ball sports. From a bending soccer ball, to a back-spinning table tennis shot, the Magnus effect explains why spinning balls curve as they fly through the air.

Although you’ve seen it in action in this activity, it’s still hard to explain exactly why it happens. Here’s one way of looking at it. As the cups fly through the air, the rotation means the air flows smoothly over the top of the cups, and eventually curves around and down. In return, the air pushes upwards on the cups, making them fly higher.

One use for the Magnus effect is in transportation. In 1930, American inventors flew a plane with rotating cylinders instead of wings, but it was not very popular. A more modern use is for powering ships – a large cargo ship known as E-Ship 1 has four giant rotors sticking up like chimneys. When the wind blows and the cylinders are spinning, the Magnus effect pushes the ship forward!

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