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My friend showed me something which he called the glider. At a first look, it looks like it will crash, but it flew quite well:

enter image description here

I was surprised to see it go about 25 meters (82 feet), as it was nothing but a big ring made out of paper. I thought of how it could fly, but I had no idea. I knew that it could go through air with least resistance, but it wasn't supposed to fly.

Can someone explain me the logic of how this flies?

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  • $\begingroup$ Was the paper heavier at the front, either by folding or by adding a paperclip? $\endgroup$
    – Peter Kämpf
    Jan 30, 2016 at 10:26
  • $\begingroup$ @PeterKämpf, nope, there was nothing. It was the same throughout. $\endgroup$
    – Box Box Box Box
    Jan 30, 2016 at 11:21
  • $\begingroup$ @mins. Mm, actually I am an absolute beginner. I don't know the difference. Basically my friend just held the ring in his hand, and threw it. He held the curved paper part in his hand. Also, it is not launched in rotation. $\endgroup$
    – Box Box Box Box
    Jan 30, 2016 at 11:25
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    $\begingroup$ @Peter Kämpf I'm wrong about that. There's a video on how to make it. Several folds in front. Half of the weight of the paper is in the front edge. $\endgroup$
    – TomMcW
    Jan 30, 2016 at 16:20
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    $\begingroup$ Demonstrated on TV show QI (2:00 onwards) $\endgroup$
    – A E
    Jan 30, 2016 at 17:14

1 Answer 1

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No takers? Then I'll bite.

This question addresses how paper airplanes generate lift. This lift is caused by higher pressure on the lower side and lower pressure (suction) on the upper side, and if both are added up, the chordwise pressure distribution is shown below:

Flat plate pressure distribution

Flat plate pressure distribution (picture source)

If you collect all the local forces in one point, the lift acts at a quarter of the wing's chord. That is why the forward half of the paper which in the end will be bent into the ring needs to be folded onto itself: The center of gravity of the wing is also at one quarter of chord. Thus, the lift and the weight will act a the same station and no pitching moment results.

To achieve a flight distance of 25 m, your friend threw the ring with some force to give it a high initial speed. This speed would allow it to create enough lift with very little angle of attack, so it did not suffer from flow separation initially. This resulted in low drag and the high launch speed gave the ring some kinetic energy.

Friction slowed the ring down gradually, but the decrease in lift at lower speed was compensated by a gradual increase of the ring's angle of attack. Why would the angle be just right to prevent the ring both from rising and dropping, you might ask? Any imbalance between lift and weight would add a vertical acceleration which would immediately change the angle at which the ring hits the flow. Its chord length and inertia would prevent it from rotating nose up or down, so it would pretty much stay on its initial path until most of the speed was eaten up and the angle of attack was so high that separation sets in. At this point, the drag would go up and speed up the deceleration process, and the ring would sink to the ground.

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