Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

The setup: two fans facing each other, distance around 1m. Both are turned on. In between them, place a simple paper plane and according to this video, it will fly.

http://www.bieraffe.com/1184/videos/papierflieger-schwebt-trick/

Is this really true? If yes, what is the physics behind it?

The fan that is blowing from behind seems to run at less RPM.

share|improve this question
    
It’s hard to judge speeds on camera with a regular frame rate like that. en.wikipedia.org/wiki/Wagon-wheel_effect –  DouglasHeriot Jul 10 '12 at 14:36
1  
I wrote that because on one fan the middle button is pressed while on the other fan the top button is pressed. –  Krumelur Jul 10 '12 at 15:23
    
I'm a bit incredulous that the plane could remain so stable for so long. In principle it seems possible, if the airflow field is the right shape. It would seem to be radially diverging, because the plane seems to weather-vane "downhill" toward a particular point. It's hard to imagine such a well-controlled airstream, but I guess it's possible. –  Mike Dunlavey Jul 10 '12 at 23:48

1 Answer 1

Intuitively, it seems entirely possible.

The "shape" of the air current blown out by both fans looks like a hollow cylinder, since the air is blown at high velocity near the ends of the blades, and lower velocity towards the center.

If you put two of these fans facing each other, the two "cylinders" of air current will collide with each other and create a "toroid" of positive pressure, in which you can suspend a light object, which is exactly what's observed in the video.

The "pressure toroid" is probably not stable (the slightest random motion of the object will lead to faster and faster motion out of the toroid), and this is also observed in the video.

I would assume that both fans should be spinning at equal RPM, if we want the point of equilibrium to be in the midpoint between the fans.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.