How does the Paper Spiral work?

[phantomvommand]

Hot air rises due to the candle below.
But how does that create a horizontal force, which is needed to produce the rotational torque?

 

[anorlunda]

Are you asking about something similar to this?

Maybe you can post a picture of the paper thing you're asking about.

 

[phantomvommand]

Are you asking about something similar to this?
View attachment 305014

Maybe you can post a picture of the paper thing you're asking about.

Yes, although the usual demonstration is less elaborate.

Some demonstrations like these https://www.ingridscience.ca/node/706 remind me of helicopter blades. But the difference I see here is that while the helicopter blades push the air in a diagonal direction such that there is a horizontal torque, in the paper spiral, when the vertically ascending air particles are stopped by an infinitesimally small paper area, the normal contact force is vertical (thus lacking the crucial horizontal component)

 

[malawi_glenn]

But how does that create a horizontal force

Because of the angle of the blades.
Forget about the air, let a particle collide with the blade instead. Seen from the side:
What is the direction of the impulse on the blade?

 

[anorlunda]

when the vertically ascending air particles are stopped by an infinitesimally small paper area, the normal contact force is vertical (thus lacking the crucial horizontal component)

No, the paper blades in the illustrations you linked are not vertical, they are diagonal. Part of the force is vertical, part horizontal.

 

[Ibix]

Are you talking about something like this, @phantomvommand? The principle is the same as the helicopter blades - the flat spiral falls into a spiral ramp, so the paper is everywhere slanted downwards in the same direction (clockwise or anticlockwise). The diagram in @malawi_glenn's post #4 then applies, and you get rotation.

 

[Lnewqban]

... in the paper spiral, when the vertically ascending air particles are stopped by an infinitesimally small paper area, the normal contact force is vertical (thus lacking the crucial horizontal component)

That statement is not correct.
The normal contact force is always perpendicular to the surface.

Please, see #1 here:
https://www.physicsforums.com/insights/frequently-made-errors-mechanics-friction/