A variable pitch rotor is used when the rotation speed stays constant, and the thrust needs to be changed. It is often used on helicopters and ship propellers.

This model demonstrates the principle in a simple mechanism.

Some gifs of the movement can be found here:

http://imgur.com/gallery/3Y5kr

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Print each part according to the number (#) of times given in the filename (..._x #).

Assemble using screws (30mm and 40mm long are recommended).