https://www.instructables.com/The-Magic-Propeller/
A magic propeller toy is a regularly notched wood dowel with a light wood bar loosely nailed to one end, free to spin. The bar is powered by raking the notches with another wood dowel, and the direction is controlled by either resting a finger on one side or the other of the notches or rotating the notched dowel such that you rake one or the other side of the “mountain”.
Why does this work? You can ELI-PhD if you need, but even with my engineering background I can’t figure out what’s happening.
In: Physics
A magic propeller was gifted to me as a child. Much later, I encountered a MP vendor at a flea market. His props were built like the inscrutable as far as I know. They worked, but with difficulty. The props spun slowly and took 10s to reverse. My toy exceeds 100 RPM in 4s and reverses inside 1s.
Why does my toy respond so much better? Close inspection reveals that a small brass washer between the prop and nail head has been modified. The washer has been sanded across a flat to create a much thinner edge one side, in other words, beveled. Pretty sure this washer amplifies the vibration effect.
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