Joseph Gwinn wrote:

Aluminum will work only as an eddy brake, and the drag will vary as the
square of speed (rpm).

A sheet of hard steel will in addition act as a hysteresis brake,
increasing the drag at lower speeds. Mild steel is too soft
magnetically to get much hysteresis effect, but a flat sheet of hardened
spring steel ought to work. It doesn't need to be pretty.


So they're two variations - eddy brake and hysteresis brake?
I think aluminum would be easier to work with, and the speed squared
characteristic of eddy sounds good.
Should the magnets be mounted either side of the disc, with opposite
poles facing each other?