According to Jordan :


An aluminum disc on the back end of the shaft running between a
pair of high strength permanent magnets recovered from dead hard disk
drives (thus cheap). The faster it goes, the more drag from the
magnetic field generating eddy currents in the disc.

It may take some playing to determine what radius and how many
magnets will be needed to get sufficient speed control.

Good Luck,
DoN.

Thanks Don
I don't know much eddy currents. Does this tend to allow the rotor to
spin easily at first?

Yes. To flatten it out, use a disk of a magnetically hard metal, such
as hardened steel. This is called a hysteresis brake.

Joe Gwinn

This is definitely of interest. Alum would work? What about mild steel?
Need to keep cost down. It'll only be up and running a few weeks.

Both will work, though I think that Aluminum would give more
braking difference between slow and high speeds.

Get a disk of each of the same size, and a pair of magnets out
of a dead disk drive (be *very* careful not to get pinched when
separating the magnets -- those are killers. :-) Once you have both
disks, try each to see what behaves more like what you want. At a dead
stop, the aluminum will produce zero drag. The faster it turns, the
more drag. I've not tried it with steel, but I would expect more
braking at slow speeds, and less at high speeds.

Thanks!

You're welcome. I hope that it works out for you.

Note -- the more wind power going into the prop, the hotter the
disc will get, so weld or secure with screws or rivets, not glue, or it
might let loose with a stiff breeze -- just the time you don't want it
to do so.

If you want to get really tricky -- add a centrifugal weight
assembly to change the pitch of the props, so you get less drive at
higher speeds.

Good Luck,
DoN.
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