Some of the curtismachine gearboxes like the one in the illustration above have this gear
configuration available......
This one has four intermeshing gears. Because there are several different optional gear
configurations and ratios for the same size case (STRAIGHT BEVEL 1:1, 1.35:1, 1.5:1),
it looks to me like it might be possible to use either the (1.35 to 1) gear sets or (1.5 to 1) gear sets
instead of the 1 to 1 ratio gears in this illustration to make a 4 blade variable pitch hub.
To do this would require some decent machining skills (and machines). It would require
boring a hole through the case/box to install another (thru) shaft and installing bearing flanges
and another gear (for a total of 5 gears). The larger gear would mount on the newly installed
thru shaft, and the smaller gears (4) would replace the four 1 to 1 ratio gears in the illustration.
Depending on how curtismachine builds these boxes, it may or may not be neccesary to do
additional machining to get the 4 gears (no longer intermeshing with one another) to mesh with
the newly installed larger gear.
Ok, even I have to admit this is getting a little too complicated, but I thought the idea was
interesting enough to throw out there even if it isn't really practical.
The curtismachine site also says they can make any custom gearbox with angles other
than 90 degrees, so I'm sure they could build a more ideal 120 degree version for a 3 blade hub.
(For a slightly higher cost)..... ;-)
Clint