There's been some talk about variable pitch hubs lately on various scales of machines. I'd like to know why more people don't build these hubs themselve's? Well everyone I ask says that they're too complicated to build and looking at some of the pictures you guys post, it's no wonder many are intimidated by it. All those weights and levers and bearings and arms and hinges...bleh...that stuff's too complicated for our generally simple mills.
Now with the lack of modern toolery I can't make very pretty gadgets, but the ones I make are pretty effective. They do make a bit of mess and require what I feel is monthly maintanance, but they work. They're a very simple screw and spring setup.
Essentially how my pitch machine is built is I take a six inch long piece of 1" OD 3/4" ID tubing and cut a 1/4" slot down it's side, diagonally, starting one half an inch from one end and ending half an inch from the other. I do this along a 90* chord...that's to say that if both ends were parallel circles, the slot started at 90* in reference to one and ended at 180* in reference to the other. This is the backbone of the machine and this is where you decide how much pitch you want your blades to be able to tackle...in my opinion a 90* slot is perfect because it will allow your blades to rotate 90*. My blades start out at at a 20*, which means in high winds, instead of a furling tail, my eight foot and bigger props reverse the pitch to fight the wind...a much more effective brake than pointing the blades directly into the wind and far superior in my opinion to a furling tail (no need, the blades' rpm automatically maintanes).
The actual mechanism is far simpler than the "sleeve" above. It's an 8" long, 1" OD steel rod with a 1/4" hole drilled 1/2" from one end. You insert this rod in the sleeve and hammer a 4140 tempered dowl into the hole, through the sleeve's slot. I then grind the dowl flush with the sleeve's surface. Then grease the hell out of it.
Then, along the hub, front and back, at intervals corresponding with the number of blades, there's eye bolts. There's one on both sides of the blades, about 3" away from their mounting surfaces. A spring is then connected between each eyebolt on the hub and it's opposite on the blades.
The driving force for this machine is the inertia of the spinning blades (faster it spins, the stronger the blades pull away from the hub), and the resistance of the springs.
The blades clamp into plates I welded at the end of the steel insert...1/8" mild on top and bottom of the blades seems to work. I'm thinking of welding the blades' eyebolts to these clamps for the sake of keeping the hub completely contained.
So far the bigger one I built (8ft prop) seems to work ok but I've only seen it at about 3 different pitch areas...only seen about 20mph winds since I put it up. I haven't seen any of the 50mph winds that inspired the invention...after the force of the blades ripped one hub apart...so it's hard to say if the springs I used were wise. It's somewhat difficult to machine-spin an 8ft prop to 750rpm lol. Trying to think of a way to do it without the blades attached if anyone has any ideas.
Screen-door spring segments seem to be working well on my 36" and 24" hubs.
I don't have pics at the moment but I'll pull the 8-footer down tomorrow and snap some...hopefully I've got a good mental painted for you guys here, though. Any thoughts?
