I ran across this set of blades while perusing Ebay, and thought it was interesting enough to post a few pictures of them here. I have no affilliation with the seller, I just thought the blades merited posting particularly because of the braking system. I obtained permission from the seller to repost his photos here.
The blades contain an overspeed brake that basically functions when the the blades speed up to a point that a spring supplying centripetal force holding a bar in place is overpowered, and the bar then moves outward away from the axis of rotation. That bar is attached to a pivot further out on the blade that causes it to lift out from the blades surface on the backside of the blade as it moves outward, disrupting the airfoil which should move the blades towards stall.
The red cover nearer the root covers the spring where the bar is attached. The spring holds the bar towards the root until it is unable to supply enough centripetal force to balance the outward force on the bar due to the radial motion. Just like those old barrel rides at the amusement park where the barrel spins around and the floor drops out. In that case, the outer wall provides the force that prevents your body from flying off in a straight line. In the case of the bar, it is the spring to start.
On the outer end, the bar is attached to a pivot. Because of this, when the bar moves outward, it is also lifted away from the back of the blade. Intuitively, I would think that this would increase the lift to an unsustainable level, separating the airflow from the back of the blade, causing it to move into stall and slowing the blades. The seller stated he pushed the bar out with a screw driver and it lifted a quarter of an inch or so. I would bet that it is pretty noisy when this occurs.
The blades were made by Wind King Electric Company, Merrill, Iowa. From their appearance it looks as though they were not used much or at all. They were apparently labeled one through three, and it would appear that the tear drop shaped red covers are balancing weights.
I would guess it was balanced on the alternator at the factory, and then labeled and disassembled for shipping to the customer for re-assembly.
The blades have copper sheeting stapled to the outer part of the leading edge, as well as a couple of bands of coper sheeting wrapping the outer part of the laminated wood blade. From this I would gather that they were designed to operate at a fairly high TSR, which would probably make sense in the days before stronger neodymium magnets.
Again, I am not trying to sell his blades for him, but I probably should post the link to the ebay ad which is here: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=260203279892&Category=121837&_trksid=
p3907.m29
Also, there are a couple of other photos in my user files of these blades.
Just thought these were interesting, Rich |
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