new blades

[phil b]

I finished a 5 phase genny from Hugh's plans 2 months ago. It produces 200 watts in a 12 mph wind. I had the unit in the air for 2 1/2 days when a 105 mph wind sheared the blades off the hub. I attribute this to inferior wood.



I made more blades from 3/4" birch plywood. I counted 13 laminations. No twist or any distortion in the blades. So far, they have held up to a 60 mph+ wind. The windward side is cut at 3 degrees along the entire length and are 8" wide at the root, 4" at the tip. I borrowed the 4/8 from DanB's 3 kw generator. Thanks Dan! I have no idea what the tsr is, maybe someone can help. They are reinforced on both sides with 3/8" steel plate. I put electrical tape over the screw holes and painted over them for easy access.

I can't leave well enough alone, so today, I added vortex generators to the blades. I have 10 1/4" triangles, set 1 1/2" apart for the first 15" from the tip. They're made from #15 magnet wire left over from the stator. I secured? them initially with nail polish on the trailing edge side, then reprimed the blades and coated them with latex house paint. Any ideas as to what effect these little critters will have on performance?

[phil b]

"I secured? them initially with nail polish on the trailing edge side...."

Sorry, should be secured on the leading edge, with the wide part of the wedge facing the trailing edge. Also, the blade is 1/2" thick at the tip and 3/4" at the root.  The 60+mph wind caused the blades to flex about 2" at the tip. Then they returned to their original position.

[Experimental]

   Hello Phil,

   I,m surprised to see plywood, survive at all, much less 60 MPH --as ply has inherent weekness to begin with !!

The number of plys, certainly helps -- problem being that the layers 90 degrees to the leingth -- make weak spots, when flexing and most ply has voids in those plys..

   You ought to try makeing some from, perhaps 1/2 X 8 boards, lamanated togeather (much stiffer)with a waterproof glue, and put twist in them  -- but if your producing 200 watts now, that,s pretty good results !!

   I doubt those vortex generators will do anything but disturb the air flow, but that is hard to say on wind gins -- these props don,t behave quite like wings , but in some respects --DO !!!

   Do you have some way of "measuring", the results ??

   Best of luck to you, Bill H....

[phil b]

    The vortex generators seem to slow the prop down some. The rpms don't fluxuate as rapidly as before.

I agree that plywood is weaker than laminates. I simply can't find comercially available wood that looks good. maybe I'm tooo picky. I've carved several props from 2X6's and they tend to bow or twist badly. Does anyone have any suggestions on where to get good straight grained wood? I hope to have fiberglass on my next set.

"Do you have some way of "measuring", the results ??"

No, I have 2 digital multimeters, and wind speed indicator. It would be nice to have a second anamometor to put behind the prop. The unit seems to produce only a couple of watts lower. I think it's worth it because of more rpm stablization.

[rotornuts]

I'm quite sure the vortex generators will bring down the unloaded rpm but in theory they should increase the width of your rotors "sweet spot". As you load the alternator the rpms come down in relation to windspeed decreasing tsr and increasing appearant wind angle. Your trying to increase how far this can go before the rotor stalls badly. It should work but my only comment is possibly related to the height of the vortex generators. They are supposed to sit within the boundry layer at low AoA's so It's possible to assume yours may be a hair tall. I would guess based on some tripwire experiments I did that an eigth of an inch should be sufficient. Could be wrong but the fun is in the experimenting.

P.S. It's nice to see someone playing around with these things.

Mike

[Experimental]

    Hello again Phil,

    You mentioned the problem with getting good wood, and I just laminated some 3/4 X 5-1/2 up, this evening !!

    I used some clear fir that has been sitting in my garage for about two years, so it should be nice and dry --  total thickness is 2-1/4"..

    I noted many guys have been laminating, side by side, in strips, but the best stiffness, is by stacking one on top of the other as aircraft propellers are made !!

    I would suggest getting some mahogany, in 1/2 or 3/4" boards and lam them together, with epoxy glue..

Mahogany is a lot like fir, to work with -- but is usually very clear -- i just used the fir, cause I had it !!   Also, the last time I bought mahogany, it was cheeper than clear fir...

    I will be doing the rough cutting on my band saw, including the twist, and final cutting with a spoke shave and wood rasp -- if it all works well -- it will be on the board, with pictures ...

    By the way, laminating also, keeps the wood from warping -- but it must be fairly dry -- The mahogany is normally kiln dried -- again, a big help !!

    Nice to see someone experimenting with vortex generators -- keep at it and have fun !!  Bill H....

[phil b]

Thanks for the info Bill H and Rotornuts. I'll probably use plywood and about 6 layers of fiberglass on my next set of blades because I have it. Afterwards, the kiln dried wood looks like a very good option.