17 footer noisy

[halfcrazy]

Ok my neighbor and I built a 17ft machine for his place from scratch. We followed the plans here exactly for the most part. I did wind it for a touch higher voltage to take advantage of the Classic MPPT Controller. The issue is that it has a wooshing sound to it when the wind slows down suddenly. anytime it falls below cutin speed or the wind slacks of rapidly basically it makes this sound it sounds like the noise you get from spinning a length of hose around almost. I suspect we did not get the blades quit right and they are just noisy when they are not loaded working.

How sharp does everyone else get the trailing edge? ours is still pretty thick maybe 3/16 to a 1/4 I was nervous about getting it to thin. I will attempt to attach a short video of it making the noise and a picture. Ok video is to large I can email it to anyone that may be interested. The turbine is on a 80ft tilt up tower and seems to make good power We where seing 2-2,500 watts yesterday threw the Classic in about 15-20 mph winds

[Hilltopgrange]

Hi halfcrazy

I had exactly this same problem with my 16ft machine when I first raised it. I tried various things inc a bead of string down the back of each blade, balance and tracking etc some of which did help but didn't fix it. In the end I run a sanding disc down the trailing edge sharpening it to a razor edge and it completely removed the noise.

I hope this helps.
Russell

[halfcrazy]

that is my suspicion I am afraid I left the trailing edge way to blunt I wonder how much performance loss this causes?

[Fused]

I have not built a 17ft machine but I can say that Ive had similar noises on smaller blade sets. I had left my trailing edge too thick as was mentioned already. I did not bring it to a knife edge but the closer I got to it, the less I noticed the noise behind the machine.
I seem to have noticed the noise behind the machine more than in front. It may be the same with yours, dont know.

Hope it helps, even just a wee bit. Others I'm sure have run into it. Hopefully they will share.

Fused

[Hilltopgrange]

To be honest I never noticed a change in performance at all but it made a massive difference to the noise, even in a gale it is almost silent. I sanded mine with a small grinder and 80 grit disc, I feathered it back about an inch or so on the back of the trailing edge to about 1mm max. I have a latice tower and did this at the tower top as I couldn't be bothered dropping the tower. You don't need to be to scientific about it, it took me a bout 10 minutes to fix. The biggest problem I had was holding the blades still while I sanded it, i forgot to bring a piece of rope up with me lol

This was about a year ago and at that time I read a lot of old posts on this very subject one in particular stood out I think it was by ULR and gave a detailed account of why it made such a noise. Do a search and it should show up! I'm not so concerned as to why it works just that it does is good enough for me lol

Russell

[Hilltopgrange]

I forgot to say I also rounded and sharpened the tips of the blades slightly but not as extreme as the trailing edge. Russell

[imsmooth]

http://www.mindchallenger.com/wind/axial13.html

If you look at the picture you can see that the trailing edge at the tip comes almost to a sharp edge.  I rounded it a little so the wood would not fall apart from the weather.  The trailing edge is very thin.  At some points it came to a point.  I thickened this up with marine epoxy for strength and protection.  I hear a slight sound at high speeds.

[freejuice]

Half,
 I get a little swooshing sound at cut in and it grows just a  bit louder as the amperage picks up, but it's really not a "loud" sound...my trailing edge is about what you described, 3/16-1/4 inch.
 I'm going to send my email address to your mailbox here..I would like to hear it.

Could it be the tracking?

[Flux]

Most props tend to be noisy just below cut in or when not loaded enough and the tsr is above the design figure. With normal stall loading the things are usually quiet above cut in.

With mppt you may find more noise in the operating range with higher tsr but if you keep it near optimum it's not really noisy.

Blunt trailing edges cause a great deal of noise but as Russell said it has little effect on performance. As sharp as possible is the best for noise, but something like 1/2mm is probably near enough. There is little loading or errosion on the trailing edge so sharp edges seem to survive ok. Some timber grain is better than others for being able to maintain a sharp edge without splitting off.

I have also found shaping the tips can help considerably with fast blades but normally with lower tsrs it doesn't seem that important.

Someone described a method of blending the tips out to a sharp edge and cutting them at an angle ( was it a sandia labs article?, cant remember where I saw it) that works really well and is worth doing if you have tsrs of 7 or over.

Thick prop sections tend to be noisier than thin ones and in that case the nylon cord or rib on the back that Russell mentioned may be worth trying, I haven't tried it but it works with some profiles used on the Chinese machines. It may only work on some of the high lift profiles.

Flux

Tracking tends to make one blade whistle if it is significant. I have not noticed any increase noise from lack of balance.

[imsmooth]

The other comment I left out was that I sanded my blades baby-butt smooth.  I worked my up to 600 and 1200 grit sand paper that I got from an auto supply.  I sanded the epoxy finish.  After it was glass smooth I sprayed it with autoclear coat.

[97fishmt]

I have also found shaping
the tips helps with noise.

You want those trailing
edges to be sharp!

[ChrisOlson]

The issue is that it has a wooshing sound to it when the wind slows down suddenly. anytime it falls below cutin speed or the wind slacks of rapidly basically it makes this sound it sounds like the noise you get from spinning a length of hose around almost

Blades that are running at a high tip speed ratio tend to make some noise, and I don't really consider that to be abnormal. At only 10 mph wind speed, and a rotor running at 7 TSR, the tips are traveling at 70 mph.  There's not many things that you can whip thru the air at 70 mph and not get some noise out of it.
--
Chris

[Fused]

Dont you buy your blades chris?

[ChrisOlson]

Dont you buy your blades chris?

I used to fiddle around making my own, but I've been using PowerMax blades for quite some time now. The blades I use run at very high TSR and they're not particularly quiet-running.  At 30 mph wind they sound kind of like whipping pine trees thru the air at 180 mph.  Standing to the side of them the noise is not that noticeable, but standing either upwind or downwind it's quite significant.

[fabricator]

On the blades for my 17" I cut about a 2.5 inch radius at the tips on the leading edge, you have to be standing within 30 feet in any direction to hear anything and mine is currently only 25 feet off the ground, 100 feet away from it you cant hear anything.
I have stainless steel leading edges and sharp formed aluminum trailing edges, I'm not sure which makes it quiet, likely it's all three together.

[97fishmt]

Hi Ryan,
It could also have to do with the prop matching the power
curve of the alternator.

I made a 12' 2 blade'r many years ago that would howl wildly
at slow speed.  The alternator was holding it back and there
wasn't enough wind to push it faster, not a good match.

Maybe with the MPPT you might need to look into different
blade profiles.

I sure would like to get one of those Classic controllers!

Mike

[Hilltopgrange]

The noise from my 16ft machine was exactly the sound you get if you cut a 4ft length of garden hose and swing it above your head. It was quite loud at all speeds above cut in, but in a storm it was like a banshee! so much so that I could hear it in the house over the TV.

I attached trimmer line with duct tape on the back of each blade and it reduced the noise level by about 50% until the tape came off. The noise was so bad that I was going to carve a new set of blades, I was convinced it was a twist or warp in one blade that was causing it. Sharpening the trailing edge was a last attempt at a fix.

After sharpening the trailing edge the noise was completely removed. I can now stand at the base of the tower and all that can be heard is a slight hum from the generator and a very gentle swoosh. In a gale the wind and the trees make more noise than the blades.

It worked like a charm for me!
Good luck with it
Russell

[freejuice]

Hi Ryan,
 I checked out the video, those blades are really moving along.
However I could not hear the sound for some reason..probably something I'm doing wrong on my end...groan.
Yet with my blades being that thick on the trailing edge  and when it hit 50 amps a time or two, It didnt get very loud.
 I remember when my 11 footer would drive out some RPM's it would get noisy...much more than the 17 footer. Iit would almost have a whistle to it.

I'm far less knowlegable than most folks who haunt this site, but  at what rpm does the generator cut in ?..Im leaning towards RPM's, in that video those blades are really moving on

[tecker]

It's the tips more than the trailing edge although I think there a little back pressure forming in the air wake .The tips are really cutting into the on coming air .

[halfcrazy]

Good news it was all in the trailing edge. we shaved them down to a little less than 1/8 inch and absolutely quite if it was dark you would not know it was spinning.

[Hilltopgrange]

Glad you got it sorted, it`s amazing how much noise and grief that little bit of extra wood makes. Mine used to scream like a banshee, now I too have to trudge outside and have a look.
Russell