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Bluey
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Joined: 31/12/2008
Location: Australia
Posts: 7

Posted: 10:14am 29 Jan 2009

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I have question regarding blade noise and where it actually comes from, am I right in asuming it eminates from the actual blade tip?

If this be the case is there a tip shape to be avoided like square as opposed to shaped ones, the ones Im writing about here have square ends.

My windmill is at ground level so I suppose the blade noise is more noticable than when I install it properly.

Blue. Edited by Bluey 2009-02-06

Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 5119

Posted: 11:02am 29 Jan 2009

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Hi Blue

What blades are you using? Is it one of those cheap chinese windmills, if so have a look here...
http://www.thebackshed.com/Windmill/articles/ChineseBlades.a sp

Blades should make very little noise. As a rule, if your standing between a tree and a windmill, the tree should be making more noise.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ

SparWeb

Senior Member

Joined: 17/04/2008
Location: Canada
Posts: 196

Posted: 07:52pm 29 Jan 2009

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Much of the noise comes from the tip, but turbulence around the generator body, tail, tower, and guy wires also make noise.

My turbine went up last year with an 8-foot diameter prop, which had square tipped blades. The prop didn't make much noise, so they stayed square-tipped for a long time.

Using the same blades, I have made successive improvements to the alternator over the course of the last year. In the end, the output went from 300W to nearly 700W. The noise, however, also increased as I went. The loading of the blades (ratio of the power they can collect versus the power demanded by the alternator) makes them operate at higher angle of attack. This in turn causes higher induced drag, especially at the tips. So far I haven't found a tip shape that fixes the noise.

The blades also turn faster, now. I currently believe that it is the combination of tip speed and induced drag vortex at the tip that causes the most noise. Profiling the tip interrupts the vortex, and slowing the tip speed decreases the acoustic energy (cubed power).

Steven T. Fahey

Bluey
Newbie

Joined: 31/12/2008
Location: Australia
Posts: 7

Posted: 12:00am 30 Jan 2009

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Hi Glenn,
My blades are wooden copied from a design by a Charles F Morrison who has modified Hugh Piggots design slightly and has put out a pdf file with the section shapes full size.

I am a complete novice when it comes to wind power so what constitutes my idea of noisy blades could be easily disregarded by an experieced builder.

Blue.

This is an update or an edit
I read where a builder had blade noise and cured it by radiusing the trailing edge at the blade tip, so I tried it and it has lessened the noise for some reason. Edited by Bluey 2009-02-06

MacGyver

Guru

Joined: 12/05/2009
Location: United States
Posts: 1329

Posted: 03:23am 15 May 2009

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I once made a 'hollow' modified "Clark-Y" blade from sheet aluminum.
Being hollow, as the blade array spun, centripetal force caused an air flow
through the interior of the blade, drawing from the hub area and spilling
at the tip. This spoiled the tip vortex and was silent.

I found also that by pulling the metal slightly one way or the other along
the long axis, the whole blade would roll and assume a feathered shape.
At that point, the leading edge was pop-riveted together with the mating
edge and I was able to produce silent-running, mammoth blades very
cheaply.

If you'd like to make a model, simply cut a piece of paper about 6" wide
and as long as paper is (11 1/2"?). Fold one of the long edges back on
itself about 1/4". Now fold what's left about 3/8" to 1/2" off center the
long way. Now, tuck the wider side into the shorter one, under the 1/4"
"lip" you have creased into it. That's a very basic "Clark Y". Play with it
and you can cut and fold the paper to make the blade (wing) tapered at
the end and thicker at the root.

Metal blades made this way can of course be squeezed and molded to
better resemble an actual asymmetric air-foil shape.

Nothing difficult is ever easy!
Perhaps better stated in the words of Morgan Freeman,
"Where there is no struggle, there is no progress!"
Copeville, Texas

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