Author Topic: Using string to check blade performance  (Read 1363 times)

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gizmo

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Using string to check blade performance
« on: April 02, 2005, 01:20:15 PM »
I remember when I was a kid ( a fair while ago now... ) my Dad was into sailing and he had wool threads all over the sails of his boat. He could tell by looking at the threads if the air was flowing over the sails smoothly or was turbulent.

Now I've been playing around with PVC blades and using the available blade calculators to work out the best angles. The blades are working well, but some are noisy at certain speeds. I'm guessing this is due to turbulence or stalling.

Its a bit hard to work out which part of the blade is making the noise, so I was going to try the old length of tube in the ear trick, where you poke one end of a length of hose in you ear then probe around with the other end. I used to use this trick to ballance carbies on my old datsun. With the hose I might be able to narrow down the noisy part of the blade.

But then I remembered the wool threads. If I attached a bunch of threads to the leading edge of a blade, on both sides and at 50mm ( 2 inch ) intervals, and then photograph the blade in action. My digital camera takes photos at 1/1000 in bright sunlight and this has in the past frozen the  propeller in any photos I take of the windmills in strong winds. So I could use these photos and see where the threads are following the blade profile nicely, or showing signs of turbulence.

I figure this methode, if it works, could be handy for wood blades as well. And it would be good to see just what happens if you used a flat blade ( no twist ) or tried changing the angle of attack, as ideally you want the blade to have the best angle of attack without stalling.

Any one else tried this?


Glenn

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« Last Edit: April 02, 2005, 01:20:15 PM by (unknown) »

TERRYWGIPE

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Re: Using string to check blade performance
« Reply #1 on: April 02, 2005, 08:40:12 AM »
Sounds like a good idea to me, they do similar things in a wind tunnel.Keep us posted.


                          Terry

« Last Edit: April 02, 2005, 08:40:12 AM by TERRYWGIPE »

RayW

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Re: Using string to check blade performance
« Reply #2 on: April 02, 2005, 10:42:21 AM »
Try it. A similar system is used to study the stall characteristics of airplane wings.

Pieces of string are taped to the top surface wing at various locations and when the air is flowing smoothely over the wing the string lays flat on the surface but as the wing starts to stall the string will lift from the surface.

            RayW
« Last Edit: April 02, 2005, 10:42:21 AM by RayW »

nothing to lose

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Re: Using string to check blade performance
« Reply #3 on: April 03, 2005, 10:22:16 PM »
That may work, lets us know when you try it how well it did.


One thing I wonder about though is will the speed of the spinning blade drag along the string behind it and the wieght of the string keep it pulled tight and not work correctly in this manner? Like if you turn a bucket of water upside down it runs out, but spin it on a rope the water stays in the bucket even while turned upside down.


The sails of a boat or airplane wings are not actually moving in a circular direction themselfs like the blades of a gennie. The wind is just moving acrossed a non-moving object, perhaps pushing or lifting the object, but the object itself is not moving. Thus the only force applied to the string is the force of the wind itself, and maybe a little gravity, but no centrifical forces are applied.


I am interested if this works.

« Last Edit: April 03, 2005, 10:22:16 PM by nothing to lose »

SparWeb

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Re: Using string to check blade performance
« Reply #4 on: September 13, 2005, 02:38:25 PM »
Any results yet?


Turbulence or flow separation would be revealed by the strings being drawn away from the surface of the blade, not out radially due to centrifugal forces.  If you position your camera at the edge, not face on, you can see the separation and ignore the displacement of the string in the other direction.

« Last Edit: September 13, 2005, 02:38:25 PM by SparWeb »
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