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
www.thebackshed.com |
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