making mill blades less noisy

[rgormley]

hi, i have a chinese built mill that uses pultruded fibreglass blades, 3 x 1.2 metres (each)
solid straight cupped squared ends.
i with to make then a little less noisy
see
PDF LINK

     
should i trim/smooth the trailing edge?
should i trim/smooth the front edge?

do a "jerry/mike mod" to the ends (round of and taper back trailing edge)

any ideas?

mill spec

Rated Power: 200W
Output Voltages: 12V or 24V
Rotor Diameter: 2.2m
Start-up Wind Speed: 3m/s (10.8kph)
Rated Wind Speed: 8m/s (28.8kph)  
Max. Wind Speed: 44m/s (144kph)

[Experimental]

  Good evening,

    I hesitate to recommend a midification on your blades, as they differ from the ones I build greatly -- insted I,ll tell you what I have done on my own,that has quieted them a great deal !!

   From the leading edge, I cut them back at a 30 degree angle -- the leading edge being shorter than the trailing edge, when finished -- the leading edge is rounded slightly, toward the angle-- the front face of the blade is sharp, and the back side rounded, toward the front ( hope that is understandable !!)

   Your blades are constant cord, and thickness so they are much different than mine, and this may not work -- but click on my "name" above (Experimental) and look at my files and the pictures of the blades I built a while back -- those pictures will help explane what I have done -- better than words !!

   You may have to taper your blades a little, from the back side ONLY to make this work -- if I can be of help, you can email me !!

    The only other thing I will mention is -- most prop noise, is made at the tips!!

   Hope something here is helpful, Bill H.......

[Flux]

Basic problem is that they run too fast.

Try Bill's suggestion on the tips. Sand the outer 2/3 of the blades to a nice smooth finish with a round leading edge. You need the trailing edge sharp, if it is more than 1/2mm thick try carefully sanding it down to a near knife edge.

Try to do the same to all three to keep it balanced. Hope this helps it may reduce swishing and whistling. If you get helicopter noises then it is just a case of too fast.

Flux

[rgormley]

i noticed the blades have a very smoth finish on them (laquer???) and i was worried if i sand them down, and round of the edges, i will destroy the smooth surface they have now. also if i cut the fibreglass wont it start to delaminate?

i presume the cuting back (30 degrees is from the outer tip front edge back along the traling edge.

the same as the "mike mods" on the jerry bades

Cheers,

Richard...

[Flux]

If the finish is already good, there's not much more you can do.

Flux

[Experimental]

Hello Richard,

   No the fibreglass will not be harmed by modifying the tips, and the smooth finish can be regained, by useing succesively finer grades of "wet or dry" sand paper, used with water -- you can go down to a 600 grit, that will give you a glass like finish -- but I recommend going to 220 and then several coats of either epoxy paint or enamel. ( to protect from UV rays, that degrade fibreglass resins)

   After looking at your picture files, it appears , your blades look to be painted white (as all composites should be)

  If you modify the tips, you could paint the outer 3 inches or so, with a contrasting color (red, blue, etc..) this puts a "halo" effect around your blades, when spinning, and helps to keep birds and such from flying thru the blades!!

   A 12 foot blade, turning at 250rpm has blade tips moveing at over 100 miles an hour -- improving the tip vortex drag of air, will not only quiet the blades, but improve there performance !!

  That is why, some jet planes, and many raceing planes at the Reno air races, sweep the wing tips back at that 30 Degree angle, I mentioned ..

   Your blades are constant cord and thickness, that makes terriable tip vortex drag and noise -- just going back a couple of inches (but only on the back side) and tapering toward the tip, may help -- but retain the curve on the back, meaning , the aerodynamic shape and thin down the tip, to maybe 3/8" at the tip -- leave the front (windward side) sharp -- rounding the back, toward the front!!

   I added a picture of one of my props, but as you can see, they taper..



   Yours, will be harder to work with, but just tapering, from the backside toward the front, may help and keeping them in balance is important!!??

   Hope something here helps, Bill H...

[Experimental]

   Just took another look at your picture, and your blades appear to be unpainted, while the pics in your files, they seemed to be !!

   If you modify the blades, I would paint them completely, with an epoxy paint after you make any mods -- and even if you don,t modify, i would definitely paint them to protect them from UV rays !!

   And the pic I sent is of the "back side" -- rotation in that pic, being clockwise !!  Bill H.....

[rotornuts]

I noticed the blades appear to have no twist as well so I could be wrong but you will always have one "section" of the blade pushing the rest around in stall making for noise no matter what you do to the tips.

If your familiar with an Ogee shape I've read about it being applied to the trailing 2/3 with the lead 1/3 being rounded. Think about the tip of a Maple seed.(not a sycamor but a maple)

Good luck.

Mike