I think I broke the "Sound Barrier" last night.

Discussion in 'General Questions' started by Andy R., May 24, 2011.

  1. Andy R.

    Andy R. WindyNation Engineer

    I bought my 1st blade 3 years ago on ebay. 48 inch wooden "Progressive Pitch" with an air foil. The guy said that it could break the sound barrier so I guess that would mean that the "Tip Speed" would go between 6 to 700 MPH. He eventually taught me how to carve blades and since my pole got damaged 3 weeks ago, (see "post my photos"), I straightened the pole and decided to make a smaller and lighter mill and carved a new 46 inch blade so last night at 1:30 in the morning while we were having 40 to 50 MPH winds, the thing woke the whole house up. Instead of a mild whirling sound, it sounded much more like a Harley Davidson. Lasted about a minuit and a half. What a "Rush"!

    Tom, is that what you meant about "Supersonic"? Anyone else ever expirence anything like that?
  2. TomT

    TomT WindyNation Engineer

    They told me it was not going supersonic. It comes from the frequency going thru the rectifier and the diodes chopping the waveform.
    Since I have hooked caps across the DC side of the rectifier it has not happened since and the turbine has been up to 56 volts at one point.
  3. windyguru

    windyguru WindyNation Expert


    Your wind turbine blades could have very likely broken the sound barrier. From what I have read, it is possible for wind turbine blades to break the sound barrier in high winds on a wind generator with no blade overspeed protection. Congratulations, my guess is that you went supersonic :D
    BTW, I think that the cracking sound a whip makes is actually the tip of the whip breaking the sound barrier.
  4. bluejay

    bluejay WindyNation Engineer

    I have heard different style blades(black swords)(very loud hum) and the ones I use now go into overspeed and sound like a fluttuering instead of a high pitched slice sound when spinning fast but not into overspeed..
  5. Andy R.

    Andy R. WindyNation Engineer

    I'm just dabeling with windmills right now and have no overspeed protection so that makes sense. Yeah, I heard that about cracking a whip. When it changes direction so quickly, it cracks the sound barrier.
  6. NEOH

    NEOH WindyNation Engineer

    TSR = 768MPH / 50 MPH
    TSR = 15
    A TSR of 15 seems unreasonably high to me.
    The most efficient TSR for a 3 Blade Rotor is between 4 and 5.
    TSR 5 x 50 MPH = 250 MPH Tip Speed
  7. Andy R.

    Andy R. WindyNation Engineer

    Good information. Thanks.

    My prop is a 2 blade and I've routinely hit a tip speed of 150 MPH in a 15 MPH wind. I rigged up a "Schwinn" Bicycle speedometer to the blade. They work with a magnet and sensor and you need to program it with the diameter of the blade just like a bike wheel. In a 15 MPH wind it tops out at 155 MPH and then as the blade speed increases, the speedometer runs backwards or lower. The device was obvious not designed to work for windmills but it's fun. LOL!

    Here's the sensor:

    Here's the speedometer


    Here it is running. I mispeak and say windspeed when should have said
    " tipspeed".

  8. NEOH

    NEOH WindyNation Engineer

    You have a TSR of (approx) 10 ( = 150MPH / 15MPH ) .
    That is a little high for a two blade rotor but reasonable especially if you need higher rpm to match genny.
    Optimum Two Blade TSR is approx 6 - 7.
    IF your turbine maintained a TSR of 10 with 50MPH wind then theoretically your Tip Speed was 500MPH.
    Tip Speed may have been very near the speed of sound - MACH 0.6 !
    Does 500MPH Tip Speed equate to 3,500 RPM ?
    At that RPM I am suprised that your rotor did not self-destruct.

    Two Blade Rotors do experience odd vibrations and sounds more often than 3 blade rotors.

    Did you happen to get a Volts & Amps or Watts reading with 50MPH winds?
  9. Andy R.

    Andy R. WindyNation Engineer

    NEOH said:

    "Did you happen to get a Volts & Amps or Watts reading with 50MPH winds? "

    No, I presently have no wires going down the tower. Except for for a few light bulbs mounted in the tail, there is no load. To keep them from blowing out, I "clamped them" (Windy Nation term), with 2 "D" size rechagable batteries.

    NEOH said:

    "At that RPM I am suprised that your rotor did not self-destruct."

    Me too! I rebuilt it many times. My original blade was 48 inches and I think by going to 46 inches, increased the speed. I just finished a 44 inch one. Can't wait to fly that one.
  10. NEOH

    NEOH WindyNation Engineer

    Andy R. said:

    "... there is no load ..."

    OK, that explains why you have a higher than normal TSR. Your genny is "free-wheeling", 0 Watts, 0% efficiency. When you "load" the genny (ie start generating Watts) then the RPMs will decrease significantly and the TSR will come way down, power captured will go up and % efficiency will go up.

    The smaller diameter rotor may DROP MORE RPM's when LOADED vs. the larger diameter rotor.
    How fast the rotor spins when properly loaded is what matters (not the unloaded free-wheeling rpm).
    If the smaller rotor spins slower when properly loaded then is it better than the larger rotor? No.

    I am guestimating that a 4' rotor at 20% overall efficiency can generate 1,650 Watts ( 125 Amps x 12 Volts ) in 50MPH Winds. WOW!
  11. Minnesota

    Minnesota WindyNation Engineer

    Anyone know the WN750 TSR at rated power?
  12. windyguru

    windyguru WindyNation Expert

    I think it hits 750 Watts into a 24 volt bank at around 700 rpm. I am just ball parking this number off the fact that generator is designed to output rated power (750W) at 700 rpm.

    I have been slowly putting together a Briggs and Stratton 5 hp motor set-up so I can generate rpm vs. power curves. One day .... one day.
  13. TomT

    TomT WindyNation Engineer

    Is that the same for the 500 watt model? I was checking Effsun and theirs are 480 and 600 RPM depends on which you buy.
    And I was just told in an email that they discontinued the 500 w. They are now selling 24 volt 600 watt 480 RPM Model.
  14. NEOH

    NEOH WindyNation Engineer

    Windy Guru,
    I am using this link for the Wind Tura 750 power curve ...
    I see approx 28MPH Wind as the 750Watt (24volt) point.
    Correct ?

    42,000 Revs/Hour = 60 Min/Hour x 700 Rev/Min
    10,555,751 Inches/Hour = (Pi x 80) Inches/Rev x 42,000 Revs/Hour
    166 MPH = 10,555,751 Inches/Hour / (5,280Feet/Mile x 12 Inches/Foot)
    166 MPH Tip Speed at 700 RPM ( 80" Rotor, 750 Watts, 24 Volts )
    6 TSR = 166 MPH (tip) / 28 MPH (wind)
  15. windyguru

    windyguru WindyNation Expert

    Tom, The 500 Watt model is designed to hit 500 Watts at 650 rpms. The specs and manufacturing of our generators are specified by us so they are different than what comes standard.

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