Focus on furling

[kitestrings]

Mitch, my son, has this all on a modeling program, and nearly detailed on CAD, but old habits die hard.  I like study models:

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Using either Hugh's or The Back Shed's methods (with derating for the prop thrust as Flux describes for the later http://www.fieldlines.com/board/index.php/topic,139284.0html
I'm coming up with similar results.  We have some boom length adjustment, and weight(s) to vary, but does this look like a reasonable starting point:

from Hugh's Windmill Wind Power Workshop               
                              
THRUST:                           
d (m)      v (mps)      kg   #   offset (in)   offest (m)     mkg    ft#
4.572   ^2   10   ^2   24   87.10   192.0   9.5   0.2413   21.02   152.01
15   (ft)   22.4   (mph)                     
                              
RESTORING MOMENT:                        
boom                              
dia (in)   #/ft   l (ft)   #   kg                  
2.5   2.004   9   18.0                     
2   1.264   6   7.6                     
vane                              
ga   sq ft   #/sf                        
20   12   1.656   19.9                     
         45.5   20.63                  
            ctr of grav               
                              
      angle (off vert)   tan   in   m   kg   #      mkg   ft#
      20   0.364   84   2.13   20.63   45.5      16.02   115.90

                        76%

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Thanks,
~ks

[XXLRay]

It looks so cute. Make a mini turbine out of it

[ghurd]

Yea.
That thing needs blades and a PMA!
G-

Edit-
http://www.otherpower.com/images/scimages/2050/VCRflying2.jpg
http://www.otherpower.com/images/scimages/2050/MiniEcmFlyHome.jp

[TomW]

[scoraigwind]

from Hugh's Windmill Wind Power Workshop               
      

I came up with the formula V^2 * D^2 /24 about fifteen years back writing Windpower Workshop first time around, and it is the basis of the stuff on EdLenz and theBackshed sites.  It's based on an idealised situation such as Betz's theorem is based on.  However I have learned to adjust it a bit.  And other real world measurements indicate that it's a bit high.  In the latest edition of Windpower Workshop I changed this to V^2 * D^2 /33 but there's nothing precise about this or any other calculation of rotor thrust.  The situation is too complex to allow for much accuracy in the theory.  But that formula should be close with the sort of blades that I make and describe.

If your offset is too small then you can calculate all you like but the centre of thrust may move from the centre of the blades (which it has a perfect right to do) and you may therefore lose your offset altogether and have no way to furl thereafter.  Mostly this happens to runaway rotors, and it's a vicious circle, because it makes them run faster and this moves the centre even further toward the windward edge of the rotor blade disk.

That's a lovely model.  I always meant to make one like that to show to people.

[kitestrings]

Thanks for the comments.  I'll have to consider building a alternator for this thing in my spare time, if I ever find that I have any.

Originally the model was just to see which way the tail needed to cant in order for it to be parallel with the blade plane when fully furled.  Mitch's computer modeling showed just the opposite of our Sencenbaugh (and what I expected).  He was right.  In Jim's design it is a single angle and the rear casting just rotates the hinge to change the furling angle (and the tial vane along with it).

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Hugh, I'm sure we'll have some fine tuning to get the thing to furl resonably consistently, and at the right speed.  As for the offset - we're 15' (4.572) diameter and planning a 9.5" (.24213 m).  Does this sound reasonble to you?  We are also designing around a MPPT controller(s), so we expect the range of speeds to be a wider spectrum then with a typical battery-tied design, otherwise I might have increased this dimension.  Though if it works properly it should always be loaded (charging or diveritng).

Thanks again for the advise.

~kitestrings

[birdhouse]

kitestrings-
i've never been one to get into the tech formulas, but do read quite a bit on what works well for others.  there seem to be a consensus that offset should be 4-6% of rotor diameter.  with your 180" diameter, 4% would be 7.2" and 6% would be 10.8".  so, your 9.5" guesstimate seems to be within the ballpark. 

with my mill, i went with the full 6%.  i figured it would be much easier to add weight or elongate the tail than to try to make it lighter.  this also gives the option of using a slightly larger tail vane to improve tracking of the wind. 

adam

[shawn valpy]

Just thought this might be worth a look its my (unsure) take on mast offset its not quite flying yet but I made the offset adjustable.

[bj]

   Nice idea Shawn.  Takes the worry out being "close" to right, but not quite.