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Pukwudji
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Joined: 10/07/2009
Location: United States
Posts: 20

Posted: 02:24am 10 Jul 2009

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I found this page: http://www.thebackshed.com/Windmill/Docs/Furling.asp

Good information but After dumping the calculations into Excel I became a bit confused.

The example given in the article says that for all of the given specs he needed a tail length of 1.378m and a tail weight of 20kg. When I do the math on 20kg I come back with a tail length of only .43m. If I use a tail length of 1.378m I get a tail weight of about 6.23kg.

The calculation is:
Tail Weight = Turbine Moment / Length of tail / Sin ( Pivot angle in degrees ) / Sin 45o
Tail Weight = 6.67 / 1.378 / Sin(20) / Sin(45)
Tail Weight = 6.67 / 1.378 / 0.913 / .851
Tail Weight = 4.84 / .913 / .851
Tail Weight = 5.3 / .851
Tail Weight = 6.23kg

Either my calculator can't calculate sin or the numbers on the page above aren't correct.

If I'm right (

) I'd be happy to send my spreadsheet to anyone who wants it. It's just a very simple thing where you put in wind speed, diameter, turbine offset, turbine pivot angle and either tail length or tail tip weight and it will give you the dimension you didn't divide (i.e., if you gave tail length it will give you the tip weight and if you gave it tail tip weight it will give you tail length needed).

-Brian V
Hillsboro, OR (USA)
KF7DUZ

montyLalor

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Joined: 17/12/2008
Location: Australia
Posts: 21

Posted: 08:01am 10 Jul 2009

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Yeah, your sin calculations are out.

sin 20` = 0.342
and
sin 45` = 0.707

I did the same thing, putting the equations into an Excel spreadsheet. Maybe you have errors in the equation's syntax in the cells that contain the sin calculations.

In Excel sin 45` looks like this: SIN(45*PI()/180)

Hope this helps. I too, could post a copy of my spreadsheet if need be.

"So are you doin' this project to make us money or cost us money?" she asks again...

Pukwudji
Newbie

Joined: 10/07/2009
Location: United States
Posts: 20

Posted: 02:09pm 10 Jul 2009

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Okay, so my problem is I just did Sin(45). My guess is the calculation you show converts 45 to degrees. That makes more sense.

Thanks,

-Brian V
Hillsboro, OR (USA)
KF7DUZ

Pukwudji
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Joined: 10/07/2009
Location: United States
Posts: 20

Posted: 03:55am 11 Jul 2009

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That helped.

Thanks,

-Brian V
Hillsboro, OR (USA)
KF7DUZ

DVSDuck
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Joined: 30/04/2010
Location:
Posts: 1

Posted: 05:48am 30 Apr 2010

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Pukwudji,

The good news is this problem is simple, easy to explain and not hard to fix. The actual cause of the problem is angular units. Excel does trigonometric functions in radians not degrees as assumed.

The conversion is based upon the fact that a radian is one radius laid along the circumference of the circle. Therefore there are 2 Pi radians in a 360 degree circle. (Remember that Pi is approximately 3.1416) Thus the conversion factor from degrees to radians is ((2*3.1416)/(360)). Note: there are a little more then 57 degrees in one radian, so 20 degrees is about 1/3 of a radian and 45 degrees is around 2/3 or 3/4 of a radian.

Where you entered the formula " Tail Weight = 6.67 / 1.378 / Sin(20) / Sin(45) "
will work if it is changed to:

"Tail Weight = 6.67 / 1.378 / Sin((20*6.2632)/360)) / Sin((45*6.2632)/360))"

I believe that this will solve the problem and help you avoid it in the future.

DVS

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