Windmill Furling Comparison

22-Jun-09

Some builders of small wind turbines (<5kw) have experienced trouble in high winds. Each has used a tail-folding mechanism, intended

to protect their windmill by turning the prop away from strong winds (called "furling"). Data from several of these windmills have been

tabulated here to attempt to show how differences in the design and construction may affect the furling mechanism. The reported

trouble usually is described as a windmill that can furl in some strong winds, but either does not stay furled, or will not furl in gusty winds.

Steven Fahey (sparweb@hotmail.com)

English units are used because all original data was supplied in feet inches and pounds etc.

Data can be changed in the blue cells. A new windmill can be compared on the right. No data will be saved here. E-mail me if something needs correction.

Description

S Fahey

D Lennox

Dan B

Janne P

Sylvain G

Bob

HillTop

Sam

Yours…

pre Oct 08

post Nov 08

Nov-08

Nov-06

Nov-08

Shau

Grange

Salomi

(toshi)

(and footnotes)

Comment

Motor Conv.

Furling

Trouble with

Otherpower

Concerned

New mill

Innovative

Still on test

Not even

Furls "early"

Corrected

late furling

workhorse

about furling

Design

stand

built yet!

Prop Diameter

8.0 feet

17.0 feet

10.0 feet

10.5 feet

10.0 feet

20.0 feet

16.0 feet

15.0 feet

12.0 feet

Number of Blades

Prop direction of rotation (looking at the front)

CCW

CW?

CCW?

Offset between generator and tower axes

8.0 inches

9.5 inches

5.0 inches

5.9 inches

4.75 inches

12.0 inches

11.5 inches

7.0 inches

6.0 inches

Offset direction from tower (looking at the front)

left

right

left

Angle of tail bracket from vertical

15 deg

16 deg

18 deg

15 deg

16 deg

20 deg

15 deg

Angle of tail bracket from the plane of the blades

45 deg

35 deg

45 deg

Length of tail arm to the vane's center of mass

53.0 inches

52.0 inches

102.0 inches

60.0 inches

79.0 inches

50.0 inches

180.0 inches

216 inches

60.0 inches

72.0 inches

Tail vane size

4.0 sq. feet

12.0 sq. feet

5.5 sq. feet

4.3 sq. feet

5.0 sq. feet

38.0 sq. feet

24.0 sq. feet

3.5 sq. feet

8.0 sq. feet

Tail vane weight

6.2 pounds

10.2 pounds

8.0 pounds

10.0 pounds

7.0 pounds

7.5 pounds

26.3 pounds

15.0 pounds

10.0 pounds

Tail arm weight

6.0 pounds

35.0 pounds

20.0 pounds

7.0 pounds

14.0 pounds

10.0 pounds

15.0 pounds

20.0 pounds

Generator peak output

340 W

500 W

3000 W

750 W

500 W

250 W

3600 W

2000 W

500 W

1800 W

Generator Efficiency

37%

36%

~50%

70%

~50%

59%

~50%

~45%

50%

RPM where peak output occurs

450 RPM

580 RPM

220 RPM

300 RPM

350 RPM

210 RPM

230 RPM

216 RPM

500 RPM

450 RPM

Wind speed where furling begins

~30 mph

25 mph

24 mph

~25 mph

~20 mph

20 mph

25 mph

Wind speed where fully furled

~35 mph

30 mph

28 mph

~30 mph

~25 mph

~24 mph

30 mph

Site Altitude

3200 feet

3000 feet

5000 feet

500 feet

400 feet

1000 feet

3200 feet

Electrical Details

S Fahey

D Lennox

Dan B

Janne P

Sylvain G

Bob

HillTop

Sam

Yours…

pre Oct 08

post Nov 08

39753

Nov-06

Nov-08

Shau

Grange

Salomi

(toshi)

Comment

Type

Motor Conv.

Axial Flux

Motor Conv.

Number of Phases

Number of Poles

Number of Coils

Number of coils in series per phase "leg"

Connection of stator phase wires

STAR

Gauge of Wire

Number of turns per coil

140

Wires "in-hand" (for Ax/Flux) or Parallel Coils (Mo/Co)

Resistance across phase "leg"

1.30 Ohm

0.83 Ohm

2.24 Ohm

0.62 Ohm

1.50 Ohm

1.40 Ohm

Resistance across phase line wires

2.60 Ohm

1.65 Ohm

4.48 Ohm

1.24 Ohm

3.00 Ohm

2.80 Ohm

Volts DC per 100 RPM (rectified but no current)

8.0 V

14.0 V

53.5 V

37.0 V

37.6 V

48.0 V

34.0 V

Battery System Voltage

13 V

26 V

52 V

13 V

52 V

13 V

52 V

Cut-in speed (where battery charging begins)

163 RPM

186 RPM

97 RPM

141 RPM

138 RPM

108 RPM

153 RPM

Triple the cut-in speed (RPM X 3)

488 RPM

557 RPM

292 RPM

422 RPM

415 RPM

325 RPM

459 RPM

Current at triple the cut-in speed

9 A

17 A

55 A

20 A

73 A

30 A

32 A

Estimated Power at triple the cut-in speed

113 W

450 W

2,835 W

1,045 W

3,771 W

1,561 W

1,673 W

Power dissipated to heat in wire (I²R)√3

338 W

1,351 W

8,505 W

3,136 W

11,313 W

4,683 W

5,018 W

Electrical efficiency (at 3x cut-in RPM)

25%

Analysis (Furling Forces)

S Fahey

D Lennox

Dan B

Janne P

Sylvain G

Bob

HillTop

Sam

Yours…

see note \6

pre Oct 08

post Nov 08

Nov-08

Nov-06

Nov-08

Shau

Grange

Salomi

(toshi)

Prop TSR before furling (RPM X Radius / wind speed)

4.3

6.6

5.6

4.3

6.6

8.2

6.2

10.7

7.7

Prop TSR at furling (RPM X Radius / wind speed)

3.7

5.5

4.8

3.6

5.2

6.8

5.1

8.9

6.4

Prop Offset Ratio (offset/prop diameter)

8.3%

4.7%

4.2%

4.7%

4.0%

5.0%

6.0%

3.9%

4.2%

Prop Swept Area (pi X diameter X diameter / 4)

50 sq. feet

227 sq. feet

79 sq. feet

87 sq. feet

79 sq. feet

314 sq. feet

201 sq. feet

177 sq. feet

113 sq. feet

Prop Aerodynamic Volume (swept area X offset)

34 cu. feet

180 cu. feet

33 cu. feet

43 cu. feet

31 cu. feet

314 cu. feet

193 cu. feet

103 cu. feet

57 cu. feet

Tail Aerodynamic Volume (vane area X arm)

18 cu. feet

17 cu. feet

102 cu. feet

28 cu. feet

21 cu. feet

570 cu. feet

432 cu. feet

18 cu. feet

48 cu. feet

Tail Area ratio (vane area / swept area)

8.0%

5.3%

7.0%

5.0%

6.4%

12.1%

11.9%

2.0%

7.1%

Tail Mass Moment (weight X length of vane + arm)

488 in-Lb

686 in-Lb

2601 in-Lb

1200 in-Lb

831 in-Lb

725 in-Lb

5634 in-Lb

4860 in-Lb

1200 in-Lb

1440 in-Lb

Tail Inertia Moment (moment X sine A1 X sine A2)

89 in-Lb

126 in-Lb

507 in-Lb

262 in-Lb

152 in-Lb

141 in-Lb

1105 in-Lb

1175 in-Lb

220 in-Lb

264 in-Lb

Estimated Peak Input Power (power output / eff)

919 W

1,389 W

6,000 W

1,500 W

714 W

500 W

6,102 W

4,000 W

1,111 W

3,600 W

Kinetic wind power (half * rho * V^3 * area)

7,035 W

4,071 W

16,334 W

6,116 W

3,798 W

3,445 W

13,806 W

8,836 W

14,985 W

9,160 W

Cp (input power / wind power)

13%

34%

37%

25%

19%

15%

44%

45%

39%

Estimated Power/Disk Load (input power/swept area)

18.3 W/sq.ft

27.6 W/sq.ft

26.4 W/sq.ft

19.1 W/sq.ft

8.2 W/sq.ft

6.4 W/sq.ft

19.4 W/sq.ft

19.9 W/sq.ft

6.3 W/sq.ft

31.8 W/sq.ft

Estimated Peak Thrust (Input Power / Wind speed)

15 pounds

28 pounds

125 pounds

30 pounds

18 pounds

13 pounds

153 pounds

100 pounds

22 pounds

72 pounds

Offset Thrust Moment (Thrust X offset)

123 in-Lb

222 in-Lb

1188 in-Lb

150 in-Lb

105 in-Lb

59 in-Lb

1831 in-Lb

1150 in-Lb

156 in-Lb

432 in-Lb

Aerodynamic Volume Ratio (prop volume / tail volume)

1.9

1.8

1.2

1.5

0.6

0.4

5.9

1.2

Furling Moment Ratio (thrust moment / tail moment)

1.4

1.8

2.3

0.6

0.7

0.4

1.7

1.0

0.7

1.6

Prop Stability Ratio (thrust moment / prop volume)

3.7

6.6

4.6

2.5

1.9

5.8

6.0

1.5

7.6

Tail Stability Ratio (tail inertia moment / tail volume)

5.1

7.2

5.0

9.5

5.4

6.8

1.9

2.7

12.5

5.5

Conclusions are difficult to draw with this much widely varying information, but the following observations can be made:

*Try to have twice as much prop volume (the product of the swept area and the offset) than the tail volume (the surface area times the arm).

*Try to have 50% more thrust moment (pushing the prop out of the wind) than tail inertia moment (keeping the tail down) to reliably activate furling.

*Some windmills rely less on furling because their props are more heavily loaded by the generator; increasing wind does not increase RPM proportionately.

Footnotes:

\1 Steven Fahey: More detailed information can be given, if desired.

\2 Dan Lennox: see Fieldlines posting "Furling Problem" November 13 2008 and GreenPower Talk

\3 Sylvain Gervais: see Fieldlines posting "Furling Problem" November 13 2008

\4 Otherpower: details of this turbine and its alternator can be found on "Homebrew 10' Wind Turbine construction" and Dan B's posting of Nov. 9, 2006

\5 Janne Peltonnen: see Fieldlines posting "Furling Problem" November 13 2008

\6 Thanks to The Backshed and to Windstuff Now where I found some tips for the analysis of the furling mechanism.

\7 Bob Shau posted detailed information about his intriguing design on Fieldlines, March 2, 2009.

\8 HillTop Grange (Russell) posted details of his build on fieldlines, June 21, 2009.