Floppy Tail

[scottsAI]

Most likely have came across some of my posts for designing a 24' 10kw wind turbine.

Issue to solve: Blade damage due to excessive yaw rate change, caused by storm, gusting etc. Consistently I heard the most frequent damage occurs during too fast turning of the wind gen.

Tail size is not consistent from the various post I can find. So, I plotted them and made a best guess. Came up with rules of thumb. Comments welcome. (table under REC what I could find:)

Tails of thumb:

Tail Length is blade dia / 2 or longer (Very consistent recommendation)

Tail area is blade area / 16 (smaller if length is longer. Not consistent data)

Tail force equation and included conversion factors.

Fd = Cd 0.5 * p * A * v^2

Fd = The drag force measured in N (Newton). 1 lbf = 4.448222 N

Cd = The drag coefficient, measured in N/ m2, i.e. the drag force per square metre frontal area of the object shape. Assumed flat suffice is = 0.9

p = The density of the fluid in kg/m3. Dry air at sea surface level at 15C = 1.225 kg/m3.

A = Frontal area of the object in m2. 1Ft = 0.3048m

v = Relative wind speed in m/s (assumed tail not moving, force less if it is) 0.440704m/s = 1mph

Force in lbf. The low speed numbers below are smaller than expected. Anybody want to check?

Sorry about the table, after messing with if for a hour I am done.

  Blade     Tail    REC   5        10      15      20      25     30      40      50     60     70

dia  Area   Area

8     50.27  3.14   5    0.18   0.72  1.63  2.89  4.52    6.51    11.57   18.08  26.04  35.44

9     63.62  3.98        0.23   0.92   2.06  3.66  5.72    8.24    14.65   22.88  32.95 44.85

10   78.54   4.91   5   0.28   1.13   2.54  4.52  7.06   10.17    18.08   28.25  40.68 55.37

11   95.03   5.94        0.34   1.37   3.08  5.47  8.55   12.31    21.88   34.18  49.22    67

12   113.1   7.07   12  0.41   1.63   3.66  6.51  10.17   14.65    26.04  40.68  58.58  79.73

13   132.73  8.3         0.48  1.91    4.3   7.64  11.94   17.19    30.56  47.74  68.75  93.58

14   153.94  9.62 10   0.55  2.21    4.98  8.86  13.84   19.93    35.44  55.37  79.73  108.53

15   176.71  11.04      0.64  2.54    5.72  10.17  15.89   22.88   40.68  63.56  91.53  124.58

16   201.06  12.57 10  0.72  2.89    6.51  11.57  18.08   26.04   46.29  72.32  104.14  141.75

17   226.98  14.19      0.82  3.27    7.35  13.06  20.41   29.39   52.25  81.64  117.57  160.02

18   254.47  15.9        0.92  3.66    8.24  14.65  22.88   32.95   58.58  91.53  131.81  179.4

19   283.53  17.72      1.02  4.08    9.18  16.32  25.5    36.71   65.27   101.98   146.86  199.89

20   314.16  19.63      1.13  4.52   10.17  18.08  28.25  40.68    72.32  113     162.72  221.48

21   346.36  21.65      1.25  4.98   11.21  19.93  31.15  44.85    79.73  124.58   179.4  244.19

22   380.13  23.76      1.37  5.47   12.31  21.88  34.18  49.22    87.51  136.73  196.89  268

23   415.48  25.97      1.49  5.98   13.45  23.91  37.36  53.8     95.64  149.45  215.2  292.91

24   452.39  28.27     1.63    6.51  14.65  26.04  40.68  58.58    104.14  162.72  234.32  318.94

The force in 5mph wind is very small, thus deterring tail size. (cut in speed for your system should be used.) Take a look at 10', 70mph, force is 55lbf  wow! Whip that baby around!

Solution: Tail is mounted to hang down from the support pipe. The attachment is a pipe in pipe to make a pivot or use bearings. The tail is made to swinging, I call it the floppy tail. Will be swinging a lot so make a good bearing of some type. Plastic, bronze, bearings of some type.

Head on winds will not effect the tails performance.

Low winds will not swing up so wind can push tail and do its yaw function without change.

Medium winds will cause the tail to swing up some, limiting the force it can push.

Gusting winds the tail will swing it up so, little force is imparted in turning the wind gen, thus protecting it from excessive yaw rate changes. Response is immediate. Noting to break. No electronics.

Secondary benefits the tail can be made with lighter, thinner materials than before.

Furling systems depent on tail weight, need to keep weight the same. Yet, Tail can pivot for protection.

Ok, experienced wind turbine builders what do you think?

Anybody want to test it? Have no idea when I will get to test.

Have fun,

Scott.

[Flux]

Your floppy tail has sometimes been used alone without a hinge for furling (may have been Jack Park?).

Mk1 Lucas freelite used conventional furling and the floppy tail.

I have only seen it on small machines, didn't seem to be much advantage. May be more use on a big one, but fortunately from my experience the bigger ones seem less frantic in yaw than the small ones.

Flux

[scottsAI]

Hello Flux,

Thanks for the info. I must admit I am a bit puzzled.

While designing my system, I came across several references to yaw rate problems.

References are to the air gap, needs to be 3/8 or so to handle yaw rate flexing the shaft causing the magnets to hit stator..

Other, state the blades can be supper flexed by too fast yaw, even to the point of hitting tower or just breaking..

So it seems to be a problem. Thus in need of a solution.

The bigger wind gen use an electronic system with a motor, good for them, good for me?

Looks like WXYZCIENCE just added one to his diary, same day.

I had searched for tail design information, tail area information data inconsistent.

One formula is (D^2) / 40 this is half the size of the sizes recommended by the experts.

I found only a few references, so eyeballed it to a equation I can handle. Blade area / 16.

After your post I went looking for floppy, swinging tail etc. Only one oddball reference could be found:

http://www.otherpower.com/cgi-bin/webbbs/webbbs_config.pl?read=14926

I had already searched, figured I must have missed something.

Found a couple cool web sights so time was well spent.

So reading the post looks like yaw rate is a problem, but nothing is being done. So, I have to ask, is yaw rate really a problem? Should I put up a fixed tail and forget it? Thanks.

Have fun,

Scott.

[Flux]

For small machines the tail is simple easy and effective, but in some ways it is not ideal. Once you get much beyond the 10kW size then it  becomes questionable.

The main virtue of the tail is that it provides a simple way to furl. On bigger machines furling becomes less attractive. Once you have a foolproof way to hold blade speed without furling then the tail becomes less desirable.

If you have a reliable way of speed control then removing the tail and its associated gyroscopic forces is a good idea. The simple fan tail at right angles is a good idea if you want simplicity. Servo yaw is not that difficult and anyone attempting things over 10kW will have much more complex worries than a yaw servo.

Tracking average wind direction to about 30 deg is perfectly adequate and no need to keep twitching about trying to track it exactly.

If I had to use a servo, it would be for yaw, I would not be worried if it failed one day.

Flux

[scottsAI]

Thank you for your feedback Flux.

The systems using the tail weight for furling can experience the gyroscopic forces (due to furling) whether or not using something like the floppy tail. The floppy tail would only help with side winds. Due to the reduced benefit they are not used. Plus the extra clearance needed while furled.

My system is right on your threshold, 24' 10kw. Active pitch control also does furling.

Looking at the design cost, incrementally going to 33' 20kw looks enticing. Winds less then 20mph are of most interest so could even stay with the same induction generator.

So the choice comes to cost. 24' dia blade.

Tail:

Boom 12' open support, 3 pipes 1' dia,

28 sqft, force at 5mph is 2lb floppy tail must weight 4 lb.

Cost maybe $100 with hardware.

Servo:

Gear and motor simple, not much cost. Big 1Mw use 1HP with 1000:1 gear. Drill motor would do it, not sure how long it would last! Some difficulty as in cost on yaw direction measurement.

Biggest cost is wind direction measurement. Weather stations with direction cheapest is $99, reviews so, so, no serial link, next up $150 only somewhat better.

As a guess, looks like this choice is a couple times more costly.

I just found out that is the way I will be going. Thanks.

Functional design is almost complete, little more work on pitch.

Then time for the detailed design.

Have fun,

Scott.