
The automatic furling system
used on small windmills is, at first, one of the hardest
concepts to grasp. But once you understand how it works,
it all makes sense as a simple and effective way to
furl the windmill in high winds.
Before I go any further, I should mention I get a LOT of emails asking me to explain the principles and mathematical formula of furling in more detail. Sorry, but this page explains the furling system as best as I can, so I will ignore any requests for more information. I do suggest you use a search engine to find more information, or build a model. There are also examples of windmills furling on YouTube. Once you see the system in action it will all make sense. Also note the math formula's used are not an exact science, they will give you a ball park figures only. The only way to fine tune the furling of your windmill is through experimentation in the real world.
There are several methods to mechanically control a wind turbine, like tilt up turbines, changing blade angle, and spoilers. On this page we will look at the most common system used on small windmills, side furling.
Why do we use furling?
Well it will save your windmill from destruction in
high winds, effectively making it "safe",
and it provides output power regulation. Manual furling
systems, like those on the old Southern Cross windmills,
use a manually operated lever or switch to turn the
turbine out of the wind. This is done by changing the
tail angle, instead of pointing straight out the back,
its turned up to 90 degrees.The tail will always be
down wind, so the wind against the tail will turn the
front of the turbine away from the wind.
Automatic furling can be
either electronic or mechanical. Electronic furling
uses wind speed and direction sensors and a small computer
to drive an electric motor, which turns the windmill
in or out of the wind. This type of furling is used
on the large wind farms.
Automatic mechanical
furling uses a clever combination of gravity and wind
force. Below is a simplified diagram
of a windmill. The Tail Pivot is just a simple hinge
that is angled back and to one side, usually with an
angle of about 20 degrees. Because the pivot is angled
back from the vertical, the weight of the tail will
want to turn the tail down. Its a bit like a fridge
door, if you tilt you fridge towards you, the door will
open because the fridge door hinge it angled off from
the vertical. There is a tail stop to stop the tail
once it is pointing straight out the back of the windmill,
at 90 degrees
to the turbine. The windmill turbine is
offset to one side from the tower/mast axis, so if you
push against the turbine, it will want to swing around
the mast axis.
In operation, the force
of the wind against the turbine will want to turn it
around the mast axis, however the tail, which is sitting
against the tail stop and at 90 degrees to the turbine
face, will want to stay down wind, so it keeps the turbine
facing the wind. But as the wind picks up, the force
against the turbine face increases until it is high
enough to lift the tail off the tail stop.

Light winds, no furling.
The tail weight is greater than
the wind force against the turbine. Tail is rested
against tail stop and pointed directly out the
back.
Note: The insert picture is what you would see if you looked directly at the turbine


Medium winds, starting to furl
The wind force against the turbine
was greater than the weight of the tail, so the
tail is lifted. This turns the turbine out of
the wind until the force against the turbine is
again equal to the tail weight. The furl system
has found a balance of wind force and tail weight.
Remember, the tail will always point down wind.


Strong winds, almost fully
furled
The wind force is so great that
the tail is almost at the same angle as the turbine.
So the furling is a balance between
the tail weight and turbine thrust.


We can use some maths to calculate how
a tail will furl, but first we need some measurements,
and all measurements are in metric ( Conversion to imperial
tables here ).
 Turbine Diameter in meters
 Turbine offset from the mast axis
in meters
 Tail tip weight in kg
 Tail length in meters
 Wind Speed in meters per second
You can measure the tail tip weight
by placing the pivot end of the tail on a fulcrum (
block of wood ) and the tail end on a set of scales.
First we need to work out how much torque
is trying to turn the windmill around the mast axis.
The turbine thrust or force can be worked
out with 
Turbine
Thrust = Diameter^{2} * WindSpeed^{2} / 24
Turbine Thrust = 2m^{2} * 20m^{2} / 24 = 66.6kg
The turbine moment ( torque ) is 
Turbine
Moment ( kgM ) = Turbine Thrust x Turbine Offset
Say our turbine has a diameter of 2
meters, and we want it to start fuling in winds above
20 meters per second ( 72kmh ). Our mast offset is 0.1
meters ( 100mm ).
Turbine Moment = 66.6kg * 0.1m = 6.66kgM
So we need a tail moment of 6.66kgM
to balance the turbine moment.
Tail
Moment = Tail Length * Furl resistance
and
Furl
resistance = Tail Weight * Sin ( Pivot angle in degrees)
* Sin 45o
Furl resistance = 20Kg * Sin20o * Sin 45o = 4.83 Kg
then
Tail
Length = Tail Moment / Furl resistance
Tail Length = 6.66KgM/ 4.83Kg = 1.378M
So for our windmill to start furling
at 72kmh, it needs a tail pivot angle of 20^{0},
a tail lenght of 1.378m and weight of 20kg.
Calculations:
Turbine
Thrust = Diameter^{2} * WindSpeed^{2} / 24
Turbine Moment = Turbine thrust x Turbine
offset
Tail pivot angle = Sin^{1} ( Turbine
Moment / Tail Length / Sin 45^{o} / Tail weight
)
Tail Length = Turbine Moment / Tail Weight
/ Sin ( Pivot angle in degrees ) / Sin 45^{o}
Tail Weight = Turbine Moment / Length of
tail / Sin ( Pivot angle in degrees ) / Sin 45^{o}
Notes:
As well as the tail stop where the tail
is pointing straight out the back, add a tail stop at
the other end of the tails travel, this will stop the
tail in extreme conditions going around so far that
it hits the turbine blades. It does happen and has ruined
many a good set of windmill blades.
A big thanks goes to Ed ( http://www.windstuffnow.com/ ) for helping with the information above, and also Gill
for his additions.
