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.

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 = Diameter² * WindSpeed² / 24
Turbine Thrust = 2m² * 20m² / 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 20⁰, a tail lenght of 1.378m and weight of 20kg.

Calculations:

Turbine Thrust = Diameter² * WindSpeed² / 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.