furling question

[bob golding]

hi all,

 i have finished my hugh type furling tail,and true to form there is no wind to speak of... i got all the angles fom flux and got it welded up. thx flux, my question is when using the formula in  windpower workshop for the tail weight where do you measure from? do you hold the tail and measure the load on the blade tips with a spring balance? wont the load vary with tail lenght? my tail is around 1.5 metes about 5 foot. is the fomula for a tail the same lenght as one of the blades.? cant  do any testing for a month as i am away working.  at least i have access to a computer again.

bob golding

[Flux]

Yes it is common to make the tail length about the same as blade length as a minimum.

1.5M should be fine for 2m prop.

The thing that matters about tail weight is the moment about the pivot. If you have the pivot end free to pivot, then the weight times the length where you weigh it is the moment that matters. ( usually easiest to weigh at the end)

This moment is reduced by the cosine of the hinge angle (20 deg) and that has to balance the moment trying to turn the machine out of the wind. That is the thrust times the alternator offset.

Life really isn't that simple as the prop will have a tendency to seek the wind and any sums you do will only be very approximate.

If you start off with the tail fairly light you can always add some weight.

My aluminium vanes usually turn out too light, probably if you use plywood it will most likely come out fairly close with a reasonable sized vane.

Flux

[windy]

Don't have a comment but I have another furling question.

 If the wind generator doesn't have a load on it will the furling system work in high winds or does the generator have to be loaded for it furl?

windy

[Flux]

It will furl at a significantly higher wind speed.

If the machine can survive the high operating speed and excessive voltage generated off load you will get away with a lot of helicopter noise.

Most of the machines used here run well below the theoretical optimum speed at full load and may not survive the excessive speed.

It will furl when the drag losses of the prop produce enough thrust, so much depends on the blades. If they are low drag and efficient at higher tsr it may shed a blade before furling.

General advice, don't let it happen.

Flux

[Shadow]

Flux, You say the prop will have a tendancy to seek the wind,I have always thought the prop had the tendancy to turn out of the wind and it was the tail that kept it facing into the wind.If I removed the tail would it still face the wind? Or am I reading and understanding this wrong?  Thanks

[Flux]

Yes the idea is that the tail keeps it into the wind and the thrust of the prop and the offset tries to make it turn away from the wind.

For a disc or high solidity rotor this simple concept works fine, but with a rotor working with lift and a reasonable tsr things are not so simple.

If this seeking effect didn't occur, you could make the offset very small and a small tail vane would keep it into the wind. There is a limit to how small you can make the offset and it is generally about 4% of the rotor diameter. If you make it less then it may or may not furl and the type of blades determine how small you can make the offset and still get it to work.

With small offsets and the right conditions it will face the wind without a tail for quite long periods.It is not a stable condition and it may suddenly pull out of the wind but with the tail helping it, it is possible to have a situation where the thing will completely fail to furl. The tail will run at an angle to the wind but you will not have true furling and the power may not reduce to protect things.

Sometimes down wind turbines get caught out and will run up wind for considerable periods, something that would seem impossible.

This seeking force is enough to mess up any calculations on the tail weight even when you have the minimum offset.

In general the larger the offset the better it follows simple theory but the big offset needs a big tail and even then the tail may need to be set beyond straight by 20 or 30 deg to keep the prop directly into the wind below furling.

Flux