Furling confusion

[SmoggyTurnip]

I am quite confused about setting up my furling system.

I have a prop diameter of 5.69 meters.

Say I want to start furling at 20 mph (9 m/s).

Windstuff Ed explains how he does it here.

http://www.windstuffnow.com/main/turbine_kit.htm

he uses.

Rotor Thrust = Diameter^2 * Velocity^2 / 24

This gives 109 Kg or 240 pounds.

This story:

http://www.fieldlines.com/story/2006/4/25/15639/4069

measures the force required to start furling at 60 pounds for a 20 ft dia turbine.

I would think it would be less for mine.

In the same story Flux suggests that you use 1/2 x rho x A x V^2. and then take about half of that. so now we're talking 120 pounds.

All these guys have built some great stuff and know what they are doing. But I have ballpark figures of 60, 120, and 240 pounds to use for rotor thrust.

Can you see why I am confused.

Also at this site

http://www.windstuffnow.com/main/turbine_kit.htm

we have

Tail moment = Rotor moment / ( sin(Angle in degrees))

There are 2 angles involved in the furling system and I would think

both would have an effect on the tail moment. Is there a formula that

takes both angles into consideration?

.

[Flux]

Yes I can see why you are confused. You are trying to mathematically analyse something that is too complicated. Most fluid dynamics problems are beyond analysis or ar at the limit of present computer modelling. Fluids equations are full of constants to make the answer fit the observed facts and when some of the constants are variable then you know you are in the world of guesswork.

I have found the thrust to be close to the 1/2 of 1/2 rho A v^2 as far as the thrust on the tower is concerned for typical high speed blades, Ed's higher figure may be nearer for a lower solidity rotor.

This will not help you a lot with furling as the rotor itself tries to stay into the wind and unless your thrust exceeds the seeking force it will not furl.

The effective force you have trying to furl it is the difference between thrust and seeking force.  I think you have no hope of calculating the seeking force as I suspect that only a few of us actually know that it exists. It depends on the type of blade, and operating tsr. The only help I can give you is that if you make the alternator offset less than about 4% of rotor diameter it will never furl, so you may be able to approximate the seeking force from that.

The main angle to consider for furling is the one that tips the tail into the air as it furls. The one deciding where the tail is attached in the horizontal plane mainly determines the bias point of the tail restoring force against its stop and slightly affects how the restoring force changes with angle of the tail to the alternator axis.

Fortunately things change so drastically with wind speed that if you aim to furl at 20 mph you would need a drastic increase in tail weight to raise it to 30 mph. If you use the usual angles and tail dimensions it will likely come out about right.

Don't kid yourself that furling is a precise process, you will never see it sitting at constant power out like a pitch control.

[SmoggyTurnip]

"You are trying to mathematically analyse something that is too complicated."

It may seem that way by the way I posted my question but I am just trying to figure out how to size my tail and set the angle.  I was only checking what people who have built sucessful machines have done.  I have my alternator constructed now and ready for paint.  I also completed my 3 blades.  I can't really proceed any farther until I have the furling system nailed down.

My blades are 5.69 Meters in diameter - tsr 7.

I will be connecting the alternator directly to AC heating elements.

I would like to set it up so it would furl at 20 mph or so to be safe for starters and then increase that if things seem OK.

Where would you start?

.

[SmoggyTurnip]

The offset is 14 inches.

.

[Flux]

You have a very generous offset, so you will not be troubled too much by the prop seeking the wind. My immediate thought is that it will be safe and you will need to make  a condiserable effort to keep it into the wind before furling. You will need a big tail and lots of offset away from the furling direction to balance the torque from thrust and your 14" offset.

If you use 20 deg inclination from the vertical  with a tail in proportion to the size of machine I think it will furl well below 20 mph, so it should be safe.

Using my estimate of thrust and Dan's method of checking things with a spring balance should bring you near.

Good luck , sorry I can't give you exact answers.

Flux

[SmoggyTurnip]

Thanks Flux,

I already cut a piece of 1/2 inch steel to make my 20 degree angle.

Do you thing setting it at 45 degrees to the wind direction be OK.

.

[Flux]

45 deg should be ok I think that is what Dan uses. I tend to use about 50 deg and I think Hugh uses 55 deg.  I have never measured it accurately I don't think it is that fussy.

Flux

[ZooT]

Hoping I'm not interrupting here, if I am please excuse this newbie.

With a 14" offset and a properly sized tail shouldn't it furl by itself given that at some point the force imposed against the rotor could overcome the force imposed by the tail to keep the rotor at 90 degrees to the wind?

[Flux]

Do you mean furl a fixed tail?

What will happen is that if the tail is in line, it will run at an angle to the wind so that the tail force balances the prop thrust. The two things are in balance, the tail restoring force also increases with wind speed so it will never furl. If the tail was a hinged vane, the it would furl as the tail vane would lift and reduce the effective angle.

I hope that was what you were asking about.

Flux