Progress Pics

[Boondocker]

After completing the tail section and installing it, I'm impressed on how furling mechanism works on these machines.   What a simple eloquent design

I cut the tail down, lowering its weight, so that 20kg of force would furl the rotor.  The furling force was measured by attaching a scale on the rotor center line and pulling while keeping the tail horizontally stationary.   I expect furling should start in an 18mph wind with the 2.8 diameter prop.  

I like the idea of banding round the magnets.   A SS pipe clamp was used by cutting off the clamp screw so there was about 3/8" overlap when wrapped around the magnets.  Took a second pipe clamp, drilled hole large enough so that a head of a screw would pass, aligned the hole over the first band's lap joint, tighten down the clamp.  This allowed a hole to be drilled through the lap joint while it is securely tight around the circumference of the magnet rotor.  Then pinned the band with a small screw and JB weld glued it in place.

[TomW]

boondocker;

Pretty slick idea on the clamp for us non welders of stainless.

Looks pretty nice all around.

Cheers.

TomW

[DanB]

20KG seems like a lot of force to furl the tail if I understand you correctly - it may not furl till fairly high winds.

I would also make very certain that the long bottom part of the tail can never hit the blades - that part will tip up and towards the blades when it does furl.

Looks great!  thanks for sharing...

[Boondocker]

Dan,

Thank you very much on your observations.    Will double check the clearance between the tail vane and the blades when the tail frame is against the stop.

This was my approach for estimating the furling wind speed based on rotor thrust:

Diameter         2.8m

Windspeed      8.0m/s     or 18 mph   

Rotor Thurst = Diameter^2 * Windspeed^2  / 24

                 = 2.8^2 * 8^2 / 24

                 = 20.9 kg

Your practical experience is priceless compared to an equation and a static measurement.   What rotor force do you recommend to begin furling for a safe conservative wind speed of 18 mph?   Once the turbine is in air, it will be trial and error adjustment of course.  Just don't want to error towards high wind furl.  I originally thought it would be in lower winds.

Also, is the above calculation not representative of real life because of gyroscopic forces?

Thanks again for bringing these points to my attention.

Boondocker

[RP]

Remember, your 20.9KG is enough to get the tail off the stop but as soon as it furls just little bit, you'll have less effective rotor facing the wind (cosine of the angle).  

If you want it to have a significant decrease in power, figure on the wind thrust with the rotor turned more than 45° out of the wind.

[Capt Slog]

The calculation you used for the rotor thrust, where does the 24 come from? I'm not trying to catch you out here, I just not familiar with the equation. :-)

I too liked your banding, a clever way of using the SS clamps.

[Boondocker]

Capt Slog

I used the thrust equation provided in the appendix of "Windpower Workshop" by  Hugh Piggot and posted on Ed's website (http://www.windstuffnow.com/main/turbine_kit.htm)  The equation has been simplified for use.  The 24 constant comes about by taking into account the rotor area, air mass, gravity and likely a square root.   For the equation to be correct the units have to be (s^2*kg)/m^4 associated with  24 constant.  

RP

The way I currently understand it, in high winds the rotor will seek the angle against the wind to maintain a thrust force i.e. 20 kg.   Wouldn't the power remain fairly constant corresponding to the thrust force seen by the rotor?   Now I'm a bit confussed......

[RP]

My thinking was based on comments I've seen here (Flux maybe?) that a HAWT will have a negligable decrease in power until it's turned significantly (like 30°) out of the wind.

In that case I'm worried that your thrust force will decrease as the cosine of the angle but your stator heating/overloading will not decrease at the same rate.  There is also the "seeking" tendency that will tend to counteract the calculated thrust force.

I suspect this is why DanB was surprised by the 20KG target force.

All in all I'm just concerned that your turbine may turn to 15-20° or so in a high sustained wind and then overheat.

[Boondocker]

Found this post that provides some insight.  http://www.fieldlines.com/story/2007/4/7/211453/2953

Going to cut the tail wt down and get the thrust force to furl in the 15 lb neighborhood.