tail boom length and weight?

[fritzblitz]

Hello all of you,

I have build two windmills with the following specs:

Nr. 1  blade Diameter 2,4 m (scoraig characteristic)

2. magnet discs 300 mm diameter 8 mm thick
   
24. magnets total  46 x 30 x 10 mm 40 grades
    
    

Stator

10. coils 125 turns 1.18mmwire coil 15 mm thick.
    
5. phase connected star
   
   

DC output at 400 rpm 54.6 Volt

Stator gap 24 mm  

Offset 120 mm, no problem to be altered

Battery block 24 Volt

Nr. 2

Blade diameter 2,4 meter (scoraig blade)

2. magnet discs, 300 mm 8 thick
   
24. magnets 46 x 30 x 10 40 grade
    
    

stator

9. coils, 1,0 mm wire three in one hand, 70 turns.
   
3. phase connected star, air gap 24 mm
   
   

DC output at 400 RPM 44,1 -Volts

Offset 140 mm , fixed.

Battery block 24 Volt.

My questions

I have planned the tail boom length 1.20 m for both machines,the boom itself is of 30 mm square tube. The tail vane area is 0,54 square meter.

Can I use this tail for both machines?

If I took 15 mm plywood instead of 9 mm (scoraig) would that make a difference in furling performance? (weight difference between 9 and 15 mm is 1.60 kg)

Fritz

[Flux]

How long is a piece of string.

I don't see why the same tail shouldn't work for both, boom half prop diameter seems normal enough. The one with a bit more offset may be able to stand a slightly heavier tail but I wouldn't do it. I wouldn't increase the tail plywood thickness beyond 9mm.

There are far too many variables that you won't be able to allow for, furling can't be reduced to mathematical things like you seem to want.

Try the same tail on both and if you find the one with more offset furling earlier you can relax that it is safer or you can add a bit of weight or even increase the tail area ( will add weight) and it will face the wind a bit better.

Flux

[valterra]

Hey, Flux, I was just asking someone privately about tail boom length and tail size.  Then I saw this post.

You said "boom half prop diameter seems normal enough."  So for my 10' machine I could reasonably try for 5 foot boom?

How would one calculate the area of the tail, and does its weight play a huge role other than balancing the rear and front of the entire assembly?  

Hope I'm not hijacking this thread.  I think the O.P. was wondering the same thing re: size and weight.

[Flux]

Yes a 5ft boom would be reasonable for a 10ft machine.

The tail weight is little use in balancing the machine about the yaw axis. As soon as it furls the balance goes completely wrong unless you add several heavy and unnecessary balance weights in strange places. The tower will be able to support the unbalanced load.

The tail weight acts through the inclined hinge angle to act much the same as a spring to keep the tail assembly rigid up to furling point, from that point onwards the tail vane stays anchored down wind but the thrust acting on the alternator offset rotates the prop away from the wind.

Area of the tail vane is not that important as long as it is big enough to effectively anchor the vane down wind against the force trying to turn the machine out of the wind.

If the tail is too small then it needs to run at a large angle to the wind to balance the force. However big you make it it will always run at some angle as there is no force on the vane until the wind hits it at an angle.

Unfortunately changing the size of the vane alters its weight and if you choose a certain tail hinge angle such as 20deg you may not have a large range of vane size available while keeping the weight about right.

Just look at the machines on Otherpower to get an idea of the vane size for your 10ft machine, assuming that you make the vane from plywood. If you use a lighter material you may need to add some weight. I  wouldn't want to use thicker or heavier material as it will be too heavy with a sensible size of vane.Too much weight raises furling speed and the stator gets too hot.

Flux

[fritzblitz]

Hi flux,

thanks  very much for your answer, it helped me a lot.

I can see now the liabilities are many and no handy formula in sight! ( I always suspected there are a lot of neat math. solutions I was ignorant of....) I am glad you share your experience!!

[valterra]

Thank you Flux.  As I suspected, the exact size of the tail isn't  critical.  That is why I haven't been able to find any formulas.

The larger the tail, the better it can "drive" the rotor around, but also the heavier I gets which limits the amount of furling movement (or more specifically, increases the amount of force required to furl).

I assumed shorter BOOMs meant more whipping around in gusty winds.  Is that right?

I made some blades from a big plastic barrel.  When I finally built wooden blades for my Ametek genny (4 foot), I reused one of the plastic blades as the tail.  It is about 3 inches at the top and 12 inches at the bottom (where it mounts to the straight tail boom).

Over time it has warped and now hangs in a sort of twisted shape to the wind.  When the wind really gets going I  think this malformed tail kicks the genny out of line with the wind.  That is the only practical experience I have with the concept of furling.  Totally unplanned and engineered to be sure.  

I don't know how critical furling is on little starter gennies like those.  I have just let it run 24x7 and it has peaked over 60 watts and survived some hellacious storms.  However, I would never consider doing that with a 10 footer.