Front - Back turbine offset

[RP]

I'm curious about something.  

Every now and then I see a posting where the blades "seem" quite a ways out in front of the yaw axis.  What affect does this have on furling?

It seems like there is a rule of thumb about how far to offset the mill to the side but what about how far the blades could/should be out in front?

[Flux]

Tricky one . In the basic theory that uses thrust alone the stick out doesn't matter as the thrust should be along the alternator axis.

As we know that this basic model is flawed it seems reasonable that this dimension does matter. My experience is that it does help to make it furl more reliably if the blades are well forward of the yaw pivot. It also seems to reduce the power when it does furl which in some cases can be a nuisance. I don't think it will hurt to bring the blades more forward than normally done but again I wouldn't go crazy with this.

Flux

[MagZ]

I acknowledge that, my experience with the 10ft design using 11ft blades and 3 inches of extra forward of the yaw pivot point gives very good and reliable furling results. I have to say also that I use 6.5 inches on my side offset. At 15 m/s wind I still get a steady 900 watts out of the alternator which I am more than happy with. TSR is ~ 6.

I tried more efficient blades and find them sometimes wind seeking, but I also have some turbulence from buildings.

Manne

[ChrisOlson]

This is an 11 foot machine that I'm building right now in my shop.  The last two of these I used 8.5" offset and the distance from the center of the yaw shaft to the stator support is 8.2".  The distance from the yaw shaft to the blade hub axis is 12.1".  Plus I tilt the mainshaft at 2° up.

I've found doing this puts a bit more load on the yaw bearing but it's not significant and it provides very good blade tip clearance on the tower.

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Chris

[ChrisOlson]

I don't think it will hurt to bring the blades more forward than normally done but again I wouldn't go crazy with this.

Very good advice, flux.

I've tried this before and if you extend it too far the tail has trouble steering it and it will continue to yaw well out of the wind when it furls.  There's no difference if it's pointed straight into the wind.  But once it gets a little out of the wind it's just like putting a long lever on the yaw axis and it really swings.  The actual lever is the length of the hypotenuse of the triangle - the distance measured directly between the yaw tube and the blade hub where it meets the shaft centerline.  The more you extend it, the longer that distance gets, and the more leverage the rotor has to turn it once it starts to swing.

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Chris

[willib]

Good question and i think this should be part of any furling equation as it does have a considerable effect on furling

[RP]

So perhaps the guideline for front offset would be to keep it not much more than the side offset?

[ChrisOlson]

My 10.8 footer has 8.5" offset and 14" of forward lead from the yaw shaft to the hub, and it works fine with blades that run at about 6.  The 11 footer that I'm building right now is going to be 12.1".  I got a head that I'm running 7.6 foot blades on that used to have a rear mount generator and that one had 20" forward.  I could never get it to steer right so I shortened the shaft up (I don't use trailer hubs and spindles in my machines) to 10" of lead and it made some difference but it was still hard to steer.  I moved the generator from the rear to the front (behind the prop hub) and that fixed the steering problem.

I think you can actually go quite a bit forward - whatever it takes to clear your tower -  because it doesn't make one bit of difference as to what wind speed it furls at.  It's just that when it starts turning it has more leverage so it doesn't steer back into the wind quite as easily.  Which to my way of thinking is good because if the wind is blowing strong enough to furl it, it should probably stay there.

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Chris

[DanB]

yes I would tend to agree Chris ~ it seems like pushing the alternator/blades forward will likely have no effect before it starts furling, but afterwards I expect it should furl more.  The 'offset' would actually increase as the machine begins to furl.

[Ungrounded Lightning Rod]

If the rotor gets turned away from the wind in the ANTI-furling direction (either because there was no fully-extended stop on the tail or because the wind shifted) the extension makes it harder for the tail to turn it toward the wind.  It also reduces or removes the furling force.

So be sure you have that tail stop and enough tail to keep the thing pointed into the wind, rather than offset in the anti-furling direction, at less than furling windspeeds.

[ChrisOlson]

Good point.  The tail has to be pretty straight with the situation and don't run the normal slight offset to the anti-furl side.

Just got my 11 footer done - I made a different type of blade hub for it and the blade hub ended up 14" ahead of the yaw shaft.  I made the tail 7 feet long.  I built the tail feather out of a piece of smoke colored 3/16" Lexan, which is very tough, light weight and looks really cool.  It ended up 2.8 lbs lighter than the 11.3 lbs my furling calculations says it should be.

If the weather is good tomorrow she's going on the tower.

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Chris