Blade tip/tower clearnace for 12' machine?

[B529]

I've put up a handful of commercial machines, finally going to build a home brew machine for one of my neighbors. Putting it on a 60' freestanding Rohn tower (used 10KW grid-tied Jacobs tower). I have few old DC Jacobs parts I want to use on it, stub tower, slip rings/brushes, post and saddle, upper and lower turntable bearings. At about the 6' mark from the top of the stub tower the leg spread is equivalent to a 20"diameter pipe. Will I have a clearance problem/risk of a blade strike? If so, could I simply tilt back the rotor? What affect would the tilt have on furling?

An alternative to the stub tower I already have would be cut a plate, weld a proper size diameter/height pipe to the plate with gussets, bolt the plate to the tower leg pads. This option would be complicated to use/access the slip rings. I really want to use slip rings, can't figure out a good/reliable way to drop a cord the will allow free-twisting down using a free-standing tower. Thought about mounting a J-box ten feet or so from the top of the tower, bringing a drooping SO cord into the bottom of the box maybe with a twist lock plug. I have no idea how often that short of a cord would need to be untwisted.  We're in a very gust/turbulent wind site.

[SnowGhost]

I've put up a handful of commercial machines, finally going to build a home brew machine for one of my neighbors. Putting it on a 60' freestanding Rohn tower (used 10KW grid-tied Jacobs tower). I have few old DC Jacobs parts I want to use on it, stub tower, slip rings/brushes, post and saddle, upper and lower turntable bearings. At about the 6' mark from the top of the stub tower the leg spread is equivalent to a 20"diameter pipe. Will I have a clearance problem/risk of a blade strike? If so, could I simply tilt back the rotor? What affect would the tilt have on furling?

A blade/tower stike is to be avoided at all costs.  It will kill everything.  So, if you think it could happen, it will happen.  If it does, it will only happen at maximum power/windspeed/rotor speed. 

Yes, you can tilt back the rotor a little.  So do that.

[silentblue1987]

Most of the smaller commercial turbines from say Home Depot have a 1'+ gap between the blades and pole, and the blades themselves are really strong. I've seen these small guys screeming in a decent wind with virtually no flex in the blades. They had plenty of room from the pole. Then again they were carbon composite construction with some companies

Most of the Hawts based on scoraigs guidance have a furling system with the blades offset from the pole, so even if the wind got out of control the offset distance had perfect clearance to keep them clear of impact.

Tilting the rotor is also a fine choice, very minimal degradation of power from straight line wind and adequate clearance to boot. My guess for that solution is no more than a 9 degree tilt.

In any case the meat and potatoes comes down to how flexible the blades are in my opinion. Too much flex will lead to impact.
Honestly, if my blades didn't have at least 8" of play room I'd adjust it, but I also don't have experience with large 20' blades which may easily see over 2' of play...

For testing sake though you could make a rough blade and bend it till it breaks. This will give you hard evidence of how much clearance you need to compensate for. Each material has different properties and I'm no expert.
There's likely a better way out there so keep trying.

[Tritium]

All the Home Depot turbines I have seen are SWP Skystream's. The blades are downwind style so if they do flex it is away from the tower.

Thurmond

[birdhouse]

tritium-
i've heard those SWWP skystrem3.7??'s are junk.  many many issues.  there are four of them flying on top of a building in downtown portland, OR.  i've never gone down there to ask about them.  they are down wind turbines. 

adam

[B529]

Probably the better question to ask is guys with actual experience with a 12' machine, what is your blade tip/tower clearance?

[nekit]

I have 16" clearance on my 17' Axial Flux Machine with GOE222 Royal Wind wooden blades.  So far that seems plenty, but have seen any super high winds yet.

[Tritium]

tritium-
i've heard those SWWP skystrem3.7??'s are junk.  many many issues.  there are four of them flying on top of a building in downtown portland, OR.  i've never gone down there to ask about them.  they are down wind turbines. 

adam

Adam I have heard the same thing and I would not drop $10 grand on one for sure. I can do a lot better with $10 grand in solar equipment. Besides that I am not allowed to grid tie without $35 dollars a month of liability insurance in place for a RE system.

Thurmond

[SparWeb]

You can incline the rotor angle for blade tip clearance.  Lots if you want.  The cosine of 10 degrees is 98.5%.

You can also set the rotor further forward, too.  There are some effects on furling by doing this, all in the conservative direction (ie. furls easier, stays furled too).

I only have an 8' rotor to relate to you, and there's 7-8" clearance if I recall correctly.  Tower stub is 2.5" diameter.  I've tried bending my carved blades; it takes a lot of force to bend the tip by even a couple inches.

It depends on the blades you will use too.  Carved wood is stiff, molded fiberglass is a bit stiffer, solid thin carbon fiber is very flexible.

3 years ago, I walked uip to a demo unit of a "residential" wind turbine at a trade show.  Pushed on the blade, bent it to within 2" of the simulated tower stub using only moderate force from one hand.  Company not in business today.

[birdhouse]

my turbine is only 8.5' in diameter.  i have 6-7" of clearance, and i have started to worry less and less about it.  my blades are 3" thick at the root, and laminated clear fir, and fiberglassed.  they are stiffer than stiff!  i'll bet i could sit 2/3s  out on a blade with minimal deflection if the hub was strong enough (which it probably is). 

i lowered my tower last time at my ranch, and yarded on the blades towards the tower, and had little success. 

adam

[B529]

Thanks for the replies.

Sounds like I'll be tilting the rotor back. I'll shoot for around 10-12" of clearance for the wood blades.