Wooden Blades- How Long Do They Last

[Yianie123.]

I am debating either to buy aluminum blades or make wooden blades.  My consern is the accuracy of what I make and how long they will really last with Chicago weather.  All help is appreciated.

[drbob]

if you put a good coat of paint on them they will last for years

[kitestrings]

I usually plan on refinishing them every ~7 years.  YMMV.  ~ks

[Yianie123.]

Its hard to believe wood can last that long and still not rot in ice, rain, snow....What finish is used?  I am thinking about using high grade pine because there are no knots or cracks.

[drbob]

my house is 110 years old with wood siding and it is in pretty good shape.   

[kitestrings]

We used a Dupont Imron primer with a one-part polyurethane top coat.

Pine is not very strong.  Sitka spruce is among the lead contenders so I'm told - nothing we can get around here though.  ~ks

[electronbaby]

Sitka Spruce would be my number one choice. Its expensive.

Ive made many blades from Doug Fir which seemed to hold up fine for years in an eastern humid climate (right on the Atlantic, however Ive since moved to the northern midwest where its still a bit humid, but nothing like the east coast).
Im now running a set of Otherpower blades made from western red cedar. I keep a relatively thick coat of linseed oil on them, time will tell how they hold up, but I suspect they will be fine. I wouldnt use soft pine for structural reasons. Honestly, many types of wood will work as long as you can keep the moisture out.

The #1 cause of blade erosion (leading edge) is a high tip speed. If you plan to run MPPT controllers with these machines, prepare to perform a lot of upkeep with leading edge erosion, especially in a humid climate. Even professionally manufactured machines such as Bergey still have issues with LE erosion if the tip speed is not kept in check.

Most people that run the axial flux turbines keep them stalled to battery voltage, and that alone will make a wooden bladeset last a relatively long time, at the expense of a bit of top end efficiency (which is not really so important if its a battery based system usually -although there are system design exceptions). If its grid tie, thats a whole different school of thought, which you can design for if you are so inclined as long as you pay attention to the top end RPM and dont get too greedy. I personally like to run my machine in stall, its more peaceful and quiet that way, and the LE doesnt suffer such a pounding.

Leading edge tape tends to work well too. Ive had good luck with stainless steel tape. On larger machines, DanB has been using copper sheet, nailed and glued, to the outer 1/3rd of the LE.

I personally would stay away from aluminum blades. Aluminum tends to fatigue and crack very easily which wood does not. Wood tends to flex without fatiguing for a very long time and thats the main reason to use it, along with the fact that its readily available and easily workable.

RoyR

[midwoud1]

 Well, Admiral Byrd set up one of the Jacobs systems at the South Pole in 1933. On June 17, 1955, Richard E. Byrd, Jr., wrote a letter to Mr. Jacobs in which he said: 

I thought it might interest you to know that the wind generator installed (by my father)...at the original Little America, was still intact this year after almost a quarter of a century...The blades were still turning in the breeze (and) show little signs of weathering. Much of the paint is intact.


Read more: http://www.motherearthnews.com/renewable-energy/wind-power-history-zmaz73ndzraw.aspx#ixzz3La7SwnDB

[gww]

Although most are saying keep away from pine, that is what I use.  The cheap stuff.   My blades are only on a 8 foot and a 9 foot turbines.  I used two coats of polyurathane but did use latex on an earlier one.  I have put a strip of alum. duct tape on the leading edge of my last one.  The most damage I have caused is due to banging the thin edge against things before install or while on test stands.  I only have a couple of years on them but they seem to be doing well.  I had one tower fail and the blades survived even though the turbine hit hard enough to bend the tail pivit pipe.  I mostly use what ever is cheapest or free but try to get the measurements correct.  Again my turbines are smaller and no glue has to be used.
So far so good
gww

[DanB]

I think wooden blades will last about forever if you take care of them.  There is one set in our canyon that's about 70 years old, they look and run like new.  Depends on the owner.

[SparWeb]

These blades are over 5 years old.  A blade was broken in a "handling" accident and replaced.
Still shiny!  They have been re-finished once.

[Yianie123.]

"Most people that run the axial flux turbines keep them stalled to battery voltage", What does this mean?

[joestue]

"Most people that run the axial flux turbines keep them stalled to battery voltage", What does this mean?

It means the rpm increases according to the torque applied to the blades by the wind.
However, the rpm-torque relationship is set by the resistance of the alternator and the transmission line, as well as the batteries.
-this means the rpm of the turbine doesn't increase very much after cut in.

With an mppt converter the rpm rises with the windspeed and maintains a constant TSR and the blades spin much faster

[Yianie123.]

"With an mppt converter the rpm rises with the windspeed and maintains a constant TSR and the blades spin much faster"

First, thank you for responding to all my questions.  The above comment brings up another question.  My understanding of MPPT controllers is that as wind increases, voltage increases, the MPPT then adds load to see if the power increases and at the same time the voltage decreases.  If the load increases, but the power decreases then the MPPT controller decreases the load.  Now here comes the part that I believe my be true, as the MPPT load increases then the RPM slightly increases but not by much.  It this true?

[joestue]

"With an mppt converter the rpm rises with the windspeed and maintains a constant TSR and the blades spin much faster"

First, thank you for responding to all my questions.  The above comment brings up another question.  My understanding of MPPT controllers is that as wind increases, voltage increases, the MPPT then adds load to see if the power increases and at the same time the voltage decreases.  If the load increases, but the power decreases then the MPPT controller decreases the load.  Now here comes the part that I believe my be true, as the MPPT load increases then the RPM slightly increases but not by much.  It this true?

the blades develop peak torque at some TSR that usually doesn't change much. as such rpm and wind speed should be proportional.
unloaded voltage follows rpm, available current follows the square of rpm, such that the total power follows the cube of both rpm and windspeed.

[Yianie123.]

Can you please give the English translation?

[dbcollen]

Can you please give the English translation?

Battery direct systems run the blades slow and limit power, MPPT systems let the blades run at a faster speed and extract more power from the wind at the cost of more blade wear.

[DanB]

There's no rule that says MPPT is going to run faster, some battery charging machines get right out of stall and overspeed w/no MPPT.  On a recent project though there is one of my 12' turbines that had 14' blades put on it....  AC coupled through a gin long inverter and it runs fast.  Furthermore....  there are a couple big loads that kick on and cuase the gin long to cut out about every half hour and the machine has to endure free spinning for 5 min.  I'm amazed it's held together.  The leading edge of the blades is trashed.  Damage to the leading edge I suppose is related to the square of the velocity (or TSR).  Lately in my normal luddite way I've been using annealed copper sheet and copper plated nails as leading edge protection, with lots of tough caulk under it. It looks neat and it's tough I think.  https://scontent-a-lax.xx.fbcdn.net/hphotos-xfp1/v/t1.0-9/10381977_10203759211412004_6098421729562355512_n.jpg?oh=20790c3c0298551cec015c220759a326&oe=55029AB7

[kitestrings]

Battery direct systems run the blades slow and limit power, MPPT systems let the blades run at a faster speed and extract more power from the wind at the cost of more blade wear.

While this is true, I think there is more to be said to really capture the differences.  Dan can much better explain direct-tied systems than I can, but broadly when you are charging batteries directly, the battery bank tends to limit or clamp the voltage.  A direct-tied machine is then 'forced', if you will, to work at a near constant speed.  This is different than what the blades want to do, which is rise directly with wind speed.  There are then some choices or compromises to be made with direct-tie.  Cut-in at a lower point and potentially stall the blades early, or raise the cut-in and give up some lower speed performance.  There are some tricks, such as adding line resistance to improve higher wind performance.  In the end you'd be hard pressed to find a direct-tied turbine that matches the ideal available blade power over a broad range of wind speeds.  The losses IMO get pretty ridiculous in particularly in moderate to higher wind speeds.  The appeal of course is it is simple, low-cost.

MPPT allows the voltage to rise directly with wind speed.  This is probably where most of the gain is - in obtaining a better match of the available power at the blades to the load on the alternator.  Allowing the voltage to rise does result in higher speed, and blade erosion.  But, particularly in higher winds you have a much higher top end.  With MPPT the controller is exchanging higher voltage for lower current.  The voltage is much higher so the I^2R losses are much lower.  This is the second area of "gain" with MPPT, and it is actually two-fold: 1) gains in higher alternator efficiency, and 2) gain in lower transmission losses.

There are some trade offs with MPPT.  It costs more for an MPPT controller, and they have losses.  IMO, however, you get much more usable power over a much wider range of wind conditions.  You can cut-in early with no penalty.  And, it's not uncommon to have an MPPT turbine increase speed 2.5 to 3.5 times that at cut-in.  The result is, conservatively, an increase two-fold that of direct-tied systems in the say 12-30 mph wind speeds.

Regarding copper flashing, I wouldn't say that's a luddite approach at all.  There are some good examples of time-proven machines where this method has held up long-term very well.

Happy Holidays, ~ks

[OldBawley]

The wooden blades of my gen where damaged last month in a thunderstorm. Wooden two blade propeller driving a Ametek over a roller chain gear.
Diameter 1,7 m.
I use the gen on a yacht, normally I take the prop off when a thunderstorm approaches, this time I was to late and then the whether was so wild I was afraid to go out. Only when hail started I went out and stopped the prop. By then the outermost feet of the leading edge was damaged. Splinters of the pine wood teared away and the varnish gone.
Repaired with a bit of epoxy filler, re-varnished, balanced, good as new.
First time damaged in two winters service.

[Yianie123.]

So, if a person like me who doesn't have the skill to carve blades out, and would rather make then buy, what is my best option?

[kitestrings]

You'll save quite a bit of frustration in the end by just making them out of wood.  If you take your time you can do it with pretty basic woodworking tools.  I've carved two bladed props on a picnic table with a clamp, a draw knife and a hand plane (two actually).  At the size you're looking at you could afford a couple of practice pieces.  Maybe enlist the help of an area/friend carpenter to help get you started?

If you search making blades on this board, or go to Hugh's site you'll find lots of tips.  I recall someone here posted "carving blades 101" or a similar title.  ~ks

[clockmanFRA]

'Hugh Piggott's book and 'Dan Bartmann's books give very straightforward procedures and techniques to make your own blades following there designs and using basic tools, saw, mallet and chisels, files and sandpaper.

[frackers]

'Hugh Piggott's book and 'Dan Bartmann's books give very straightforward procedures and techniques to make your own blades following there designs and using basic tools, saw, mallet and chisels, files and sandpaper.

I'll second that - in fact to get an idea of what each stage looks like, I took pictures the last time I carved some blades. Each process took an evening (couple of hours) for all three blades, so about 5 days all up.
Start on this one http://gilks.ath.cx/gallery/index.php/windy/DSC04740 and use the right arrow to get to the next few pictures.

[gww]

Frackers
good totorial.  bummer on the bent tower. 
I usualy use an electric chain saw for the deep portion on the front and a circular saw for the thinning on the back and then fine tune it with a electric hand plainer and angle grinder with 35 grit sandpaper wheel and then just use sand paper for the final finnish.  My last set took two full days but turned out as well as I have done yet. My largest set so far has only been for a 9 foot turbine.  Two coats of polyurithane and all is good.  Never perfect but flying well and looks good no the tower.
Cheers
gww