Wooden Blades

[Yianie123.]

Hello again.  I hope all is well with everyone.  I was wondering what the life of wooden blades are and what little can be done to extend the life, wood type, coatings...ect.  Thank you always.

[brandnewb]

I can't say much other than that wood seems more durable than aluminum More experienced members amongst us made me trust that recently after my alu tries went scray.

Also one good thing about wood is that it is easy to detect early signs of pending failure. Something I do not know how to do with steel or alu.

I guess my advice will be go wood

But please understand that I know absolutely nothing about anything ;(

[Bruce S]

Yianie123;
Have a look at some of the post from ClockmanFRA. He's done a fairly new posts about this very subject.

There are others and have recently posted about water damage. Do a search on this forum for Wood Blades, there's tons of info.
Happy Reading :-)
Cheers
Bruce S

[Adriaan Kragten]

Blades made out of good quality hardwood and covered with two layers of epoxy and two layers of aluminium paint can have a lifetime in between 5 and 10 years depending on the climate and the maximum tip speed. Blades made out of stainless steel sheet and provided with a 7.14 % cambered airfoil have an almost endless lifetime if the rotor and the safety system are designed such that the blade is free from flutter at high wind speeds. My VIRYA-2.2S rotor has run for more than 10 years and the blades still look new. Aluminium blades are sensible for fatigue and aluminium blades can snap off suddenly after a certain number of revolutions.

[MattM]

Keep it simple, stupid.  Take a 1x6 and add stainless steel ogee blades.  Yes, it really works.  Quite well actually.

Your wood absorbs the shock.  The blade may bend down in high winds but that only decreases its angle of attach which actually helps in two ways.  Under some load its lower angle of attack helps accelerate it.  Under enough load it will stop producing an angle of attack and decelerate.  Depending on the material is how much it bends.  You can get by with as low of thickness as 24 gauge for extremely responsive acceleration.

[Yianie123.]

Thank you everyone for your input and advice.  I am a little confused with Ogee blades.  Where can I get more info on these blades?

[MattM]

Trial and error.

[Adriaan Kragten]

Trial and error.

Yes, error

[MattM]

Trial and error.

Yes, error

It's too simple for you, I get it.  But you have never explored it so its very easy to dismiss.  No worries.

It works on no different relevant principles as your constant chord blades you've repeatedly promoted.  Its very generic and is good enough without much hassle to construct.

[brandnewb]

What software is it you use Matt, that you use to convey ideas?

I like the idea that it can shorten the time if one just wants a point across.

If I need to model something with my current workflow it will usually take a bit of time ;(

[MattM]

I just rough draw using mspaint.exe on the PC.  I use the open source paint.net version for fancier work.

[Adriaan Kragten]

Trial and error.

Yes, error

It's too simple for you, I get it.  But you have never explored it so its very easy to dismiss.  No worries.

It works on no different relevant principles as your constant chord blades you've repeatedly promoted.  Its very generic and is good enough without much hassle to construct.

My constant chord blades are designed according to the aerodynamic theory and are not based on an airfoil which no one would use with a minimum knowledge of aerodynamics. Your airfoil with sharp edges must have a lot of drag and will stall at a small angle of attack. Constant chord blades with a normal airfoil use a low lift coefficient at the blade tip and a high lift coefficient at the blade root and this is allowed if you use an airfoil which has a low Cd/Cl ratio for a large alpha-range. I have measured a maximum Cp of more than 0.4 for a 3-bladed rotor with a design tip speed ratio of 6 and constant chord blades in the open wind tunnel of the University of Delft. Any rotor with blades with whatever airfoil wil rotate in the wind if the blades are put at a certain blade angle but for claiming that it has an acceptable maximum Cp at a reasonable tip speed ratio, it must have been measured in a correct windtunnel. That is what I am missing in your story.

[MattM]

You do move goal posts, Adrian.  I will give you that.  You really think its a design nobody would use because its not perfected in a wind tunnel now.  Got it.  You are the expert after all.  You do at times offer some great information on this forum.  But you really should stick to what you know through your science.  But once you wander into opinions you are extremely out of your realm.

You have absolutely no basis save for opinion on this design.  When you get into the science about how much force it takes to bend the blade down into an even plane to the wood its on the extreme side where you already needed to back away from the limits of strain being applied to the wood.  That thin strip of metal actually adds quite a bit of strength to the wood at that point, and metal under a tensile load is not violating any science.  As for the angle of the blade, the manufacturer sets their angle.  So it would be easy to put too much angle on it, but it doesn't take much angle to have a working blade.  And that steep angle on the trailing edge is for enhancing the flow the air across the leading section that has less angle.  The air cannot change direction down the steeper angle without creating a pull on the air ahead, that air which is coming across the lesser angle.  If you need to use a wind tunnel to disprove it, be my guest because you will not.  Because it works just fine.  Actually it works more than just fine.  And its inexpensive, simple, and easily reproducible.  And in no way does it violate any rules of aerodynamics.

[Adriaan Kragten]

Betz and Glauert have develloped the aerodynamic theory aready in about 1930 and this theory is given in my public report KD 35. To get the maximum power coefficient, the wind speed in the rotor plane must be reduced to 2/3 of the undisturbed wind speed V. This requires a certain rotor thrust. A certain rotor thrust requires a certain lift on all blades together. The lift increases quadratic to the local speed and this means that the higher the tip speed ratio, the smaller the local chord must be. This is the reason why you need a rather high design tip speed ratio for a 2-bladed rotor. The higher the design tip speed ratio, the smaller the Cd/Cl ratio must be to get an acceptable high maximum Cp (see figure 4.6, KD 35). I don't believe that the Cd/Cl ratio of this strange airfoil is low enough for a 2-bladed rotor which must have a design tip speed ratio of about 7 for the given solidity. I can't understand why people design rotors by try and error as the aerodynamic knowledge to design a correct rotor is so easily availabe these days.

Aerodynamic characteristics of more than hundred airfoils are given in Report R 443 D which is given on my website at the bottom of the list of KD-reports. There is a good reason why airfoils have the shape they have and why the used airfoil as given in your photo must perform badly. Sorry, but I don't believe in fairy tales. It is true that manufacture of wooden blades with a increasing chord and increasing blade angle for a decreasing radius is difficult but constant chord and constant blade angle blades are much easier to manufacture from massive wood and the maximum Cp which can be obtained is high enough. Tests have been performed in the windtunnel of TU-Delft by Nienke Hosman which demonstrated that constant chord blades are even better than tapered blades. The reason isn't clear. It might be that the used rotor with tapered blades didn't have the optimal geometry. The summary of the report of Nienke Hosman is given on my website at the bottom of the list with KD-reports.

One should read the note "Sequence of KD-reports for self-study" given at the top of the list with KD-reports on my website. At point 5 of this note, I specify 38 KD-reports in which detailed rotor calculations are given for many different rotors with different number of blades and different airfoils. So even if one can't follow the aerodynamic theory, one certainly can find a calculated rotor which has the wanted characteristics. At the menu VIRYA-folders on my website, you find information about KD-reports and manuals of eleven different small VIRYA-windmills including detailed drawings of rotor, head and tower.

[SparWeb]

Hi Yiannie,

Back to the wooden blades question you asked first.

I have researched and studied blade fabrication techniques many times and I wanted to share what I learned.  Here's a summary of what I have found so far.

https://www.fieldlines.com/index.php/topic,149797.msg1049337.html#msg1049337

These aren't just personal opinions, they are based on my personal experience and on the performance of a number of materials in other WT's I have examined and on aircraft and helicopters, too.  There are enough similarities that we can learn lessons from them.

I am not impressed by sheet-metal blades, although they aren't the worst.  WT's such as Dunlite and Parris-Dunn used them.  They might hit the point of being the longest-lived for the cost to fabricate, which is a good economic baseline.

I also support what Adriaan has said above about wooden blades and variable chord and twist.  You really have to want the last few % of performance and good-looks to invest the time and money in wooden blades like the ones I have made.  If it was pure economics I have to admit it's overkill.  This is my hobby so I can ignore economics, and this WT stands beside my home so I absolutely want it to look good and be quiet.

Long life is just one factor, out of many, to consider.

[Adriaan Kragten]

Blades made out of massive wood mostly use airfoils with a flat lower side like the NACA 4412 or the Gö 623. A disadvantage of such airfoils is that a Reynolds value of at least 10^5 is needed to get an acceptable low Cd/Cl ratio. The local Reynolds number can be calculated with formula 5.5 of my public report KD 35. For small 3-bladed rotors with blades with small chords, the Reynolds values can be much lower than 10^5 at low wind speeds and for these wind speeds small rotors therefore perform badly. The advantage of the 7.14 % cambered sheet airfoil is that it has a sharp nose and the airflow is therefore turbulent. A turbulent airflow gives a higher Cd/Cl ratio at high Reynolds numbers than a laminar airflow but a lower Cd/Cl ratio at low Reynolds numbers. The advantage of a turbulent airflow is also that stalling starts at a larger angle of attack which results in a higher maximum lift coefficient. In figure 2 of report KD 398 it can be seen that the Cl/Cd curve of the 7.14 % curved airfoil for Re = 1.2 * 10^5 gives a minimum Cd/Cl ratio of about 0.03 and that this curve is even better than the curves for higher Reynolds numbers. This is the reason why for small wind turbines, it can be better to use a 7.14 % cambered airfoil than a NACA 4412 or a Gö 623 airfoil.

You see the same in nature. Wings of big birds have a normal asymmetric airfoil. Wings of insects have a flat or cambered sheet airfoil.

[kitestrings]

The fellow at Royal Wind... DaveB I think it was, I believe was carving & selling blades with a CNC machine with the 4412 profile IIRC.  It was not a flat lower side anyway.

A few observations, biases:

We used to work on Quirks/Dunlites a fair bit, and often when the blades failed it was pretty ugly; James Taylor would sing about "...in pieces on the ground."  Some of the result however, could have followed maintenance neglect.

Regarding twisting blades, I had always thought one minor advantage was that they were better at the initial start-up; getting the thing spinning sooner when the wind starts up.

I'm not convinced that wood blades have to be hardwood.  Many of the better, longest lasting blades that we encountered were Sitka spruce, or some similar straight-grained, knot-free material.

Spar's blades are IMO a work of art.  Very nice.

[Adriaan Kragten]

The fellow at Royal Wind... DaveB I think it was, I believe was carving & selling blades with a CNC machine with the 4412 profile IIRC.  It was not a flat lower side anyway.

A few observations, biases:

We used to work on Quirks/Dunlites a fair bit, and often when the blades failed it was pretty ugly; James Taylor would sing about "...in pieces on the ground."  Some of the result however, could have followed maintenance neglect.

Regarding twisting blades, I had always thought one minor advantage was that they were better at the initial start-up; getting the thing spinning sooner when the wind starts up.

I'm not convinced that wood blades have to be hardwood.  Many of the better, longest lasting blades that we encountered were Sitka spruce, or some similar straight-grained, knot-free material.

Spar's blades are IMO a work of art.  Very nice.

The NACA 4412 and the Gö 623 have both a thickness of 12 % of the chord c and have almost the same geometry and aerodynamic characteristics. However, the difference is that the zero line of the Gö 623 is the flat lower side and that the zero line of the NACA 4412 is the line which connects the nose and the tailing edge. These lines differ about 2° (see figure 5.7 report KD 35). But if you look at the real geometry of the lower side of the NACA 4412, you will see that it is almost flat as long as you are at a certain distance from the nose. A mistake which is often made for the NACA 4412 is that the calculations are made for the zero line but that the real blade angle is set from the flat lower side. So if you do that, you make a mistake of 2°. The geometry and characteristics of the NACA 4412 can be found on page 3-97 in report R 443 D given at the bottom of the menu KD-reports on my website.

[kitestrings]

I believe I may have miss-spoke.  I believe Royal blades were using the GOE222 profile.  Sorry for the confusion.

[SparWeb]

A mistake which is often made for the NACA 4412 is that the calculations are made for the zero line but that the real blade angle is set from the flat lower side. So if you do that, you make a mistake of 2°.

The potential for this error is there, yes, but even a rudimentary reading of airfoil aerodynamics emphasizes the importance of the chord line as the definition used for all parameters including angle of attack and force coefficients.  I trust that anyone who is taking the time to study these airfoils will come across the chord-line definition, and understand that the "flat bottom" is just a surface, not the reference.

There is a lot to be gained, for those interested, in the original work, still available in print and on line. 

https://ntrs.nasa.gov/search?q=abbott%20von%20doenhoff%20Summary%20of%20Airfoil%20Data

The follow-up book Theory of Wing Sections is still in print by Dover Publications.
https://store.doverpublications.com/0486605868.html

Even with high-school mathematics you don't have to be a passive user of airfoil designs.  Instead, you can select and configure them for specific uses. Even the simplest NACA 4-digit airfoils permit this to some degree (admittedly the 5-digit and 6-digit airfoils are more versatile).  Airfoil systems contemporary with NACA like Goettingen ("GOE") do not have systematic configurations quite like the NACA did (to my knowledge - correct me with documentary references if you can).  Subsequent airfoil designs like Wortmann, Lissaman and Selig develop higher performance, but are less transparent in their means of development, configuration, or selection.  Today it seems, even to many engineers, that airfoil selection and design is now a dark-art, while 80 years ago it was plain science.

(I was clicking at the same time as Kitestrings - you fixed it first!)