Simple pitching system for downwind high wind turbine

[Timbersawz]

 I have been been looking at various designs of wind turbines and am trying to work out how to build a rugged turbine that will cope with high winds while being simple to produce.

 This idea is rough and not to any mechanical scale. The idea is that the blades are held at there normal position by the tension of a spring on the shaft (which is threaded so the spring compression can be adjusted) against a backing plate.

When the wind gets to a certain force, the blades fold back, pushing the actuator arms which is connected to the sliding ring (linear bearing) which is against the spring, lessening there sail surface and lowering the RPM of the turbine.



Its similar to the proven or kingspan design.

I have not factored in the centrifugal force of the blades vs the spring tension, the "snap back" force of the spring without dampening or anything else at this point. I dont know how far back the blades would need to pitch back to be effective.

 I have done quite a bit of searching but haven't found anything similar looking about this site or the net in general so I figure theres a pretty obvious reason its not been done

I would probably look at using goe222 blades with a 3.2 to 3.8 diameter 3 blade rotor (although Im in awe of the beautiful blades I see made on here) on a 2-3kw generator. But am open to your collective thoughts!

If the wind force against the blades tilts them back, do they still create rotational force off the wind or would they just stall?

[Adriaan Kragten]

This idea won't work. The centrifugal force in the blades is very high and it will give a forwards moment as soon as a blade is hinged a little backwards. The angle which the blade takes if there is no spring and only a hinge, is about 2 - 3 degrees depending on several factors. The angle can be calculated for a blade with a constant chord by formula 20 given in my free report KD 614 (see www.kdwindturbines.nl). So you will never reach angles that large that the rotor area is reduced that much that you get reduction of the power.

[Timbersawz]

 Thank you!

I knew I had to be missing something major, and that certainly would be major! Thanks for the info. I have followed links to your impressive site a few times, and felt a little lost, but im going to have to learn quick!

 Although that does bring be to wonder how the proven/kingspans worked as they had big springs in them that seem to attatch to the blades. I guess its got alot to do with the unique blade form or do they pitch with that set up? Im guessing they would have to!

I did wonder about the centrifugal force but obviously underestimated its force on the rotor 

Awesome, thanks again!

[bigrockcandymountain]

Hello again Timbersawz. 

I'm following your project fairly closely since we have slightly similar locations and goals I think.
I am planning on a simple furling system like everyone uses with fixed pitch blades.  It will be an induction motor conversion.  The one extra thing I'll be adding is a linear actuator to manually fold the machine up when the thunder storms are coming.  I'll run a string of 14-3 cable from the house so I can fold it up from the living room window.  The old water pumpers we use have this sort of arrangement with a lever on the tower base to turn off and they work great. 

Don't let me talk you out of variable pitch as your winds are way higher than here, but for a first effort, I'm thinking simple is good.  The old water pumpers use springs to hold the tail out instead of the angled hinge and I think I'll do a spring as well.  By adjusting the tension and attach point, you can fine tune them very nicely. 

Hopefully someone with more experience will chime in with a way better idea than these and help us both out. 

[hiker]

Might give a thought about a .downwind. Turbine..no tail..linkage....it's just offset from the pole a bit and turns out  of the wind...turns out of the wind according to how much offset you have and wind speed..

[Adriaan Kragten]

In my free report KD 485 I give an overview of the main characteristics of five different safety systems which turn the rotor out of the wind. The so called ecliptic safety system is generally used in traditional water pumping windmills. For every safety system there is a separate report (see references in KD 485) in which the system is described in detail using the moment equations. For certain systems it appeared to be possible to derive a formula for the delta-V curve (delta is the yaw angle, V is the undisturbed wind speed in m/s). If the delta-V curve is known, it is possible to predict what effect the safety system has on the power generation of the rotor at high wind speeds when the rotor is yawing out of the wind (see KD 35, chapter 7).

[Adriaan Kragten]

In the period 1975 -1990 I have worked for the Wind Energy Group of the University of Technology Eindhoven. During the first years we did research to existing books, reports and magazines about water pumping windmills. I now remember that I have seen a picture in a magazine (probably a very old copy of The Engineer) of a traditional multi-bladed water pumping windmill with a special safety system for which I will now try to describe the functioning by memory.

The blades had a lenght of about 0.6 R. Each blade was hinged about in the middle. So if the outer part of the blade moves backwards, the inner part moves frontwards. The distance in between the rotor and the tower was chosen that large that a blade could never hit the tower. If the mass distribution of the blade is chosen correctly, it can be realised that the frontwards centrifugal moment of the outer part is compensated by the backwards centrifugal moment of the inner part. The swept area of the outer part of the blade is larger than that of the inner part of the blade and the thrust on the outer part is therefore larger than the thrust on the inner part. So the thrust will cause a moment such that the outer part will move backwards. The backwards movement is counter acted by a torsion spring in each blade. Up to a certain wind speed, the spring moment is larger than the thrust moment and the blade is in the rotor plane. Above a certain wind speed, the thrust moment becomes larger than the spring moment and then the outer part of the blade moves backwards. For larger movements this results in decrease of the swept area and so in decrease of the power coefficient. At very large wind speeds, the outer part of a blade has moved about 70 degrees backwards.

May be someone can try if this system also works for a modern blade of a fast running rotor meant for generation of electricity.

[Timbersawz]

Thanks Adriaan. the pendulam safety system outlined in kd485 is really interesting, chapter 7 in kd35 makes me wish I had done alot better in school!
Such a great resource, thank you.

Im trying to envisage what you describe and I have tried drawing it out and using cut pieces of card to understand, and im afraid im failing. In this design, you mention the blades are hinged in the middle, so are the hard fixed or hinged at the hub to to allow them to move forward?

I am quite aware that im revisiting obviously flawed and long proven to be hopeless concepts. The problem is that straight away its not obvious to me, (or guys I know) why they wont work and obviously flawed designs arent easy to find examples of! So i do apologize if it seems Im insulting the long established work of the likes of Jacobs, Piggott, Bergley, Proven etc by submitting these. 
 But they stick in my head and sometimes keep me from sleeping until I can see why they wont work.
 Im currently reinventing the wheel (cutting wheel on one of my stump grinders) but thats a much simpler thing than a wind turbine.


When you talked about the centrifugal forces countering the blades pitching backwards and how little they had to pitch backwards to loose power, it got me to thinking.




Again not to any real scale or sense of mechanical reality.
Under high wind speed, centrifugal force forces the blade from its "rested" between the "elbow" and hub plate to a pitched position

1) Where the blade elbows, there would be alot of stress as centrifugal forces try to straighten it (the elbow could be much longer or different angle or braced of course)

2) There would be bending stress on the blade as the centrifugal force trys to counter the pitch angle forced on it by the elbow. dependant on the stiffness, weight and rotational speed, the force could well win.

Both of these would possibly be a cause of metal fatigue over time.

Third issue would be that the blades would not pitch far enough back as required to control RPMs or wind force on the blades to prevent damage.

The more I look at this the more variable pitch systems such at found on the windspot seems simpler to build!



   

[Adriaan Kragten]

The system which I described for the rotor of the traditional water pumping windmill had a hinge at about 0.7 R, so very far outside the rotor centre. But with this place of the hinge it is realised that the centrifugal force on the outside part of the blade is counterbalanced by the centrifugal part on the inner part of the blade. For a modern blade with a real airfoil it seems impossible to me to create a hinge at such a large distance from the centre. Another point is that a traditional water pumping windmill has a very low design tip speed ratio of about 1.2 and the effect of the centrifugal force acting on the blades is therefore rather small. I think you should forget this idea of designing a safety system this way. When you don't want a pitch controled safety system, you can better chose one of the systems as described in report KD 485. But understanding the exact functioning of certain systems requires some mathematics. The pendulum saftey system is the system with the simplest mathematics.