axial generator with lamination core

[kitestrings]

Thanks, and again does (-) just indicate a count or measure, as for example:

"B    number of blades    (-)"?

[Adriaan Kragten]

Thanks, and again does (-) just indicate a count or measure, as for example:

"B    number of blades    (-)"?

(-) means dimensionless like the lift coefficient Cl, the power coefficient Cp, the torque coefficient Cq or the number of blades B.

In my previous post I mentioned the curves with the drag coefficients of a pipe depending on the roughness and the Reynolds number as given in figure 54 of report R 999 D. I have made a scan of this part of R 999 D and added this as an attachment.

[ Specified attachment is not available ]

[kitestrings]

I may have missed your third paragraph (post #923, 02/07), or perhaps that was the edit, but regardless, thank you for clarifying.

mbouwer, I saw this recently, so I guess there are challenges at all scales, huh:
https://www.rechargenews.com/wind/two-blades-break-off-worlds-most-powerful-wind-turbine-in-china/2-1-1752939?zephr_sso_ott=hUQRfr

[mbouwer]

Maybe mount guy wires on the blades after all?

[Adriaan Kragten]

Some of the first big Danish wind turbines, like the Gedser wind turbine, had guy wires in between the blades and in between each blade and a central pole. But this turbine had a rotor with fixed blades and only the blade tip could rotate. Modern big wind turbines all have blades with pitch control for which the whole blade can rotate about 90°. If you would use guy wires for these blades, the guy wire would wind itself around the blade if the blade rotates around its axis. The tension in the wire will also change resulting in bending of the blade. Apart from this problem guy wires give a lot of extra drag and noise. Guy wires make the air behind the wire turbulent and turbulent air wil cause more aerodynamic drag if this air makes contact with the airfoil of the blade.

Another effect is that a guy wire has a strong influence on the natural bending frequency of the blade. This frequency will become much higher than for a non guyed blade and will have different higher harmonics too. The same effect happens for guyed towers. If the natural frequency becomes the same as the rotational speed of the rotor in rev/s, you get strong oscillations. So there are some good reasons why the blades of big modern wind turbines have no guy wires.

The much higher natural frequency of a guyed tower is the reason why I don't like them. A slender free standing tower has a very low natural frequency and the rotational speed of the rotor in rev/s is equal to this frequency at very low wind speeds for which oscillation gives no problem. At normal wind speeds, the rotational speed of the rotor is always higher than the natural frequency of the tower.

[Adriaan Kragten]

Concerning the influence of Reynolds on the aerodynamic characteristics of airfoils, a lot of information is available in report R 443 D: "Catalogue of Aerodynamic Characteristics of Airfoils in the Reynolds number range 10^4 - 10^6". A scan of this report can be found on my website: www.kdwindturbines.nl at the bottom of the menu KD-reports.

In the end of this report at pages A-2 and A-3 one has put the minimum Cd/Cl value of a lot of different airfoils together depending on the Reynolds number. Unfortunately, the 7.14 % cambered airfoil isn't part of this comparison. But the Gö 417-A is taken into account at page A-3. The Gö 417-A is an airfoil made out of sheet but cambered such that it has about the shape of the upper side of a normal airfoil. It has a sharp nose which makes the boundery layer turbulent just as it is also the case for a 7.14 % cambered airfoil. The geometry and the aerodynamic characteristics of the Gö 417-A are given at page 3-66 of R 443 D

If you look at page A-3 for the Gö 417-A, you see that this airfoil has been measured for very low Reynolds values of about 4 * 10^4 and 1 * 10^5 and that the minimum Cd/Cl ratio is about 0.03 for both Reynolds values. The NACA N 60 has also been measured for Re = 4 * 10^4 and for this airfoil, the minimum, Cd/Cl ratio is about 0.2. The Gö 625 has also been measured for Re = 4 * 10^4 and the minimum Cd/Cl ratio is about 0.4, so very much higher than for the Gö 417-A. The figure at page A-3 shows clearly that normal airfoils like the NACA N 60 or the Gö 625 perform very badly if the Reynolds value is lower than about 1 * 10^5. The Gö 417-A is even better than normal airfoils up to about Re = 4 * 10^5. The minimum Cd/Cl ratio for this Reynolds value is only 0.014.

[mbouwer]

The Reynolds number seems to me to be especially important if you are going to plane fixed blades.
But how should I see that if I apply blade adjustment?

[Adriaan Kragten]

(Attachment Link)

The Reynolds number seems to me to be especially important if you are going to plane fixed blades.
But how should I see that if I apply blade adjustment?

No, the Reynolds number isn't more important for fixed blades than for blade adjustement. And turbulence is not always bad. Turbulence is good at low Reynolds numbers when it is caused by a sharp airfoil nose or by a wire which is mounted before the airfoil nose of a normal airfoil. This is sometimes done for scale models of airplanes. The turbulence makes that there is interchange of energy in between the layers. This makes that a turbulent layer keeps longer in contact with the upper side of the airfoil and this makes that a larger angle of attack is allowed untill the airfoil stalls. So at low Reynolds numbers, airfoils with a sharp nose like cambered sheet airfoils are better than normal airfoils and the minimum drag lift ratio is about 0.03. But at high Reynolds numbers, normal airfoils are not easily stalling and if the airfoil isn't stalling, a laminar boundery layer gives a lower Cd/Cl ratio than a turbulent boundery layer. At high Reynolds numbers, normal airfoils can have a minimum drag lift ratio of less than 0.015. So at high Reynolds numbers, a normal airfoil, can have a large angle of attack without slalling. But once it stalls, the flow behind the point of stalling becomes turbulent and this suddenly gives a lot of extra drag. 

The difference in Cd/Cl ratio depending on the Reynolds number and the turbulence can also be seen in the different wings of big birds and small insects. Big birds use wings with airfoils similar as used for airplanes. Small insects use wings like curved plates and have a turbulent flow along the wings.

[mbouwer]

Back to what it's all about:
Meeting self-builders who want to brainstorm about a well-functioning design for our yard.

[Adriaan Kragten]

Back to what it's all about:
Meeting self-builders who want to brainstorm about a well-functioning design for our yard.

(Attachment Link)

If you want to design a wind turbine with other people you have to be aware that everyone has different ideas and different skills. You will get endless discussions about the starting points. For instance, you are fascinated by a pitch control safety system but I believe that this is too complicated for a small or medium size wind turbine. At my website at the top of the menu KD-reports you find the folder: "Sequence of KD-reports for self study" in which I mention all the subjects about which a decision has to be made. At the bottom of this menu, you find the folder: "Development procedure of a range of wind turbines" in which I give the steps which have to taken to realise a range of good wind turbines. For only one type, you can cancel some steps but it is a lot of work and you must know very well what you are doing. A badly designed or manufactured wind turbine can be very dangerous.

During about ten years I have sold licencies of the tested VIRYA-2 and The VIRYA-3.3 to people who want to build their own wind turbine. However, I have stopped doing this because most people don't follow the drawings and the manuals. But at the menu VIRYA-folders, you find several free small VIRYA-designs which I think can be built by someone with enough skills. But the question remains, who is reponsible for the mistakes? If someone builds his own wind turbine, he is always responsible for the proper functioning because the designer can never be sure that manufacture was done properly or there may be mistakes in the design if it wasn't tested.

[mbouwer]

Around us we see that there are many possibilities with wind energy when different areas of knowledge find each other and work together.
That is also what you write about in: Ideeën over een 4-bladige windturbine met een rotordiameter van 30 m

[Adriaan Kragten]

The Dutch note: "Ideeën over een 4-bladige windturbine met een rotordiameter of 30 m" translated as: "Ideas about a 4-bladed wind turbine with a rotor diameter of 30 m" was especially written to show that there are alternatives for megawatt turbines with rotor diameters and tower heights of more than 100 m. This note is my contribution to the discussion about the energy transition in my town. The municipality only takes solar energy in consideration but ignores the big imbalance in between the gain in the summer and the need in the winter if houses are heated by electricity and not by natural gas.

The note starts with an enumeration of the disadvantages of wind turbines and what can be done to reduce these disadvantages. These disadvantages are the reasons why people don't like big wind turbines in their back yard. The 30 m diameter wind turbine which I have described, meets the objections of most people and the profit of the wind turbine is not for the shareholders but directly for the neighbours. One wind turbine can supply the energy in December for the heat pumps of 60 well isolated houses. If you use only solar panels on the roofs of the houses for heating, there will always be a big shortage in December.

[mbouwer]

Living in the countryside in a simple well-insulated house with a fairly undisturbed wind flow, and a battery like Clockman is now fabricating it may be possible to meet the energy needs year-round with a wind turbine.

[mbouwer]

If you want to design a wind turbine with other people you have to be aware that everyone has different ideas and different skills.

A nice starting point:
building on this beautiful design.

[Adriaan Kragten]

If you want to design a wind turbine with other people you have to be aware that everyone has different ideas and different skills.

A nice starting point:
building on this beautiful design.

(Attachment Link)

Being beautiful isn't enough. This design is far from optimal. Tapered wooden blades are difficult to manufacture and the used hinged side vane safety system isn't stable at fast changing wind speeds and wind directions. The generator has good characteristics but it isn't closed and so the magnets and the coils are directly in contact with rain and dust. The closed axial flux generators of Hefei Top Grand are much better but making such a generator yourself is almost impossible.

[mbouwer]

But it is a good starting point because based on various construction descriptions many well-functioning versions have been built.

[Adriaan Kragten]

This is not the design of a new wind turbine. I you want to build one, you can buy a manual written by Hugh Piggott and build one. Many people have done that. I have that manual and studied it. But I was consulted by several companies in Africa and India who did this and tried to start up serial production and all meet similar problems with this design. So if you use these drawings, you will meet the same problems too. It is impossible to change the design once you have started and you must have a lot of knowledge to modify something without making it worse.

There is also a big difference in design criteria if you want to build one wind turbine for yourself or if you want to start serial production. For one wind turbine for yourself, you don't count the invested time. But for serial production, the cost of manufacture are not only the costs of the materials but also the costs of the labour. Tapered wooden blades are very labour intensive if you use only hand tools. Industrial manufacture is possible but this requires a complex machine like used by EAZ-wind for their tapered wooden blades. Constant chord and no twist blades are much easier to manufacture and the maximum Cp is almost the same if the correct airfoil is used. But the bending stress in the blade root is higher and therefore you need a safety system which limits the thrust very well.

But the biggest problem with this design is the choise of the inclined hinge main vane safety system. Everyone copies this system without fully understanding how it works. It is very difficult to describe the functioning accurately especially if the vane is positioned behind the rotor and if the self orientating moment of the rotor is taken into account. I have designed two wind turbines with this system, the water pumping windmill CWD 2740 and the electricity generating windmill Wesp and I have written report R 999 D first and KD 431 later in which I have tried to describe the system. My advise is: don't use it because the dynamic behaviour is very unstable and therefore you can get very high peaks in rotational speed and thrust. The hinged side vane safety system as described in KD 213 is simpler and much more stable. Therefore I use it in almost all my VIRYA designs. But every system has its advantages and disadvantages and mistakes are easily made.

[kitestrings]

Adriaan suggested earlier, that "If you want to design a wind turbine with other people you have to be aware that everyone has different ideas and different skills."  I agree with this, and that a starting point of developing a back yard turbine versus something that serves a neighborhood, or larger, would likely steer the design in a different direction.  I think we also agree that variable pitch is probably better suited to a larger turbine, than to a one-off, back yard turbine.

Most of what followed however, I disagree with.  Most of it is opinion; nothing more.  The side-furling turbine that you depicted has been built and copied, and designed, and tweaked, re-designed for decades.  Hugh's turbines have been built for years.  I think he might say that he revised and refined some the design over time.  The Dan's built, wrote about and continue to build them.  If you google Dan's Shop, you can find numerous turbines with tapered, twist-profile blades, with practical limits between what is ideal and what is sufficient at the root.

Going back a few years, Jim Sencenbaugh successfully manufactured turbines with this design for many years in the 70's and 80's.  One of two models were thoroughly tested by NREL.  North Wind made a unit intended for harsh environments with a side-furling design.

I don't think you can present design theory taken to 3-decimals on the one hand, and then carte blanche write off a time proven design, like the side-furling, or the down-wind coning design presented earlier in this thread, in some cases pointing to comparable experience with a water pumper on a 5m tower.

[mbouwer]

The question is:
Are we going to accept that it is too difficult to build a DIY mill?
Or are we going to look for windmillfriends who want to create a peace of work that is safe and also generates energy?

[Adriaan Kragten]

The question is:
Are we going to accept that it is too difficult to build a DIY mill?
Or are we going to look for windmillfriends who want to create a peace of work that is safe and also generates energy?

(Attachment Link)

If you want to create a new DIY wind turbine with a group of people, at least you need someone who can make the required calculations, someone who can make the detailed drawings and someone who can make the manual. If you don't have such people in the group, all work is a waste of time. I have joint several of those groups in the past but next week I become 78 and I am no longer available. But all my KD-reports are available for free and if the right ones are studied, this may prevent that the wrong choises are made.

My negative experiencies with the inclined hinge main vane system are mainly obtained by the testing of the 1 m diameter Wesp using a Sturmey Archer hub dynamo. I have described these experiencies in an earlier post. There are several reasons for instability. Assume that the moment which turns the rotor out of the wind is gained by an eccentricity e in between the rotor axis and the tower axis and by the rotor thrust. So it is not gained by a side vane like it was done for the CWD 2740.

The first reason for instability is the fact that the moment of inertia of the head is small because the vane arm is not a part of the head. The smaller the moment of inertia, the faster the head will move if a certain moment is exerted at it. So the head and the vane can make fast oscillations. The gyroscopic moment is proportional with the angular velocity of the head and the angular velocity of the rotor and the bending stresses in the blades and the rotor shaft can be very high at high yawing speeds and high rotational speeds.
The second reason can be a too small eccentricity in relation to the rotor diameter. If the eccentricity is chosen too small (less than 8 % of the rotor diameter) the self orientating moment has a too strong influence to keep the rotor in the wind.
The third reason is mounting of the vane blade in the rotor shadow. The wind speed behind the rotor is much lower than the undisturbed wind speed and therefore, to get a certain aerodynamic force acting on the vane blade, you need a much bigger vane blade than for a vane blade which is jutting outside the rotor area. But if the rotor turns away, the vane blade comes in the undisturbed wind and then the force suddenly increases. Sudden change of the force on the vane blade also causes instability. The best position of the vane blade is above the rotor. But then the vane arm must point upwards.
The fourth reason is that the moment caused by the vane weight around the vane axis increases sinusoidal if the vane arm moves away from its lowest position. So the vane arm works as a kind of pendulum which keeps moving if there is no damping. There is some damping because of the aerodynamic force acting on the vane blade. There can also be demping because of bearing friction but too much bearing friction results in hysteresis in the delta-V curve. Hydraulic damping would be the best solution by this requires an extra mechanisme in between the vane arm and the head.

In chapter 2 of my report KD 485, I show that to get the ideal delta-V curve, the counter acting moment of the vane around the tower axis must be constant above Vrated. But for the inclined hinge main vane system, this moment increases sinusoidal. This means that the rotor starts turning out of the wind directly from zero wind speed. This effect is normally compensated for low wind speeds by giving the zero position of vane arm a pre angle phi1 of about 25° in the horizontal plane (see KD 431 figure 2). But at moderate and high wind speeds this results in substantial loss of power. This is an inherent disadvantage of this system. The former windmills of the Dutch company LMW use a chain in between the vane arm and the head such that the vane arm can't turn back to its lowest position. So they used the part of the sine in between about 30° and 115° for which the value of the moment changes less than for the part in between 0° and 115°. The normal system needs a stop for about an angle of -65° and 115° te prevent that the vane can touch the rotor if it moves very strongly.

The vane moment for a certain rotational angle of the vane arm depends on the angle of the vane axis with the vertical and the vane weight. It is very difficult to calculate the correct angle for a certain rated wind speed. So the optimum angle for a certain vane weight is normaly found by try and error and this requires a lot of testing on a windy test side.

It is possible to find a geometry which is working acceptably but even the best geometry can have high peak values of the rotational speed and thrust if the wind speed and the wind direction are fluctuating strongly. To prevent rotor damage, the rotor blades and shaft must be made rather strong. So I stay with my advise to not chose for this system. But if you don't agree with me, you must do what you like and learn from your own experiencies.

[mbouwer]

Nice valuable comments.
I see it as a plea to make a driven yaw bearing instead of a tail.

[Adriaan Kragten]

Nice valuable comments.
I see it as a plea to make a driven yaw bearing instead of a tail.

(Attachment Link)

If you want that the head is kept perpendicular to the wind by an electric motor, you need a small vane mounted on the head which determines the angle in between the wind direction and the direction of the rotor shaft. As this vane can move rather fast, this angle has be averaged over a certain time to prevent that the head is constantly moving. The vane must have a 360° potentio meter and the measured angle determines in which direction the head must turn. Such a system is used for big wind turbines. Another option is to use a mechanical wind servo with two rotors perpendicular to the main rotor (see KD 671).

There must be a large reducing gear ratio in between the yaw motor and the head to allow a motor with a low power. An advantage of such a system is that the head is turning only slowly and this limits the gyroscopic moment in the blades and the rotor shaft. Another advantage is that it can also be used to turn the head 90° out of the wind. However, this requires energy which has to be taken from an external source as the rotor isn't rotating if it is turned 90° out of the wind. I think that turning the rotor out of the wind goes too slowly to use it as a safety system.

So you still need a safety system which limits the rotational speed and thrust if the rotor is perpendicular to the wind. You may think about using the pendulum safety system with a torsion spring as described in my report KD 439. The rotor turns out of the wind around a horizontal axis. A torsion spring is used to make that the counter acting moment is about constant above Vrated. This system therefore has a delta-V curve which is lying close to the ideal curve. I expect that this system can be used up to a rotor diameter of about 30 m but its has never been tested in practice. It would be good if someone tests it first for a small wind turbine.

[mbouwer]

Where have all the wind enthusiasts, who were so eager to build their own windmill, gone?

[mbouwer]

Of course, we are also increasingly realizing that building a turbine is not that easy.
Even if you make a small version.

[mbouwer]

Wind energy is booming.
Where are the new windmill builders like Piggott and the Dan's?

[mbouwer]

Reading about journal bearings in modern wind turbines it seems that it could be very advantageous for homebuild to apply to the main shaft.
Large diameter, low weight double wall construction ( T ) for shaft and hub, low surface pressure, grease lubrication.

[mbouwer]

How can we more exchange ideas on this forum with enthusiasts who want to build a small mill.
And discuss new nowadays possibilities.

[mbouwer]

We can talk about problems that self-builders encounter and discuss solutions.

[mbouwer]

Starting with a small model: Fun and not complicated.