Report KD 377 reviewed

[Adriaan Kragten]

Chapter 6 of public report KD 377 about the pendulum safety system has been reviewed. A new figure 7 has been added which shows an alternative for a rotor mounted far behind the tower. This new figure is added as an attachment.

[SparWeb]

Interesting.

What arrangement would you suggest for the power cable(s)?

[Adriaan Kragten]

Interesting.

What arrangement would you suggest for the power cable(s)?

I would use a hollow hinge axis and put the cable through the hole. This results only in some twisting of the cable at the hinge if the rotor is swinging. For the head bearing at the tower, I use a hollow head pin for all my VIRYA-windmills and I accept twisting of the cable in the tower. So I use no brushes at that point. But you have to use a flexible cable and a loop at the bottom so that you can easily see if the cable has been twisted too much. The cable is disconnected and twisted back if it is twisted too far.

[kitestrings]

Adriaan,

This is interesting.

Have you considered adding a manual or power furling tail option as a means of high-wind, overspeed control and/or means to shut down a small turbine for maintenance, inspection or just to avoid dangerous winds?  There are at least a couple of us doing this here.  Personally, it has been by far the very best feature we've incorporated, and I would not build another turbine without some means of shut-down from the ground.  In our case, a button press inside the house and we can bring the turbine to a near stop in any wind conditions in about 30 seconds.  And, the charge controller can allow this to happen automatically.

[Adriaan Kragten]

For the original construction as described in KD 377, the whole pendulum is mounted on top of the tower. For this construction, I claim that it is tornado proof. This is realised because there is a clamp which locks the movement of the pendulum if it has rotated 90°, so if the rotor is in the so called helicopter position. The helicopter position is not reached for stationary conditions but only during strong wind gusts. Once the pendulum is locked, it must be unlocked manually. This unlocking is done by climbing in the tower up to about 2/3 of the height and unlocking is done by a long pole with a hook at the end. I might also be possible to push the pendulum by hand in the helicopter position for maintenance if the windmill isn't very big.

An about similar construction but with a much smaller eccentricity, a torsion spring and a rotor turning in front of the tower is described in report KD 439.  This construction is especially meant for a big wind turbine and detailed calculations are given for the VIRYA-10 windmill. Also for this construction, the pendulum is locked automatically in the helicopter position. But the VIRYA-10 is that big that pushing it in the helicopter position for maintenance isn't possible. It migh be possible to add a motor driven construction to realise this but this construction must be disconnected for normal wind conditions.

[kitestrings]

Trying to follow:

In KD 377 fig 2, is there an off-set balance weight, perhaps not shown, at G that counter-balances moment of the rotor and head assembly?

Is the rotor in effect a downwind design, albeit with structural support that is downwind of rotor?  Otherwise, what keeps the rotor steering into the wind is I guess my question.


 

[Adriaan Kragten]

In KD 377 it is written that the real balancing weights are not given in figure 2 but that G is the total weight of all moving parts positioned in the centre of gravity. The real balancing weights are mounted at two long levers which move along the tower pipe. Drawing of the real balancing weights in figure 2 would make the drawing too complicated for the explanation of the balance of moments. Formula 13 for the balance of moments around the horizontal hinge axis is rather simple and therefore it is rather simple to derive the delta-V characteristic for this safety system as given in figure 3 for a design wind speed of 7 m/s.

The hinge axis coincides with the tower axis and the rotor plane coincides with the hinge axis. So the rotor is not upwind or downwind. The rotor is kept in the wind by a so called double vane. This is explained chapter 5 of KD 377. A double vane is also used for the version with a torsion spring as given in KD 439. I have tested a double vane on one of my first windmills and it works really nice. A photo is given in figure 11 of KD 439. This vane has a high moment of inertia and this slows down the maximum angular velocity of the head. Therefore it reduces the gyroscopic moment.

[kitestrings]

So, with the two levers and associated structure, this more than doubles the weight of a given turbine.  Seems like that would be a consideration.

[Adriaan Kragten]

So, with the two levers and associated structure, this more than doubles the weight of a given turbine.  Seems like that would be a consideration.

Yes, this is true and that is one of the reasons why I wrote KD 439. For the option as described in KD 439, the eccentricity is much smaller and the weight of the balancing masses is compensated by a torsion spring. But the version of KD 377 with a very large eccentricity is easiest to describe mathematically because the selforientating moment can be neglected.

I have added a new part to chapter 6 of KD 377 in which a version is described for which the rotor is turning far before the tower to prevent vibrations due to passing the tower shadow.

[MattM]

Hurricane winds move like windings on a toroid coil, giving some vertical component rather than just horizontally.  I cannot imagine tornadoes do not do the same as they have a scouring action along the point of contact with the ground.  I wonder if the theory holds up in practice.

[kitestrings]

That's an interesting, unconventional (double vane) steering method.  Does it ever get caught with the rotor in a downwind position?  It would seem like it could.  Downwind designs sometimes had issues with this, when the wind stopped in one direction and came up in the opposite (180 deg) direction.

It also looks like there are tips brakes on the blades in the photo, or are those spoilers of some sort?

I don't know that I'd ever claim something to be tornado proof even if I thought it were.  I'm probably superstitious, but it would seem to challenge Mother Nature in rather defiant way ;>].  In news coverage of such events, it seems everything in the path is destroyed.

It does seem to me that, in your list of five, that you are missing at least two or three proven methods of control and/or shutdown: manual or power furling, and automated blade pitching/feathering like what Midwoud has illustrated here.  I think the other method is a motorized yaw, but this is usually in conjunction with blade pitch designs.

[Adriaan Kragten]

Hurricane winds move like windings on a toroid coil, giving some vertical component rather than just horizontally.  I cannot imagine tornadoes do not do the same as they have a scouring action along the point of contact with the ground.  I wonder if the theory holds up in practice.

If the rotor is locked in the helicopter position and if there is an upwards wind caused by a tornado, it moves through the rotor in an opposite direction as normal. So the rotor will turn backwards and this means that now the relative wind streams the airfoil from the tailing edge and not from the nose. The airfoil will have a lot of drag for this direction of the wind speed and the rotor will therefore turn only slowly. So no dangerous high rotational speeds will be reached. So I think a locked horizontal position is the safest position imaginable and the rotor will therefore be able to resist very high wind speeds.

[Adriaan Kragten]

That's an interesting, unconventional (double vane) steering method.  Does it ever get caught with the rotor in a downwind position?  It would seem like it could.  Downwind designs sometimes had issues with this, when the wind stopped in one direction and came up in the opposite (180 deg) direction.

It also looks like there are tips brakes on the blades in the photo, or are those spoilers of some sort?

I don't know that I'd ever claim something to be tornado proof even if I thought it were.  I'm probably superstitious, but it would seem to challenge Mother Nature in rather defiant way ;>].  In news coverage of such events, it seems everything in the path is destroyed.

It does seem to me that, in your list of five, that you are missing at least two or three proven methods of control and/or shutdown: manual or power furling, and automated blade pitching/feathering like what Midwoud has illustrated here.  I think the other method is a motorized yaw, but this is usually in conjunction with blade pitch designs.

The double vane was only tested for an upwind rotor. It had two square vane blades size 0.5 * 0.5 m mounted at both ends of a six m long pipe under an angle of 20° with the rotor shaft. A square vane blade has about a linear Cn-alpha curve for alpha in between 0° and 40° (see figure 5 report KD 551) and the vane therefore accepts variations of the wind direction of 20° without stalling of one of the vane blades. The head bearing was a big one row ball bearing of a trailer with very little friction and the vane was therefore turning the head in the wind for very low wind speeds.

The 3-bladed rotor had a diameter of 4 m and a design tip speed ratio of 6. However,  the generator housing was empty and the rotor was therefore running unloaded. The blade tips were provided with elastic air brakes made out of 1 mm stainless spring steel. The air brakes bend more to the outside as the rotational speed is higher. A scale model of the rotor with a diameter of 1.8 m was tested in the wind tunnel also with elastic air brakes. It appeared in the wind tunnel, that the air brakes were very effective in reduction of the Cp at increasing wind speeds. The braking effect on the real rotor was also effective but the rotor was very noisy at high rotational speeds. It sounded like if a helicopter was flying over. This was the reason why the idea finally was cancelled.

In my report KD 485, I describe five different systems which turn the rotor out of the wind; three around a vertical axis and two around a horizontal axis. There are more systems but those are less common. Pitch control is another way but pitch control is not turning the rotor out of the wind and is therefore described in several other KD reports (see for report numbers note "Sequence of KD-reports for self-study" at the top of the list with KD-reports).

Turning the rotor out of the wind manually is no safety system as it isn't working automatically. Anything with a motor to turn the rotor out of the wind isn't fale-safe as it fails if the energy needed for the motor is gone or if anything in the steering of the motor fails. I prefer only systems which are directly steered by the rotor thrust or by the rotational speed.

Big wind turbines have pitch control systems steered by several different signals like wind speed, rotational speed, vibrations and power and regulated by a computer but several big accidents have happened because of a fail somewhere in the circuit resulting in an unloaded rotor turning at extremely high rotational speeds.

[kitestrings]

Enertech, a manufacturer of induction turbines used tip brakes as a fail-safe.  The smaller machines had an aluminum plate that was pre-loaded such that above a given rpm they bent out and slowed the turbine; one and done.  The larger turbines - up to 60 kW - were hinged at the leading edge tip and had a centrifugal latch where they hinged forward, and down against the leading edge.  They required regular maintenance but were quite effective.

A manual or automated means of shut down from the ground is complimentary to the trust governing of a furling tail; not instead of - and, Hugh has proven this method to be among the most effective long term.

[MattM]

If the rotor is locked in the helicopter position and if there is an upwards wind caused by a tornado, it moves through the rotor in an opposite direction as normal. So the rotor will turn backwards and this means that now the relative wind streams the airfoil from the tailing edge and not from the nose. The airfoil will have a lot of drag for this direction of the wind speed and the rotor will therefore turn only slowly. So no dangerous high rotational speeds will be reached. So I think a locked horizontal position is the safest position imaginable and the rotor will therefore be able to resist very high wind speeds.

But for every downdraft you'd have an equally strong updraft somewhere else.

[Adriaan Kragten]

If the rotor is locked in the helicopter position and if there is an upwards wind caused by a tornado, it moves through the rotor in an opposite direction as normal. So the rotor will turn backwards and this means that now the relative wind streams the airfoil from the tailing edge and not from the nose. The airfoil will have a lot of drag for this direction of the wind speed and the rotor will therefore turn only slowly. So no dangerous high rotational speeds will be reached. So I think a locked horizontal position is the safest position imaginable and the rotor will therefore be able to resist very high wind speeds.

But for every downdraft you'd have an equally strong updraft somewhere else.

What goes up must go down. But if the wind goes up in a tornado it is concentrated in a very narrow area. So you can get a high upwards wind speed. But the wind which goes down somewhere else is spread over a very large area and the downwards speed is therefore very low and certainly not equally strong.

[Adriaan Kragten]

A new chapter 7 has been added to report KD 377 about the pendulum safety system. The title of this new chapter is: "Using a tension spring instead of balancing weights". This new chapter contains a new figure 8 which is added as attachment.

[Mary B]

There is a tilt back wind turbine near me, sounds like a gunshot when it bangs back down on the stops. I suspect a short lifespan is going to result. Just went up this summer...

[kitestrings]

It seems like it might also require some means of dampening of the motion, like a shock absorber.  Otherwise, I'd be concerned that you could end up with unwanted oscillations in the tower.  I wonder too, if you might get situations where it is returning back to position, only to meet the next untimely gust?

It's interesting to explore various approaches.  I guess the thing that misses for me in the overall direction of this chapter is that it doesn't allow you to pre-emptively shut the thig down.  If, for example, you know a violent storm is coming and you want to "batten down the hatches" and ride through it... or if you don't like the sounds of something and just want to shut it down for a closer look.  I think there are better ways.

[SparWeb]

Similar to a Parris-Dunn.
There is on near my house (deactivated).

Video of one furling:
https://forum.allaboutcircuits.com/threads/1930s-parris-dunn-model-44-wind-generator-rare-find.141317/

[kitestrings]

Similar except the Paris-Dunn is an upwind machine with a tail.

One thing I hadn't noted initially is that in Adriaan's latest drawing, the entire rotor is above the yaw point.  It's neither in front of it, as with an upwind design, or behind it as with many downwind designs.  This puts the full weight of the turbine above the hinge point.  Something structurally to consider.

[Adriaan Kragten]

The pendulum safety system described in this report KD 377 has no stop around the normal working position. So no shocks will be heard when the rotor comes back after a strong wind gust. The original system with two balancing weights which are moving along the tower pipe as shown in figure 2, has a large moment of inertia around the hinge axis. This large moment of inertia slows down fast movements. The rotor itself also has a damping effect as forwards movement of the rotor increases the thrust. So the movement of the whole assembly around the hinge axis will be very fluent. The rotor has a negative yaw angle delta of -20° for V = 0 m/s and at wind speeds in between 4 m/s and 7 m/s, the yaw angle is smaller than 10° (see delta-V curve figure 3). So for the most general wind speeds, the power loss due to yawing is negligible.

There have been made systems with a spring and a stop around the normal working position which are very unstable if the spring moment decreases at increasing yaw angle. I have even seen a system which turned backwards in one big stroke and didn't come back to the normal working position automatically. For such systems there will be a shock if the stop is hit and if there is no damping of the movement. I have also designed a pendulum safety system with a torsion spring and a much smaller eccentricity (see report KD 439). The combined effect of the weight and the torsion spring results in an almost constant counteracting moment and this system therefore has almost the ideal characteristic. But there is a stop at the normal working position and therefore you need some damping of the return movement or at least an elastic stop.

The advantage of using a very large eccentricity as used in figure 2 and figure 8, is that the hinge axis can be laid in the rotor plane. So the side force on the rotor gives no moment. If the eccentricity is very large, the so called self orientating moment can also be neglected. So now the rotor moment is only determined by the thrust. This simplifies the moment equation and therefore it is rather simple to derive a formula for the delta-V curve.

The main advantage of turning the rotor out of the wind around a horizontal axis is that the rotor can be turned out of the wind 90° to the so called helicopter position. If the rotor is kept in this position by a hook after a big backwards swing, the rotor is very safe at extremely high wind speeds.

[MattM]

May be easier to tie the tail to the furl with an arm.  As the turbine furls about 45 degrees the tail can hinge, turning it out of the wind the other 45 degrees.

[Adriaan Kragten]

May be easier to tie the tail to the furl with an arm.  As the turbine furls about 45 degrees the tail can hinge, turning it out of the wind the other 45 degrees.

It is not clear to me what you mean. Do you want to turn the rotor out of the wind around a horizontal axis for the first 45° and then turn the head out of the wind around a vertical axis with respect to the vane for another 45°? If you do so, the rotor is never 90° out of the wind because you can only add angles if the planes rotate around in parallel axes. Once you have a horizontal axis of rotation, you should only rotate around this axis because this is the only way to reach the safe helicopter position.

The disadvantage of rotation around a vertical axis is that even if the rotor is turned 90° with respect to the vane, it is only in parallel to the wind if there are no sudden changes in wind direction. During heavy storms the wind direction can change very fast. If a rotor is locked after turning 90° with respect to the vane it will stop to rotate but the forces on non rotating blades can still become rather high for fast changes of the wind direction. 

[MattM]

No, I want the rotor and tail to act together to do that 90 degree effective turn.  If each turns 45 degrees the rotor should move to a perpindicular final position.  Your connecting rod would need to link them together.  Just thinking its easier for both sections to work together.

[Adriaan Kragten]

No, I want the rotor and tail to act together to do that 90 degree effective turn.  If each turns 45 degrees the rotor should move to a perpindicular final position.  Your connecting rod would need to link them together.  Just thinking its easier for both sections to work together.

The pendulum safety system has no normal vane. It has a double vane which consists of a long pipe in parallel to the rotor plane with a square sheet at each end. The sheets make an angle of 20° with the rotor axis. A photo of such a vane is given in figure 11 of report KD 439. The advantage of this vane is that it gives the head a very large moment of inertia around the tower axis. This slows down the angular velocity of the movement of the head around the tower axis and this reduces the gyroscopic moment. Reduction of the gyroscopic moment is required for the pendulum safety system because the gyroscopic moment influences the head movement around the horizontal axis. So it isn't possible to combine rotation of the rotor with rotation of such a vane.

Reduction of the angular velocity of the head reduces the gyroscopic moment in the rotor shaft and therefore also the bending stress in the blades. The disadvantage of safety systems with a vane arm which has a hinge close to the centre of the tower is that the vane arm is not contributing to the moment of inertia of the head. So if there is a wind gust, the head can turn rather fast but the vane arm stays in its position. The hinged side vane safety system, which is used in my normal VIRYA windmills, has a vane arm which is a part of the head and the moment of inertia of the head is therefore rather large. Only the vane blade is moving and light door hinges can be used to connect the vane blade to the vane arm. But for a certain rated wind speed, there must be a certain ratio in between the weight of the vane and the area of the vane. This results in a very flexible vane blade if the vane blade is big. The hinged side vane safety system is therefore limited to rotor diameters of maximal 5 m or 6.5 m depending on the size of plywood sheet available. The pendulum safety system was meant for bigger rotors. The system as described in report KD 439 is used for the VIRYA-10 as described in report KD 715.

[kitestrings]

I would think one could pretty easily fashion a means of deploying this type furling from the ground with a simple pull line.  This would allow you to shut it down by choice, or to more practically release it once it is latched.  The idea of climbing the tower with a pole to release it doesn't seem very practical, especially for larger units, and adds an unnecessary risk element.

Interesting to ponder, but I don't see any clear advantage over side furling, and I do see considerable added challenges to the build and structure.  This is probably why there aren't any examples of these designs except in a drawing/concept, though I'm not discouraging exploring alternate approaches.

[Adriaan Kragten]

I would think one could pretty easily fashion a means of deploying this type furling from the ground with a simple pull line.  This would allow you to shut it down by choice, or to more practically release it once it is latched.  The idea of climbing the tower with a pole to release it doesn't seem very practical, especially for larger units, and adds an unnecessary risk element.

Interesting to ponder, but I don't see any clear advantage over side furling, and I do see considerable added challenges to the build and structure.  This is probably why there aren't any examples of these designs except in a drawing/concept, though I'm not discouraging exploring alternate approaches.

Every safety system has certain advantages and disadvantages. In my report KD 485 I describe five different systems. For three of them, the head turns out of the wind around the tower axis and for two of them it turns out of the wind around a horizontal axis. But independent of what system is chosen, you always need two axes. I have practical experience with the inclined hinge main vane system used for the water pumping windmill CWD 2740 and also used for the water pumping windmills of Southern Cross and for the designs of Hugh Piggott and with the hinged side vane system which is used in all VIRYA designs. The third system is the Ecliptic system, mainly used for water pumping windmills. But systems which turn the rotor out of the wind around a vertical axis give problems if the rotor is very big. I think that the pendulum system which is decribed in KD 439 can be used up to a rotor diameter of 20 m. The pendulum system which is described in KD 377 can't be used for very big rotors as it would result in a very big mass om top of the tower. But it is always moving in fluctuating winds and therefore it has a high artistic value. An advantage of both pendulum systems is that it is rather simple to find a mathematical description for the delta-V curve for certain geometry parameters. So the behaviour can be predicted rather accurately which eliminates a lot of try and error.

I think that the main reason why most people use the inclined hinge main vane system is that this is a rather old system and that everyone makes copies from earlier designs. But it is very difficult to describe the behaviour theoretically (see KD 431) and predict the delta-V curve. Therefore a lot of try and error is needed to get an acceptable result.

[kitestrings]

I think the reason the inclined hinged is used is that it is comparatively easy to build, and time proven.  Sometimes there is a tendency to over complicate things.  At a basic level, we swing the rotor around and the circle presented to the wind shifts to a progressively shrinking ellipse until at 90 degrees, the wind "sees" only line parallel to the rotor.  If we fully furl our turbine, and then short the windings, it is a very passive, soft-stop.  Unless the wind event rises to a level that threatens the tower structure itself, it can withstand most anything thrown at it.  Pretty simple IMO.

[Adriaan Kragten]

Yes, technically the inclined main vane system is simple but the description of the moment equations is rather difficult, especially if the vane is in the rotor wake. If the vane is jutting above the rotor, description is easier because then the vane always feels the undisturbed wind speed. No stop is needed if the vane is in the lowest position but you need a stop at both end positions to prevent that the vane can hit the rotor in very turbulent winds. I also advise a pre-angle in between the zero line of the vane arm and the rotor shaft to make that you have a small negative yaw angle at very low wind speeds, a yaw angle of about zero degrees at a wind speed of about 5 m/s and positive yaw angles at higher wind speeds. If you do so, the delta-V curve looks about similar to the delta-V curve of the pendulum safety system as given in figure 3 of KD 377. At moderate wind speeds this is okay but at high wind speeds, the power is reduced more than required.

It is important that you use proper bearings at the vane hinge axis and at the tower axis. I have seen versions of this safety system with too much friction at the vane hinge axis and then the rotor isn't going out of the wind at moderate wind speeds. You also need a rather large eccentricity of at least 8 % of the rotor diameter if the safety system is used in combination with a fast running rotor because such rotors have a large self orientating moment. Multi bladed slow running rotors of water pumping windmills have almost no self orientating moment and then you can use a much smaller eccentricity of about 3 % of the rotor diameter. I have seen several safety systems with fast running rotors and a too small eccentricity which would not turn out of the wind at high wind speeds.

[SparWeb]

I think that the main reason why most people use the inclined hinge main vane system is that this is a rather old system and that everyone makes copies from earlier designs. But it is very difficult to describe the behaviour theoretically (see KD 431) and predict the delta-V curve. Therefore a lot of try and error is needed to get an acceptable result.

I think you have it summed up well, here.  Those who are concerned about the exact behaviour of a mechanism, and having a need to model it before putting it into service, will find the side-furling choice hard to deal with.  Those who can accept approximation or trial-and-error will copy the past practices and find it close enough.  I have done the latter, and successfully to my standard.

[Adriaan Kragten]

To my opinion the hinged side vane safety system is technically the most simple system of all systems. It needs only light door hinges for the vane blade, it has a large moment of inertia of the head so it strongly limits the gyroscopic moment and the obtained delta-V curve is close to the optimum curve. The main disadvantage is that the vane arm must be very stiff and there must be a stop for the vane blade at almost horizontal position to prevent flutter of the vane arm at very high wind speeds. Theoretical description of the behaviour is rather complex, especially because the self orientating moment of the rotor is take into account. But it is possible and it is done in report KD 213 for the VIRYA-4.2 windmill. But the theoretical description of any other system also becomes complex if the self orientating moment is taken into account. The self orientating moment can only be neglected if the eccentricity is very large and this is the reason why description of the pendulum sytem as given in KD 377 is that simple. So if people wants to study safety systems, I always advise to start with KD 377 even if one wants to use another system.