Active Pitchcontrol

[Bruce S]

Frans;
There are quite a few new faces here.
Perhaps you could give us an update on these beauty?

https://www.youtube.com/watch?v=U9DdA5LWs08

Many Thanks
Bruce S

[Adriaan Kragten]

Yes, and it is not just the bearings to consider.  The blades have to rotate in unison, so there is linkage between them.  There is also the center shaft and slides of some sort that limit the range of motion.  It may go without saying, but the entire mechanism needs to be protected from the elements, and even condensation can introduce corrosion or frost.  I've had sliprings on my engine driven generator glaze over or stick the brushes momentarily, preventing output, when the wrong combination of humidity and and temperature drop have combined.  (Stop and restart it, and all is good once a bit of heat is present).

The blades have to be coupled mechanically to each other if the system contains centrifugal weights. This is because apart from the centrifugal force, also the own weight is acting on a centrifugal weight. The direction of the moment caused by the own weight depends on the position of the blade and so if it is positive for one position, it will be negative if the blade has rotated 180°. This fluctuating moment causes a torsional vibration in the blade. But if the blades are coupled mechanically, the resulting weight moment of all blades together is zero.

Some traditional Dutch windmills have air brakes at the blade tip of two opposite blades which are activated by a centrifugal weight and for which there is no mechanical coupling. I have observed a windmill with this system and it could be seen that the brake goes out for a certain position of the blade and goes in if the blade has rotated 180°. As traditional Dutch windmills have a rather low design tip speed ratio of about 2.5, the tip speed is rather low and the force because of the own weight of the centrifugal weight is therefore rather large with respect to the centrifugal force acting on the weight. The oscillating system must cause some variation of the balancing of opposite blades and some variation of the blade thrust but as the blades are very heavy and very stiff, this isn't hindering the functioning. The positive effect of automatic limitation of the maximum rotational speed is more important than the negative effect of the vibration.

If the blade movement is only activated by the aerodynamic moment, there is almost no torsional vibration as the aerodynamic moment is almost constant for every position of the blade. The aerodynamic moment is determined by the relative wind speed W which is mainly determined by the blade speed for the most important outer part of the blade where the blade speed is high (see formula 5.11 of report KD 35). So even if there is some variation of the wind speed in the rotor plane, this results in almost the same relative wind speed W for every position of the blade. Therefore mechanical coupling of the blades isn't necessary if the blade movement is only activated by the aerodynamic moment. But as the aerodynamic moment is rather small, the bearing friction must be low and every blade must have its own spring and its own stop for the working position. Cancelling of the coupling mechanism makes the system more simple, especially for a 2-bladed rotor. This system is described in detail for the VIRYA-5 rotor in chapter 9 of report KD 622.

[MattM]

It sounds like there are more than two categoriess to control pitch.  And it sounds like weight and the harmonics vary by choice.  I'd argue that furling is a form of pitch control.

[Adriaan Kragten]

It sounds like there are more than two categoriess to control pitch.  And it sounds like weight and the harmonics vary by choice.  I'd argue that furling is a form of pitch control.

It depends on the definition. I say that there are two main categories for pitch control systems but within each main category you have several sub categories. For pitch control, it is generally meant that the blade angle varies and so the blade can turn around the blade axis. Furling means that the whole rotor is turned out of the wind and if this is done, you can use a rotor with fixed blades. But there have been built windmills which have both pitch control and furling like the big Windcharger. For these windmills, the pitch control system is normally used to limit the rotational speed and thrust. The furling system is used to stop the rotor by turning the whole rotor 90° out of the wind if no power is needed, if a big storm is expected or if maintenance is required.

If the rotor is furled, it turns with a certain yaw angle delta in between the rotor axis and the wind direction. If the blade is followed during one revolution, you will see that the angle of attack alpha varies. However, the blade angle beta, which is the angle in between the neutral line of the airfoil and the rotor plane for a certain station, is constant and therefore furling is no form of pitch control.

[kitestrings]

Yes, I'd also interpreted this discussion to be focused on blade pitch/pivoting strategies.

One additional consideration -

Most of the pivoting blade designs I've seen have the rotor mechanism enclosed (e.g. Dunlite, Jacobs).  Some designs have the blades mounted to a flange-plate, so they are just thru-bolted to a pivoting assembly.  Others, however, have a shaft that extends into the root of the blade.  For this approach you have to consider carefully how the blades are mounted and removed.  I've seen some where corrosion makes them a bear to remove.  The pockets themselves are also a place where water can enter, be trapped, and cause imbalance over time.

[MattM]

I wouldn't be surprised someone uses Arduino or Raspberry Pi to create an active pitch control mechanism on the wing tips.

[Adriaan Kragten]

I wouldn't be surprised someone uses Arduino or Raspberry Pi to create an active pitch control mechanism on the wing tips.

In stead of turning the whole blade, it is also possible to turn only the blade tip. 3/4 of the power is generated by the outer half of the blade. So a relative small outer blade section can generate enough drag to consume all energy generated by the inner part of the blade. However, such a construction has some disadvantages. One is that the outer blade section can become rather noisy if it isn't streamed at the optimal angle of attack. Another disadvantage is that it is more difficult to connect the movement of the different blade tips as this requires a rod which is moving in the fixed inner part of each blade and a coupling mechanism at the center of the rotor.

About 30 years ago the University of Delft has tested a pitch control system of the blade tips on a 8 m diameter windmill which was using the rotor of a helicopter. They used a mechanism for which the blade tip moves outwards because of the centrifugal force and for which it rotated because of a helical twist in the blade shaft. What I remember was that the system worked nicely and I think that there was no mechanical coupling in between the blade tips.

About 40 years ago I have tested elastic air brakes on each tip of a 3-bladed windmill rotor with a diameter of 4 m. I have also tested a scale model in the wind tunnel and the air brakes were very effective in limitation of the rotational speed. There was no mechanical coupling in between the three air brakes. However, at high wind speeds, the noise production was enormous for the real windmill and therefore this idea was cancelled. Windcharger has made small 2-bladed rotors with air brakes at rods which make a 90° angle with the blades. But as the speed at the position of the air brakes is much lower than at the blade tip, you need much larger air brakes to get enough braking torque than for my rather small air brakes at the blade tip. The advantage of the lower speed is that the noise production is much lower.

So the advantage of turning the whole blade is that there is no part of the blade which generates a lot of energy which has to be destroyed by the outer part of the blade.

[MattM]

An elastic airbrake was the basis to conclude active blade tips are noisy?

[Adriaan Kragten]

For an elastic air brake at each of the blade tips, the whole blade is active, so the airbrake has to destroy all power generated by the rotor as the system has to work also for an unloaded generator. The field tests have been performed for an unloaded rotor. The blade speed is maximal at the blade tip so you need only a small air brake area to generate a lot of drag. The air brake was made out of 1 mm stainless spring steel and had a chord of 200 mm (the same as the blade) and a width of 150 mm. The air brake was connected to the blade at the front side and bent outwards due to the centrifugal force acting on it. It produced a very large tip vortex which sounded like a helicopter was coming over.

If you turn only the blade tip, a smaller remaining part of the blade is active than for an air brake at the blade tip but this active part of the blade will still generate a lot of power. This power can only be destroyed for a small blade tip if this blade tip is stalling. So you need negative pitch control for the blade tip. A stalling blade tip will also be very noisy, probably less noisy than my elastic air brakes but I won't chose this option if you have neighbours.

The advantage of positive pitch control of the whole blade is that the lift coefficient of the whole blade is reduced and so there is no part of the blade which generates a lot of energy which has to be destroyed by another part of the blade. Negative pitch control of the whole blade can also be rather noisy but it can be used for constant chord blades as for those blades, stalling starts at the blade root. This is because for those blades, the lift coefficient is low at the blade tip and high at the blade root.

In about 1980 there has been a France company (I can't remember the name) which supplied a windmill with a rather big 2-bladed rotor with negative pitch control. They used a clever pitch control mechanism with two springs in it with different stiffness. This made that the blade angle was large during starting and the weak spring was compressed already at low rpm which makes that the rotor was turning with the normal blade angle at moderate wind speeds. At high rotational speeds, so at high wind speeds, the strong spring was compressed and then the whole blade was stalling. So an advantage of negative pitch control is that you can have a large blade angle during starting if you use two springs with different stiffness.

[Mary B]

I wouldn't be surprised someone uses Arduino or Raspberry Pi to create an active pitch control mechanism on the wing tips.

In stead of turning the whole blade, it is also possible to turn only the blade tip. 3/4 of the power is generated by the outer half of the blade. So a relative small outer blade section can generate enough drag to consume all energy generated by the inner part of the blade. However, such a construction has some disadvantages. One is that the outer blade section can become rather noisy if it isn't streamed at the optimal angle of attack. Another disadvantage is that it is more difficult to connect the movement of the different blade tips as this requires a rod which is moving in the fixed inner part of each blade and a coupling mechanism at the center of the rotor.

About 30 years ago the University of Delft has tested a pitch control system of the blade tips on a 8 m diameter windmill which was using the rotor of a helicopter. They used a mechanism for which the blade tip moves outwards because of the centrifugal force and for which it rotated because of a helical twist in the blade shaft. What I remember was that the system worked nicely and I think that there was no mechanical coupling in between the blade tips.

About 40 years ago I have tested elastic air brakes on each tip of a 3-bladed windmill rotor with a diameter of 4 m. I have also tested a scale model in the wind tunnel and the air brakes were very effective in limitation of the rotational speed. There was no mechanical coupling in between the three air brakes. However, at high wind speeds, the noise production was enormous for the real windmill and therefore this idea was cancelled. Windcharger has made small 2-bladed rotors with air brakes at rods which make a 90° angle with the blades. But as the speed at the position of the air brakes is much lower than at the blade tip, you need much larger air brakes to get enough braking torque than for my rather small air brakes at the blade tip. The advantage of the lower speed is that the noise production is much lower.

So the advantage of turning the whole blade is that there is no part of the blade which generates a lot of energy which has to be destroyed by the outer part of the blade.

There is a commercial unit near me that uses blade tip brakes. Looks to be an induction type and has twin out swept tails(to make it track better?). No clue if it is functional still, has been up for 30+ years. See the tips rotated 90 degrees all the time...

[midwoud1]

Pitchcontrol.

Rotorhead gear



https://youtu.be/MRVGYVrgsdc

electronic control  tach

tacho-chip LM2917 >  Arduino Uno  software in C   

[Adriaan Kragten]

In the photo I see that the turning point is lying about at half the blade chord. This results in a large aerodynamic moment coefficient which has tendency to increase the angle of attack alpha and so to decrease the blade angle beta. The mechanism which turns the blades has to be that strong that it can over power the total moment of all three blades at high wind speeds. The influence of the position of the turning point is discussed in chapter 4 of public report KD 463 for the Gö 623 airfoil and in chapter 5 for the Gö 624 airfoil. The relation in between the moment coefficient Cm (-) and the Moment M (Nm) is given by formula 3.3 of public report KD 35.