Homebrewed Electricity > Wind
Active Pitchcontrol
Adriaan Kragten:
--- Quote from: MattM on July 28, 2021, 08:00:18 AM ---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.
--- End quote ---
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:
--- Quote from: MattM on July 29, 2021, 01:13:57 PM ---I wouldn't be surprised someone uses Arduino or Raspberry Pi to create an active pitch control mechanism on the wing tips.
--- End quote ---
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?
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