Homebrewed Electricity > Wind
Active Pitchcontrol
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:
--- Quote from: Adriaan Kragten on July 31, 2021, 02:14:56 PM ---
--- 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.
--- End quote ---
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.
Navigation
[0] Message Index
[*] Previous page
Go to full version