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In the last class we have seen that the main force that drives a wind turbine is the lift
force and we had drawn this diagram.
You had the airfoil and at some point we drew the direction of the motion, u and the direction
of the incoming airflow, v and so we obtained the parallelogram, minus u and v, add them
together we have the vector for the relative wind, w and based on this relative wind, we
have a force that is created in the direction of the relative wind, which is the drag force
F D and a force that is created perpendicular to the relative wind which is larger than
the drag force, which is F L. So, you have a projection of the lift force in the direction
of motion and a projection of the drag force in the direction opposite to the motion and
since this force overwhelms this force, you have a net force that tries to push it further
in the direction it is moving. So, essentially this is the basic principle.
But, engineers are often intent on obtaining some numerical predictions. We should be able
to predict how much will a wind turbine produce. How much power wind turbines will produce?
How much torque it will produce? Not only that, a wind turbine if it is rotating like
this and wind is coming, they will also be a force that tries to topple it, a thrust
force, right. That means the wind is making it rotate, but it rotates in a direction perpendicular
to the, to the direction in which wind is coming and that tries to topple the tower.
I will, I will illustrate it by this means.
So, you have the wind coming and the wind turbine, it is rotating like this. Therefore,
the direction of u, direction of v, wind is coming and the direction of u will be perpendicular
to each other, right. So, we can draw this like this.
You have the v force here and you have the u force here, the wind direction is like this
and the direction of the rotation like this. As a result, we will construct the parallelogram
like this, minus u and in the last picture we drew a general picture, but now we are
referring specifically to wind turbine in which these two vectors are orthogonal. You
have the w here, so this vector, this vector added together gives the w or the relative
wind vector. Now, you have the drag force in this direction, F D and the lift force
in this direction, F L. They added together give you the total force. This will be the
total force. Let us call it F, the total force.
Now, this will have a component in the direction and this direction. So, there will be a component
like this and there will be a component like this. Now, what does this component do? Notice
that it is in the direction of v. That means this is the force, this is the force that
tries to topple the tower and there is another force that acts in the same direction in which
the blade is moving and therefore, that produces the moment, so this force is called the F
T, the thrust force and this force is called F M, the moment producing force. So, this
is a force. When we calculate all these, it is not difficult to see that we cannot do
this calculation for this whole blade in just one go. Why? Just imagine that the blade is
like this.
I am drawing one, one blade and it is rotating like this. What will be the u here? u means
the velocity of the blade, linear velocity of the blade. If it rotates at a certain angular
velocity, it is not difficult to see that the linear velocity here will be different
from the linear velocity here. So, the linear velocity along the blade will change and therefore,
these vectors will be different, both in magnitude as well as direction as you go along the blade,
right. So, in order to do this calculation, what we do is we make the blade divided into
small sections, then take one of them and for that you can draw this kind of a force
diagram. As you go to another, the magnitudes will change. The v magnitude will remain the
same, but the u magnitude will change. Naturally, the direction of the w will change, direction
of all these forces will change. So, we need to do this calculation for each section.
While doing the calculation by hand, we often divide the blade into a small number of sections
say, 3 or 4 and we can do the calculation by hand. But often, the actual calculations
are designed, done by computers, where you define these into many small sections and
for each section the calculation is carried out and then you can add up. For example,
if this one produces a certain amount of thrust, this force, this one produce another certain
amount, the whole thrust experienced by the blade will be simply the summation of them.
Similarly, the moment; the moment produced by this one, moment produced by this one,
moments produced by this one, all these will have to be added together to obtain the final
moment.
So, we say that here is a particular element that is at a distance say r and having the
width of dr and for that we can do the calculations. Now, what calculations? Now, the calculations
essentially start from the expression for the F L and F T. What is F L here and F D
here? Normally, since these are all elements, therefore these are denoted as dF D, dF M,
dF T, dF L, dF and all that. Why? To signify that these are related to the blade elements,
not the whole blade. Now, what will be the lift force produced? The lift force produced,
dF L, its expression is half rho dA b, dA b is the area of the blade, then w square
times a very important factor that is called the lift coefficient, C L. This lift coefficient
depends on the shape of the blade, the roughness of the blade and things like that.
So, essentially that depends on, when I drew this particular shape, you might ask what
is the, what is the geometry of this, what is equation for that? Now, in practice, various
geometries exist. People have experimented on various geometries. You see the aeroplane
wings, you can see the curve, all right, but in each aeroplane, each design this curve
is different which means that these equations are different. What actually happens is that
people have experimented with various designs. The experiment is normally carried out in
wind tunnels. They have constructed large wind tunnels in which this particular shape
of the airfoil is put, wind is blown and they make accurate measurements on the amount of
drag produced and amount of lift produced and from there, this lift coefficient and
drag coefficient can be calculated.
So, this is the expression for the lift produced, F L, dF L and the expression for the dF D
is exactly similar, another coefficient here, another coefficient that is a drag coefficient.
Now, if these two are known, so what is known? This is known and this is known and if, we
need an angle in order to refer to that, this angle is said to be I, you see, this angle,
then we can resolve this into the proper directions in terms of the I. So, we can write, we are
interested in actually the thrust force, because the tower has to withstand that amount of
force. We are interested in the moment producing force, because the power produced is dependent
on that. So, we can resolve this and that in this direction and in this direction to
obtain this. You can easily see that, then the dF T, the thrust force will be dF L cos
I plus …, so that is the thrust, fine.
Now, dF M will be this force, yes, dF L sin I minus, yeah, dF L sin I minus dF D cos I,
right. Now, dF M is the moment producing force. How much will be the moment? Moment will be
the moment producing force times r. So, dM is r times that, so r into this. We can simply
calculate from here, we can simply do the calculations from here, clear, because the
dF L and dF D, if we know all these terms, rho is the density of air, it varies all right,
because the air pressure varies. But often we take it as rho is 1 point say 225 or something
like that. Area we can, we can measure, w square is obtained from these two, u is obtained
depending on the distance and C L, C D, these things are obtained from the air foil characteristics
that are available. That means for every airfoil people have measured that.
Now, these are often available, these days available on the net. Earlier they were not
available on the net, but these days if you find, if you search for the airfoil characteristics,
there are a few places, few universities, for example the University of Stuttgart has
a large wind turbine, wind tunnel and their data are available on the net. University
of Wisconsin, they have a large wind tunnel, their data are available on the net. So, these
things you can easily calculate. Now, in what form are they available? I will, I will show
you the representative graphs of this. For example, for certain air foil, the air foils
are sometimes named after NACA, National Aeronautical Council of America, there, they have a series
of airfoils, they have numbers. There is a few airfoils that are designed in Germany
by Votman, so they have separate numbers.
So, each airfoil has a number and the graphs are often available, data are often, often
available in the form of a, the angle of attack versus C L C D. Now, what is this angle of
attack? You can keep the airfoil, if the air is blowing you can keep the airfoil like this,
you can also keep the airfoil like that, so that is the angle of attack, the angle at
which the …, for example, here this is the let me, let me draw it then it will be clearer.
If you have this, then you can draw a line of chord and if the relative wind is coming
from this angle, then this is the angle of attack and this angle is denoted by small
i. So, for every angle of attack you have the C L and C D listed. They have calculated
found and they have made it available on the net. So, you have the graph something like
this.
If you draw the graph they will look something like this that small i verses C L will have
some kind of a characteristic, small i verses C D will have some kind of a characteristic.
These are very specific to that particular airfoil, there is no general characteristic.
So, from here depending on the angle of attack for that particular blade element, you have
to read the value of C D, you have to read the value of C L and that you have to substitute
here.
It is clear that for each blade element the angle of attack will be different. In what
sense? The capital I that you have drawn here, this is the angle between the lift force and
the wind velocity, this is not the angle of attack though. Why? Because, this blade, the
blade is not aligned; its chord, its chord is not exactly aligned with the direction
of motion. It has an angle called a pitch angle and therefore, you have to subtract
that from the I, in order to find the angle of attack. From there you have to read the
values of C L and C D and then everything follows. So, from there you calculate the
moment. The moment times the rotation is the power, the moment times omega, rotational
speed is the power. So, you can calculate the power also. Under any given condition,
you can calculate the power also. So, this gives a very simple way of calculating the
power as well as the thrust that will be produced by the wind turbine, clear.
So, you see that because along the blade, the u, the linear velocity changes there would
be a few implications. One, the implication will be that at this point and at this point
the moments will be different, clear. Why because, at this point u is large. As u is
large, w is large and therefore, the lift, lift force is large and therefore, this component
will overwhelm this component further. As a result, this part will try to fly off faster
than this part, clear. Therefore, there will be a sort of a shear force produced that tries
to break the blade and this is one of the major reasons of blade failures. Blades do
fail because of this reason. There will be another force that is here is a blade and
there is a thrust force produced. There will be larger thrust force to the tip and there
will be smaller thrust force to the inside of the blade. As a result, there will be a
bending kind of force produced and that bending force will also, might be a cause of the failure.
Generally, people try to avert this problem by two means. One, by using a taper, so that
the area inside is bigger than the area outside; to the tip it sort of tapers. As a result,
this point, this number, this particular dA b becomes smaller at the tip and so, we can
adjust to keep it within the, keep the breaking force, bending force within the limit that
can be withstood by that material. The other thing is that v generally produces a twist.
Twist means this pitch angle at which it is inclined that need not be the same. In fact,
that should not be the same. Why? Because, you see, inside the u will be small, outside
the u will be large and therefore, this w will be more inclined to this side, toward
the inside, more towards that, towards the outside.
Now, normally you would like to have as little drag produced as possible. So, just, just
look at this.
How much is the drag produced that is the amount of area that is seen by the wind because,
notice that it is, it is like this. This is the area, this is the w. So, this is the area
that is seen by the relative wind and that is the area which it tries to push back and
therefore, the F D that is why is proportional to this, the area that it sees, w sees. So,
in order to reduce that drag what you have to do? You have to orient it along the direction
of the relative wind. That is the most natural thing to do. Now, the relative wind changes
from the inside to the tip of the blade, the direction changes and so, you will need to
change the angle from the inside to the tip of the blade. It means that it will have a
twist. So, most of the modern wind turbines have
a twist. That means the pitch angle is not the same, inside the blade and the outside.
So, by these two means we try to have sort of a balance between the power produced this
side and that side. So, this is how we often adjust it, clear.
So, now there are a few very important numbers associated with any wind turbine.
One of the important numbers is the tip speed ratio. Tip speed ratio is the speed of the
tip divided by the unaffected wind velocity. So, what will it be? That will be 2 pi R,
the radius into the n, the speed in rpm divided by V infinity. Now, notice that as the wind
speed changes if the speed of rotation remains constant, then the tip speed ratio changes.
There is another important identifier. One thing is that there are very slow turbines.
The turbines that are mainly used for say water pumping, they are slowly rotating, those
will have a low tip speed ratio and the ones that are used for electricity production,
they are fast rotating turbines, they will have a high tip speed ratio. You might ask
how much? Well, the tip speed ratio of a slow water pumping wind turbine is often of the
order of 1.5 to 2, while that of an electricity production wind turbine will be of the order
of 6 to 9. So it, that rotates very fast, the blades rotate very fast in comparison
to the wind speed.
There is another important factor, important number that is associated with wind turbine.
That is the power coefficient that is essentially the efficiency, the aerodynamic efficiency.
So, that is power output of the wind turbine divided by a power contained. The only distinction
is that this power output is not the actual electrical power output of the whole wind
turbine generator system. This is the power output of the aerodynamic power that means
the power that has been converted from wind into the mechanical domain. So, this is the
power coefficient. Now, it so happens that for every wind turbine there is a distinct
relationship between the power coefficient and the tip speed ratio and the graph would
be something like this.
So, this is the tip speed ratio TSR in the x-axis and the power coefficient C P in the
y-axis. What is the maximum C P? No, we have derived in the last class that the maximum
C P possible is 16 by 27; that we have derived in the last class. So, that is the maximum
C P possible. So, that is what we tried to achieve. Now, we can say 1, 2 3, 4, 5, 6,
7, 8. Now, the modern horizontal axis electricity production wind turbines will have the curve
something like this. That means the peak goes very close to the Betz limit. The Savonius
rotors that we have talked about will have somewhere here. The one Savonius rotors which
is a vertical axis wind turbine with two cylinders cut and put together, so that will have it
here.
So, every wind turbine will have this kind of a characteristic and this kind of a characteristic
means that the maximum power coefficient occurs at a specific TSR, at a specific tip speed
ratio, which means that as the wind speed changes you would like to keep the TSR constant
and since the tip speed ratio is the speed of the tip divided by the wind speed, wind
speed is varying, therefore the speed of the tip has to vary proportionally which means
that the speed of rotation, rotational speed has to be, has to vary in proportional to
the wind velocity.
Now, that does not happen automatically. We need to do something in order to make that
happen and the lever in our hand is often, in case of large wind turbines, the pitch
angle. So, we vary the pitch angle as the wind speed changes. Why would we need to vary
the pitch angle?
Because, all the time we would like to be aligned with the relative wind and as the
v changes, as the v changes, then the w’s direction also changes, so we would like to
rotate the blade, so that it is always aligned and that is how it produces the maximum amount
of energy. This is called the pitch control. That is why most modern wind turbines, the
ones that are used for electrical power generation, now there are electrical power generation
wind turbines of the size 1 megawatt, huge wind turbines and they have this pitch control
mechanism.
There is another type of wind turbine which is called, rather peculiar looking wind turbine,
they are called the Darrieus rotor.
The Darrieus rotors are peculiar in the sense that just by looking at it you will never
understand how it rotates. They look something like this. So, this is the shaft. It is a
vertical axis wind turbine and here you have the generator system and the shaft actually
goes up some length and then, you have the …. connected that is tied down, to the down.
So, this is the shape and these are, these blades are thin and air foil shaped. Now,
you might say that when wind comes and hits it, it will of course try to push this one
as well as this one and therefore, there will be no force produced, no torque produced.
Yes, that is true, but the point is that your argument is coming from the, from the point
of view of thrust.
How much it pushes? No modern wind turbine works on the push principle. So, this works
on, again the lift principal of a, of an airfoil. How it actually works? In order to understand,
let us draw in this way.
Suppose this is the shaft at the center and suppose this is the, not a bad circle, so
this is how it rotates. So, at every point we will draw the airfoil structure and imagine
that the wind is coming from this direction, fine. So, now when it is here, then your v
is like this, u is like that and therefore, minus u is like this. So, this is your v,
this is your u, minus u and therefore, you will construct the parallelogram like this
and we will say that this is my w. If this is the w, then this is the drag force produced
and this is the lift force produced, right. This is the w.
You would notice that the lift force has a component in the direction of motion. So,
if it rotates, then this lift force aids the rotation and therefore it produces a positive
torque. When it comes here, then your v is in this direction, u is in this direction,
therefore minus u is in that direction and therefore, you will complete the parallelogram
and say that here is my w. So, your drag force will be like this and the lift force will
be like that. I am always drawing the lift force larger than the drag force, because
that is the characteristic of the most airfoils. Now, here also there is a component of the
lift force that aids the motion.
Come here, this is the direction of the v and this is the direction of minus u and therefore,
this is the direction of w. In this position also this is the direction of F D and this
is the direction of F L. Still F L has a, has a component in the positive torque direction.
Come here; here you have the direction of the velocity, wind velocity here, v and here
is minus u and therefore, this is the direction of w. Again, you have the F D like this and
F L like that. Do you notice that everywhere, well almost everywhere there is a component
of the lift force in the direction of the motion and as a result there will be a positive
torque produced and it will keep rotating.
So, do you now notice how it looks?
This is the airfoil that is actually here. All this is the airfoil structure. So, as
you look at the cross section it would be this airfoil, fine. Can you see the structure
and as it rotates, well one thing is clear that if there is no u, sorry, yes, if there
is no u, then you might again draw these parallelograms and you will find that no net torque is produced,
which means that this particular wind turbine, the Darrieus rotor has no starting torque.
It has to be started somehow by some means and then the moment it has a starting torque,
it starts rotating on its own, because there is a positive torque.
Sir, I could not understand the structure of the …
You could not understand the structure of the Darrieus rotor, right. So, the Darrieus
rotor structure is the axis is vertical. At the, at the ground level there is the generator
and gear box. At the top there is a, there is a hinge and which is connected to a …. that
means these …. keep the shaft vertical and does not allow it to move. Now, on the shaft
you have these structures, the airfoils are bent like this and like this. Now, you might
ask why is it this shape, why could not it be anything else? The reason is that, okay
what is the shape? This is the geometry. Is it parabola?
No, not exactly parabola, its geometry is called Troposkein, it is exactly that shape.
You see, if you hold a flexible wire and rotate it like this, it takes a shape, right and
that shape is called Troposkein. That means it is exactly the shape that a flexible wire
would take if it is rotated like this which means that as it rotates, the only force that
can be, only stress that can be produced in these blades could be just a tensile stress.
Do you understand? There can be, cannot be any bending stress, because if it were flexible
and if it were rotated, then you have to take the same shape and if you make it in that
shape, then the only force that can be produced is the tensile stress that stretches, that
is all. So, if you have, if you build into it strength
along the direction of the length of the blade, then obviously it becomes more stable. It
is very difficult to, for such a blade to break, which is not true for horizontal axis
wind turbines. There can be this kind of forces which can make it break, but here nothing
will happen. So, the structure is that you have the vertical axis and you have the blades
that are shaped like this and this cross section is the airfoil cross section, clear, this
cross section is the airfoil cross section. So, as it rotates, so the airfoil goes around
like this and that is what I have drawn here, the airfoil goes around.
So, once it is here, after sometimes goes there, after sometimes it goes there and it
rotates. As it rotates, you would notice from this diagram that everywhere it experiences
a positive torque. This, this particular shape, this particular design was invented by Darrieus
that is why is called the Darrieus rotor.
Now, this Darrieus rotors, if you draw it on this diagram, their position would be somewhere
here slightly lower than the efficiency of the horizontal axis wind turbines, slightly
into the lesser TSR side. But, as far as efficiency is concerned, since it also works on the principle
of the lift force, therefore it does not really is very much off. Its maximum efficiency is
also very large. So, the question is how do they compare?
The one is the horizontal axis wind turbine which have a structure like this
and the vertical axis wind turbine which has structure like this. How do they compare?
Now, you will notice that by virtue of being the, blade being up at a height, by virtue
of being at a height the horizontal axis wind turbine is able to access higher wind velocities,
but this vertical axis wind turbine has to be close to the ground and therefore, the
wind speed it can experience is not as high. But at the same time, there is a very large
expenditure in constructing this tower and since the whole gear box generator assembly
has to be here, the whole turbine and its weight has to be here, therefore the tower
has to be strong and naturally that incurs a lot of expenditure.
Here there is no tower; the generator is on the ground, easily accessible, so naturally
the cost of production of this kind of turbine is far lower. So, this can access higher wind
speeds and as a result, as I told you, the amount of power contained in the wind is cubically
proportional to the wind speed, so it has higher amount of energy available to it. This
has a lower amount of energy available to it, but its production cost, the cost of installation,
these things are far smaller.
Now, the places where you can expect a reasonably high amount of wind velocity close to the
ground, where are they? Very close to the sea, because the sea surface is more or less
flat and therefore, there is no obstacle, up and down, high and low obstacle and therefore,
there the wind speed close to the ground is also very high and there the Darrieus, Darrieus
axis wind turbines, Darrieus, Darrieus rotors are economically competent. But, the ones
that are produced, that are installed in land there the wind has to cross a lot of obstacles
to reach the position of the wind turbines. There it is definitely more meaningful to
have the wind turbines at an elevation, so there the horizontal axis wind turbines are
more meaningful. So, in terms of axis, this is the horizontal axis wind turbine and this
is the vertical axis wind turbine.
Notice that the horizontal axis wind turbine has to be oriented in the direction of the
wind, while the vertical axis wind turbine it does not matter. So, another complication
and another control system can be avoided in case of the vertical axis wind turbine.
This control, the controlling to face the wind is called Yaw, Yaw control. I will come
to the details a little later. So, Yaw control is necessary for horizontal axis wind turbines
and Yaw control is not necessary for vertical axis wind turbines. So, there are essentially
two types of wind turbines, nowadays used for bulk electricity production - the horizontal
axis wind turbine and the vertical axis wind turbine, though right now in India all the
wind turbines that have been installed, India has now a very large wind turbine install
capacity, mainly in the states of Gujarat and Tamilnadu, these two places have really
gone ahead with installing wind farms. That means large tracks of land that are otherwise
barren, now covered with wind turbines and that produces in totality very large amount
of power. So, that can be compared with the power production capacity of a standard thermal
power plant. So, India has this kind of and all these are at present horizontal axis wind
turbines, though you can easily see that under certain conditions, certain circumstances,
the vertical axis wind turbines will be also very meaningful. It is just that the companies
which have started operating here they specialize in the horizontal axis wind turbines.
So, you have learnt about three different types of wind turbines. The ones that are
used mainly for water pumping, they are the vertical axis Savonius rotor types. There
are also another type of horizontal axis wind turbines that are used for water pumping.
Their structure would be something like this.
There would be a tower, obviously any horizontal axis wind turbine will have to have a tower
and here there would be a large number of blades, a large number of blades and there
will be another ring to give it the strength and the whole thing will be, why do you need
a large number of blades? Because, you need to produce a large torque in water pumping
wind turbines and naturally from here there would be a shaft coming down and down here
would be the pump. This water pumping wind turbines were very popular in USA, early part
of this century, of the last century. But nowadays, with the availability of electricity
even in very remote locations, these have gone out of favour.
But in India, this makes sense because, there are large areas in which there is a, there
is a large availability of high wind speeds, places where you need irrigation and the places
where you can install these. These are horizontal axis wind turbines, relatively more expensive
than the Savonius rotor. Generally, these are industrial products, while the Savonius
rotors can be constructed on site. These have some inherent mechanism. For example, how
would it orient itself to the direction of the wind that means Yaw control?
Simply by means of, suppose this is the blade, by means of a tail vane like this, a vane,
so that if the wind comes from wrong direction, it pushes and makes it orient itself to the
direction of the wind, simple. Not only that, some of them have a built in protection against
high winds. When very high winds or cyclonic winds come, then most of the wind turbines
have to be shut down. I will come to those things little later, but for the water pumping
wind turbines you cannot have the sophisticated mechanism of disorienting it. So, there has
to be a very simple mechanism. What is done is something like this.
Suppose this is the blade and here is the axis of the blade, of the turbine and there
is the axis of the tail vane, which is slightly made off and in between the shaft is here.
Earlier you would, you would think that this is in the same direction, right. If you look
from the plane view, then it is not really in the same direction, slightly off. As a
result, when the wind speed is less, then the force on this one dominates and it orients
it around the direction of the weight. But, when the winds are large, then the thrust
on the, on the blade will dominate and it will disorient it away from the direction
of the wind. So, this kind of simple mechanisms of, automatic safety mechanisms are sometimes
built into the water pumping wind turbines. So, you have, now, now let us concentrate
on electricity production wind turbines. As I told you, there are two types - the Darrieus
rotor and the horizontal axis and the most popular at present are the horizontal axis
wind turbines. In horizontal axis wind turbines you have, you need a few types of control
mechanisms.
One, suppose the wind is starting to blow at a certain point of time, obviously it cannot
start below a certain wind speed, so if you draw the power output versus wind speed characteristics,
then it will start only after certain time, not time, only beyond a certain wind speed.
This wind speed is called the cut-in wind speed. It is called the cut-in wind speed.
Beyond the cut-in wind speed, it will start producing power. Now, if all the things that
I just mentioned like properly controlling the direction of the pitch, so that it, the
blades always face the relative wind and this kind of mechanisms are followed, then the
power output will be proportional to the power contained and the power contained in the wind
is cubically proportional to the speed. So, the power output will grow like this as the
wind speed increases.
But, after sometimes you reach the power generation capacity, the rated power of the generator.
Even if the wind speed increases, you cannot allow the generator power output to increase,
because then it will over heat and burn. So, there has to be some mechanism then of keeping
the power output constant as the rated power. So, it will be kept constant at the rated
power till a certain wind speed, beyond which it is dangerous to operate the wind turbine.
So, at that point it will be closed down. This is called cut-out or furling wind speed.
So, normally the power output versus wind speed characteristic of a generator would
be something like this. I will come to more details about how this is maintained, how
this is obtained in the next class. Fine, that is all about it today.
This is a small model wind turbine. As you can see, it is a very small diameter thing
and it is being blown with artificial wind from the front and so you can see it is rotating.
Now, we will stop the wind and see the actual structure and construction of the blades.
What you can see now is the airfoil profile, which is as you can see, as it is zooming
back you can see that it is inclined at an angle to the plane of rotation and actually
this is the plane of rotation. As you can see, it is the plane of rotation and these
airfoils are inclined at an angle, so the chord is inclined at an angle to the plane
of rotation which is the pitch angle. So, as you can see, the wind is blowing from this
side and it is rotating this way. So, the relative wind is along this direction and
this pitch angle has to be so aligned, so that the blade is, blade chord is aligned
to the direction of the relative wind.
So, this is the front view of the wind turbine, model wind turbine and normally this is not
rotating now, because wind is not blowing and normally it will rotate in that direction
and here you can see the tail vane which is used to orient to the turbine in the direction
of the wind. So, as the wind blows, this will feel a force and that will automatically orient
it to the direction of the wind. Since it is a very small wind turbine one can use a
tail vane for this purpose. For larger wind turbines, you will need an automatic motor
control mechanism for this orientation.