Why are wind turbine blades long and skinny instead of short and fat?

[Guido]

Wings are "pulled" by lift? And are also "pushed" ?

What drugs are you on?

Reality.

Wings are pull up from the top and pushed up from the bottom. The
combined force is called lift.

Wings must be moved forward by engine thrust in order to generate lift.
Wings do not generate any sort of imaginary force in front of them that
aids their forward motion, and certainly they experience drag forces
that must be overcome by engine thrust in order to keep them moving
forward.

Then explain why a sailplane can stay aloft for hours, even climb,
without engine thrust then? I'll tell you. A combination of inertia,
thermals, and wind speed, that overcomes an extremely low drag
coefficient. (along with a pilot skilled enough to make use of these)

You will note that airplane propellers are not shaped like airplane
wings (they do not have the cross-sectional profile of an airplane
wing).

In fact, they precisely do! The only difference being that the angle of
attack changes with the radius on a propellor.

 

[Bob F]

But your premise in that case is that you take an existing tower, with
it's 3 long, skinny blades, and replace them with 3 fat blades. That
wouldn't really happen. You scale down the 3 fat blades (they
wouldn't be as long as the typical long, thin blade) and the tower
wouldn't need to be so tall either.


And as mentioned above, the new tower (with it's 3 fat blades) isin't
going to be as tall, so it wouldn't be as visible from a distance.

I would even argue that 3 fat blades turning in the distance wouldn't
be as visually noticable (from a motion or movement pov) as 3 long
and thin blades.

Clearly, you are way smarter than all the designers of wind turbines in this
world. Why they haven't come to you for your superior knowledge is beyond
comprehension.

A number of people here have suggested good reasons your theory is wrong. But
clearly, you aren't. It is obvious that designers of cheap home fans know way
more than the designers of multi-million dollar wind turbines, and that their
ideas will solve America's energy crises, with your help.

Thank you for correcting the errors of those stupid engineers, who clearly don't
have your intuitive insight into wind turbine design.

 

[Energy Guy]

In fan design, there is a measurement known as the "solidity factor".

If a fan blade has a width of FB inches at it's tip or end, and if the
blade is BL inches in length (from the center of rotation to the tip of
the blade), and if there are N blades, then the solidity ratio is:

Solidity ratio = (FB x N) / (BL x 2 x pi)

The work done by a fan blade increases the further out you go from the
center of the fan out to the tip of the fan blade and is proportional to
the solidity ratio. The wider that a blade is at any given radius, the
more work it can do. The further away that a blade section is from the
center of rotation, the more work it can do compared to a section that
is closer to the center of rotation.

One way to increast the solidity ratio is to simply increase the number
of blades (N).

If a blade is tapered (wider near the center of rotation, narrower at
the tip) it will have a changing solidity ratio (higher at the center,
smaller at the tip) but can have equal work done by every blade section
as a function of distance from center of rotation.

Blades that are wider at the tip have an increased solidity ratio and
ability to do work as a function of radius compared to a straight or
tapered blade.

The work performed by a fan blade at any radial position is a function
of:

a) Blade chord width (solidity ratio)

b) blade or airfoil twist

c) tangential velocity squared

 

[Guido]

A number of people here have suggested good reasons your theory is wrong. But
clearly, you aren't. It is obvious that designers of cheap home fans know way
more than the designers of multi-million dollar wind turbines, and that their
ideas will solve America's energy crises, with your help.

Thank you for correcting the errors of those stupid engineers, who clearly don't
have your intuitive insight into wind turbine design.

Your sarcasm is funny, but it makes me wonder if Christofer Columbus was
subject to the same sort of thing. However, I do believe the "stupid
engineers" ;-D

 

[Guido]

Blades that are wider at the tip have an increased solidity ratio and
ability to do work as a function of radius compared to a straight or
tapered blade.

Apples and Oranges. Moving air with a fan is not the same as moving a
fan with air. Like trying to move water up hill instead of down takes
entirely different thinking.

But the way you seem to be thinking, if we shine light on a light bulb,
it should produce electricity. But it clearly doesn't. I am sure there
are other factors that need to be considered as to why fat blades aren't
used. Things like the weight and the friction that weight causes, drag
coefficients, cost to Watt ratios, etc..., that makes the skinny blades
preferable, or they'd be using the fat blades instead.

I won't insult you like many do here, and in fact, applaud your
curiosity and courage to ask why in a public forum where ridicule seems
to be relished.

Then again, there is nothing so hidden as the absolute obvious ;-)

 

[Scott]

Apples and Oranges. Moving air with a fan is not the same as moving a
fan with air. Like trying to move water up hill instead of down takes
entirely different thinking.

As has been mentioned, the shape of a windmill blade is rooted in the same
design sensibilities as the wing of a sailplane -- to extract wind energy as
efficiently as possible. In a sailplane, efficiency yields better climbs,
greater range, longer glides, etc., all desireable traits. In a windmill,
efficiency is simply a way to maximize your return on investment.

Some of the same principles apply to fans. The prop on the front of my
Cessna 172 is certainly designed to develop as much thrust as possible from
the available power, and sure enough it's got long skinny airfoils. Quiet,
it is not.

I would submit for consideration that in e.g. a residential or commercial
fan (commercial = think office, not factory), one of the prime
considerations would be NOISE. In these applications, a fan that operates
near silently is much preferable to a fan that moves more CFMs per watt but
makes a lot of noise doing it. I'm thinking that the wide blades (and low
RPMs) we see in these applications is as much about noise control as
anything else.

 

[Energy Guy]

As has been mentioned, the shape of a windmill blade is rooted in
the same design sensibilities as the wing of a sailplane -- to
extract wind energy as efficiently as possible.

That is, more or less, completely wrong.

When a glider is moving forward through the air, 99% of the air flow
that the wings experience is parallel or in the same plane as the wing.
Wings generate lift - not propulsion. A glider has no engine, and the
wings don't give the glider any propulsive forward thrust. Gravity is
what causes a glider to move forward (after it has been released by a
tow plane).

A glider gains altitude (or potential energy) by flying into and with
thermal updrafts along the sides of hills or mountains. If those did
not exist, then a glider can do only one thing: Lose altitude while
moving forward.

In a wind turbine, wind flows directly into the face of the blades -
perpendicular to the blades - not in the same plane as the blades.

If turbine blades were to function as wings, then they would generate a
lift force that would operate perpendicular to the surface of the blade
in the same way that a wing generates a lift force that operates on the
top surface of the wing and in a direction that is perpendicular to the
wing surface. Such a lift force is, naturally, desirable for a wing,
but would perform no function for a wind turbine blade. The blade wants
to see a force operating in the plane of rotation, which happens when it
converts the perpendicular wind force into a tangential force because of
it's twisted shape.

I would submit for consideration that in e.g. a residential or
commercial fan (commercial = think office, not factory), one
of the prime considerations would be NOISE. In these
applications, a fan that operates near silently is much
preferable to a fan that moves more CFMs per watt but makes
a lot of noise doing it.

Noise represents energy loss caused by turbulance or non-laminar flow.

A fan that is quiet is also a fan that moves air efficiently, without
energy loss caused by noise.

Noise for wind turbines is also an important consideration, and
especially since phenomena such as infrasonic noise is becoming a factor
in public acceptance of these large wind plants.

 

[Bob F]

That is, more or less, completely wrong.

When a glider is moving forward through the air, 99% of the air flow
that the wings experience is parallel or in the same plane as the
wing. Wings generate lift - not propulsion.

Wings generate all the propulsion when gliding. A component of their list is in
the forward direction.


A glider has no engine,

and the wings don't give the glider any propulsive forward thrust.
Gravity is what causes a glider to move forward (after it has been
released by a tow plane).

A glider gains altitude (or potential energy) by flying into and with
thermal updrafts along the sides of hills or mountains. If those did
not exist, then a glider can do only one thing: Lose altitude while
moving forward.

In a wind turbine, wind flows directly into the face of the blades -
perpendicular to the blades - not in the same plane as the blades.

Sorry. The blades are turning. They are working just like a gliders wing to
produce forward motion. If "wind flows directly into the face of the blades -
perpendicular to the blades" The blades would be stalled, and no motion would
result.

If turbine blades were to function as wings, then they would generate
a lift force that would operate perpendicular to the surface of the
blade in the same way that a wing generates a lift force that
operates on the top surface of the wing and in a direction that is
perpendicular to the wing surface. Such a lift force is, naturally,
desirable for a wing, but would perform no function for a wind
turbine blade. The blade wants to see a force operating in the plane
of rotation, which happens when it converts the perpendicular wind
force into a tangential force because of it's twisted shape.

An efficient blade will be sliding smoothly through the air, with no stalling.
It is not just a wind deflector. It is a wing gliding through the wind, just
like if you dropped it from altitude straight down. The lift it produces is what
makes it turn with force. The angle of attack of the blade is the same over the
length of the blade. The blade twists along its length to accomplish this, since
the tip of the blade is moving way faster than nearer the hub. So the angle at
the tip is less than the angle at the hub. This keeps the blade operating in its
aerodynamic mode where it produces lift and thrust efficiently along its full
length. Big fat flat blades like a window fan would have huge losses, as the tip
of the blade is driven faster than the wind is going through it, causing it to
accelerate the wind going through it, while the part near the hub would do the
reverse.

Window fans just operate slow enough that they are quiet. Try to get the most
out of them and they would scream. Efficiency is not a serious consideration on
cheap fans.

Noise represents energy loss caused by turbulance or non-laminar flow.

A fan that is quiet is also a fan that moves air efficiently, without
energy loss caused by noise.

Or just a fan that isn't turned very fast.

Noise for wind turbines is also an important consideration, and
especially since phenomena such as infrasonic noise is becoming a
factor in public acceptance of these large wind plants.

That's one reason they design very efficient blades for them.


Why do I bother?

 

[[email protected]]

Two blade turbines, apparently, produce just as much power as multi-bladed
ones.

However, I understand two bladed turbines can stall and not self start again
so three blades have become popular.

josepi/gymmy bob/solar flare/john p benji trying to correct
energy-nitwit by promoting concepts that are equally ill-considered,
is as funny as Usenet can get.

Wayne

 

[Bob F]

josepi/gymmy bob/solar flare/john p benji trying to correct
energy-nitwit by promoting concepts that are equally ill-considered,
is as funny as Usenet can get.

OK. Why don't you go ahead and enlighten us. Make it all perfectly clear.

 

[[email protected]]

OK. Why don't you go ahead and enlighten us. Make it all perfectly clear.

You shouldn't need any help from me. Just look around at all the
stalled two-bladed turbines, and all the manufacturers who continue to
produce them despite the lack of self-starting ability. As for my own
two-bladed turbine, I assume that it stalls regularly, but that the
invisible turbine fairy has been visiting as necessary to keep it
going for the last 12 years. My friend says that his turbine has a
third blade to minimize unequal blade loading. Why do people like him
persist with such strange ideas when they could come to Usenet and get
edumacated about the real reason for the third blade?

Now, I know you're thinking "Wayne, why not patiently explain some of
these things". Sometimes I do, to sincere newbs. But josepi is a
nym-shifting time waster. Most recently he argued that average daily
insolation numbers were phony because the sun doesn't shine 365 days a
year. It took what, a half dozen lengthy posts to dissuade him. Not
long ago he wanted to know about turbine calculations for a 10' rotor
with a 6' hub. Before that he claimed that one needs high-voltage
safety gear to measure voltage on home battery-charging turbines. I
don't have a lot of time for people like that, other than to see the
humor in some of these discussions.

Wayne

 

[daestrom]

When you have those huge wind plants, a lot of air is going to flow
right between the relatively slow-moving blades without touching them.
There's a lot of potential energy that sailing right between those 3
narrow blades.

Okay, try looking at it this way. A GE 1.5 MW wind turbine gets
1,500,000 watts from the wind when it blows at 14 m/s

http://www.gepower.com/prod_serv/products/wind_turbines/en/downloads/GEA14954C15-MW-Broch.pdf

A 14 m/s wind has energy density of about 1631 watts per m^2.

It's a basic law of physics that if you extract all the energy from each
kg of air, the air won't have any velocity left. And if that were to
happen, the air would not be moving. Betz calculated the absolute
maximum power you can extract from a wind turbine at 59% of the total
power flowing.

That turbine has three blades that are about 38 m long and about 1 m
wide. So that's a 'blade area' of just 114 m^2. So it's developing
over 13,000 watts per m^2 of blade area.

Pretty neat trick since the wind only has 1,631 watts per m^2.

So there's a whole lot of wind that is *not* 'sailing right between
those 3 narrow blades.'

But still have a higher blade-area to swept-area ratio then the massive
commercial wind generators.

And still work off much large differential pressures than windmills.
Still apples / oranges.

An aircraft wing is designed to generate lift.

If wind power blades were exactly like aircraft wings, then they would
have no twist, would be perfectly flat relative to the plane of
rotation, and wouldn't turn at all if wind was flowing directly
(perpendicularly) into their top (curved) surface.

If you start turning such a blade arrangement using external power, the
blades will experience a bernoulli force that would tend to bend the
blades forward (the same way that a plane wing wants to bend upward and
results in lifting the plane).

That's why only an idiot would orient the blades as you suppose. The
blades are oriented so that the apparent wind flows along both sides of
the airfoil, not just 'flat'.

Blades are twisted because the outer end of the blade is moving much
faster than the inner part, so the vector sum of the blade's speed and
the wind speed always create an 'angle of attack' that has the air
flowing smoothly over the air foil.

Near the hub where tangential speed is low, the blade is angled to face
nearly straight into the wind. Near the tip, where the blade tip speed
is high, the combination of blade speed and wind speed cause the air to
meet the blade at a very shallow angle with respect to the axis so the
blade is twisted to nearly the same shallow angle.

Re-read what I said. I did not say that lift pulls airplane wings
forward.

I said that if you take an airplane wing (or 3 wings) and mount them
vertically on a pinwheel, then their rotation would generate a force
that would pull the wings forward (forward means out of the plane of
rotation).

And that is *not* how wind turbine blades are angled. So your statement
is pointless. If you take your 'airplane wing' and turn it 90 degrees
to your mounting so the wind is flowing over both surfaces just like the
air flows over an airplane's wing, now the 'lift' generated by the air
foil doesn't pull it 'forward' but 'around' the axis of your pinwheel.

This analogy with a plane wing or an airplane should not consider the
specific cases of take-off and landing, but instead should consider the
situation the wings are in during cruise flight, when the attitude of
the plane and wings are configured for low drag (and essentially no
angle of attack). That is what the wing is really designed for.
Optimal low drag and optimal lift for cruise flight.

That particular mode of operation of aircraft wings (cruise flight) is
of no use for the blades of a wind power plant.

Turn the wing sideways so the 'lift' turns the axle and you'll see that
it makes a lot more sense.

daestrom

 

[Energy Guy]

Okay, try looking at it this way. A GE 1.5 MW wind turbine gets
1,500,000 watts from the wind when it blows at 14 m/s

A 14 m/s wind has energy density of about 1631 watts per m^2.

It's a basic law of physics that if you extract all the energy from
each kg of air, the air won't have any velocity left. And if that
were to happen, the air would not be moving. Betz calculated the
absolute maximum power you can extract from a wind turbine at 59%
of the total power flowing.

That turbine has three blades that are about 38 m long and about
1 m wide.

So using a radius of 38 meters, we have a total area of 4,536 square
meters that we've devoted to energy extraction.

A 14 m/s breeze has an energy density of 1631 watts per square meter.
So we have a total potential of 7.4 MW of energy. Taking 59% of that is
4.37 MW. And what is our GE generator doing? It's extracting 1.5 MW,
or 34% of the realistic maximum energy, while 66% is sailing right
through those long skinny blades.

So that's a 'blade area' of just 114 m^2. So it's developing
over 13,000 watts per m^2 of blade area.

Pretty neat trick since the wind only has 1,631 watts per m^2.

Pretty neat trick to not do what I did - which is to consider the total
swept blade area. Like I said, I've just proved that 2/3 of the
extractable energy is sailing right between these skinny blades.

So there's a whole lot of wind that is *not* 'sailing right
between those 3 narrow blades.'

Even to a non-engineer, it's obvious that a lot of wind *will* sail
right between those blades. Why isin't that obvious to you?

And still work off much large differential pressures than
windmills. Still apples / oranges.

Wrong. Wind speed is wind speed. Differential pressure (when expressed
in terms of square inches or meters) is of the same scale, whether we're
talking about a huge blade or a house fan.

Blades are twisted because the outer end of the blade is moving
much faster than the inner part, so the vector sum of the blade's
speed and the wind speed always create an 'angle of attack' that
has the air flowing smoothly over the air foil.

A propeller blade (and I suppose a wind-generator blade) will have a
cross-section that looks like the profile of a wing (camber) for only
one reason: To improve the lift-to-drag ratio. A flat profile blade
will have a similar lift-to-alpha ratio as a cambered profile (alpha =
angle of attack).

And that is *not* how wind turbine blades are angled.

Yes, I know that.

I'm responding to the argument that a wind blade is shaped like the wing
of a plane (specifically - a glider).

My point is that aircraft wings are designed for cruise flight, where
the angle of attack is very small - if not zero.

A wind blade will not have such a small angle of attack. Quite the
opposite.

 

[Bob F]

Pretty neat trick to not do what I did - which is to consider the
total swept blade area. Like I said, I've just proved that 2/3 of the
extractable energy is sailing right between these skinny blades.


Even to a non-engineer, it's obvious that a lot of wind *will* sail
right between those blades. Why isin't that obvious to you?

Have you considered the effect of the blade on air that is not directly hitting
the blade? Is the air an inch from the blade affected? 2 inches? A foot? 2 feet?
If so, then the blade is also affected by that air. And therefore, the blade, as
it sweeps around, may well be extracting energy from all of the air going
through it.

Wrong. Wind speed is wind speed. Differential pressure (when
expressed in terms of square inches or meters) is of the same scale,
whether we're talking about a huge blade or a house fan.


A propeller blade (and I suppose a wind-generator blade) will have a
cross-section that looks like the profile of a wing (camber) for only
one reason: To improve the lift-to-drag ratio. A flat profile blade
will have a similar lift-to-alpha ratio as a cambered profile (alpha =
angle of attack).

Lift to drag ratio is critical to efficient operation. The more drag you have,
the less energy is left to generate power. So a good airfoil design is very
important. Also, an airfoil has more lift than a flat shape, so again, can
develop more power, since the lift is what makes it turn.

Yes, I know that.

I'm responding to the argument that a wind blade is shaped like the
wing of a plane (specifically - a glider).

My point is that aircraft wings are designed for cruise flight, where
the angle of attack is very small - if not zero.

A wind blade will not have such a small angle of attack. Quite the
opposite.

Really? The blade must have a small enough angle of attack to prevent stalling.
If it stalls, energy is wasted in increased drag and lost lift. Sure, a farm
pump may get away with this, but it is far from optimal for power production.
Remember, the angle of attack is lessened by the speed of the blade through the
air.

All my comments relate to horizontal axis windmills. Vertical axis windmills
depend on drag, it seems.

Note the performance graph for the horizontal axis unit with airfoil blades
compared to others on the following report.
http://www.copernicusproject.ucr.edu/ssi/Science Fair/5/Optimizing Windmill Blade Efficiency.doc

Another article about blade design.
http://www.mh-aerotools.de/airfoils/windmill.htm

And more technical.
http://digital.library.okstate.edu/OAS/oas_pdf/v56/p121_124.pdf

This one is a little more fun.
http://www.kidwind.org/Presentations/BladeDesign 8.13.09.pdf

 

[Energy Guy]

Have you considered the effect of the blade on air that is not
directly hitting the blade?

Have you considered the math that I posted (which you did not quote)
which clearly showed that the GE turbine was only 34% efficient at
extracting the realistic available power from the wind that it's exposed
to?

Is the air an inch from the blade affected? 2 inches? A foot? 2
feet? If so, then the blade is also affected by that air.

What is this new, mysterious force that you are inventing?

Does it have a name?

I just proved to you, mathematically, that the GE turbine is only 34%
efficient, so therefore it obviously IS NOT extracting anywhere near
100% of the practical or useable power that is available to it given the
area that occupies.

So you don't have to invent any new mystery force to explain the power
that the blades *are not* capturing.

Lift to drag ratio is critical to efficient operation.

I agree with that. But whether or not a blade has a flat profile vs a
cambered profile is a different issue vs the area of the blade, which
brings us right back to why use a narrow blade if a wide blade can
capture more wind force and convert it into rotational motion.

You don't question the fact that sailing ships have the largest
sail-area they can structurally manage, because more sail area = more
wind energy capture and conversion into motion.

Also, an airfoil has more lift than a flat shape,

The lift component of the camber of a wind-energy blade does not aid in
increasing the force vector in the direction of forward rotation. It
decreases the drag vector in the direction that opposes forward
rotation.

You people keep confusing the lift vector from the camber of an airfoil
as used by a plane in cruise flight with the angle of attack that a
wind-blade uses when it's rotating.

A wind-blade has an angle of attack with respect to the wind in a manner
similar to a plane that's taking off from a runway. The plane is
gaining altitude more because of this angle of attack than it is because
of the camber of it's wing cross-section. But during cruise flight,
it's not an efficient way to maintain altitude if you maintain this
angle of attack, so this angle is reduced and the camber provides the
necessary lift to maintain constant altitude.

Really? The blade must have a small enough angle of attack to
prevent stalling.

A wind blade with a 45 degree angle of attack is theoretically the most
efficient at converting the force of a wind vector into perpendicular
rotational motion. It doesn't matter how long the blade is, or how fast
it's moving. 45 degrees will always give you the most rotational
force. Vector math will tell you that.

If you reduce the angle because you want to minimize drag or turbulance,
then you might do that, and you might find that a shallower angle (25
degrees? 30 degrees?) might give you more net power (after drag is been
considered) but to have a blade with a 5 or 10 degree angle of attack is
absurd - because your goal is to not simply swing a blade in a circle
with as little drag as possible (what's the point of that?) - the goal
is to have the wind "push" the blade out of the way, and a blade does
not get pushed out of the way if it's angle of attack is low (or high -
if you consider the direction of the wind in relation to the orientation
of the blade).

A blade that is oriented at a 45 degree angle to the wind will get
pushed by the wind. A blade that is 90 degrees will not.

If you could stand on the leading edge of a wind blade as it's rotating,
and you think you are experiencing a wind that's blowing into you, you
would be correct. But that is a fake wind. It's not real. It's caused
because the blade is turning and is being pushed through the air. That
"wind" is pushing back against the blade's leading edge as it's
turning. It's a drag - it is opposing the blade's forward motion. You
don't want it to be there. It does you no good. You can't extract
energy from it. You can only do things to help the blade move through
it. That's where the lift component of the airfoil camber comes in. It
helps to reduce this drag force.

Some people here think that a glider is somehow pulled forward by it's
wings and that a wind blade is also pulled forward because of this
mysterious force as this fake wind flows into the leading edge as it
rotates.

There is no pulling. There is only drag. Wings don't pull airplanes or
gliders forward, and a rotating wind blade is also not pulled forward by
the sake of or as a consequence of the fake wind that it is created by
it's own rotation.

 

[[email protected]]

You mis-spelled 'gymmy boob'. I remember him as the self proclaimed
expert on everything. It seems to me he was posting out of Edmonton,
Alberta.


mike

I remember it as Southern Ontario, but IIRC he claimed to have a place
in Michigan as well. His ISP referred to him a troll and it looks like
they finally got rid of him. As solar flare said he had a big
solar/wind setup. Now as josepi it's a dead Ebay special, dead 125AH
batteries, and PV that needs mounting. It's about time for him to
change nyms again. I wonder what his new story will be?

Wayne

 

[Scott]

There is no pulling. There is only drag. Wings don't pull airplanes or
gliders forward, and a rotating wind blade is also not pulled forward by
the sake of or as a consequence of the fake wind that it is created by
it's own rotation.

I take it you are a self-educated man, am I right?

 

[Bob F]

Have you considered the math that I posted (which you did not quote)
which clearly showed that the GE turbine was only 34% efficient at
extracting the realistic available power from the wind that it's
exposed to?

And how efficient is your proposed design?

What is this new, mysterious force that you are inventing?

Does it have a name?

It is a really advanced concept called air pressure. If you increase it one
place, it causes movement of nearby air, increasing pressure farther from the
source, with the effect lessening with distance.

You don't really think that only the molecules of air that hit the wing are
involved in giving it enough lift to fly? The pressure, or lack thereof, of air
more distant from the wing affects the pressure of the molecules that actually
lift the wing.

I just proved to you, mathematically, that the GE turbine is only 34%
efficient, so therefore it obviously IS NOT extracting anywhere near
100% of the practical or useable power that is available to it given
the area that occupies.

Extracting 100% is impossible.

If you did, the air on the back of the blade would not be moving, and no more
air would get through the blade.

So you don't have to invent any new mystery force to explain the power
that the blades *are not* capturing.

I didn't invent air pressure.

I agree with that. But whether or not a blade has a flat profile vs a
cambered profile is a different issue vs the area of the blade, which
brings us right back to why use a narrow blade if a wide blade can
capture more wind force and convert it into rotational motion.

You don't question the fact that sailing ships have the largest
sail-area they can structurally manage, because more sail area = more
wind energy capture and conversion into motion.

Ever notice that they don't put one sail directly leeward of the 1st.

The lift component of the camber of a wind-energy blade does not aid
in increasing the force vector in the direction of forward rotation.
It decreases the drag vector in the direction that opposes forward
rotation.

You people keep confusing the lift vector from the camber of an
airfoil as used by a plane in cruise flight with the angle of attack
that a wind-blade uses when it's rotating.

And you keep forgetting that the aerodynamic lift of the airfoil has a
rotational component that increases the rotational force.

A wind-blade has an angle of attack with respect to the wind in a
manner similar to a plane that's taking off from a runway. The plane
is gaining altitude more because of this angle of attack than it is
because of the camber of it's wing cross-section. But during cruise
flight, it's not an efficient way to maintain altitude if you
maintain this angle of attack, so this angle is reduced and the
camber provides the necessary lift to maintain constant altitude.

The angle of attack increases the pressure on the bottom of the wing. The
airfoil shape decreases the pressure on the top of the wing. The difference
between these two pressures causes the lift.

A wind blade with a 45 degree angle of attack is theoretically the
most efficient at converting the force of a wind vector into
perpendicular rotational motion. It doesn't matter how long the
blade is, or how fast it's moving. 45 degrees will always give you
the most rotational force. Vector math will tell you that.

Force, maybe. Power, no. You need speed to get power.

If you reduce the angle because you want to minimize drag or
turbulance, then you might do that, and you might find that a
shallower angle (25 degrees? 30 degrees?) might give you more net
power (after drag is been considered) but to have a blade with a 5 or
10 degree angle of attack is absurd - because your goal is to not
simply swing a blade in a circle with as little drag as possible
(what's the point of that?) - the goal is to have the wind "push" the
blade out of the way, and a blade does not get pushed out of the way
if it's angle of attack is low (or high - if you consider the
direction of the wind in relation to the orientation of the blade).

A blade that is oriented at a 45 degree angle to the wind will get
pushed by the wind. A blade that is 90 degrees will not.

If you could stand on the leading edge of a wind blade as it's
rotating, and you think you are experiencing a wind that's blowing
into you, you would be correct. But that is a fake wind. It's not
real. It's caused because the blade is turning and is being pushed
through the air. That "wind" is pushing back against the blade's
leading edge as it's turning. It's a drag - it is opposing the
blade's forward motion. You don't want it to be there. It does you
no good. You can't extract energy from it. You can only do things
to help the blade move through it. That's where the lift component
of the airfoil camber comes in. It helps to reduce this drag force.

Some people here think that a glider is somehow pulled forward by it's
wings and that a wind blade is also pulled forward because of this
mysterious force as this fake wind flows into the leading edge as it
rotates.

Nonsense. Noone here thinks that.

There is no pulling. There is only drag. Wings don't pull airplanes
or gliders forward, and a rotating wind blade is also not pulled
forward by the sake of or as a consequence of the fake wind that it
is created by it's own rotation.

Wings produce the force that keeps the glider moving forward. The lift they
produce has a forward component as the glider descends, just like the wheel on a
car rolling down a hill does. A wind blade takes advantage of its lift the same
way.Part of the lift is in the rotation direction.


OK. You know everything, and all the windmill designers in the world are
absolute idiots.

Is this really what you think?

 

[Bob F]

I take it you are a self-educated man, am I right?

Home schooled maybe?

 

[Ade]

Airplane propeller blades do not even have the same length-to-width
profile as commercial wind plants.

And look at the 4-blade propellers from WW2 bombers - wide and short.

Both of the above use power to drive the blades, which is a completely
different ask to moving blades by air.

If fat blades were so efficient, then a prop-powered aircraft wouldn't
need to feather the prop in the event of an engine failure (the prop is
feathered to reduce drag).

And look at the turbines of jet engines. Total surface area of the
turbine blades is more than half the swept area.

Turbines (and turbos) use a massive pressure drop (relative to
atmospheric pressure differences, i.e. wind) to drive their blades.
Again, it's a completely different ask.

If wide and fat blades are efficient when it comes to making air move,
then the converse must also be true.

Why? Explain, with maths please.