Posted by harry on December 16, 2009, 7:04 pm
The way drag works is this. There are two forms of drag. One is
caused by the shape of the object air is passing over. The other is
caused by friction of the air on the surface of the object air is
Remarkable as it might seem as the air speed increases drag REDUCES
with the latter and INCREASES with the former.
This means that if you combine the two, you get a curve with a
distinct low point, ie minimum drag. So the aerofoil works most
efficiently at this speed.
Now, an aircraft wing can and is designed to cruise at this speed, the
whole airframe is designed to have minimun drag at this speed.
However, there is a problem here with wind turbines, they have to work
at various wind speeds.Ideally they would be allowed to speed up as
the wind speed increases but this is not possible for electrical
reasons (if generating AC).
The main way of reducing drag with aerofoils is to make them narrow
(the chord) and make them long. Hence a glider has long narrow wings
as it needs to be efficient. And modern wind turbines have long
narrow blades. You never see "winglets" on a wind turbine, they only
work well at one particular speed.
Posted by Curbie on December 16, 2009, 9:30 pm
Your winglet and glider analogies make sense, but still am have
trouble figuring out where you're going with drag.
It is my foggy recollection that air on the high-drag side of an
airfoil is slowed by drag, creating a higher air pressure at lower
velocities, where the air on low-drag side of an airfoil having less
drag, causing higher air velocities and lower pressure, thus lift from
high-drag to low-drag sides.
In drag type device the high-drag side of an airfoil has higher wind
velocities and pressure, and the low-drag side of an airfoil has lower
air velocities and pressure.
The difference between lift and drag type machines being the relative
air-velocity on low-drag side of the airfoil.
Posted by Michael B on December 16, 2009, 10:44 pm
As you have said before, think anemometer.
If it's done right, it should travel as fast, or slow,
the air going past.
Potentially, it could have essentially no friction,
suspended by a magnetic field without any flux
But the main reasons for my interest in such a
device, rather than with a traditional airfoil
(and even the Dutch windmills have an airfoil,
even though it's in the form of a sail), is that
prevailing winds are low speed, up to 6-9 M/Hr,
but can be over 20M/Hr. I want something I can
have at ground level, put plenty of size on without
having to perch it on a mast, not be concerned
about it hurting someone, not have to go to a
lot of activity to pull maintenance on it, many
other reasons. Biggest thing is that I want it to be
turning when a light breeze happens. Rather than
sitting idly by until the wind speed gets up to 6 miles
Traditional problem has been actually generating at
the low RPM, but that means I need to make my own
generator. Even though I do like the format of the ones
that will be sold at the Ace Hardware stores.
No problems with overspeed, it won't be going faster
than the wind.
Posted by Michael B on December 16, 2009, 11:27 pm
Although I'm sure most of you are aware of this
Honeywell unit, I'll give site info for those who aren't.
Posted by Curbie on December 17, 2009, 12:42 am
Interesting link, any clue what those units cost?