response 1 of 13:

Apr 12 23:02 UTC 2001

The BBC article is short on details, such as the amount of power
the power station is expected to generate.  It also appears to
confuse general upper-altitude winds with the jet stream.  On the
other hand, I've spent the last week watching the wind data for
the 500 millibar level over North America; it's obvious to me that
there is a whale of a lot of power there for the taking.  As a
for-instance, today (a windy day) winds at Detroit have been running
around 30 knots; that's a total wind power of about 2400 W/m^2.  At
the 500 millibar level, winds have been about 85 knots, and the
total wind power roughly 27 KW/m^2.  (Available power is around
half that.)

This idea deserves expedited consideration, IMHO.

response 3 of 13:

Apr 14 00:48 UTC 2001

Re #2: It'd have to be light, but a good conductor.  Aluminum might be a 
good choice, maybe with a core of kevlar or some other 
high-tensile-strength material for strength.

Realistically, you can only put this up somewhere uninhabited, so if one 
of the contraptions sheds a blade it doesn't fall on someone's head.  
Over the ocean would work.  Australia's a handy place to test it in that 
it has lots of uninhabited land.  (Woomera was once an atomic bomb test 
site, IIRC.)

response 5 of 13:

Apr 15 01:42 UTC 2001

Re #2:  I did a bit of number crunching based on what seemed
like reasonable assumptions and got some answers I liked.

According to sources I found, Kevlar-49 has a density of 1.44
and an ultimate tensile strength of 3.6 GPa.  Assuming that it
can be used at a working strength of 1/3 its breaking strength,
and that a 2.2 megawatt generator system could have 100,000 lbs
(445 kN) of drag and 200,000 lbs (890 kN) of total force
(sqrt(lift^2 + drag^2)), I got a required cross-sectional area of
7.42 cm^2 of Kevlar.  This would have a mass of about 1.07 kg/m
of cable.  (Kevlar might not be a good idea due to its lack of
water and UV resistance, but better fibers exist or it could be
jacketed for protection.  I'm just using Kevlar as an example.)

Conductors are the other half.  According to my pocket reference
by Glover, #3 copper wire has a resistance of 232 micro-ohms per
foot, or 761 microohm/meter.  (This is equivalent to 8 12-gauge
strands, which could be woven as part of a cable.)  The area is
0.2664 cm^2, and the weight is 0.159 lb/ft or 0.236 kg/m.  If the
wire were divided into half the conductors going up and the other
half down, the circuit resistance would be 3.04 milliohms per meter.

The total mass of the cable would be about 1.31 kg/meter.

If the "gyromill" were flying at an altitude of 5000 meters with a
total cable run of 7000 meters one way, the total mass of the the
cable would be about 9200 kg and it weight under 90 kN.  This is
far less than the lift force of 385 kN.  The circuit resistance
would be 14000 m * 3.04 mohm/meter = 42.6 ohms.  If you transmitted
2.2 MW from the top at 50,000 volts at 44 amps, you'd have less than
1900 volts drop across the cable, or less than 4%.  The heat loading
of the cable would be about 2.9 watts per meter per conductor (5.9
W/m for both directions).  This appears unlikely to cause undue heating.

I think the fundamentals are very solid, all this requires is a bunch
of engineering (and financing, and politicking) to take care of the details.

response 7 of 13:

Apr 18 12:49 UTC 2001

Re #6:  I suggested copper because it works and it's relatively
corrosion-resistant.  Aluminum builds up a very highly insulating
oxide layer, which can present problems.

By my numbers, the load-bearing part of the cable would have to be
rather thick (over an inch across).  Using metal as an outer jacket
would cause it lots of stretching and compression when it was reeled
in and wound around a takeup drum.  It seems likely that a solid
outer jacket wouldn't be solid after a few cycles.

The obvious response is to go to a stranded conductor.  Using stranded
wire eliminates much of the flexing problem.  It would also allow a
cable to be made by adding some spools of wire to the machine which
twists yarns of the load-bearing fiber (in other words, cheap).  The
issue of skin effect is largely eliminated if the strands do not
interconnect much and weave in and out of the body of the cable.
This is the principle behind Litz wire.  (Skin effect is not an issue
if the power transmission is DC - though I doubt that would be done
because it would require power converters on the flying platform.)

An insulating coating could present a problem.  If one of the strands
breaks, load will be re-distributed when the wires touch above and
below the break.  The better the insulating properties of the oxide,
the more heat would be generated at these points.  Too much heating
could result in additional conductor failures or burning of the
load-bearing fibers, culminating in breakage of the rope.  I don't
like this idea, and after the problems associated with aluminum wire
in housing I wouldn't want to spec it for a first-cut design.

Right now I'm trying to find my drawings of a motor housing to get
an idea of the power/volume ratio of small motors.  No luck so far.

response 9 of 13:

Dec 24 20:08 UTC 2001

#2 reminds me of a technique for model sailplanes called "dynamic slope
soaring"  Rather than fly in the updraft on the windward side of a rigdge,
the sailplane loops in the fast moving air behind and above the ridge, and
the slow moving air behind and _below_ the top of the ridge.  A simple
explaination is available at
http://ourworld.compuserve.com/homepages/dlstone/dsoar.htm .  No relation
to power generation, but I think you might find it interesting!  -Alex

response 11 of 13:

Dec 26 09:25 UTC 2001

Boy, and I thought the Altamont Pass windmills were ugly...  Now this!

There's an interesting technique I saw presented at a Mars Society convention
a couple of years ago for constructing tethers - have several vertical strands
connected by a lattice of smaller diagonal strands, around in a circular
arrangement, somewhat like this:

           |  |  |  |
           |\ | /|\ |
           | \|/ | \|
           | /|\ | /|
           |/ | \|/ |
           |\ | /|\ |
           | \|/ | \|
           | /|\ | /|
           |/ | \|/ |
           |  |  |  |
 
If one, or several, of the strands breaks, the load is taken up by the
lattice:

           |  |  |  |
           |\ | /|\ |
           | \|/   \|
           | /|\   /|
           |/   \|/ |
           |\   /|\ |
           | \|/ | \|
           | /|\ | /|
           |/ | \|  |
           |  |  |  |
 
This type of assembly, constructed with Spectra fiber (ultimate tensile
strength of 3.25gpa), was the basis of experiments in the use of tethers for
orbital maneuvering and space power generation.  (Spectra is resistant to
chemicals, water, and UV.)

response 13 of 13:

Jan 18 20:51 UTC 2003

russ; why are you a homophobe?