Tethered
Airfoils: An Enabling Technology
By Wayne German,
wlgerman@verizon.net. October 22, 2003
Occasionally,
new technologies are developed that meet global needs and generate considerable
revenues in the process. Widely
recognized examples are the light bulb, transistor, radio, television,
computer, automobile, and airplane. The
intent of this paper is to introduce another technology, Tethered Airfoils,
whose potential to generate revenue
exceeds all of these. The development,
marketing, and deployment of this technology could yield the cheapest and cleanest means of: 1) electrical power
generation, 2) shipping, 3) transportation, and 4) communication (radio signal
relaying).
Each of
these four areas could be revolutionized by the introduction of products that
incorporate Tethered Airfoils. For the
purpose of this paper, Tethered Airfoils are aerodynamically efficient
inflatable kites in the shape of wings that have lift to drag ratios of ten to
one or greater. Unless stated otherwise,
they are extremely light when inflated with air and lighter-than-air when
inflated with helium or hydrogen. These
airfoils have on board power and autopilots for stable, remotely controllable
flight. Most importantly, they provide a
means of harnessing wind power to provide the mechanical power required to
generate electricity, synthesize fuel, or provide propulsion.
The
potential applications for Tethered Airfoil technology are numerous. Some of the applications that should be
possible are listed below. The
applications that could most easily be developed are listed first followed by
those that would require more skill and experience.
Wind power generators that use reciprocating
airfoils to produce electricity on the ground.
Water pumps that use reciprocating airfoils to
pump water for irrigation.
Sailing craft that have a Tethered Airfoil to
tack into the wind or with the wind -- the airfoil being held aloft by aerodynamic lift,
or buoyancy (helium or hydrogen), or both.
Recreational airships that fly over water
without fuel by tacking in the air while being attached by tether to submerged
hydrofoils.
Paraglider wings and ultralight aircraft that
could use buoyant lift, and/or the methods of manufacture that are discussed in
a separate paper entitled, Making Tethered Airfoils and Air Tensioners, would
greatly reduce cost.
Passive self-regulation of altitude using highly
pressurized lighter-than-air structures.
Ship and vessel propulsion assistance with minor
retrofitting.
Energy conserving tugs that could deploy
Tethered Airfoils to pull unmodified vessels across oceans.
Land Based High altitude wind power generators
that use reciprocating Tethered Airfoils to tap winds as high as the jet stream
to produce electricity at a generator on the ground.
Sea
Based wind power generators (low or high altitude) to produce electricity at a
boat or barge.
Synthesizing
Hydrogen at Sea Using Tethered Airfoil Generators
Flight
without fuel over land or water by using an airfoil at lower altitude tethered
to another airfoil at a higher altitude to harness the power available in the
differential velocities of the two altitudes.
Radio
signal relaying by hovering indefinitely in the air while using excess wind to
generate electricity to relay radio signals.
Wind Power Generators
Wind power generating systems can be developed using
reciprocating Tethered Airfoils. Using
two airfoils and a tether that passes from one airfoil through an electrical
generator on the ground to the other airfoil, power could be generated if one
airfoil flew at a high angle of attack (nose up) while the other flew at a low
angle of attack (nose into the wind or slightly down). The airfoil flying at a high angle of attack
would have greater lift and drag, which would cause it to be blown downwind and
upward while pulling the other airfoil upwind and downward. Electricity would be generated as the cable is
pulled and the generator is forced to spin.
As the airfoil having the lower angle of attack approaches
sufficiently close to the generator, remote control could cause it to assume a
high angle of attack and cause the airfoil further downwind to assume a low
angle of attack. This would cause the
upwind airfoil to fly downwind and the downwind airfoil to fly upwind. Periodically changing the angles of attack
would, therefore, cause the two airfoils to reciprocate in the sky producing
power on the ground. Between strokes, as
the airfoils change their angles of attack, and as the cable changes its
direction of travel, there would be a brief time when no power would be
generated. Therefore, in Tethered
Airfoil wind farms the flights of all the airfoils should be synchronized so
that as few as possible would change direction at the same time. This would ensure that the power generated at
the farm would be as even and continuous as possible.
Note that only the pitch, or angle of attack, would have to
be controlled remotely -- not the yaw and roll.
This should make the design and development straightforward. Adjusting the tether bridle position fore and
aft should provide the level of control required for this application. The Tethered Airfoil could be designed to
passively correct for yaw and roll -- much the same way that single string
kites do today.
A single Tethered Airfoil could produce electricity if a
flywheel or external electrical power is used to winch the airfoil in on the
upwind stroke. The airfoil would produce
more power on the downwind stroke flying in a high lift, high drag mode than
would be required to winch it back in on the upwind stroke.
The amount of power that a Tethered Airfoil could generate
is not proportional to the size of the airfoil.
It is proportional to the area swept by the airfoil per unit time --
just as in wind turbines. A small
airfoil that quickly traverses a large area would generate more power. But Tethered Airfoils could generate far more
power than wind turbines because they could sweep a greater area for an
equivalent cost since they would not have the cost of the tower, nor be limited
to the sizes that towers can accommodate.
Unlike standard wind turbines, Tethered Airfoils would not
require expensive towers, specially designed low speed generators, and would
not be subject to the strong vibrations that cause premature failures. Most importantly, they could fly at higher
altitudes to harness more powerful winds.
On average, over flat land, the wind is twice as powerful at every
five-fold increase in altitude. So a
Tethered Airfoil flying at only 500 feet would encounter twice the wind power
as a wind turbine 100 feet off the ground.
At a half mile the Tethered Airfoil would encounter more than four times
as much wind power. This effect can be
greatly magnified by terrain that causes the air to be funnelled -- as is
generally found at the best wind farm sites.
Obviously, Tethered Airfoils that fly at high altitude would
need to be assigned their own airspace a safe distance away from commercial
flight paths. They might obtain
permission to fly in the restricted airspace over wilderness areas because they
do not pollute or make noise.
Alternatively, the vast areas that exist offshore would provide excellent
sites for both low and high altitude wind farming (as will be discussed)
later. But initially, windy rural areas
would provide good lower altitude proving grounds.
Inflated with helium, these Tethered Airfoils would simply
float up in exceptionally calm winds.
But in places, such as Minnesota, where the winds are constant and
strong close to the ground it may prove practical to develop Tethered Airfoil
Generators that rely exclusively on aerodynamic lift rather than buoyant
lift. Inflated only with air, they could
be developed to automatically launch from a stand when the winds blow
sufficiently strong and be winched down quick enough to maintain controllable
flight when the winds are exceptionally calm.
While the jet stream offers the greatest potential power per
unit area, it may be more practical to fly larger Tethered Airfoils at lower
altitudes. This would reduce the cost
and drag of the tethers, but would require larger or more numerous airfoils to
generate a like amount of power.
Even in typical installations, wind power used in
conjunction with hydropower or fossil fuel plants could reduce the long-term
rates at which these plants use water or fuel.
These plants on the other hand, could provide backup power during
periods of calm winds when these wind power generators would produce little or
no power.
Tethered Airfoils can be used to pump water as well as to
generate electricity. The specific
application of pumping water is mentioned here for three reasons. First, it would not require a generator. Pulling the tether could drive the pump
directly. Second, water pumps do not
require a consistent power source. If
the winds cause short-term variations in the amount of water that is pumped
there is no problem provided that daily or weekly quotas are met. Third, many nations require or could benefit
by the use of good cheap water pumps.
Many underdeveloped nations need power to pump irrigation
water. Studies conducted in Sri Lanka,
Kenya, Cape Verda, and the Sudan show that windmills can be cost effective
compared with diesel engines for pumping water.
If windmills are considered cost effective, Tethered Airfoils should
prove superior because they can extract power from much stronger winds and
sweep through a far greater airspace.
(As mentioned previously, the power that may be generated is
proportional to the area swept per unit time).
A lighter-than-air Tethered Airfoil and a watercraft having
a small wetted surface could be tethered together to make a very fast and
efficient sailing craft. Canoes and
kayaks with centerboards or catamaran hulls would make good choices. Tethered Airfoils suitable for this purpose
would need to have remotely controllable pitch and roll so that they could fly
"out to the side" as well as downwind. These Tethered Airfoils would not require
remotely controllable yaw. These
airfoils could be designed (perhaps with a delta wing shape) to ensure that the
Tethered Airfoil would always fly with nearly zero yaw with respect to the
wind. (The purpose for flying "out
to the side" is to generate a force perpendicular to the direction of the
wind just as sails do when tacking into the wind.)
The Tethered Airfoils that have been discussed previously
require pitch control only (nose up or down) The purpose of this control is to:
1) generate varying tether tensions by adjusting the lift and drag
characteristics of these airfoils, or 2) to adjust the height of the Tethered
Airfoils in the sky. Tethered Airfoils
that could be used to provide propulsion into the wind (as well as with the
wind) require roll control as well.
These airfoils must be able to fly out to the side as well as overhead
and downwind. The best Tethered Airfoil
for this purpose would be one that could be directed to assume a relative
position in the sky with respect to a hull -- in response to remote control --
and then hold that position indefinitely without requiring power. It appears that such control may be possible
(and patentable).
A Tethered Airfoil should be able to passively maintain a
new relative position in the air in response to a single radio control request
to change the tether bridle position, flaps, wing warping, or center of
gravity. Using this technique changing
the attitude of the airfoil would cause the airfoil to select a different
position in the sky. This, in turn,
would cause the tether to be pulled in a different direction -- causing a new
tack to be taken. If the airfoil could
maintain this new position indefinitely after it had made these changes, it
would be highly desirable, because power would only be required when changing
tacks -- not to maintain the course of a tack.
Even more important, is the fact that if it could passively self-correct
it's own position it would be immune to brief system power failures or
shutdowns. It would still continue to
fly just as well on the same tack.
Members of the Flight Research Institute have demonstrated
the feasibility of water skiing upwind or downwind with a Tethered Airfoil at
the Columbia River Gorge. They also won
first place in a speed sailing competition in England -- racing against craft
having similar sail area. Even though
the airfoil and hydrofoil were inefficient off-the-shelf kites and skis, they
won by the greatest margin of the day.
While the principle of tacking into the wind with Tethered
Airfoils may sound unique, it has actually been accomplished and documented as
early as 1827 by G. Pocock. (The Samoans
used it even earlier.) It appears that
as soon as Orville and Wilbur Wright showed that it was possible to fly without
a tether, virtually all scientific research into the applications of Tethered
Airfoil flight ceased. Back then, the
only way that an operator could remotely control a Tethered Airfoil, was by
applying varying tensions on additional drag-inducing cables. The winds that kept the airfoil aloft also
acted upon these control cables. When a
wind gust would cause an airfoil to start diving to one side, different
tensions would result in the control cables.
Often, these different tensions would cause the airfoil to dive even
more. These airfoils often flew out of
control and crashed. What is surprising
is that in 176 years nothing has changed.
To the best of my knowledge, no one has yet put an
inexpensive autopilot and an aerodynamically efficient Tethered Airfoil
together. I hope to work with others to
be the first to achieve this goal. With
such equipment there is no reason why Tethered Airfoils would not be every bit
as stable, controllable, reliable, and useful as standard aircraft.
Tethered Airfoils could provide propulsion for small
boats. Attached to the gunwales
negligible listing moment would be generated.
In fact, traveling with the wind, the airfoil could help pull the hull
of smaller boats out of the water, thereby reducing drag. Motorboats, sailboats, hydrofoils, canoes,
kayaks, sailboarders, skiers (both water and snow) -- all could be accommodated
with a handful of different models.
Unlike sails, Tethered Airfoils need not be custom made for each boat or
application. No heavy masts, ballast,
special ship design, or expensive retrofitting would be required. Like sails on a sailboat, Tethered Airfoils
could provide power for all points of tack except dead into the wind. They would be better than sails because they
would have an aerodynamically superior shape -- higher lift to drag ratios --
and therefore be able to tack much closer into the wind. They would also have access to the stronger
winds aloft. They would have one cable,
requiring one winch, and take up no deck space (mounted externally to a track
on the gunwales).
Over land, the available wind power doubles with every
five-fold increase in altitude. This
factor can be much greater over water when the wind causes the waves to crest
and the waves cause more pronounced boundary layer effects. So Tethered Airfoils could tap much more
powerful winds than sails.
If a motorboat were outfitted with a Tethered Airfoil that
flew at 500 feet (where the winds at sea are often three to four times as
strong as at the top of most masts and towers) it could outrun most sailboats
-- without engine power. Naturally, If
the winds became too strong the airfoil could be tied down or deflated. For example, fishing fleets could race to
their fishing grounds with their airfoils at high altitude and troll with their
airfoils slightly overhead.
Motorboats under power could use Tethered Airfoils to
provide a component of thrust in the direction they wished to travel. Suppose that a captain desired to travel east
and decided to use an airfoil to help reduce fuel consumption. Suppose further that the wind was blowing
such that his Tethered Airfoil pulled strongest in a northeasterly direction. He could accomplish his goal by directing the
motors to cause an equally powerful thrust in a southeasterly direction. If the captain wished to travel east at 20
knots, the motors would only need to propel the boat at 14 knots. Depending on the ship and the sea conditions,
this thirty percent reduction in motor propulsion speed could result in a fifty
percent reduction in fuel consumption -- yet he could travel just as fast as if
he had used motor power only.
It is typically reported that by assisting propulsion with
standard sails, fuel consumption can be reduced by a fourth. But since Tethered Airfoils can harness winds
having greater power, and since they could be much larger, Tethered Airfoils
could save much more fuel. Since Tethered
Airfoils could be attached at the gunwales they could never pull the boat over
-- just along. So, unlike sails,
Tethered Airfoils would never need to be furled to prevent capsizing. Tethered Airfoils should always be able to
make use of the best winds -- at altitudes where there is over four times as
much power available.
The Tethered Airfoils for sailing applications could be
inflated with lighter-than-air gases such as helium or hydrogen so that they
would simply float up in exceptionally calm winds. Alternatively, they could be inflated with
air in which case they would need to launch and land as the winds would
permit. As the winds would become strong
enough, or as a boat having a propulsion source would pull, an air inflated
Tethered Airfoil could be launched by letting out the tether. To land the airfoil when desired, or in the
event of exceptionally calm winds, a winch could pull the Tether back in again
at a sufficient velocity to maintain stable flight.
Airfoils that are inflated with air would be advantageous
because they could readily be deflated and conveniently stored on board when
not in use. Also, there is additional
cost and logistics involved in obtaining, storing, and transferring
lighter-than-air gases. As elegant as it
would be to have lighter-than-air Tethered Airfoils pull boats, in general it
would probably be more practical to use air inflated Tethered Airfoils.
As soon as Tethered Airfoils are developed that can pull
hydrofoils reliably, passengers could fly in gondolas attached to airfoils rather
than sail in hulls over the water. The
principles of operation would be just the same.
The only difference is that the hydrofoil would now be remotely
controlled rather than the airfoil. Such
a craft should have a much smoother ride.
The tether would dampen Wave action before it was transmitted to the
gondola. In the event that the wind
stopped, the gondola would simply float -- being held up by the buoyant lift of
the lighter-than-air airfoil.
This configuration could render a truly efficient sailing
craft because a lighter-than-air airfoil could support the passengers, cargo,
and all other components of the craft except for the hydrofoil that would be
required for tacking. In other words,
the craft could be made very efficient by the elimination of the hull and all
unnecessary water drag. Having a high
sail, very little drag, and always being "up on the hydrofoils" such
a craft could sail even in the lightest of winds. For truly high speed, the airfoil could fly
at high altitudes. For passenger comfort
without cabin pressurization, the gondola could be attached to the tether a
reasonable distance above the ocean.
Nearly this same level of comfort and efficiency could be
obtained by using Tethered Airfoils that are inflated with air. In this case, the Tethered Airfoil and
gondola would have to launch and land as the winds would permit. But this would probably not be a very big
penalty because they would land when the winds would provide little or no propulsion
and when the water would be calm. The
one disadvantage in using air rather a lighter-than-air gas to inflate the
airfoil is that some of the aerodynamic and hydrodynamic lift generated by the
airfoil and hydrofoil would have to be used to lift the gondola and wing. Normally, a relatively small percentage of
the power would be required to lift the gondola and wing. The vast majority of the power would still be
available to provide propulsion.
As the winds would start to pick up, this craft could be
launched by releasing tether from a spool in the hydrofoil. In many cases this would be sufficient to
cause the gondola and wing to take to the air.
But if the winds at low altitude were insufficient, the gondola and the
airfoil would float on the water downwind from the hydrofoil. When the tether would be let out
sufficiently, the tether could be winched back in briefly and strongly to cause
enough tension in the tether between the hydrofoil and the airfoil to pull the
airfoil into the sky. Once in the sky,
under the influence of greater wind power, the winch could stop pulling and
gradually let out more tether so that the gondola and airfoil could ascend to
the altitudes that would allow tacking.
Tethered Airfoil construction techniques should enable the
construction of high performance inflatable paraglider wings and ultralight
aircraft. Standard Paraglider wings are
ram-air inflated. This causes drag to be
generated at the leading edge. Also
during flight, standard paraglider wings can easily be deformed into less
efficient shapes. Tethered Airfoils
should be at least as light, but they should form much more rigid and
well-defined airfoil shapes. It should
also be possible to use these techniques to make inflatable ultralight
aircraft.
Using the proprietary construction methods that are
discussed in the paper “Making Tethered Airfoils and Air Tensioners”, highly
pressurized lighter-than-air airships (airfoils, aircraft, or balloons) could
be manufactured that could passively stabilize their altitudes in free flight
without being restrained by tethers.
These construction methods could be used to make lighter-than-air
airships that would prevent the internal gases from expanding as they rise.
These would be constant volume airships.
As a consequence, if they were free to ascend or descend they would come
to rest at the altitude that would have the same density as the over-all
airship. If these balloons rose higher
-- perhaps due to momentary gusts -- they would be heavier than the surrounding
air so they would settle back down.
Likewise, if they were lower, they would be lighter than the surrounding
air so they would rise. They would
always passively return to the altitude whose density is equal to that of the
airship. In short, they would require no
monitoring, control, or power to automatically self-regulate their own
altitudes. If they were in no hurry they
could float to destinations downwind consuming no power. This might be a useful plan in hauling
freight inexpensively.
This technique was used by NASA in the Ultra Long Duration
Balloon that launched March 16, 2003, and which was designed to circumnavigate
the globe for 100 days. Interestingly,
this technique has never been used to maintain the altitude of lighter-than-air
man-lifting balloons or airships.
To date, all lighter-than-air man-lifting balloons require
continual monitoring and adjustments of altitude. This is because the air in these balloons
expand during ascent and compress during decent. If they start upward, they continue upward at
an accelerating rate, until helium is released to cause them to descend again
to the desired height. But once they
start to descend they continue to descend at an accelerating rate, until
ballast is released to cause them to ascend again. These balloons continually rise and fall
requiring continual releases of helium and ballast to compensate.
In standard airships or blimps, the lifting gas is free to
expand or compress to come to equilibrium with the surrounding air. So as the airship descends, the gases
compress. This would cause the airship
envelope to become limp were it not for ballonets. Ballonets are special internal air pressure
compensating balloons that inflate during descents to maintain a small but
uniform positive pressure in the airship.
Unfortunately, a ballonet requires a fan to maintain a slight positive
pressure. The fan in turn requires a
power source. Present day airships do
not regulate altitude by alternately releasing helium and ballast like
balloons. That would be too costly. Instead, they use the aerodynamic forces of
thrusters to maintain altitudes when the airship has a different density than
the surrounding air. These thrusters are
used to provide an upward force when the airship is heavier than the
surrounding air and a downward force when the airship is lighter. This method requires engines that continually
consume fuel.
It would be better if airships were designed to withstand
high internal pressures (such as up to 5 psi).
To ascend, air could be released from an internal ballonet. The loss of this air, and the expansion of
the helium that would result in the adjacent chambers, would lower the overall
density of the airship, which would cause it to rise to the altitude having the
same density -- and no higher. To
descend, a compressor would be required to draw air back into the
ballonet. This additional air, and the
compression of the helium that would result, would cause the airship to descend
to the altitude that would have the same density -- and no lower.
Such an airship would never need to discard helium or
ballast, or consume fuel to maintain a specific altitude. It could also be smaller because it would not
need the extra buoyancy required to lift ballast or the additional fuel
required to maintain altitude. In the
course of adjusting altitude, this airship would only need to consume power
when using the compressor to draw in additional air to descend. It would require no power to maintain a
specific altitude or ascend. It could
float indefinitely downwind at a specific altitude without requiring any
altitude monitoring or control.
If freighters and ocean going vessels used even relatively
simple and inefficient Tethered Airfoils they could realize dramatic reductions
in the costs of fuel. When traveling the
direction that the jetstream blows (eastward in the Northern Hemisphere) the
vessels could pull large Tethered Airfoils into the jetstream. Once in the jetstream, these airfoils could
simply pull the vessels downwind. A 50
percent reduction in the cost of fuel one direction on a large freighter would
save hundreds of thousands of dollars annually.
Efficient Tethered Airfoils might be able to save significantly more
because they could provide propulsion assistance on the return upwind trip as
well.
Some freighters have been designed to use metal sails to
provide propulsion assistance with the wind or into the wind. They are designed to save as much as 60 percent
of the cost of the fuel. Like all sails,
these metal sails cause the vessels to list to one side when the winds
blow. Listing causes all decks and cargo
bays to have sloping floors. To prevent
capsizing, the metal sails are "furled" by folding. They require special ship designs to
accommodate the masts, ballasts, and the forces that the sails generate.
Tethered Airfoils in contrast could provide greater power
from higher altitudes and yet cause negligible listing. Little or no retrofitting would be required
because Tethered Airfoils could pull the vessels at the same attachment points
that tugs would use. Even if these
Tethered Airfoils were not lighter-than-air they could be self-launched into
the apparent wind generated by these ships at sail.
Between territorial waters there are no governmental bodies
that regulate how high Tethered Airfoils would be allowed to fly. As low as a ten percent reduction in the
worldwide consumption of fuel by freighters would save billions of dollars
annually -- not to mention the environmental benefit of reduced pollution and
less global warming.
Special tugs could be designed for the express purpose of
manipulating Tethered Airfoils to pull ships across oceans. This would have the advantage that the large
vessels would not have to manipulate the Tethered Airfoils directly. All the tasks associated with providing
propulsion assistance could be handled by a tug specially designed to do the
job. Tethered Airfoils suitable for this
purpose would probably not have to be lighter-than-air. The tug could sail into the wind, pulling
even a heavier Tethered Airfoil into the air.
A heavier-than-air airfoil would have to fly exclusively by aerodynamic
lift, but it could still land safely even in calm winds by being pulled in fast
enough to ensure stable flight back down.
Most appealing is the prospect of harnessing winds in the
jetstream where the wind power is often hundreds of times greater than at the
top of masts and towers. Technical and
political hurdles would have to be overcome, but as Tethered Airfoil technology
matures and gains acceptance jetstream wind farming may prove practical.
At each site, the local terrain and the proximity to the
jetstream will determine whether it would be best to fly more airfoils at lower
altitude or fewer airfoils at higher altitude.
Mountains or other land formations that funnel wind may favor lower
altitudes. One such mountain range
exists in Hawaii. This range runs
perpendicular to the prevailing winds and funnels winds up and over. (Hawaii also has expensive electricity and a
state government that has recently invested millions in wind energy development
in a single year.)
Obviously, Tethered Airfoils that fly at high altitude would
need to be assigned their own airspace.
They could be assigned airspace far from the commercial flight
paths. In rural Kansas, for example, strong
constant winds at ground level would assure that the Tethered Airfoils could
self-launch and self-land inflated only with air. Alternatively, they might obtain permission
to fly in the restricted airspace over wilderness areas because they do not
pollute or make noise.
Many Third World countries are crossed by the jet streams of
the northern and southern hemispheres.
They might desire to relinquish airspace to produce inexpensive
electrical power. If the winds at ground
level are insufficient to launch these Tethered Airfoils, they could be filled
with helium or hydrogen so they would always be in flight even in calm winds.
(Ever since the Hindenburg blew up, people have been
reluctant to use hydrogen in lighter-than-air aircraft, but it should be noted
that the Hindenburg contained the hydrogen in "gold beater's skin" --
the intestines of calves beaten thin -- nothing to be compared with today's
multi-layered plastic films.)
A number of articles have been written about the feasibility
of developing wind power generating systems that could tap the power of the
jetstream. But the systems described in
these research papers consist of wind turbines mounted on large metal wings
that are tethered with special power conducting cables. The wings use the turbines as thrusters for
launching and landing. The complexity
and manufacturing costs are staggering; yet the amortized costs of the
electrical power generation are considered favorable (in the 7.5 - 9.5 cent per
kilowatt range nearly twenty five years ago).
However, it would be much simpler and less expensive to
design a system that would:
1) Have an ordinary land based generator,
2) Have inexpensive inflatable fabrics that can
be quickly deflated and stored away during periods of excessive wind,
4) Bounce rather than crash in an accident,
5) Contain virtually no costly and fragile high
tech components,
6) Require no heavy turbines or metal cables to
conduct lightning,
7) Never need to land during light winds,
8) Provide a much greater return on investment
because the same costs could be used to construct larger Tethered Airfoils that
could extract power from a greater area.
Over much of the United States the
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