Tethered wings.

On Fri, 22 Oct 2004 20:52:39 +1300, "Pete Lynn"
wrote:

For the same reason that yachts can sail upwind. There is also a degree
of inertia in the "kite" speed, such that with AOA control you can even
out the pull and power through the circle. Consider vertical axis wind
turbines as an extreme example.

Oh, I see... hmm... I don't really understand how lift-type vertical
axis wind turbines work either, but I accept that they do work

What percentage of a helicopter's weight goes into the rotor
blades as is? I thought they were fairly light?

It is very scale dependant, though 5% might be a starting point, which
is quite light. I expect it would be somewhat greater at 747 scales...
The blade is held out in tension by the centrifugal loads which are
typically at least an order of magnitude greater than the lift force.
Hence the blade is ten times heavier than a pure lift load would
otherwise necessitate.

Does that matter, though? Is it worth changing the design to save even
90% of something that's only 5% of the weight in the first place? (Is
there a market for a 747-sized helicopter?)

Also, is _area_ the factor that determines lift?

L = 0.5*air density*lift coefficient*area*velocity^2

Why do gliders have thin wings?

I suspect the main disadvantage at this stage is that the tethered wing
is effectively a UAV, with the reliability issues that infers.
Presently UAVs typically crash something like every few hundred flights.
This would have to improve by a couple of orders of magnitude before
people could fly on them.

I don't understand this part - why do UAVs crash every few hundred
flights? And what's the connection between being unmanned and using a
tethered wing?

--
"Always look on the bright side of life."
To reply by email, remove the small snack from address.

Interesting - I have seen proposal for kite flown wind turbines. These
use a kite shape to provide lift and channel the air into a central
how with a turbine. The main advantage is the more reliable winds at
greater height. Two main issues are landing and take-off in strong
wind for large non rigid structure, and the amount of area that each
structure needs, given it can rotate 360 degrees around a pivot.

Yes.

If you then start moving the kite around, sweeping more of the sky and
building up apparent wind then your power output increases by a few
orders of magnitude and strong winds become but a small proportion of
flight speed such that they stop being an issue.

The reason a kite sweeps so fast acros the skyis that the lift vector
that pushes it foward only has to over come the drag on the strings.

As soon as you add a wind tubine on it to use the increased apparent
wind, the drag goes WAY up and the kite will slow down.

You won't actueally actually get the "power output increases by a few
orders of magnitude". The power output achievable is related purely to
the swept area of the turbine and the wing area of the kite

Launching and landing is the big problem, but you have enough thrust for
VTOL, so why not?

Pete.

If you then start moving the kite around, sweeping more of the sky and
building up apparent wind then your power output increases by a few
orders of magnitude and strong winds become but a small proportion of
flight speed such that they stop being an issue.

Thought of this just after I posted. To test out for yourself some of
what I said. get one of the kites, and attach a drouge chute to it
with an area say 1/10 of the wing, and see how much the performance
changes. I bet it won't sweep across the sky nearly as fast

"Hamish" wrote in message
om...

The reason a kite sweeps so fast acros the skyis that the
lift vector that pushes it foward only has to over come the
drag on the strings.

Yes, the same is true of a wind turbine, glider, yacht, aircraft, etc.,
hence glide or lift to drag ratio. Though you should really be thinking
of total drag, wing, line and propeller, the last being where you
extract/add energy from/to the system.

As soon as you add a wind tubine on it to use the
increased apparent wind, the drag goes WAY up and the
kite will slow down.

Of course, this is also how you extract energy from a standard wind
turbine and prevent over speed and over load. Though to clarify, lift
and drag coefficients are relatively independent of speed over this
range. As drag goes "WAY up" lift also goes "WAY up". Lift and drag
forces are proportional speed squared, hence power is proportion to
speed cubed, (mostly).

You won't actually get the "power output increases by a
few orders of magnitude".

Standard wind turbines would dispute that. A typicalish tip speed to
wind speed ratio of 10 is sort of a three order of magnitude power
increase, (for a given blade area), over say a drag type wind turbine
where the tip speed is limited to wind speed. Assuming swept area is
not a constraint, which it mostly is not for the kite system. Pushing
the Betz limit like wind turbines have to results in a lower effective
wind speed at the blade/kite, lower blade/kite speeds, a bigger and
heavier blade/kite, less power, etc.

Say your kite and lines has an overall L/D of 20, maximum power can
generally be extracted at an effective L/D of about half this, say 10.
Say the wind speed is 10m/s, max kite speed, (over speed condition with
no power take off), will be 200m/s, optimal power extraction speed will
be around 100m/s. The power roughly equals propeller drag force times
kite speed, (lift force *(1/10 - 1/20)*100m/s).

The power output achievable is related purely to the swept area of
the turbine and the wing area of the kite

Yes, swept area being far less of a constraint for the kite. It is far
cheaper to use a longer tether and sweep more of the sky than try to
extract the last bit of energy out of a small area as a wind turbine
must.

I think you are getting some of the details confused, think of a wind
turbine come kite, instead of the other way around. Kites perform like
wings, or wind turbine blades, not drag chutes.

Pete.

"Hamish" wrote in message
om...

Thought of this just after I posted. To test out for yourself
some of what I said. get one of the kites, and attach a
drouge chute to it with an area say 1/10 of the wing, and
see how much the performance changes. I bet it won't
sweep across the sky nearly as fast.

Many years ago my father was flying a two line stunt kite around the
sky, backwards and forwards, up and down, looping it left and right. An
old man comes up to him, grabs him by the arm and says, "Young man,
young man, that kite would fly much better with a tail." :-)

Sorry, your statement while quite correct, left me kind of speechless.

Pete.

"Pete Lynn" wrote in message ...
"Len" wrote in message
om...

Thanks for the post, Pete. I've always like the
simplicity of Peter Lynn Kites' arch kite. I tend
to vacillate between the soft wing and a large ultralight
rigid wing with carefully distributed loads through tethers/
shroud lines--or propellant tanks in the wing. Our
Space Van 2009 carrier stage features the latter approach.
As I have noted before, much or our work is inspired by
the work and suggestions of you and your brother.

As you know I have spent many years developing the soft variety, I am
not sure I can push them much further, they have a number of inherent
weaknesses, like tendency to collapse which forces performance
compromising. I am increasingly thinking of rigid designs which need
not weigh more and have superior performance and handling. In the last
few years I have come to understand that it is not their softness that
makes a kites/parachutes, etc., light, but their structure.

Pete, this is a very interesting--and reinforcing--
statement on rigid designs, coming from a member of
perhaps the world's premier kite family. I am encouraged
to continue to pursue the "huge ultralight" aircraft
approach for the design of a carrier aircraft for space
launch. Allowing an orbiter to start life above much of
the atmosphere appears to be a surprisingly large advantage.
And--as you have pointed out--getting there does not have
to be as difficult or expensive as generally thought.

Best regards,
Len (Cormier)
PanAero, Inc.
(change x to len)
http://www.tour2space.com

"Len" wrote in message
om...

Pete, this is a very interesting--and reinforcing--
statement on rigid designs, coming from a member of
perhaps the world's premier kite family. I am encouraged
to continue to pursue the "huge ultralight" aircraft
approach for the design of a carrier aircraft for space
launch. Allowing an orbiter to start life above much of
the atmosphere appears to be a surprisingly large advantage.
And--as you have pointed out--getting there does not have
to be as difficult or expensive as generally thought.

Hypothetical question Len. If it were possible to reduce
the vortex/induced drag on a very low aspect ratio wing
to the point that it was competative with high aspect ratio
wings, would it be possible that a rigid wing vehicle
could beat the kite wing concept? This is assuming that
an aspect ratio of 2 could perform at the levels of aspect
ratio 10 or so.

I'm not suggesting I have the answer now, just a lead that
may or may not be worth investigating.

Best regards,
Len (Cormier)
PanAero, Inc.
(change x to len)
http://www.tour2space.com

You won't actually get the "power output increases by a
few orders of magnitude".

Standard wind turbines would dispute that. A typicalish tip speed to
wind speed ratio of 10 is sort of a three order of magnitude power
increase, (for a given blade area), over say a drag type wind turbine
where the tip speed is limited to wind speed. Assuming swept area is
not a constraint, which it mostly is not for the kite system. Pushing
the Betz limit like wind turbines have to results in a lower effective
wind speed at the blade/kite, lower blade/kite speeds, a bigger and
heavier blade/kite, less power, etc.

Say your kite and lines has an overall L/D of 20, maximum power can
generally be extracted at an effective L/D of about half this, say 10.
Say the wind speed is 10m/s, max kite speed, (over speed condition with
no power take off), will be 200m/s, optimal power extraction speed will
be around 100m/s. The power roughly equals propeller drag force times
kite speed, (lift force *(1/10 - 1/20)*100m/s).

The power output achievable is related purely to the swept area of
the turbine and the wing area of the kite

Yes, swept area being far less of a constraint for the kite. It is far
cheaper to use a longer tether and sweep more of the sky than try to
extract the last bit of energy out of a small area as a wind turbine
must.

Funny thought. My initial thoughts were that the "swept area " you
extract power from would essentilly only be the area of the device,
not the area the device swings through ( presumably a horizontal
figure 8 ). then i though about

http://groups.google.co.nz/groups?hl...gy.rene wable

James Jones cant seehow a standard turbine works and " that most of
the wind goes straight through the huge gaps between the blages "

Almost found my self falling into the sam trap !!!!!!!

I dont think you will get any where near the betz limit for the entire
area, but even some of it would add up because it is a HUGE area


I think you are getting some of the details confused, think of a wind
turbine come kite, instead of the other way around. Kites perform like
wings, or wind turbine blades, not drag chutes.


The drag chut bit was only to simulate the performance change of the
kite a generator would have.

Been thinking on the whoe idea some more, Getting a Mw back to the
ground would require some pretty heavy " kits strings " ,and no one
would want to live within the swept area or within a fair distance of
it just in case the " string " broke, and a deacent sized one of these
would have a big footprint. Go off shore ? landings a lot softer too
but coastal and tidal flows could push it to directly upwind for the
new breeze.

Interesting idea,but risks and development costs make it very unlikly
any company would want to build it. Only way to start this off is to
build a say 10 Kw device on your own as a demenstrator and test it out
in the middle of nowhere

Pete.

"Hamish" wrote in message
om...

I dont think you will get any where near the betz limit for
the entire area, but even some of it would add up because
it is a HUGE area

If need be you will get as close to the Betz limit as a wind turbine,
though the economics do not favour it. Standard wind turbines have to
push the Betz limit because they are so swept area constrained, this is
one of the reasons why they are so expensive.

Been thinking on the whoe idea some more, Getting a Mw
back to the ground would require some pretty heavy " kite
strings " ,and no one would want to live within the swept
area or within a fair distance of it just in case the " string "
broke, and a deacent sized one of these would have a big
footprint. Go off shore ? landings a lot softer too but
coastal and tidal flows could push it to directly upwind for
the new breeze.

The kite strings have to take the same load as a standard wind turbine
tower, by such a more meaningful comparison it would be difficult to
consider them as heavy.

Blades have been known to "fall off" wind turbines on occasion, even the
big ones. Due to their heavier weight,(even with generators on the kite
come UAV), and lack of independent aerodynamic control, they are far
less inclined to flutter to the ground in a relatively benign fashion.
Other safety systems are also possible, still, I do favour off shore
operation for big systems.

Pete.

"Len" wrote in message
om...

Pete, this is a very interesting--and reinforcing--
statement on rigid designs, coming from a member of
perhaps the world's premier kite family. I am encouraged
to continue to pursue the "huge ultralight" aircraft
approach for the design of a carrier aircraft for space
launch. Allowing an orbiter to start life above much of
the atmosphere appears to be a surprisingly large
advantage. And--as you have pointed out--getting there
does not have to be as difficult or expensive as generally
thought.

Thank you.

Have you given any thought to the idea of taking a cheap powered
hanglider or ultralight, and putting a small rocket on the back and
going for a few altitude records?

From what I can tell, you should be able to get to 20-30 km very cheaply
indeed, (a few tens of thousands of dollars?), though I am not so sure
about the pilot life support systems. I expect it would be a very
interesting ride and that people would be sure to take notice.

I would be interested to hear any quick numbers, technical design
comments or possible applications. (supersonic skydiving, sounding
rockets, photography, etc?) To my knowledge this is a very cheap and
constructive stunt that has not been done before. A few obvious
potential sponsors come to mind.

Pete.