"pragmatist" wrote in message
om...

An interesting concept!
Modern computer and control technologys might just be able to handle
it.

The last time it was looked at seriously was thirty years ago during the
oil crisis, at that time the control systems were almost non existent,
and steel cables were the order of the day, even so it was not far off
being practical. Fortunately these problems have become far easier to
solve.

The cables would have to be very long to accomodate a transfer from
VTOL to standard flight mode, however, (you would have to reverse half
your 'kites`), and those cables could not be as light, or offer the
low wind resistance you seem to contemplate.

Not sure what you mean here by reversing kites, the kite is a
sufficiently rigid flying wing that can seamlessly transition from
airplane to kite mode and back again, this is necessary for lift
control.

This is all quite simple to model, which obviously I have done, and
continue to do, I have also built small prototypes. Line length can
actually scale with size, somewhat, a 100MW unit might optimally have
around a 1000m line. High strength materials allow for a smaller line
diameter, line faring makes a huge difference and really makes the
numbers add up, such systems are very sensitive to overall lift to drag
ratio, as this equates to speed. As a side note, the world altitude
record for kites is around 5km, in my professional capacity I have
investigated designs capable of 20km, (without even using line faring).

a. You have to carry electrical power to your engines, (pumping enough
fuel to run engines through even moderately long thin lines is out of
the question, you would very quickly reach the point where pressure
would require too much wall thickness), and both insulation and cable
size are not insignificant. You can decrease the weight by operating
at very high voltage, but then the insulation grows in size.

All true, and within limits, available areas are sufficient for
electrical and fuel transmission. Another trick that one can use is
multiple cables, allowing electrical transmission lines to be physically
separated, though this might cost slightly in terms of line drag.

b. Critical loadbearing components of aircraft are overdesigned by
neccessity, (gust loads can pile on the Gs don't ya know), and a kite
design with fast moving elements would be especially vulnerable.
The connections between the 'kites` and fuselage, (or groung station
in the generators), would be troublesome. You would need to carry
electrical power at very high voltage, and control circuits, (you
don't want to try 'wireless` for critical flight control), through
rotating, (you can't let your cables twist), loadbearing, critical
assemblies.
Have you looked at these details?

Yes. :-)

A wireless backup is likely desirable, (this is what I have used in
prototypes), but I expect that using the power cable might be more
secure.

The Gs you talk of are not a big issue, the mass, (an hence force), is
an order of magnitude less than for an airplane, and loads are carried
directly by tensile members when in kite mode. The Gs that are an
issue, (gyroscopic loads with regard to the generator), are those
sustained by circling, for small units this can easily exceed ten Gs,
(wing speed squared over circle radius), this is not a problem at larger
scales. This is one of the reasons that I favour circling instead of
following a figure eight pattern, although this necessitates a rotating
anchor point. Such a rotating anchor point should be easier than it is
for a standard horizontal wind turbine, (no bending moments).

Pete.

Pragmatist