Space related applications might be far better air launch platforms and
vehicle recovery systems, even planes for Mars. Energy related
applications include the possibility of far more effective and efficient
wind turbines and aircraft.
The ongoing development of traction kites and unmanned air vehicles has
raised the possibility of some alternate solutions to some of the most
basic of aerodynamic design problems. Possible advantage lies in the
separation of wing from body via a load carrying tether, this allows the
wing to otherwise fly, and generate lift, independently of the body.
It also enables loads to be distributed in tension, avoiding much of the
need for heavy structure and the weight and cost so incurred. Powered
free flying wings of this kind will likely have sufficient thrust to
weight ratios to take off and land vertically, and by circling, take off
and land their payloads vertically. The following is a outline of some
of the possible applications of such free flying tethered wings to
aircraft, wind turbines, kite sailing, and other technologies, and some
of the potential advantages there of.
An airplane utilising the free flying wing concept might consist of a
streamlined body, in which payload and fuel are stored, with retractable
landing gear sufficient for VTOL and taxiing purposes, and perhaps
landing pads on the top of the body suitable for holding free flying
wings when not in use. Likely multiple wings will be used so as to
balance rotating tether loads and for the purposes of redundancy. Fuel,
electrical power, and control, will likely be transmitted within the
line fairing with the wing capable of a degree of autonomy in case of
emergency, numerous safety features could be added. Such aircraft could
be built with payloads ranging from a few grams, to a few thousands of
Compared to a standard air plane such aircraft would have VTOL and a
significantly lower mass fraction, resulting in much greater range,
payload, efficiency, and much lower cost. Primarily this is due to the
elimination of a large part of the aircrafts structure and weight, which
no longer serves any purpose.
Such an aircraft would have similar advantages over a helicopter with
additional advantages in the elimination of the heavy gear box and the
additional capacity for high speed flight. For a given amount of lift a
free flying wing is far lighter than a rotor, and by being far less
constrained by effective rotor diameter greatly improved hover
performance, and much higher efficiency is possible. In effect, such an
aircraft might even hover more efficiently than it could fly
horizontally, due to the reduction in body drag. The heavy lift
capacity, perhaps into the thousands of tons, of such a large helicopter
might be particularly useful.
The free flying wing approach might offer especially great advantage
with regard to power generation from the wind. With VTOL, such a wing
could be developed to launch and land autonomously and due to the high
flying speed should be able to survive extreme wind strengths without
even needing to land. In comparison to a standard wind turbine, the
tower is replaced by a line with the free flying wing replacing the
rotor tip, eliminating most of the blade. The large low speed generator
and gearbox are replaced by a small high speed generator/motor direct
coupled to a small propeller or ducted fan. This is sufficient for
VTOL, electrical power is transmitted via a cable within the line
fairing. Even with the much higher speed operation the generator is the
dominant cost, there is significant advantage in using lower performance
generator designs of far lower cost. The dominant generator cost also
favours the use of a larger wing that can generate in much lower wind
speeds, further, such a system is able to operate at much higher
altitude where the wind is generally stronger. Wind turbines typically
have a utility of around 25%, with the capacity to generate in much
lighter winds, this system would operate far more of the time. A
further advantage is the capacity to scale up to very large sizes, units
in the hundreds of megawatts at least, should be possible, this is not
possible with current wind turbines.
A comparative analysis would tend to infer that this system should be
able to generate electricity for about a tenth the cost of standard wind
turbines, direct cost analysis would tend to confer with this. It has
the potential to be significantly less expensive than other mainstream
electricity production. This does not, however, account for the cost of
a site, power transmission, and social and environmental costs.
An interesting application for an unpowered free flying wing is as a
parachute or even paraglider. While structurally similar such a wing
can be made much smaller than a parachute due to the much higher flight
speeds, and can be made of high performance materials. For these
reasons an arch style wing system especially, can be made much lighter
than a comparable parachute. By using such a free flying wing as a
gyrocopter reasonable glide rates are possible, in effect the gyrocopter
mode trades glide rate, or lift to drag ratio, for lift. With this
system it is possible to combine a flared landing, as per a paraglider,
with pitch control and energy storage in the wing's speed, for highly
effective and controlled vertical landings.
In recent times high performance kite development has been greatly
pushed by kite traction and kitesurfing in particular. Considerable
effort is now going into the development of kite sailing, this is in
many ways driving the development of the free flying wing concept.
Traditional problems for kite sailing are launching, landing, power
control and light wind operation. Light wind operation is particularly
difficult because it necessitates extremely light weight construction.
One possibility is to use the wind turbine type solution, this enables
launching and landing, and the capacity to motor the wing in light
winds. It also enables power generation for use on board and if used in
conjunction with a diesel electric type ship, the capacity to sail
directly into the wind, avoiding the need to tack. The free flying wing
enables a comprehensive solution to power control and lends itself to
control by autopilot.
Another possible application for free flying wings is as aerostats. For
example, a wing might be flown high over a city providing everything
from communications to surveillance services. Power might be
transmitted up the line, enabling it to generate power when the wind
blows, and to be powered when it does not.
It would seem possible to use free flying wings to generate power from
water currents, rivers, tides, even ocean currents, in much the same way
as a wind power generation system would work. While the free flying
wing approach is very effective at extracting energy from such flows the
available energy is not as great as one would think. While water is far
more dense than air it is the speed of the flow and available area that
is really important. The power available is proportional to the speed
of the flow cubed, this makes wind power more attractive, especially as
the available areas are much greater. There are also issues with regard
to impacting submerged objects, even so, this could be a significant
application for free flying wings.
This system might also be used in place of water propellers, they would
be particularly useful in applications requiring high thrust at low
speed, they might also be useful with regard to manoeuvring. Pitch, yaw
and roll mitigation might also be possible, this can actually use the
energy of waves to power forward motion. In this way wave energy power
schemes could also be developed.
The lift of a wing is proportional to the velocity squared, as such a
wing is very speed dependent, generating little useful lift at low
speed. For helicopters and wind turbines this means that the inner part
of the blade, which is travelling at a proportionately slower speed, is
generating significantly less lift, in effect, the tip does most of the
work. For airplanes this means that a high takeoff speed is generally
necessary. Sail craft likewise have little lift at low speed, being
slow to accelerate and build up apparent wind.
Within typical operating ranges, the lift to drag ratio, or efficiency,
of a wing is largely independent of speed, such that higher speed wings
can be smaller, for a given lifting force, without costing efficiency,
though power increases in proportion to speed. The general design
difficulty, is in overcoming this low speed regime without compromising
overall design. Generally this design compromise requires a wing which
is too small for efficient low speed operation, and to large for
effective high speed operation.
The other significant advantage of a free flying wing is that
structurally they can use distributed support from beneath via tensile
members. Such bridles, as in a paraglider, parachute, or kite, can be
very light and inexpensive, providing distributed support and largely
eliminating the need for internal wing spars. This is a very
significant thing enabling major weight reductions. An interesting
consequence of this is the avoidance of this scaling constraint which
currently limits larger aircraft. Free flying wings capable of lifting
a thousand ton would seem theoretically possible, with the possibility
of using multiple wings, partially for redundancy, truly large payloads
should be possible, though perhaps not optimal.
A further limitation of rotors as per helicopters and wind turbines, is
the slow rotational speed which comes with large diameter, requiring
complex and heavy power transmission systems. Because a free flying
wing, can be flying at high speed, a small high speed propeller or
ducted fan can be used. While there are inefficiencies involved in
doing this, they are not great, and this avoids the low speed gearing
problem. In this mode, a free flying wing can to some extent be thought
of as a free flying rotor tip, without the same limitations in diameter.
In addition to enabling the predominate use of tensile load carrying
members where heavy structural members in compression or bending were
previously often required this approach enables the speed, and hence
lift, of the wing to be actively controlled independently of the body.
For an aircraft this might enable vertical takeoff and landing, also,
using a bridle to distribute load a much lighter wing, less limited by
scale, should be possible. Compared to a helicopter this might allow
the elimination of the gearing necessary for low rotational speed and
much of the inner rotor, also, the adoption of much larger rotor
diameters. Wind turbines might be similarly advantaged with the added
capacity of self erection and operation at much higher altitudes,
without a tower.
The lift generated from a free flying wing is utilised some distance
beneath the wing via tensile members, this enables the spanwise lift to
be supported via tensile members instead of the traditional and heavy
wing spar. Paragliders, parachutes, and kites exemplify this form of
wing and so provide considerable insight into what their design and
construction might entail. The task at hand is to transfer the lift
force from the skin of the wing through to the payload in a light and
effective fashion which little compromises the overall aerodynamics.
The first step in this load transmission is in collecting this lifting
force from the skin of the wing, this raises a number of possibilities.
Generally kites support this lifting force by transmitting it in tension
along the wings skin, while this generally distorts the skin shape,
seriously compromising the aerodynamics, it has the advantage of being
very light weight, a necessity for low speed flying. Aircraft tend to
use a rigid skin structure which is internally supported by heavy
structural members, this does not aerodynamically compromise the skin
shape, at the expense of weight. Obviously, there are also a number of
hybrid solutions to this problem, for example the use of ram air
inflation to support the skin structure in a paraglider, and the use of
a fabric skin stretched over a rigid spar and rib internal wing
structure in many older and lighter weight aircraft.
The two wing types that suggest themselves are a standard type rigid
flying wing, bridled much like a paraglider, though likely with fewer
fared bridles, and the arch style wing where the bridles are effectively
internalised with load distributed spanwise under tension from each tip.
The advantage of the arc style wing is that the skin can function in
tension with out compromising the skin shape, this avoids the need for a
rigid skin structure and the majority of the weight and cost there of.
The major disadvantage of the arc style wing is that conventional wisdom
would infer that lift coefficient corresponds roughly to the projected
area when flying, which is somewhat less than the wing area when laid
out flat. Interestingly, for a given aspect ratio, the arch style wings
tend to have a higher lift to drag ratio than conventional soft wings,
likely, this is due to the elimination of bridle drag. Initial
calculations would infer that a five to ten fold weight and cost
reduction might be possible over a standard type bridled rigid wing, but
there are a number of uncertainties, this is a field in need of further
study. A bridled rigid wing might ultimately achieve a weight of 2%
that of the load carrying capacity, an arc style wing might get well
below one percent. Note that some applications favour low wing loadings
that can invoke a skin thickness below the minimum gauge constraints of
some materials, this can constrain such designs.
For a rigid wing, to first approximation, doubling the number of bridles
halves the internal structure required. This eventually reaches a point
of diminishing returns as bridle drag scales with line diameter while
the bridle load scales with line diameter squared. Experimental and
theoretical evidence to date would infer that wings with high lift to
drag ratios are going to require a degree of bridle and line faring in
order to realise high efficiency. Current high performance kites and
paragliders are already constrained by this limitation. The development
of fared lines that are aerodynamically stable might be interesting.
The dynamics of line twist in conjunction with aerodynamic feedback will
need to be mitigated to stop strumming. The centre of line tension will
likely want to be significantly forward of the centre of pressure of the
fared section to aid this stability, though there are other
possibilities like the addition of tail planes to active control
systems. These problems will likely govern the design and construction
of such fared lines. Some basic construction methods are to use a
standard line with a foam trailing edge faring, or a pulltruded glass or
carbon fibre section in which the trailing edge is hollow, so as to keep
the centre of tension forward. Interestingly, such a hollow trailing
edge section is sufficiently large for the insertion of high voltage
power cables or fuel lines sufficient to power such flying wings.
The numbers infer that such wind turbines should produce electricity for
around 0.1-0.25 cents per unit.
Such aircraft, in addition to having VTOL and being half the price,
would have around twice the range, (or payload), and twice the
efficiency, being half the weight including payload. This makes a great
many things possible, one could directly fly people, cars, small ships,
and all sorts.
Heavy lift into the thousands of tons should be quite possible, not that
this is really desired for air launch for which this approach seems
almost ideally suited.
As a recovery system for launch vehicles, weights can be far lighter
than an equivalent parachute, less than one percent by weight, with high
cross range, gentle vertical landings, and full control. Power could
also be added far more cheaply than alternatives.
Development cost and time should not be great, perhaps a year or two for
most applications, effectively they are just UAV's.
Thanks for an interesting series of posts. I believe I understand how a
free-flying wing as you describe might be applied to aircraft, but I
don't see how it applies to wind turbines. OK, you could use a kite
structure instead of a tower to hold your turbine aloft (and hope the
wind never dies for even a few seconds!), but you still need rotors to
actually crank the generator, right? How do the principles you're
describing impact rotor design at all?
| Joseph J. Strout Check out the Mac Web Directory: |
| j...@strout.net http://www.macwebdir.com |
Yes, this method has been suggested in the past, so have similar systems
to what I am suggesting, I am just taking it one step further. Consider
a controllable kite which is continually looped within the centre of the
wind, an electric motor/generator and propeller are attached to extract
and supply energy to the system. The energy used to keep the kite aloft
is a small proportion of that which the kite generates. This much has
been suggested before. There are very important reasons why such
systems should exploit the apparent wind of the kite, which is many
times that of the true wind speed. By analogy with a standard wind
turbine, this is equivalent to driving the generator via a very small
high speed propeller mounted at the wind turbine blade tip.
Envisage an airplane with electric motor driven propellers, similar in
scale to a turboprop. These electric motors are powered from the ground
via an electric cable within the line faring. It just so happens that
there is sufficient thrust available for VTOL, this simplifies launching
and landing, which traditionally have been problematic. After launch
this airplane circles in the sky, like a wind turbine blade tip, (fast,
say 100m/s or more), loading up against the tether and the wind like a
kite. At these speeds the propellers are very effective wind turbines
and the electric motors become generators that feed electricity back
down the cable to the tether point. The area that the airplane sweeps
as it circles is equivalent to the swept area of a standard wind
turbine, though obviously it can be much greater.
The trick is that the propeller is operating at the speed of the
airplane, which is many times greater than that of the true wind speed.
At an overall lift to drag ratio of ten the air plane speed is ten times
that of the true wind, with power proportional to wind speed cubed, the
propeller can have a thousandth the swept area of a comparable wind
turbine for the same power. This makes for a very compact and effective
unit, it is important to exploit this apparent wind directly as it
allows for much higher specific speed of the propeller and generating
unit, (no gearing).
When the wind is light the propeller can revert to being driven,
powering the airplane along, if this continues for a period of time, the
airplane can be landed. Note also that the speed and kinetic energy of
the airplane is sufficient to coast through short lulls in the wind.
Such airplanes can fly and generate useful electricity in true winds
significantly lower than that required of standard wind turbines.
An interesting concept!
Modern computer and control technologys might just be able to handle
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.
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.
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
Have you looked at these details?
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
> 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
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
> Have you looked at these details?
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
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).