Tethered free flying wings

The following is an outline of a concept that I have been developing for some time, for the sake of brevity and future intellectual property, I have omitted numerous details.

It is possible to free a wing from the shackles of a fuselage, rotor hub or boat hull, so as to gain numerous advantages. The wing in effect becomes an unmanned air vehicle with lift transferred to the main body by a load carrying tether. Conceptually this is a motorised computer controlled rigid kite with all the performance of an aircraft wing. It is the ultimate aerodynamic sky crane, and has significant application to aircraft, wind turbines, and kite sailing.


Left: Prototype motorised tethered free flying wing. Right: Kiteboarding kites in use.

There are four major advantages to be found in the separation of wing and body:

1. Tensile load distribution is possible, typically enabling an almost tenfold reduction in wing weight and cost, and the adoption of yet higher aspect ratio wings of greater performance. One example of such tensile load distribution is the bridling used on a kite, parachute or paraglider, although arch shaped wings that distribute this load internally are probably preferable. This is a very significant thing. For example, it enables the economically viable development of aircraft and wind turbines ten times larger than are currently possible.

2. Wing speed, and hence lift is independent of the body, for an aircraft this enables constant lift independent of forward speed. By circling, or figure eighting the wing it is possible to generate maximum lift at zero forward speed. In this fashion VTOL is possible with the capacity to transition to full forward flight. Not only does this enable a lighter, cheaper and more efficient helicopter but it combines this with the attributes of a lighter, cheaper, and more efficient fixed wing aircraft. For kite sailing this enables constant sail force independent of wing size or wind speed, no sail changes are required.

3. Propulsive efficiency is generally best at high speed, low speed operation requiring large, heavy, expensive and problematic gearboxes and propellers, a helicopter and wind turbine being extreme examples. By mounting the propulsion system on the free flying wing the propulsion operates at a constant and efficient high speed all the time, from vehicle takeoff, through to maximum speed. This is tantamount to placing small jet engines on the tips of a helicopter rotor blade, though much easier.

4. The natural wing weights and required engine thrust levels for normal flight are such that the thrust to weight ratios of practical free flying tethered wings are invariably greater than one. That is, free flying tethered wings with appropriate control, are capable of VTOL in their own right. They can be made to automatically launch themselves without recourse to fancy launching systems. This enables the likes of self erecting wind turbines systems.


Arch style wings
Arch style kites are favoured as traction kites where they have been found to have superior performance to traditional bridled traction kites like parafoils. In some regards this is counter intuitive as to first approximation both lift coefficient and lift to drag ratio are decreased by 2/pi. One of the reasons for this is that there is an interesting dynamic between centre and tip whereby the wing automatically twists to maintain constant angle of attack across the span. Another consequence of this is that a less rigid wing is required, making higher aspect ratio much easier. This effect greatly improves the efficiency and stability of such wings such that the reduced lift to drag ratio can be more than recovered by operation at lower angle of attack. Reduced bridle drag is another significant advantage.

Structurally Arch style wings are analogous to a cylindrical pressure vessel, by comparison rotor blades and conventional wings are analogous to square pressure vessels. Not surprisingly there is an approximately tenfold weight and cost difference. With a little analysis it is quickly seen that the aerodynamic deficiencies of the arch wing are easily compensated for by increased aspect ratio, at little structural cost. Indeed the utilisation of still higher aspect ratio arch wings would seem probable, increasing fuel efficiency over conventional aircraft. Such a wing is not directly possible on a conventional aircraft as like a paraglider it requires a considerable bridle length to efficiently transfer the lift force from the wing to the payload.


Tethers
The drag of the tether is very significant and seriously effects the performance of a free flying wing. While this drag is not prohibitive, it is highly desirable to fair the line. A fully faired line will reduce line drag more than tenfold, however doing this is not as straight forward as it sounds. Even with the situating of the centre of tension forward of the centre of pressure, (such that the fairing naturally feathers into the wind), dynamic oscillations and the high drag they create are still possible. While active solutions will solve this problem at large scale, perhaps simple air bleed approaches will even be sufficient, a low cost and perhaps not quite as effective passive solution would be desirable. There are various profiles that might be capable of this, though this is an area in need of further investigation.

While there are many applicable construction materials for the tether, perhaps the best is simply S-glass. It has very high strength, is comparatively inexpensive, and can be cheaply pultruded in a faired shape. In order to keep the centre of tension near the leading edge, such that the profile will be inclined to naturally feather, it is necessary to make the aft section of the fairing hollow. This is not difficult and allows the running of electrical transmission lines, fuels lines, and such like up inside the tether.


Specific aircraft design
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 with 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 to provide multiple 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 backup safety features could be added. Such aircraft could be built in sizes from a few grams, to a few thousands of tons.

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. Obviously the wing no longer needs the many complicated flap arrangements required for takeoff and landing, for that matter, nor does it need a tail plane.

Such an aircraft would have similar advantages over a helicopter with additional advantages in the elimination of the heavy gear box and the 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. The heavy lift capacity, perhaps into the thousands of tons, of such a large helicopter would be particularly useful.


Wind turbines
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 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. This favours the use of a larger wing that can generate in much lighter winds, further, such a system is able to operate at much higher altitude where the wind is generally stronger. Wind turbines typically have a capacity factor of around 25%, with the ability to generate in much lighter winds, this system would operate far more of the time, this has major infrastructural advantages. A further advantage is the capacity to scale up to very large sizes, units in the hundreds of megawatts should be possible, this is well beyond 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. This should be apparent from the advantages previously stated, a direct cost analysis seems to confirms 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.


Kite sailing
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.


Parachutes
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 arc style wing 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. 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. Controlled vertical landings should also be possible.


Aerostats
A free flying wing might be flown high over a city providing everything from communications to surveillance services. Power might be transmitted up and down the line, enabling it to generate power when the wind blows, and to be powered when it does not, also powering onboard equipment.


Water application
It would seem possible to use free flying wings to generate power from 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 should be particularly useful in applications requiring high thrust at low speed as propeller diameter is effectively unlimited. Low speed trawling or the manoeuvring of large ships are obvious applications. Pitch, yaw and roll elimination should also be possible with the appropriate attachment of such free flying wings. This can also serve to replace keels and rudders on large yachts, as they work far better at low speed and can be directed to conform to a lower draft as required. Such a solution would be highly desirable on large kite sailing yachts which would otherwise require very large and expensive keels.


Current status
I have modelled via spreadsheet the performance of free flying wings, with emphasis on various wind turbine systems. This agrees closely with what the more obvious and easily comprehended comparative analysis would infer. However experimental verification is required before this is taken further. Within given assumptions the numbers strongly infer that such a wind turbine system can produce power for around a fifth to a tenth the cost of current generation systems, depending on site costs. This is roughly US 0.5 cents a kilowatt hour, with the likelihood that this will reduce further with mass production.

I am currently attempting to develop a basic prototype in order to demonstrate the practical working of a tethered free flying wing in order to justify a more ambitious development program. This system is not just a short step into the unknown, but a fundamental redesign at the most basic level. As such it is asking a lot for people to trust the theory and assumptions on which it is based without recourse to a practical demonstration.


Prototype details
I am currently experimenting with the development of a small computer controlled flying wing as seen in the photos, though I am yet to achieve stable flight. The arch wing is taped polystyrene as per a model glider, it is half a meter span with three brushed Speed 400 motors attached. The base wing weight is approximately 20g and can easily sustain 2kg of lift, it could probably sustain 20kg of lift if necessary. This would infer a lift to weight ratio of a thousand. Although this is small scale this figure should give some indication of the advantages of this type of wing. The motors weigh around 70g each and have a thrust to weight ratio in excess of three. The line length is currently around 2.5 meters with individual wiring to each motor, obviously this is heavy and has a lot of drag, especially considering the low voltage, though it is expedient for now.

The rig is computer controlled by a basic stamp with potentiometer line angle and line tension sensing. I am using a PID approach with speed control of the individual motors. The centre motor is controlled so as to maintain a constant line tension with the left and right motors controlled in response to line angle. The anchor point swivels so that the wing can circle continuously without twisting the lines, though initially I only wish to demonstrate a stable hover.

As I discover what does and does not work I am upgrading various parts of the system, currently I am upgrading to a more accurate PWM motor speed control system. The previous approach caused too many line oscillations which in turn effected the angle sensing and control. If this does not work I have also purchased an accelerometer so that with the addition of an extra stamp and wireless modem I can control the wing more directly. While this will weigh more it will enable me to do the speed control onboard such that only two power transmission wires will need pass up the tether, this will be much lighter. More powerful brushless DC motors are also expected in the long term.

I may also construct a larger wing that should function better on such short lines. The lines are currently short so as to reduce line weight and sag and for the sake of convenience. However, there is a line scale effect where at short distances centrifugal forces from turning dominate lift forces from the wing. Both forces scale with velocity squared and the only real solution is a reasonable line length, unfortunately wire weight due to very low voltage currently limits this. Obviously I could use a DC-DC converter on the wing but ideally I wish to allow electricity to pass both ways as I would like to eventually demonstrate power generation with this model. I will perservere a little longer until I see what my next step should be, there are many options.

Peter S Lynn
12 October 2004