Since its inception, the idea of the flying windmill has had several spin-offs that include a wind-powered toy helicopter that featured a spinning rotor while it flew like a kite. Hobbyists developed a method whereby multiple wind-powered toy helicopters could be flown in a line up of progressively higher altitudes when attached to a single control line. Kite builders in China developed a similar method of simultaneously flying multiple kites at progressively higher altitudes using a single control line.
Over a century ago, telephone pioneer Alexander Graham Bell built a kite able lift a grown man. Bell's precedent indicated that if the height and width of a kite were increased arithmetically, its lifting capacity could increase geometrically. In more recent years, kite hobbyists have used computer technology to design super kites that fly while controlled by multiple control lines. They have developed a method of using a single set of control lines to fly a line-up of several such kites at progressively higher altitudes.
Model Airplane Development
A model airplane hobbyist in the UK recently built a scale model aircraft that uses a "paddle wheel" that rotates on a transverse axis and is mounted within the upper side of the wings. The rapidly rotating paddle wheel accelerates a stream of air over the upper surface of the sharply curved wing and generates substantial lift at very low flight speeds. The wing curvature (angle of attack) is adjustable to over 45-degrees and that the aircraft can be made to hover like a helicopter.
This technology can be adapted for stationary flight using wind power and control lines and where the paddle-wheel would be replaced by a magnus rotor that would use the boundary layer effect to re-direct a fast-flowing wind stream over the sharply curved upper surface of the wing to generate lift. When built to a large scale (wingspan of over 200-feet), the concept may be able to carry a light windmill and electrical generation equipment.
Unpowered Flight developments
During the 1970's a popular disc-shaped flying toy called a Frisbee appeared on the market. When thrown by hand in a way that produced a spin, a spinning Frisbee could remain aloft for up to a minute. A machine-launched, fast-spinning Frisbee could remain aloft for several minutes. The spinning motion of the Frisbee induced a boundary layer effect in the air on its curved upper surface while centrifugal forces pushed air outward and over the sharply curved outer rim of the Frisbee to produced lift. When the centre of a Frisbee-shaped disc is mounted on a powered axle (e.g.: a toy flying saucer held by a tether), the spinning disc could remain aloft for extended periods of time.
Over the past 30-years, numerous aeronautical hobbyists have designed, built and tested a variety of unpowered flying ultra-lights, paraplanes, parasails and kites that can carry a person. These technological developments can be adapted for stationary flight using multiple control cables in perpetually windy locations. A battery of such units would then be able to carry the weight of wind-powered electrical generation equipment at high altitude over extended periods of time. The main drawback of such designs is that they are restricted in the weight carrying capacity because they only use the lower surface of the paraplanes, parasails and kites to maintain lift.
Applying the Precedents
The aforementioned advances that have occurred in kite and model aircraft technology can be combined into a revised flying windmill concept. One such concept could involve a series of up to 12-airborne windmills being flown at progressively higher altitudes from 3000-feet to 5000-feet (below the altitude of propeller-driven commercial aircraft). This battery of flying windmills would all be controlled by one common set of cables that are well secured to the ground and controlled from a ground-based control station. The control station may be computer controlled and also have a degree of rotational freedom. The control cables may be made from a lightweight high-tensile strength metallic alloy or a modern composite fibre. The design of the flying windmill could incorporate kites as well as lightweight rotating machinery.
The original flying windmill concept that was envisioned by Dr Roberts was designed to use one or two pairs of counter-rotating rotors to simultaneously provide lift and drive lightweight electrical generating equipment. The use of multiple control lines would allow a battery of such windmills to fly "in formation" at progressively higher altitudes of between 1000-feet and 2000-feet. Rotors of 100-feet diameter may be mounted with their centres 105-feet apart on either side of a single upper cable and above 2-lower cables that will provide vertical and lateral control. Lightweight vertical-axis wind turbines that feature helical blades may be mounted below the rotors. These wind turbines could drive the electrical generation gear and rotate independently of the rotors that only provide lift.
A variation of this concept could use automotive-type Rzeppa constant-velocity joints between the counter-rotating, helically bladed wind turbines and the rotors. The wind turbines would drive the rotors that in this case could be replaced by giant Frisbees that will use the boundary layer effect to remain aloft. Birds that bump into the giant spinning Frisbees may be none the worse off after such an encounter. Large spinning Frisbees would produce powerful gyroscopic effects that could strain bearings. A 4-cable system may be used to control the tilt of the flight rotors, the tilt of the vertical-axis bladed turbines, manage system torque reactions and also provide lateral control to the system.
Transversely Mounted Wind Turbine
Large wind turbines that were designed to operate as vertical-axis units may be transversely mounted as single units below the flight rotors and operate in a horizontal-axis mode. The helical blades of the turbine may be set so to produce power would from below the horizontal rotational axis. Such operation may produce some extra lift. An airfoil/kite may also be placed ahead of the transverse mounted turbine and above its rotational axis to produce additional lift and deflect a faster airstream over the working surfaces of the turbine. Torque reactions from the transversely mounted wind turbines would easily be restrained by the upper and lower control-cables.
Take-off and Landing
Modern electrical multi-plexing technology could enable the electric power cables to simultaneously perform multiple functions. During take-off, the tilt of the rotor blades could be adjusted (via solenoid) while power is provided to induction motors that may be used during system take-off and landing. When the system reaches an optimal altitude, the induction motors will cut out while the rotor blades tilt (via spring pre-load) to maintain wind-powered flight and spring-loaded clutch mechanisms (previously dis-engaged by solenoid) would simultaneously engage the electrical generation sub-system.
The landing sequence could follow standard helicopter practice when after a mid-flight engine shutdown. Pilots are trained to re-adjust the tilt of the rotors to accelerate them during descent, then reverse the tilt of the rotor blades at low-altitude so as to enable a gentle landing. Electrical signals sent along the multi-plexed power cables could activate solenoids to re-adjust the tilt of rotor blades and disengage power generation during the descent and landing sequence of a flying windmill battery.
In parts of Alaska, Siberia and Northern Canada, the wind speed is estimated at 45-feet/second at over 3000-feet above ground. Remote communities in these regions could use flying windmills to generate power during the winter months. Canada may have a potential over of 30,000,000-Kw of wind power at altitudes between 3000-feet and 5000-feet above ground. During a typical Northern Canadian or Alaskan winter, the air density at minus 40-degrees F is 0.09447-lb/cu.ft. At a wind speed of 44-feet/second (~30-miles per hour), an area that is 1000-feet wide by 3000-feet in height would have a power generation potential of just over 1000-Mw in wind energy. The potential would rise to over 8000-Mw in a wind blowing at 60-miles/hour (88-feet/second). (Equation used for power = 50% x air density x 1/gravity x cross section area x velocity cubed).
If the total swept area for a flying windmill battery covered 1,000,000-square feet, its power output at 25% efficiency would be 42.2-Mw of power (wind speed at 30-miles per hour) for an installation that could supply power to a community of some 8,500-homes. Some designs of wind turbines can operate at 35% efficiency in high winds and raise power output to 59-Mw, enough to supply power to a community of over 11,000-homes (5-kw per home). During the cold northern winter months, winds frequently exceed 30-miles per hour at elevations of over 3000-feet above ground and can exceed 60-miles per hour for prolonged durations in some regions. The power output could rise to 338-Mw at 25% efficiency and to 473-Mw at 35% efficiency.
As an alternative the bladed rotor, the flying windmill could also use a Magnus rotor that is transversely mounted into the upper control surface of a sharply curved wing. This approach would be derived from the British "paddle wheel" propelled model aircraft. Air that flows over the upper surface of the wing-kite would spin the Magnus rotor and activate the boundary layer effect. The boundary layer effect could re-direct the airstream on the upper surface through an extreme "angle of attack". The moving smooth surface of the spinning Magnus rotor would prevent the separation of air from wing that would otherwise cause a stall on the wing of conventional aircraft.
The combination of the Magnus rotors and specialized airfoils could generate sufficient lift to carry the weight of power generation equipment. In addition, the hollow spinning Magnus rotors would produce a gyroscopic effect around the transverse axis and provide a measure of stability. The electrical generation equipment may be driven by a separate Magnus rotor that may also be transversely mounted and near the rear of the under side of the wing. There are a variety of arrangements as to how additional Magnus rotors that spin on a transverse axis may be installed below the wing. The main wing and additional kites mounted below it may alternatively carry vertical-axis counter-rotating Magnus rotors to generate power.
Lightweight, counter-rotating Magnus rotors that spin on a vertical-axis could be designed to mimic the flying behavior of an array of box kites so as to provide additional lift while generating power. The main advantage of using Magnus rotors is that they are quite harmless to birds. The helical-blade, vertical-axis wind turbine can become more bird-friendly if the surface that travels against the wind is shielded by a kite, an airfoil or by a wind deflector that directs the airstream on to the blades' working surfaces. Such turbines may be mounted under the lift-enhanced wing that contains the Magnus rotor.
Suitable locations for batteries of flying windmills would be perpetually windy areas that are fat away from population centres. There are several suitable regions in Northern Canada that include Nunavut, the mountainous regions of British Columbia, the mountainous regions of Newfoundland and Labrador, the highland regions of Northern Quebec, much of Northern Ontario, Northern Manitoba and Northern Saskatchewan. There are many other suitable regions around the world where flying windmills could be used and where they would contribute to the amount of electric power that may be generated there in the future.
The powerful Northeast Trade winds blow over the Lesser Antilles in the Eastern Caribbean. Batteries of flying windmills may have to land and take-off from the west coasts of the islands of the Lesser Antilles. These island nations lack available space on land. Powerful Southeast Trade winds blow on to Brazil's sparsely populated coast between Natal and Salvador. Strong winds called Westerlies blow over the high altitudes and sparsely populated southern regions of Chile and Argentina. A range of suitable locations exists around the world where batteries of flying windmills could be installed to generate electric power.
Perhaps the main conclusion that can be drawn about batteries of flying windmills is that it is a concept that is open to further research and development. It does offer the promise of reducing the cost of installing multiple wind turbines. It offers the potential to economize on land area as far as generating large amounts of power is concerned. The capital cost of wind power generation is high when compared to the amount of power being produced. These costs need to be reduced while the power output needs to be raised. The flying windmill battery of multiple wind turbines is one possibility in that direction in that it could be developed further over the long-term future to improve the cost to power ratio for wind power generation.
The main concern about flying windmills is the altitude at which they can be flown (between 15,000-feet to 35,000-feet). There are efficiency gains to be had from flying windmills at such altitudes provided they fly well away from commercial flight paths. When they are flown at lower altitudes, the risk of collisions between large commercial passenger aircraft is greatly reduced.