Abstract:
A helicopter includes a main rotor guided and protected in minor impacts by a guide-ring. In selected embodiments, the rotor is powered by an electric motor employing a magnetic field generator carried by the main rotor and stator coils installed on the guide-ring. The helicopter&#39;s main rotor thus effectively functions as the rotor of the electric power plant of the helicopter. In selected embodiments, the rotor&#39;s blades are arranged so as to create an opening in the center of the rotor, allowing pilot ejection or deployment of a parachute capable of safely lowering the helicopter in case of an emergency.

Description:
REFERENCE TO RELATED PATENT DOCUMENTS 
   This application is related to Disclosure Document Number 540675, entitled T IP -D RIVEN  S HAFTLESS  R OTATING  W ING  A IRCRAFT  W ITH  G UARD -R ING,  filed on 24 Oct. 2003. The Disclosure Document is hereby incorporated by reference in its entirety, including all figures and matter incorporated in the Disclosure Document. This application claims priority benefit of provisional U.S. Patent Application Ser. No. 60/552,092, entitled ROTATING WING AIRCRAFT WITH TIP-DRIVEN ROTOR AND ROTOR GUIDE-RING, filed 10 Mar. 2004, which application is hereby incorporated by reference in its entirety, including all figures, tables, claims, and matter incorporated therein. 

   FIELD OF THE INVENTION 
   The present invention relates generally to rotating wing aircraft, and, more particularly, to helicopters, gyroplanes and other flying apparatus with tip-driven rotors and guide-rings. 
   BACKGROUND 
   Helicopter is a highly complex aircraft. It is said that a helicopter is two thousand parts flying together in one formation. This complexity contributes to the cost of production and operation and to the danger inherent in operating a helicopter. One of the factors contributing to the complexity of the helicopter design is the necessity to connect numerous control mechanisms, such as variable pitch angle control, collective control, etc., to the rotor blades via a rotating shaft. This complexity is further exacerbated by the flexible connection of the rotor blades to the shaft to allow for varying pitch angle and blade articulation necessary to compensate for lift deferential in the advancing and retreating phases of the cycle of rotation. The flexibility considerations dictated by the design of helicopter control mechanisms must be combined with structural strength dictated by the fact that the helicopter fuselage is supported in the air by the rotating rotor blades flexibly attached to the rotating shaft. These divergent factors greatly contribute to the complexity of design and operation and to the cost of production of helicopters. It would be desirable to provide a shaftless rotating wing aircraft or a rotating wing aircraft with a simplified or stationary main rotor shaft. 
   Helicopters are both dangerous and fragile. When on the ground, rotating rotor blades, which typically extend far beyond the fuselage of the aircraft, are akin to giant swinging swords creating a dangerous zone in the vicinity of the aircraft. Helicopters often operate at low altitudes and near trees, power lines, antennae, buildings, and other structures. Rescue helicopters often operate near mountain cliffs, even at high altitude. For well known reasons of basic physics, the main rotor of a helicopter is relatively large, typically extending beyond the helicopter&#39;s body in most horizontal directions. Therefore, objects in the immediate vicinity of an operating helicopter present danger to the helicopter: if the main rotor strikes an object, the rotor is likely to disintegrate, and the helicopter will probably crash. It would be preferable to provide a way to safeguard the rotor, at least in low-impact contacts with external objects. 
   Another problem with rotary wing aircraft is that rotor tips generate rather violent eddies of air. This turbulence increases power drag on the engine and the resulting parasitic losses, decreasing aircraft efficiency. The problem is aggravated at higher rotation speeds, as well as at higher cruising speeds, because the losses increase at higher air speeds of the rotor tips. It would be beneficial to provide a rotor with reduced tendency to generate eddies at its tips. 
   Yet another problem inherent in typical helicopter designs, and particularly in military helicopter designs, is that pilot ejection is impracticable: ejecting in the vertical direction obviously does not work because the pilot would be killed by the rotor. Helicopter pilots, however, are subject to manifold dangers on the battlefield, possibly even more so than pilots of fixed-wing military aircraft. It would be desirable to provide helicopters with pilot ejection capability. 
   Still another problem of helicopter design is related to the fact that linear velocity of a point on the rotor blade is proportional to the radius of rotation of the point, increasing towards the tip of the blade. Therefore, more lift is generated at the tips of the blades than near the rotor shaft. This causes the rotor blades to bend up at the tips during rotation, causing the disk of rotation to curve and assume a concave shape. This creates aerodynamic inefficiencies. It would be desirable to keep the blades horizontal and maintain a relatively flat disk of rotation to improve aerodynamic efficiency. It would also be desirable to have blades wider at the tips and narrower at locations closer to the center of rotation, to generate more lift. 
   Unlike airplanes, helicopters often operate at low airspeeds. Low airspeeds make possible, at least theoretically, deployment of a parachute in case of mechanical failure or another emergency that would otherwise result in a freefall of the aircraft. A parachute could thus safely lower a disabled helicopter to the ground—if only the parachute could be deployed. But much like the problem of ejecting a pilot from a disabled helicopter, the rotor is in the way of the parachute. It would be advantageous to enable parachute deployment in a helicopter. 
   Similar problems are pertinent to gyroplanes. As used by the Federal Aviation Administration (FAA), the term “gyroplane” refers to an aircraft that gets lift from a freely turning rotary wing (rotor blades), and which derives its thrust from an engine-driven propeller. Historically, this type of aircraft has also been known as the autogiro and the gyrocopter. 
   A need thus exists for rotary wing aircraft with a simplified design of the rotor shaft, or with shaftless design. Additional need exists for rotary wing aircraft that provide some protection to the rotor during impacts with external objects. A further need exists for rotary wing aircraft with reduced power losses due to eddies generated by rotor tips. Still another need exists to maintain rotor blades relatively straight during rotation and to avoid rotor curling up at the tips. A still further need exist to have rotor-blades wider at the tips than at the center of rotation. Another need exists for rotary wing aircraft with pilot ejection capability. Yet another need exists for helicopters that can deploy a parachute in an emergency. 
   SUMMARY 
   The present invention is directed to apparatus and methods that satisfy one or more of these needs. One aspect of the invention herein disclosed is embodied in a rotary wing aircraft with a fuselage, one or more rotor guide-rings (also known as guideways, guide-rails, and guard-rings) attached to the fuselage with a plurality of telescoping supports, one or more main rotors rotatably disposed in the guide-rings and guided by the guide-rings, and an inductive electrical main power plant driving the one or more main rotors from rotor tips. Various embodiments of the rotary wing aircraft in accordance with the invention include helicopters, gyroplanes, vertical take-off and landing aircraft, aerial tactical vehicles, and personal air transportation devices. Various embodiments of the rotary wing aircraft in accordance with the invention further include features on the following partial list:
         anti-torque tail rotor;   anti-torque tail rotor suspended in a second guide-ring;   tailless vector stealthy (TVS) propulsion;   propellers, jet, and turbine engines for facilitating forward motion and directional control;   transfer systems for transmitting power from one of the main rotors to the tail rotor or propeller;   duct systems for guiding the air pushed down by one of the main rotors to a nozzle used to control yaw; and   rotor center concentric guide-rings defining an opening used for ejecting the helicopter&#39;s pilot or for deploying a parachute capable of safely lowering the helicopter in an emergency.       

   
     BRIEF DESCIRPTION OF THE FIGURES 
       FIG. 1  illustrates a helicopter with a rotor guiding and protecting ring; 
       FIG. 2  illustrates a cross-section of the rotor guiding ring of the helicopter of  FIG. 1 ; 
       FIG. 3  illustrates top view of a rotor center portion of a helicopter in accordance with the present invention; 
       FIG. 4  illustrates side view of a rotor center portion of a helicopter in accordance with the present invention; 
       FIG. 5  illustrates a helicopter with a rear-mounted pusher propeller; 
       FIG. 6  illustrates a helicopter with twin coaxial counter-rotating rotors of varying diameter; 
       FIG. 7  illustrates a helicopter with an upper rotor disposed above the fuselage, and a lower counter-rotating rotor disposed below the fuselage; 
       FIG. 8  illustrates a helicopter with twin coaxial counter-rotating rotors of substantially equal diameter; 
       FIG. 9  illustrates a helicopter with yaw control provided by an anti-torque tail rotor; 
       FIG. 10  illustrates a helicopter with twin coaxial rotors of varying diameters, rotor guiding rings located by separate sets of supports, and an anti-torque tail rotor; 
       FIG. 11A  illustrates a helicopter with a rotor having six blades and a center ring defining an opening through which a pilot can be ejected or a parachute deployed in an emergency; 
       FIG. 11B  illustrates the helicopter of  FIG. 11A  with a deployed parachute; 
       FIG. 12  illustrates a helicopter with an anti-torque rotor and dual propulsion propellers in addition to a main rotor; 
       FIG. 13  illustrates a helicopter a helicopter with an S-shaped rotor within a guide-ring; 
       FIG. 14  is a side view of a helicopter rotor that acts as a rotor of an electric power plant of the helicopter; 
       FIG. 15  is a top view of a four-blade helicopter rotor that simultaneously acts as a rotor of a squirrel-cage induction electric power plant of the helicopter; 
       FIG. 16  illustrates a helicopter with a power transmission system that transfers power from the helicopter&#39;s main rotor to the tail rotor; 
       FIG. 17  illustrates a helicopter with an S-shaped rotor; and 
       FIG. 18  illustrates a helicopter with a guide-ring integrated with the helicopter&#39;s fuselage. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to several embodiments of the invention that are illustrated in the accompanying drawings. Wherever feasible and convenient, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts. The drawings are in a simplified form and not to precise scale. For purposes of convenience and clarity only, directional terms, such as top, bottom, left, right, up, down, over, above, below, beneath, rear, and front may be used with respect to the accompanying drawings. These and similar directional terms should not be strictly construed to limit the scope of the invention. In addition, words such as couple, connect, and similar terms with their inflectional morphemes are used interchangeably, unless the difference is noted or made otherwise clear from the context. These words and expressions do not necessarily signify direct connections, but include connections through mediate components and devices. 
     FIG. 1  illustrates a helicopter  100  in accordance with the present invention. The helicopter  100  includes a fuselage  110  attached to a rotor guide-ring  120  with a plurality of supports  130 . A main rotor  140  revolves within the ring  120 , which both guides and protects the main rotor  140  in some impacts. 
     FIG. 2  illustrates a cross-section of the ring  120  of the helicopter  100  of  FIG. 1 . It should be understood that different cross-sections are employed in alternative variants of the illustrated embodiment, and in other embodiments. 
   The lengths of the supports  130  determine the relative position of the ring  120  with respect to the fuselage  110 . Because the plane of the ring  120  is substantially the same as the plane of the disk of rotation of the main rotor  140 , the lengths of the supports  130  also determine the plane of the disk of the main rotor  140  and the axis of lift generated by rotation of the main rotor  140 . As a person skilled in the art would know, the relationship between the axis of the lift and the fuselage  110  affects the attitude of the helicopter  100  providing forward thrust and directional control. To change the attitude, the relative lengths of the supports  130  are varied. In the illustrated embodiment, the supports  130  are hydraulically-activated telescopic supports, controlled by the helicopter pilot from the cabin located within the fuselage  110 . In another embodiment, the pilot controls the lengths of the supports through electromechanical actuators. Other variable-length supports for the ring  120  can also be used for controlling the attitude of the helicopter. 
   The lift generated by the rotor  140  is transferred to the helicopter  100  through the tips of the rotor  140 , the guide-ring  120 , and the supports  130 . To facilitate rotation of the rotor  140 , its tips include rollers traveling on the inner surface  121  of the guide-ring  120  akin to a monorail. A sample inner surface  121  is illustrated in  FIG. 2 . Note also lips  122  formed on the inner surface  121 ; the lips  122  prevent the rollers and the tips of the rotor  140  from disengaging from the ring  120 . In one alternative embodiment, the inner surface  121  and the tips of the rotor  140  are covered by a low-friction finish, such as polytetraflouroethylene, also known as PTFE or by trade name Teflon®. 
   In another alternative embodiment, the tips of rotor  140  are suspended between the lips  122  supported by the magnetic field (magnetic levitation or “maglev”) using, for example, a form of linear induction motor. 
   Several approaches to magnetic levitation systems may be used in accordance with the present invention. Two of the approaches are briefly discussed herein. The first approach, called electromagnetic suspension (EMS), uses conventional electromagnets. It is employed by German Transrapid® magnetic levitation trains, and in Spanish-designed Maglift™ Monorail. The second approach, called electrodynamic suspension (EDS), is employed to levitate trains. In known applications, the approach uses the opposing forces between superconducting magnets on a train and electrically conductive strips or coils in a guideway to levitate the train. This approach is used in maglev trains in Miyazaki, Japan, developed by the Railway Technical Research. 
   Magnetic support of a rotating member is also illustrated and described in Horton, U.S. Pat. No. 4,807,830, which is hereby incorporated by reference in its entirety, including all figures, tables, and claims. 
   Persons skilled in the art may also find the following documents useful in understanding and designing magnetic levitation systems: 
   U.S. Pat. No. 6,816,052, entitled Track litz rungs and shorting bar design for urban maglev inductrack and method for making the same; 
   U.S. Pat. No. 6,777,833, entitled Magnetic levitation stage apparatus and method; 
   U.S. Pat. No. 6,629,503, entitled Inductrack configuration; 
   U.S. Pat. No. 6,450,103, entitled Monorail system; 
   U.S. Pat. No. 6,357,359, entitled Integrated high speed maglev system utilizing an active lift; 
   U.S. Pat. No. 6,085,663, entitled System and method for magnetic levitation guideway emplacement on conventional railroad line installations; 
   U.S. Pat. No. 6,057,620, entitled Geometrical structure configuration of maglev forces in a maglev rotational bearing apparatus; 
   U.S. Pat. No. 6,044,770, entitled Integrated high speed MAGLEV system; 
   U.S. Pat. No. 5,953,996, entitled System and method for magnetic levitation guideway emplacement on conventional railroad line installations; 
   U.S. Pat. No. 5,862,028, entitled Method and apparatus for detecting quenching of a coil for a superconducting magnet; 
   U.S. Pat. No. 5,668,090, entitled High-temperature AC superconducting magnets for a magnetic levitation system; 
   U.S. Pat. No. 5,602,430, entitled Superconducting electromagnet arrangement for a magnetic levitation system; 
   U.S. Pat. No. 5,560,589, entitled Active vibration damping arrangement for transportation vehicles; 
   U.S. Pat. No. 5,479,145, entitled Superconducting electromagnet for levitation and propulsion of a maglev vehicle; 
   U.S. Pat. No. 5,448,213, entitled Electromagnetic shielding concept for superconducting levitating magnets; 
   U.S. Pat. No. 5,445,347, entitled Automated wireless preventive maintenance monitoring system for magnetic levitation (MAGLEV) trains and other vehicles; 
   U.S. Pat. No. 5,433,149, entitled Composite reaction member for lateral guidance and levitation control of parellel electrically conductive and ferromagnetic strips; 
   U.S. Pat. No. 5,388,527, entitled Multiple magnet positioning apparatus for magnetic levitation vehicles; 
   U.S. Pat. No. 5,249,529, entitled Self-nulling hybred MAGLEV suspension; 
   U.S. Pat. No. 5,215,015, entitled Track system and vehicle having both magnetic and aerodynamic levitation, with wings on the vehicle carrying the whole weight at normal operating speeds; 
   U.S. Pat. No. 5,170,715, entitled Aeromagnetic control of maglev vehicles with turntable mounted hinged control surface having two degrees of motion; 
   U.S. Pat. No. 5,146,566, entitled Input/output system for computer user interface using magnetic levitation; 
   U.S. Pat. No. 5,052,309, entitled Track carrier for a high speed magnetic levitation transport system; and 
   U.S. Pat. No. 4,709,883, entitled Launch and ascent system. 
   In various embodiments of rotary wing aircraft in accordance with the present invention, both EMS and EDS systems use a traveling magnetic wave along the circular guideway within the guide-ring  120  to propel the rotor  140  while the rotor is suspended between the lips  122  of the guide-ring  120 . 
   The helicopter  100  is illustrated without an anti-torque tail rotor. Horizontal control of the helicopter  100  can be achieved by providing an air intake at the top of the fuselage  110  and an internal ducting system that channels the air pushed down by the main rotor  140  into a nozzle at the aft portion of the fuselage  110 . In one variant of the embodiment  100 , the air is directed to a fixed nozzle so as to counteract the torque exerted on the fuselage  110  by the motor that causes the main rotor  140  to rotate. In this variant, the helicopter&#39;s yaw is controlled by a valve regulating the volume of air that can exit through the nozzle. In another variant, the nozzle is movable, so that the direction of the exiting air can be controlled by pointing the nozzle through a control provided in the cabin. 
   Top and side views of center portion of the main rotor  140  are illustrated in  FIGS. 3 and 4 , respectively. As can be seen in these Figures, blades  141  and  142  are flexibly coupled by a joint  143 , in order to allow for articulation of the main rotor  140  during flight. 
     FIG. 5  illustrates a helicopter  500  in accordance with the present invention. The embodiment of  FIG. 5  includes a fuselage  510 , a guiding and protecting ring  520 , telescoping supports  530 , and a rotor  540 ; these components are similar or identical to corresponding and similarly-numbered components illustrated in  FIG. 1 . An additional aspect of the invention is a rear-mounted pusher propeller  545  that facilitates or causes forward propulsion of the helicopter  500 . In some variants of this embodiment, the propeller  545  is powered by a separate power plant, for example, one or more electric motors, reciprocating internal combustion engines, or turbines. Propulsion of the helicopter  500  may be caused by the pusher propeller  545 , by a horizontal component of the lift generated by the rotor  540 , or by a combination of these propulsion sources. The pusher propeller  545  may also be used to counteract the torque of the rotor  540 . In alternative embodiments, one or more jet engines cause forward propulsion of a helicopter. 
     FIG. 6  illustrates another helicopter,  600 , in accordance with the present invention. The helicopter  600  is also similar to the embodiment of  FIG. 1 , but includes dual guide-rings  620 A and  620 B that guide and protect twin, substantially coaxial rotors  640 A and  640 B. The guide-ring  620 B is attached to a fuselage  610  by a set of guide-ring supports  630 B; the guide-ring  620 A is attached to the guide-ring  620 B by a set of telescoping supports  630 A. The rotors  640 A and  640 B are counter-rotating. They substantially cancel the torque, i.e., a tendency of the helicopter  600  to rotate about its vertical axis, and obviate the need for an anti-torque tail rotor or other means for countering torque-caused changes in yaw. As illustrated, diameter of the rotor  640 A is slightly larger than the diameter of the rotor  640 B. In some variants, the two coaxial rotors and the corresponding guide-rings have the same diameters. In other variants, the upper rotor is smaller than the lower rotor. 
   Helicopter  700  illustrated in  FIG. 7  is similar in concept to the helicopter  600 , with two counter-rotating rotors  740 A and  740 B canceling the tendency of the helicopter to rotate about its vertical axis. In this embodiment, however, a lower guide-ring  720 B, which guides and protects the rotor  740 B, is disposed below a fuselage  710  and is attached to the fuselage  710  by a set of lower supports  730 B. An upper guide-ring  720 A is attached to the fuselage  710  by a set of upper supports  730 A. 
     FIG. 8  illustrates a helicopter  800  with twin, substantially coaxial rotors  840 A and  840 B. The twin rotors  840  have substantially equal diameters and are guided and protected by guide-rings  820 A and  820 B. Telescoping supports  830 B attach the two guide-rings to each other. Guide-ring supports  830 A locate the lower guide-ring  820 B. In an alternative embodiment, the supports  830 B are fixed, while the supports  830 A are hydraulically telescoping. In this alternative embodiment, the supports  830 A tilt the assembly formed by the rings  820 , the rotors  840 , and the supports  830 B as a single unit. A tail boom  855  and a vertical stabilizer  856 , which is disposed at the end of the tail boom  855 , enhance horizontal stability of the helicopter  800 . 
   In the helicopters in accordance with the present invention horizontal (yaw) control can be realized through a conventional anti-torque tail rotor.  FIG. 9  illustrates a helicopter  900  with a single main rotor  940  within a guide-ring  920 , located by a set of supports  930 . For yaw control, an anti-torque tail rotor  957  is disposed at the tail boom  955 . In a variant of this embodiment, the tail rotor  957  is powered by a separate motor, for example, an electrical motor. In other variants of the tail-rotor design, the main power plant of the helicopter  900  also drives the tail rotor  957 . One such variant is illustrated in  FIG. 16 , which is discussed further below. In some embodiment in accordance with the present invention, the tail rotor is also disposed in a guide-ring, similarly to the main rotor. 
     FIG. 10  illustrates a helicopter  1000  with coaxial rings  1020 A and  1020 B that guide and protect substantially coaxial main rotors  1040 A and  1040 B. The helicopter  1000  also includes an auxiliary anti-torque tail rotor  1057 , which allows the helicopter  1000  to operate with the main rotors  1040  rotating in either the same direction or in opposite directions. In this embodiment, the rings  1040 A and  1040 B are attached to a fuselage  1010  by separate sets of supports  1030 A and  1030 B, respectively. 
   Rotor design need not be limited to any particular number of blades, such as two-blade designs illustrated up to this point. Moreover, rotor blades need not pass through the center of the guide-ring within which the rotor rotates.  FIG. 11A  illustrates a helicopter  1100  with a rotor  1140  that includes six blades attached to a center ring  1144 . Otherwise similar to the helicopter  900  of  FIG. 9 , the helicopter  1100  is capable of receiving pilot ejection equipment or a parachute that can safely lower the helicopter  1100  in case of emergency. Both the ejection equipment and the parachute can be deployed through the opening created by the center ring  1144  of the rotor  1140 .  FIG. 11B  illustrates the helicopter  1100  with a deployed parachute  1160 . 
   An aircraft in accordance with the present invention may include both an anti-torque rotor and a rotor/propeller used for propulsion. One such aircraft is illustrated in  FIG. 12 , which shows a helicopter  1200  with an anti-torque tail rotor  1257  and dual propellers  1245 A and  1245 B used for propulsion. 
   Various shapes of the main rotor also fall within the scope of the invention.  FIG. 13  illustrates a helicopter  1300  with an S-shaped rotor  1340  within a guide-ring  1320  attached to a fuselage  1310  by four telescoping supports  1330 . 
   Power to the main rotor of a helicopter in accordance with the present invention can be provided in several ways. In certain embodiments, a helicopter&#39;s main rotor is powered by an electric motor. For example, the main rotor can also act as a rotor of an electric motor, while the guide-ring acts as a stator of the electric motor. Many different electric motors are known in the art. Some aspects of electrical motors are described below. 
     FIG. 14  is a greatly simplified illustration of a side view of a rotor  1400  that acts as both a helicopter&#39;s main rotor and a rotor of a synchronous alternating current (AC) electric motor. The rotor  1400  includes two blades,  1401 A and  1401 B coupled to each other at the center of the rotor  1400 . Inside the blades  1401  reside small direct current (DC) sources of electric energy  1404 A and  1404 B for energizing windings  1405  and creating the field circuit used in the operation of the electric motor. As illustrated, the windings  1405  are partially embedded inside the blades  1401  and pass through the coupling  1403 . Alternatively, the windings  1405  can be either completely embedded inside the blades  1401 , or be substantially outside the blades  1401  and the flexible joint  1403 . In still other embodiments, the rotor  1400  carries permanent magnets on its blades  1401  to create the DC field used in the operation of the electric motor that is the main power plant of the helicopter. 
   In another embodiment, the electric motor is a squirrel-cage induction motor. Top view of a four-blade rotor  1500  of this embodiment is illustrated in  FIG. 15 . Each of the rotor blades, designated by reference characters  1501 A-D, is located by a center ring  1506  and an outer ring  1507 . Rotor windings are not needed in this embodiment, because the rotor&#39;s field is induced in embedded conductors  1508 , which move in the field generated by the stator coil of the electric motor. Note that only cross-sections of the embedded conductors  1508  appear in  FIG. 15 , because the conductors  1508  are disposed substantially perpendicularly to the plane in which the rotor  1500  rotates. 
   The stator of the electric motor—be it an AC, DC, synchronous, induction, single phase, multiphase, or another kind of electric motor—may be included in the guiding and protecting ring surrounding the rotor. In one embodiment, several coils are built into the ring, with stator windings being disposed substantially perpendicularly to the plane in which the rotor rotates. The power to the stator windings is provided by a main energy source on board the helicopter, through the telescopic supports that hold the stator/guide-ring. In particular helicopter embodiments in accordance with the invention, the electric energy for the helicopter power plant is provided from batteries, fuel cells, solar cells, primary cells, secondary cells, capacitors or any combination thereof. When the power plant is AC-powered, the helicopter may include a DC-to-AC converter in order to adapt the DC power stored in a cell to the power plant requirements. 
   In some embodiments, electromagnets are disposed circumferentially in the guide-ring and a linear induction motor is formed by the main rotor and the guide-ring. The main rotor is magnetically levitating in the guide-ring supported by electromagnetic suspension forces generated by the electromagnets. A traveling magnetic wave generated by the linear induction motor and the electromagnets propels (rotates) the rotor inside the guide-ring, producing lift. 
   In some embodiments, electrically conductive strips or electromagnetic coils are disposed in the guide-ring, and superconducting magnets are disposed at the tips of the rotor. A linear induction motor is formed by the main rotor and the guide-ring. The main rotor is magnetically levitating in the guide-ring supported by electrodynamic suspension generated by opposing forces between the superconducting magnets and the electrically conductive strips (or coils). A traveling magnetic wave generated by the linear induction motor propels (rotates) the rotor inside the guide-ring, producing lift. 
   Persons skilled in the art may find the following documents useful in understanding and designing electric motors: 
   U.S. Pat. No. 6,800,973, entitled Stator winding connection arrangement for electric motor; 
   U.S. Pat. No. 6,598,693, entitled Electric motor-driven vehicle; 
   U.S. Pat. No. 6,504,275, entitled Electric motor having rotor-embedded sensor; 
   U.S. Pat. No. 6,147,423, entitled Electric motor having improved rotor assembly, and method by which the rotor assembly is made; 
   U.S. Pat. No. 5,808,387, entitled Electric motor for an electric vehicle; 
   U.S. Pat. No. 5,548,167, entitled Inductive linear electric motor; 
   U.S. Pat. No. 5,117,138, entitled Stator for an electric motor and motor equipped therewith; 
   U.S. Pat. No. 5,013,990, entitled Energy conserving electric motor power control method and apparatus; 
   U.S. Pat. No. 4,928,051, entitled Electric motor; 
   U.S. Pat. No. 4,774,448, entitled Reversible variable-speed 2-phase electric motor; 
   U.S. Pat. No. 4,503,346, Electric motor having a stator iron of a bent shape; 
   U.S. Pat. No. 4,467,231, entitled Permanent-magnet d.c. electric motor with resilient stator yoke 
   U.S. Pat. No. 4,291,248, entitled Electric motor; 
   U.S. Pat. No. 4,281,263, entitled Hybrid stator and a two-axis induction electric motor constructed therewith; 
   U.S. Pat. No. 4,194,178, entitled Electric motor with internal wireless load monitor; 
   U.S. Pat. No. 4,114,060, entitled Electric motor; 
   U.S. Pat. No. 4,081,726, entitled Electric motor; 
   U.S. Pat. No. 4,081,724, entitled Variable reluctance electric motor; and 
   U.S. Pat. No. 4,063,135, entitled Electric motor having controlled magnetic flux density. 
   All of the above-listed patent documents are hereby incorporated by reference in their entireties, including all figures, tables, and claims. 
   Additional guidance regarding electric motors may be found in Irving M. Gottlieb, Practical Electric Motor Handbook (Buttrerworth-Heinemann 1997) (ISBN 0 7506 3638 2), which book is incorporated by reference in its entirety, including all figures and tables. 
   As described in this document, a main rotor of a helicopter can be driven from the rotor&#39;s perimeter or blade tips, such as in the cases of the electric-powered rotors of  FIGS. 14 , and  15 . (By driven from the rotor&#39;s perimeter or driven from the rotor&#39;s tips I mean that the power/torque is delivered to the rotor not from the center of the rotor by a central shaft connected to a motor, but from point or points closer to the tips of the rotor&#39;s blades.) If the helicopter is equipped with an anti-torque tail rotor or a propeller used for forward motion, the tail rotor or the propeller may be powered by a second, independent power plant, as has already been discussed. Alternatively, the tail rotor or propeller may be powered by the main power plant, for example, through a power transmission system of shafts and gears that transfers the power from the main rotor to the tail rotor or propeller.  FIG. 16  illustrates a helicopter  1600  using one such arrangement. 
   The helicopter  1600  is similar to the helicopter  900  illustrated in  FIG. 9 . Here, however, a power-tapping shaft  1670  forms part of the system for transferring power to a tail rotor  1657 . A main rotor  1640  and a guide-ring  1620  form, respectively, rotor and stator of an electric power plant of the helicopter  1600 . The shaft  1670  connects to a joint  1643  at the center of the rotor  1640 . When the rotor  1640  turns under influence of the electromagnetic forces generated between the stator/guide-ring  1620  and windings or permanent magnets of the rotor  1640 , the shaft  1670  turns together with it. At its lower end, the shaft  1670  is attached to a transverse gear set that changes the direction of the power flow, directing the power flow towards the rear of the helicopter  1600 . In some variants of the embodiment illustrated in  FIG. 16 , the gear set also changes the rotational velocity of its output shaft, scaling the rotational velocity of the main rotor to the rotational velocity requirements of the tail rotor. 
   So far, main rotor blades have been illustrated as relatively straight elements. This need not always be the case.  FIG. 17  illustrates this point by showing a helicopter  1700  with an S-shaped main rotor. 
   The guide-rings need not be structurally separate from the helicopter fuselage. In some embodiments, one or more guide-rings are integral with the fuselage. For example, a fuselage may have a cylindrical duct, with a rotor guide-ring situated on a cross-section of the duct. A helicopter  1800  with this arrangement is illustrated in  FIG. 18 . 
   This document describes in some detail the inventive rotary wing aircraft and methods for powering them. This was done for illustration purposes only. Neither the specific embodiments of the invention as a whole, nor those of its features limit the general principles underlying the invention. In particular, the invention is not necessarily limited to specific arrangements of rotors, to the number of the rotors employed, to particular power plants, or to the use or non-use of anti-torque tail rotors, propellers, or flexible rotor joints. The specific features described herein may be used in some embodiments, but not in others, without departure from the spirit and scope of the invention as set forth. Many additional modifications are intended in the foregoing disclosure, and it will be appreciated by those of ordinary skill in the art that in some instances some features of the invention will be employed in the absence of a corresponding use of other features. The illustrative examples therefore do not necessarily define the metes and bounds of the invention and the legal protections afforded the invention.