Rotary wing aircraft such as helicopters have found many application due to the vertical flight and hovering capabilities of such craft.  These capabilities are achieved through the use of rotary wings, i.e., rotor blades having an airfoil cross-section.  As used herein, the term "airfoil" refers to shapes capable of generating lift due to airflow thereabouts from a leading to a trailing edge.  Rotary wing aircraft are thus capable of generating lift even in vertical flight or while hovering because the rotary motion causes airflow about the surfaces of the rotary wings. 
A disadvantage of conventional rotary wing aircraft, i.e., helicopters employing a single main rotor blade assembly in their principal lift generating systems, is that such aircraft generally employ a heavy and power consuming tail rotor for torque compensation and yaw control.  Torque is exerted on conventional rotary wing aircraft due to the rotation of the main rotor blade assembly which would result in rotation of the aircraft body if not counteracted.  Typically, this torque is counteracted by use of a tail rotor which generates a torque equal but opposite to that of the main rotor blade assembly.  The pitch of the tail rotor blades may also be adjustable to vary the torque generated by the tail rotor thereby providing helicopter yaw control.  Thus, in conventional helicopters, a significant amount of power and weight is dedicated to the tail rotor for torque compensation and yaw control.  This extra mechanism makes it far more difficult to fly and to maintain.  This also makes the aircraft more unsafe not only because its harder to fly, but also because the spinning tail rotor could strike someone or something. 
Another disadvantage of conventional rotary wing aircraft is the inefficiency and complexity of forward flight relative to fixed wing aircraft.  In conventional rotary wing aircraft, a forward thrust is provided by angling the main rotor blade assembly relative to vertical so that a component of the force generated by the assembly is directed forward.  By contrast, in a fixed wing aircraft, substantially all of the force generated by a propulsion assembly, such as a propeller or a jet engine, may be directed to provide a forward thrust. 
In addition, conventional rotary wing aircraft generally employ a cyclical pitch control assembly to compensate for varying relative air speeds experienced by the rotor blades in forward flight.  In rotation the rotor blade has an advancing portion, where the blade is rotating into the "wind" resulting from forward movement of the aircraft, and a retreating portion where the blade is rotating away from the wind.  The speed of air relative to a rotor blade section and the force generated by the section in forward flight depends in part upon two components: the speed of forward flight and the speed of the rotor blade section due to rotation of the rotor assembly.  As can be understood, these components will be generally additive during the advancing portion of a rotation and generally subtractive at the retreating portion.  A complicated cyclical pitch assembly is generally employed in conventional rotary wing aircraft to vary the pitch of the rotor blade over a rotation cycle so that a substantially symmetrical lift and thrust distribution results.  To facilitate forward flight, the rotor of conventional rotary wing aircraft is therefore complex in design and operation and generally inefficient in comparison with fixed wing aircraft.  Another disadvantage is that it is very difficult to use a propulsion launched parachute to afford a soft landing, in an emergency, for pilot, crew and aircraft. 
Thus, it would be advantageous if the positive attributes of fixed wing and rotary wing aircraft could be combined.  Desirably, such a rotary wing aircraft could combine the hovering and vertical flight capability of rotary wing aircraft with the efficiency and simplicity of fixed wing aircraft in forward flight.  Additionally, such a craft could preferably eliminate the need for a tail rotor to compensate for rotary wing torque thereby enhancing aircraft weight and power efficiency.  Also great advantages would be attained if the aircraft had gyro stability, in that it would be a smoother, safer, flying platform.  Further efficiencies would result if such a craft were provided with a fixed wing capable of generating lift in both forward and vertical flight.  Finally a further and most important advantage would be attained if this aircraft could have the redundancy of two drive mechanisms and power plants either of which could safely fly the aircraft if one system failed; could be landed by auto-rotation or by use of the fixed wing and further if all systems failed could be landed safely by the use of a parachute (aircraft, occupants, and all). 
According to the present invention there is provided a helicopter main rotor, comprising:
a hub having an axis of rotation; PA1 a plurality of rotor blades radiating from the hub; and PA1 a circular airfoil mounted on the rotor blades, coaxially with the hub, the airfoil having a chord of up to 20% of the rotor radius and a thickness about 15% of the airfoil chord. 
The circular airfoil (CA) may be attached to the rotor blades in line with or slightly down stream from and coaxial with the rotor blades and in a plane essentially parallel to the plane of the blades for capturing vortices produced in the wash of the rotor blades.  The CA shelters the rotor blades thus reducing air resistance while in forward flight.  The CA increases lift, acting as a fixed wing while in forward flight.  The CA ties the rotor blades together so they work together more like a disc.  This also allows the inter-rotor distance to be reduced, thus further enhancing the lifting efficiency of the counter rotating rotors; which because of the added flow, would, through the synergistic effect, increase the lift of the CA.  The CA further provides Gyro stabilization to the aircraft. 
The cross section of the airfoil preferably has a cambered configuration.  In the preferred configuration the shape is similar to that of the AEROBIE aerodynamic toy ring.  The cross section of the airfoil is uniform all around the circle, i.e. the chord will be the same all around the circular airfoil and will give equal lift no matter what radial direction the edge of the circular airfoil is moving.  In addition to the preferred cross sectional shape mentioned above, the airfoil could have a shape like an NACA 00012 or NACA 23015 i.e. fairly thin with little wind resistance.  In other embodiments it could have a thicker cross section for slower speed, very stable operation. 
In operation, the downwash produced by the main rotor is directed through the circular opening in the airfoil, capturing the vortices at the tips of the rotors.  This stops the interference caused by the air re-circulating from down below the rotors and short circuiting back up to the top of the rotors.  This allows more air to be drawn across the circular airfoil, causing additional lift, and moving down through the center of circular airfoil, displacing more air and enabling the rotors to also be more efficient & giving more lift, less vibration and less noise.  Additional air is drawn across the upper surface of said airfoil as well as from the region adjacent to the downstream surface of the airfoil to reduce boundary layer build up, thus enhancing the lift of the airfoil. 
The rotor structure provides gyro stabilization and equal lift in all flight directions.  This permits flat turns versus normal banking turns, which further simplifies the operation of this aircraft. 
The CA, gives the aircraft the stability and versatility to operate in three modes and allows a change to an age old method of flying, i.e. flat turns and no throttle operation.  This in turn yields the following simplifications: flat turns versus needle and ball banking turns; one speed for normal operation of the rotors and a forward thrust propeller (i.e. a simple automatic control just like a cruise control).  In other words, for normal operation, it is not necessary to touch the throttle.  It is only necessary to use the pitch controls for the rotors and the propeller along with the rudders, and possibly the stick control at times for the flaperons as a trim control.  Both for flying and for maintenance the CA configuration is simpler.  None of the controls and equipment for a tail rotor are necessary, nor are the cyclical controls.  With the whole aircraft being a Gyro stabilized platform, the simplicity of just up, down, and turn, type of operation makes it a real possibility for simple auto-pilot operation. 
The CA equipped rotor provides reduced noise level compared to a conventional helicopter caused by interference of the rotor vortices and the high speed tail rotor vortices and also the noise caused by the chopper rotor blades cutting their own sound waves during high speed forward flight.  With this invention the rotor blades, during high speed forward flight, are either feathered or almost feathered.  Also the noise caused by cyclical operation is eliminated. 
An aircraft equipped according to the invention has a high speed capability greater than conventional helicopters.  It overcomes the standard problems with conventional helicopters: 1) unequal lift due to different airspeeds of the wind over the rotating rotors.  2) Drag due to high pitch angle of blades and due to the drag caused by the rotor tilted into the wind to give forward flight (versus flat level rotors with feathered blades), this is possible because of the circular airfoil acts like a fixed wing and gives the required lift, allowing the majority of the power to be used to drive the propeller for forward thrust. 
An aircraft equipped according to the invention may have twin engines and drive systems, and a projectile launched parachute system. 
The method of operation, gyro stabilized with flat turns and no need for throttle control, the circular airfoil which protects a projectile launched parachute system, and the almost zero need for use of the stick control make an aircraft equipped according to the invention a very safe airplane.  It has been stated by many experts that a large majority of gyroplane accidents have been caused by pilot induced oscillation.  This is also true of some helicopter accidents.  This is caused by over control on the stick.  Since, because of gyro stability and the new method of operation with its flat turns, there is little need to use the stick, which should eliminate these accidents.  Many other accidents are caused by stalling in banked turns and improper use of throttle control.  These two functions are unnecessary and therefore these accidents should be eliminated.  The dual engine and drive systems give enough redundancy to eliminate engine and drive system failure caused accidents.  The circular airfoil will stop a projectile launched parachute from being entangled or cut by the rotors.  Therefore in a last ditch, emergency situation, the pilot could launch a parachute that would give pilot, crew and aircraft a soft landing. 
A second CA may be used on a second, counter-rotating rotor.  This structure would be smaller and lighter than the structure on the main rotor.  When used the primary function would be to connect the blades in that rotor and make them function more like a disc.  It would also function in a similar fashion to that of the main rotor, but to a lesser extent, being smaller. 
The invention allows aircraft to be more versatile than the prior art.  An aircraft equipped according to the invention can function as a very easy to fly, easy to maintain helicopter; gyroplane and fixed wing airplane; in other words it is a convert-a-plane that can convert to any of the three cited modes while flying. 
The AEROBIE type airfoil profile, when attached to the rotor blades gives a balanced lift which allows the gyro stability to function in such a manner that the circular airfoil will not tend to precess and therefore will maintain a horizontal orientation, normally, without use of other trim controls.  Other airfoil profiles, without the spoiler, may be used to give more efficient lift.  To overcome the consequent reduced stability, the pilot would have to use the trim controls to maintain stable horizontal flight. 
The rotor drive system may allow the pilot to stop the rotor from being driven by either or both of the two engines.