This invention relates to helicopters, and more specifically to helicopters for achieving high speed flight.
In conventional helicopters, lift propulsion and control are provided by one or more rotating airfoils or rotors blades. The rotational plane of the rotors blades is substantially horizontal. In forward flight, the velocity of air flowing over the rotor airfoils differs between the left and right sides of the helicopter; because the airfoils on one side are advancing in the direction of flight and on the other side the airfoils are retreating. Consequently, the maximum air speed of the advancing rotor blade at its tip is the sum of the tip speed due to rotation and the forward speed of the helicopter. Similarly, the minimum speed of the retreating rotor blade on the other side of the helicopter is the difference between the tip speed of rotation and the forward speed of the helicopter. Consequently, the air speed of the advancing rotor blade is greater than that of the retreating rotor blade by twice the forward air speed of the helicopter.
This difference in the velocity of air flow over the advancing and retreating rotor blades is the factor which limits the maximum air speed of a helicopter, primarily because, as the helicopter's forward air speed increases, the air flow over the retreating airfoil or rotor blade becomes too low to support the required weight; and the retreating airfoil stalls. On the advancing side, the blade tip approaches supersonic speed; and the resulting compression of the air against the blade (compressibility effect) can cause blade flutter and greatly increased power requirements as well as creating unstable operating conditions of the helicopter. As a consequence, the maximum forward speed of a conventional helicopter is limited because of the effects of retreating blade stall and advancing blade compressibility effects. Without auxiliary lift or propulsion devices in the helicopter, a practical maximum speed for a conventional helicopter is approximately 250 miles per hour.
Another problem which is encountered in conventional helicopters, and which is increasingly noticeable at high speed, is that the helicopter tends to roll to one side, the retreating side, due to unequal lift of the rotor blades on the advancing and retreating sides. To compensate for this tendency and to maintain stable flight at relatively high forward speeds, attempts have been made in the past to compensate for this tendency by equalizing the lift on opposite sides of the rotor hub (that is on the advancing and retreating sides). To some extent this can be accomplished by raising the pitch of the retreating blades while simultaneously lowering the pitch of the advancing blades with a swash plate mechanism. A problem arises, however, if the blades are already operating at pitch angles near their stall limits for hovering efficiency. In this situation, the procedure of increasing the pitch of the retreating blades can result in the stalling of the retreating blades significantly decreasing their lift capability instead of increasing the lift. Compromises therefore must be made between the rotor efficiency of the helicopter and the rotational speed of the rotor. At higher speeds, the rotor blades can operate at a lower average pitch angle to accommodate the asymmetry of lift.
In the past, three methods for avoiding the stalling of the retreating rotor blades in high speed forward flight have been tried. One of these methods is the "advancing blade concept" which consists of the utilization of two coaxial counterrotating rotor hubs. Rotor airfoils or rotor blades then are mounted rigidly on these respective hubs so that there are always advancing blades on both sides of the helicopter. As a consequence, the retreating blades do not need to provide any lift to keep the helicopter in balance. Helicopters employing this advancing blade concept have been constructed and successfully operated, but these helicopters are very expensive. In addition, the blades must be extremely rigid because large moments must be transferred across the two counterrotating hubs.
A second method is an "X-wing aircraft". This type of aircraft has a substantially horizontally mounted multiple blade rotor which is stopped in flight. Propulsion then is achieved by a conventional propeller to either pull or push the aircraft through the air, and the rotor blades become the fixed wings of the aircraft. Theoretically, an aircraft of this type would be the ideal aircraft since the rotor could be rotated to provide lift, but is not required to provide forward propulsion. In practice, however, an aircraft of this type is also very expensive; because the horizontal rotor blades must be rigid enough to support the aircraft without the benefit of centrifugal force. This means that the blades must be large and rigid. In addition, the control system and the hub are very complex. Consequently, such aircraft have not achieved any degreee of practical acceptance.
A third method for avoiding the retreating blade stall of helicopter airfoils or rotors is the "tilt rotor" type of aircraft. This aircraft essentially comprises a conventional airplane in which the two wing-mounted propellors are rotated, so that the axis of rotation is vertical for take off. The propellors then are slowly rotated into a conventional horizontal position for high speed flight. The tilt rotor, however, also is expensive due to mechanical complexity and the necessity of cross-linking the two propellors in case of engine failure, since a failure of one of the two propellors could be catastrophic.
Some systems have been developed in the past to compensate for the difference in lift between the advancing and retreating blades of a helicopter to minimize or offset the tendency of this lift differential to cause the helicopter to roll over toward the retreating blade side. One system, directed to a resolution of this problem, is disclosed in the patent to Garfinkle #3,921,939. In Garfinkle, the single rotor hub is mounted on a pair of transverse slide rods; so that the lateral position of the hub may be moved toward the retreating blade side. This shifts the center of lift of the rotor system toward the retreating blade side of the helicopter fuselage. There is no speed advantage which is achieved by this shifting, however, because the hub itself does not create any moment on the shaft of rotation. It is desirable to provide a helicopter or rotary wing aircraft capable of attaining flight speeds or air speeds substantially higher than have been attainable by such aircraft in the past. In addition, it is desirable to provide a high speed helicopter in which the retreating blades are unloaded while maintaining control of all six degrees of freedom of the helicopter (roll, pitch, yaw, up-down, left-right, fore-aft) by using a combination of forces and moments created solely from a single main multiple blade rotor.