A functional, preferably single rotor, hybrid aircraft capable of carrying a plurality of passengers with a substantial payload has not been successfully developed. The military, commercial and private benefits of a vehicle that can take off vertically, transition from low to high speed forward flight and back to low speed flight for vertical landing are well known.
There are two types of heavier than air aircraft that achieve lift by movement through the air: (1) The airplane, which has stationary wings that create lift when propelled through the air by a thrust mechanism such as a propeller or jet engine, and (2) The rotorcraft or rotary wing aircraft in which blades rotate to describe a disc above the aircraft to create lift.
There are three types of rotorcraft that utilize a blade to provide lift: (1) The helicopter, in which the rotor blade provides vertical thrust and, because the rotor disc can be tilted on a supporting and rotating vertical mast, a horizontal thrust component. (2) The autogyro, in which lift is provided by a rotary wing and forward thrust provided by a propeller or a jet. Autogyration is achieved by tilting the rotor disc back relative to the airflow so that some air flows up between the blades and through the rotor disc rather than down through the rotor disc as in a helicopter. As the air flows up through the rotor disc, the rotor is driven much like a windmill is driven by the wind. (3) The gyroplane, in which a rotor is used for vertical and slow speed flight, but at high speed cruise the rotor is unloaded (provides almost no lift) and the wing provides nearly all the lift.
High-speed vertical takeoff and landing aircraft, particularly the tilt-wing, tilt-rotor, stowed-rotor, and X-wing are well known in the aerospace industry. These concepts may be considerably different in design, yet attempt to achieve similar operational goals of vertical takeoff and landing, and high speed forward flight. Large congested cities as well as military operational sites lack nearby landing sites from which fixed wing aircraft can operate from. Conventional helicopters can operate from such sites, but are limited in forward speed and range. Helicopters have high hovering efficiencies due to their low-disk-loading, yet forward speed is limited to about 250 miles/hour. Speed is limited by the advancing (upwind) rotor blade tip speed, which cannot approach or exceed the speed of sound without incurring unacceptably high drag. Current vertical and short takeoff and landing (V/STOL) aircraft designs attempt to have both qualities; efficient hovering and high-speed forward flight. Demonstrated designs appear to be an unsatisfactory compromise between the two.
One V/STOL aircraft, the tilt-wing, uses large oversized conventional propellers driven by engines. These engines are attached to a wing that can be tilted from the horizontal position for forward flight, and to the vertical position for vertical takeoff and landing. The tilt-rotor, such as the test flown Bell/Boeing V22 Osprey, behaves similarly except that only the rotors and engines tilt; not the wing. These aircraft compromise both hovering efficiency, because of high-disk-loading, and forward speed. When the rotors are tilted forward for forward flight they become inefficient impellers above about 350 miles/hour.
Another concept known as the stowed-rotor behaves like a conventional helicopter for vertical takeoff and landing. For high speed forward flight the rotors are slowly stopped and stowed out of the way of the airstream to reduce drag, while a set of conventional fixed wing airfoils assume primary lift. Considerable complexity is involved in stopping and folding the rotors. These aircraft have hover efficiencies approaching a helicopter, yet require the additional weight of a fixed wing, as well as complexity and aerodynamic drag of a stowed rotor.
Still another concept known as the stopped rotor X-wing aircraft behaves like a conventional helicopter for vertical takeoff and landing, having low-disk-loading. To achieve high speed forward flight the four main rotor airfoils are slowly stopped and fixed in an "X" position in the horizontal plane, forming 45 degree swept wing angles (two airfoils are forward swept 45 degrees, and the other two airfoils are aft swept 45 degrees). The stopped rotor airfoils provide primary lift for forward flight, eliminating the need for additional fixed wings. Since two of the four main rotor airfoils are essentially flying backwards in the fixed wing position (relative to their rotary wing airfoil position), a complicated air circulation control system is required for each airfoil to achieve lift in both the rotary and fixed wing operation. This causes the airfoil leading edge to be identical to the trailing edge.
The rotor systems research aircraft X-wing (RSRA/X-wing), built by Sikorsky Aircraft Corporation, was never able to demonstrate rotary wing flight or transition from rotary wing to fixed wing flight, or vise versa. Complexity and number of the mechanisms associated with main rotor airfoil circulation control, and questionable reliability of successfully transitioning between rotary wing and fixed wing flight, and vise versa, caused the program to be abandoned.
The quest for faster rotorcraft has been ongoing ever since. One basic problem is that a rotor's lift is limited by the lift that can be produced by the retreating blade, since the aircraft will roll if the total lift moments on the advancing blade and retreating blade are not equal. At high aircraft forward speeds, the retreating blade tends to stall and lose lift, because the rotor RPM cannot be increased without the advancing blade tip going faster than the speed of sound. Because of this problem, the ratio of aircraft forward speed to rotor tip speed, known as Mu, is limited to about 0.5 in helicopters and autogyros.
To achieve the highest speed flight with a gyroplane or helicopter it is necessary to reduce rotor lift during horizontal flight, to reduce the problems with retreating blade stall. The English Frairey Rotodyne, which had a wing and tip jet autorotating rotor, used for take off and landing, set a closed course speed record for rotorcraft of 191 mph in 1959. The Russian KAMOV KA-22 broke this speed record in 1961 with a speed of 221 mph. The current record is approximately 250 MPH. All these aircraft reduce lift on the rotor by having some lift provided by a wing or by providing auxiliary thrust with a separate engine so that the rotor provides lift but no thrust. However, none of them exceed a Mu of 0.5.
The drag of a rotor blade increases with the cube of the rotation rate. Therefore, it is a great advantage if the rotation rate can be reduced. The ratio of aircraft forward speed to rotor tip speed, known as Mu, must be increased as much as possible, probably over 1.0. The challenge, then, is to maintain autorotation and rotor stability at high Mu.
Vibration, structural and aerodynamic barriers presently practically prevent the use of high speed rotary wings. The primary barrier to high speed helicopter flight is that the retreating rotor blade in high speed forward flight will stall because its effective airspeed approaches zero at a set rotational velocity. Conversely, the advancing rotor blade sees a higher airspeed. Because lift vanes as the square of airspeed, the lower airspeed on the retreating blade requires a larger pitch angle of attack than the advancing blade. As the retreating blade airspeed velocity vector sum approaches zero the pitch angle will approach an angle of attack where a blade stall will occur. One way to avoid these stall barriers is stopping the rotor blades in flight when a sufficient forward air speed has been obtained.
The challenge in stopping the rotor is similar to that of high speed flight. Sufficient forward speed of the rotor relative to the aircraft must be achieved and maintained to lift the aircraft as the rotor is stopped. The retreating blade will see a reduction in airspeed to zero and then the airflow will reverse before the rotor system stops. Because of this the control sequence for the retreating blade must also be reversed while the rotor system is rotating before it is stopped.
There will be a period of time when the retreating rotor blade cannot provide any control or lift to the aircraft because of its near zero relative airspeed. The range of vehicle airspeed in which the rotor blade cannot provide control or lift is bound by the retreating rotor blade stall speed, and its stall speed when the airspeed reverses as the net airspeed increases and the blade rotation slows. For control, the aerodynamic lift characteristics must be the same in both directions, therefore the blade must be effectively symmetrical in cross section at all times.
The aerodynamic requirements of high speed fixed rotor flight contrast with the aerodynamics of low speed rotary wing flight. During fixed rotor flight a given surface area is required at a given airspeed to avoid stalling. The slower the airspeed the larger the surface area required. It is advantageous to transition from fixed wing flight to rotational flight at low to moderate airspeeds for control and structural reasons. Therefore, a larger surface area is preferred. However, during rotational flight higher airspeed is typically seen over the rotor blades because of the added angular velocity. Thus a smaller surface area is sufficient to provide lift. Therefore, a larger surface area during rotational flight is detrimental to efficiency and control responsiveness.
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