Aeronautics and astronautics – Aircraft – heavier-than-air – Airplane and helicopter sustained
Reexamination Certificate
2000-12-18
2002-05-07
Poon, Peter M. (Department: 3643)
Aeronautics and astronautics
Aircraft, heavier-than-air
Airplane and helicopter sustained
Reexamination Certificate
active
06382556
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to vertical takeoff and landing (VTOL) airplane. More specifically, it pertains to a type of tilt-rotor VTOL airplane wherein large diameter helicopter-type of rotor is used for vertical lift, and which may be tilted 90 degrees forward to provide horizontal thrust during cruising flight when the aircraft is supported by conventional wings. In this invention, there is only one large tiltable rotor mounted on top of the aircraft fuselage at the aircraft longitudinal axis to provide both vertical lift when the rotor is pointed upward and horizontal thrust when the rotor is pointed 90 degrees horizontal.
2. Discussion of the Prior Art
Shortly after the airplane was invented, its disadvantage of requirement of a significant runway for takeoff and landing was quickly noticed, which significantly limit the airplane's utility. The helicopter was introduced afterward in order to overcome the limitation of the airplane. However, the helicopter has not received wide spread use but only in special roles that strictly require VTOL capability, and the helicopter numbers is but {fraction (1/10)} that of the airplane. The helicopter flies too slowly and too inefficiently, with speed and range ½ to ⅓ that of the airplane, with 2 to 3 times the fuel consumption and cost of operation per passenger-mile. The helicopter is far less safe per passenger-mile basis. According to NTSB statistics, the fatality rates for piston helicopters is 3-4/100,000 hrs and for light turbine helicopters 2-3/100,000 hrs where as the rates for a typical high wing airplane such Cessna 172 is 0.5 and Cessna 182 is 0.7/100,000 hrs. Light turbine helicopters have purchasing cost 2-4 times that of comparable piston airplane, but recently, the Robinson piston helicopters with their simplified rotor head design has brought down their purchasing cost to a level comparable with piston airplane.
In order to maintain the VTOL advantage of the helicopter while overcoming the helicopter's inefficiency, there have been at least 50 different projects experimenting with high-speed VTOL aircraft by a large numbers of well known aerospace companies, proposing at least 12 different configurations in the last five decades. See, for example, “An Introduction to V/STOL Airplanes” by Iowa State University Press, 1981. Today, there is only one VTOL transport airplane that has sufficient merits to achieve production status, the tilt-rotor Bell-Boeing V-22 Osprey, after 40 years of experimentation, 40 billion dollars spent, and intense congressional lobbying effort after repeated congressional attempts to cancel the program, due to huge cost over-run, weight increase and increase in complexity. The V-22, with a pair of tiltable rotors mounted on each wing tips, is so expensive and complex that it cannot replace conventional high-lift helicopter such as the Boeing Chinook CH-47D with twice the payload at ½ the cost, nor can it be cost-competitive with the Sikorsky UH-60L Black Hawk single rotor helicopter with ¾ of the useful vertical load capacity at a small fraction of the purchasing cost. The civilian derivative of the V-22 is the Bell-Agusta BA-609 tilt-rotor having less load capacity as compared to a LearJet 45 yet costing twice as much, and flies at ⅗ the speed of the LearJet 45 with half the range.
There are many obvious reasons for the high cost of the tilt-rotor VTOL airplane. Current tilt-rotor VTOL airplane requires two large helicopter-type of rotors, which are expensive to make due to their complexity in incorporating flapping rotor blade design, complex linkage for combining cyclical pitch control together with collective pitch control. The requirement of two large rotors significantly increases the cost of the aircraft over that of a single-rotor heavy-lift helicopter. Mounting those two rotors at the wing tips means that a significant degree of thrust asymmetry exists should one engine fails, thus necessitating complex transmission mechanism connecting the two rotors together. Since the two rotors are very large, they produce significant degree of vibration necessitating rigid wing construction with expensive materials further increasing the cost. In term of safety, a twin tiltable-rotor VTOL airplane cannot be as safe as a single tiltable rotor design, because a twin rotor design will have twice the risk of a one-sided rotor/transmission/control mechanism failure, which would be fatal. Even in the absence of mechanical failure, a sudden reduction in lift of one rotor with respect to the other can lead the aircraft to flip over and out of control. This can happen due to vortex-ring effect or power settling when descending too rapidly in the hovering mode or upon turbulence. Attempting to increase lift by increasing throttle and pitch will only worsen the problem. This is what was blamed for the crash of the Marine MV-22 in Apr. 8, 2000, wherein a tight turn to the right at high rate of vertical descent at low airspeed causing the right rotor to lose lift, leading to a roll to the right. The pilot applied more power and collective pitch to the right rotor as correction only worsen the problem, causing the airplane to roll over and crash. Power settling in a single-rotor helicopter usually does not lead to a crash because no asymmetrical lift is involved, only a high sink rate at high power. The pilot simply increases the forward speed to overcome the problem. For all the above reasons, the vast majority of current helicopters now have a single centrally-mounted main rotor, even for very large helicopters, as opposed to multiple rotor designs that originated since the days that the first rotary wing aircraft were invented.
Another disadvantage of a twin-tilt-rotor airplane would be the necessity of using computer stability augmentation system in the VTOL mode, due to the very large roll and yaw moment of inertia with the engines and rotors mounted at the wing tips, and also due to the low cyclical pitch control moment of the smaller size rotors. This requires a fly-by-wire system with triple redundancy system which would be very expensive for a smaller size aircraft, and a digital flight control system has been known to be vulnerable to electromagnetic interference, system conflicts and crashing, and other factors making it unreliable in actual practice. By contrast, a single rotor helicopter due to its large diameter thus more dynamic stability, can be controlled by a trained pilot without requiring stability augmentation system, thus has been proven to be much cheaper and more reliable.
Clearly, the VTOL tilt-rotor airplane, the most successful VTOL airplane configuration, must be designed with a single main lifting rotor in order to be simpler, cheaper and safer than current twin wing-mounted tilt-rotor design. It is only then that the VTOL airplane can make a significant impact on commercial air transportation, thereby reducing the problem of airport congestion at large commercial airport and increasing rate of closure of small airports due to local political pressure favoring real-estate developers.
To my knowledge, there has not been any disclosure of a tilt-rotor VTOL airplane utilizing a single tiltable rotor. A related U.S. Pat. No. 5,395,073 by Rutan et. al discloses a drone type of aircraft with a propeller mounted in front of a fuselage, a pair of “free wings” mounted on both sides of the fuselage, and tail booms attached to the wings for supporting the tail surfaces. The entire fuselage is pivotable transversely with respect to the wings and the tail surfaces. When the fuselage points upward, the front-mounted propeller is then pointed upward in the take off and landing phase thus providing vertical lift. After takeoff, the fuselage is pivoted to its horizontal orientation to provide horizontal thrust for cruising flight. The wings and the tail are always maintained in parallel with the direction of travel. Rutan's design is intended primarily for a robotic drone aircraft, therefore its desi
Jakel Kevin
Poon Peter M.
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