Aeronautics and astronautics – Aircraft sustentation – Sustaining airfoils
Reexamination Certificate
1999-07-20
2001-07-10
Poon, Peter M. (Department: 3643)
Aeronautics and astronautics
Aircraft sustentation
Sustaining airfoils
C244S215000
Reexamination Certificate
active
06257528
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to aerodynamic or hydrodynamic control systems and methods for aircraft or watercraft. The invention relates more particularly to such control systems and methods employing pivotal trailing edge control surfaces that include pivotal tabs for reducing hinge moments on the control surfaces and/or for increasing effectiveness of the control surfaces.
BACKGROUND OF THE INVENTION
Such tabs have been used on some aircraft for providing power to assist the movement of a control surface. For example, as shown in U.S. Pat. No. 3,363,862, an elevator is disclosed having a control tab pivotally mounted on the aft end of the elevator. The tab is mechanically linked to the pilot's controls such that the pilot mechanically moves the tab. Deflection of the tab results in aerodynamic forces being exerted on the tab, and these forces in turn cause the elevator to pivot in the opposite direction until the elevator and tab system reaches an equilibrium condition.
More recently, aircraft control surfaces have generally been actuated by hydraulic devices. In some hydraulically powered control systems, geared tabs have been used in order to reduce the hinge moments of trailing edge control surfaces. In a geared tab arrangement, a tab is pivotally connected to the trailing edge of the control surface and the tab is mechanically linked to aircraft structure such that pivotal movement of the control surface in one direction causes a pivotal movement of the tab in the opposite direction. The aerodynamic moment about the control surface hinge axis is reduced by the moment exerted by the forces on the tab. A drawback of a geared tab arrangement is that the gearing ratio between the control surface and the tab is fixed and thus cannot be optimized for all flight conditions.
This drawback was remedied in part by the development of a “reversing” geared tab for an elevator as described in U.S. Pat. No. 4,431,149 issued to Brislawn et al. The reversing geared tab described in the '149 patent provides tab deflections that are inversely proportional to (i.e., in the opposite direction to) the elevator deflection for all trailing-edge-down (TED) elevator deflections, and for trailing-edge-up (TEU) elevator deflections from zero to about 13°. This inversely proportional tab deflection reduces the elevator hinge moments and over-sensitivity for elevator deflections typical of high-speed flight. However, at 13° of elevator deflection, the tab gearing ratio changes sign so that the tab starts to move in the same direction as the elevator for TEU elevator deflections greater than 13°, resulting in the tab returning to a faired position at the maximum TEU elevator deflection of 35°. This provides normal elevator effectiveness at maximum TEU elevator deflection (i.e., no tab losses) as required for low-speed flight.
While the reversing geared tab of the '149 patent is an improvement over a non-reversing geared tab, the fixed schedule of gearing ratio provided by the reversing geared tab system is not capable of providing optimum tab hinge moment balancing over the full range of operating conditions. Furthermore, the geared tab of the '149 patent does not function as a “leading tab” wherein, for example for takeoff rotation, the tab deflection is in the same direction as and exceeds that of the elevator so as to provide increased elevator control power.
Next-generation Boeing 737 aircraft employ a reversing elevator tab that functions both as a balancing tab to reduce hinge moments in high-speed cruise operations, and as a leading tab to provide increased elevator control power for takeoff rotation. The gearing ratios in both the balancing mode and the leading mode are fixed. If hydraulic power is lost at any time, the tab automatically reverts to the balancing mode in order to minimize hydraulic power requirements.
The present invention was developed at least in part to address the unique control needs presented by the Boeing Blended-Wing-Body (BWB) aircraft that is under development by The Boeing Company. The BWB aircraft is essentially a large flying wing. To provide longitudinal stability of the aircraft, the wing requires large trailing edge control surfaces across the entire wing span, and the control surfaces must be capable of deflecting at substantially higher rates than those of moreconventional aircraft. Accordingly, the BWB aircraft presents some technical challenges in terms of providing sufficient hydraulic power to drive the control surfaces in high-speed cruise operations. Additionally, in low-speed operations, especially at takeoff, increased pitch control power may be needed beyond that which the elevator control surfaces are able to provide. A further challenge is to assure that the control surfaces can be driven as needed to provide a minimally acceptable amount of control power during a hydraulic power failure such as when all of the engines fail. The present invention seeks to provide a solution to all of these challenges.
SUMMARY OF THE INVENTION
The above needs are met and other advantages are achieved by the present invention, which provides a control system employing a reversing geared tab that, in one preferred embodiment of the invention, has a gearing ratio that can be varied between zero and a predetermined limit such that the gearing ratio can be optimized over a broad range of vehicle operating conditions. Preferably, the gearing ratio can be varied from zero down to a negative gearing ratio limit, and from zero up to a positive gearing ratio limit, so that the gearing ratio can be optimized for both high-speed and low-speed flight.
To these ends, a control system in accordance with a preferred embodiment of the invention is provided for a vehicle of the type having a trailing edge control surface mounted for pivotal movement about a control surface pivot axis fixed relative to the vehicle, and a control surface actuator connected between the vehicle and the control surface and operable to pivotally move the control surface about the control surface pivot axis. The control surface comprises a tab connected to the control surface so as to be pivotable about a tab pivot axis located aft of the control surface pivot axis, and a linkage having an aft end connected to the tab and extending forward therefrom to a forward end of the linkage. The forward end of the linkage can be placed in various positions including a neutral position in which the forward end of the linkage is in line with the control surface pivot axis, and other positions in which the forward end lies on one side or an opposite side of the control surface pivot axis. When the forward end of the linkage is in the neutral position, the gearing ratio is zero and the tab deflection is one-to-one with (i.e., the tab is faired with) the control surface. When the forward end of the linkage is moved out of the neutral position on one side or the other thereof, the gearing ratio becomes positive or negative such that a control surface deflection causes a proportional or inversely proportional deflection of the tab relative to the control surface.
For varying the gearing ratio, the control system further includes a gearingcontrol actuator connected to the linkage and operable to move the linkage so as to cause the forward end thereof to move along the path extending out of the neutral position. The gearing-control actuator is further operable to position the forward end of the linkage at selectively variable distances on at least one side of the neutral position, and more preferably on one side or the other side of the neutral position, so as to vary an effective gearing ratio between the control surface and the tab.
Preferably, the control system includes a gearing controller operatively connected to the gearing-control actuator and operable to control the gearing-control actuator so as to vary the gearing ratio as a function of vehicle operating condition. Thus, for example, the gearing ratio can be varied as a function of aircraft Mach number, altitude, and/or other pa
Alston & Bird LLP
Jakel Kevin
Poon Peter M.
The Boeing Company
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