Main landing gear having vertical main gear pivot pins

Abstract: A main landing gear system which employs Ackermann type steering utilizing kingpins (28) and tierods (30) wherein a plurality of paired wheels (22,24) are employed and wherein each truck axle (20) is adapted for independent steering. Electronic control means along with hydraulic directional valve means are utilized. A main landing gear having single wheel or king pin steering. Pivot pin tilting is utilized in truck type main landing gear, e.g. where a truck axle is adapted for independent steering or where single wheel or king pin steering is utilized.

Description: Background of the Invention 1. Field of the Invention The present invention relates to aircraft steerabable main landing gear. More specifically, the invention relates to a landing gear for an aircraft, including a plurality of wheels mounted in pairs on the at least one truck, at least one of said pairs of wheels being steerable. Such a landing gear is know from US-A-5,242,131. 2. Description of the prior art In the patent literature, US-A-2,567,074; US-A-2,682,311; and US-A-3,516,625 are illustrative of steering which is accomplished by pivoting the entire truck about a vertical axis as a single unit. Control of relative rotation between trucks is accomplished by a variety of means in these systems. US-A-2,630,285 shows a means of coupling pairs of wheels on a common axis using kingpins and tierods. Systems are shown for both single and multiple axle landing gear trucks. Castered wheels are shown for self alignment. There is no mechanism for steering the wheels. GB-A-904,783 is exemplary of load equalization techniques wherein there is shown wheel truck where wheel pairs articulate about a longitudinal axis so as to equalize wheel loads when an aircraft encounters transverse terrain contours. US-A-4,917,334 shows a multi-wheeled trailing type landing gear. In this configuration, the forward axle (two wheels) is mounted directly to the shock strut. The aft two axles, two wheels each, are mounted on a truck which is cantilevered from the aft side of the strut on a pivoting radius arm. Shock absorbers are used to react the vertical loads. As aircraft get progressively larger, wheels per landing gear (and/or gears per aircraft) increase the amount in order to offset the problem in flotation and tire loading limitations. However, as the number of wheels and gears increase, so does the reluctance of the main landing gear to allow the aircraft to turn during steering. The main landing gears increasing adhesion to the ground (tracking) causes a reduction adhesion to the ground at the nose gear. When the ground adhesion at the nose gear approaches zero, the directional control of the aircraft during taxi is greatly decreased. During tractor towing operations, the nose gear steering angle is governed by the tractor but heavy torsional loads can be induced into the main landing gears resulting in heavy designs. The above problems have resulted in the incorporation of the "body gear steering system". However, without the advantage of load equalization the turning moments due a one tire flat situation, are reacted by the steering actuators. This results in exceptionally large and heavy actuators. The above-identified prior art document US-A-5,242,131 already discloses a multi-wheel, multi-axle truck type main landing gear in which the rear wheels are steerable to improve manoeuverability. The steerable pair of wheels is mounted on a common axle that is connected to the truck beam by a single pivot pin. The steering action is driven by two actuators coupled between an extended part of the truck beam and both sides of the steerable axle. Summary of the Invention The invention has for its object to provide an improved steerable main landing gear for an aircraft. In accordance with the invention, this is achieved in a landing gear of the type described above, in that each steerable wheel is connected to the truck by a pivot pin that is substantially vertical when the landing gear is on the ground, the steerable wheels of a pair on opposite sides of the truck being connected by a tie rod and a steering actuator being coupled between the truck and one of said pair of steerable wheels. By mounting each steerable wheel to the truck by a separate vertical pivot pin, the steerable wheels do not exhibit any forward or aft movement during turning, so that interference between wheels of adjacent axles in case of different steering angles is prevented. Preferred embodiments of the landing gear of the present invention form the subject matter of the dependent claims. Brief Description of the Drawing(s) FIG. 1 is a plain view illustrative of Ackermann system components for paired wheels on a truck axle; FIG. 2 is a view as FIG. 1 however looking forward and showing a flat tire; FIGS. 3A, 3B and 3C are descriptive and illustrative of the problem of wheel load equalization in multi axled landing gear; FIGS. 3D, 3E, 3F and 3G are illustrative of the solutions to the problems shown in FIGS. 3A, 3B and 3C; FIGS. 4A, 4B and 4C utilize the arrangement of FIG. 1 utilized for steering a 4 wheeled truck; FIGS. 5A, 5B and 5C incorporate the steering arrangement of FIG. 1, however applied to a 6 wheeled truck; FIG. 6 is illustrative of a Boeing 747 type aircraft body gear; FIG. 7 is illustrative of the present single wheel steering main landing gear system; FIGS. 8A, 8B, 8C and 8D show steering arrangements for steering an 8 wheeled truck utilizing the present single wheel steering main landing gear systems concept; and FIG. 9 is illustrative of how the present single wheeled system can be utilized in a 8 wheeled truck to offset the effect of a side drift landing. Description of the Preferred Embodiment(s) The present invention provides a main landing gear steering system which uses the "Ackermann Steering System" (Automotive type with kingpins and tierods). Each truck axle is a candidate for independent steering, depending on the amount of control desired. When combined with electronic control of hydraulic directional valves, the axles can be turned ih opposite directions to give minimum turning radius, or all turned the same direction to compensate for a side drift landing, or any combination in between. Current landing gears do not provide independent steering of each axle on multiple axle trucks. This limits the minimum turning radius of the aircraft and induces large loads on the truck structure, thus requiring correspondingly heavy construction. This invention as exemplified in the embodiments hereinafter described is directed to a main landing gear steering concept which employs Ackermann type steering which employs kingpin and tierods wherein a plurality of paired wheels are employed and wherein each truck axle is adapted for independent steering. The term "single wheel steering" where utilized in the following description relates to main gears having a minimum of two wheels (twin) as shown, for example, in the embodiment of FIGS. 1 and 2. The present main landing gear single wheel steering system reacts to all movements due to a brake drag and side loads provided these loads are equally distributed between the two wheels. Small, unequal movements due to unequal tire pressures or other incidental rolling friction variations can be accommodated by the steering acuator. Large, unequal movements due to "one burst tire case (see FIG. 2)" and small deviations in frictional coefficients can be minimized by the use of the load equalization system solutions of FIGS. 3D, 3E, 3F and 3G. Such usage of load equalization in the present main landing gear single wheel steering systems is an option which could reduce reaction on the steering actuator by e.g., 80 percent. Turning now to FIG. 1, a plan view of a first embodiment of the present invention can be seen in which a main gear truck 20 having two wheels 22 and 24 comprises further a bell crank 26 with vertical pin 28 per wheel, each bell crank 26 being connected symmetrically with a tie rod 30. Steering actuator 32 is coupled between bell crank 26 and main gear truck 20 by means of vertical pin 28. The forward direction is represented by arrow 40 and drag forces by arrows 42. This main landing gear single wheel steering system reacts to all movements due to brake drag and side loads provided these loads are equally distributed between two wheels 22 and 24. Small unequal movements due to unequal tire pressures or other incidental rolling friction variations can be accommodated by steering actuator 32. Apart from the "one burst tire" case shown in FIG. 2 and small deviations in frictional coefficients, the movements about vertical pins 28 are equal and opposite. Each pin 28 is located in a bell crank 26 which are then connected symmetrically with a tierod 30 by means of a tensile load, a steering system used and known in the automobile industry as the "Ackermann" steering system. With the aforementioned conditions of small deviations in frictional coefficients, the load in actuator 32 would only be that required to move wheels 22 and 24 to the required steered position. However, in the case of a flat tire (FIG. 2) wheel load equalization such as shown in FIGS. 3D, 3E, 3F and 3G must be used to enable the wheel rim (in the absence of the blown tire) to contact the ground. This maintains as much as possible the movement created by the rolling resistance of the tire prior to blowing. Any difference in rolling resistance of an unblown tire (24 in FIG. 2) and a wheel rim (tire 22 side in FIG. 2) will increase the steering actuator accordingly, "roll on rim" requirements however tend to keep this increase to a minimum. Turning now to the wheel load equalization problem, it can be appreciated that due to depressions and humps in the surface of runways it is difficult to achieve equal loading of wheels associated with multi-axled landing gears. Equal loading in a fore and aft direction for 3 or 4 wheeled trucks is achieved as shown in FIG. 3A. For trucks consisting of 5 (or more) wheels, load equalization cannot be achieved completely by the single continuous truck beam as shown in FIG. 3B. Equal loading in the transverse direction is not achievable on conventional wheel axles as shown in FIG. 3C. This statement applies to all landing gears having twin type multi-axles. Note: The problem of unequal loading is less with landing gears having 4 (or less) wheels, The deflection has a beneficial effect, The problem becomes an issue with landing gears having 5 (or more) wheels, Unequal tire loading would adversely affect brake performance, tire wear, and fatigue life of related component parts. FIGS. 3D, 3E, 3F and 3G show proposed solutions to the problem of load equalization. (a) Load Equalization - Fore and Aft Direction Figures 3E and 3F illustrate the arrangement of multiple truck beams 220, 221; 320,32 in order to achieve load equalization. This system could be expanded to utilize a 3rd order truck but is considered too complex and unnecessary for a landing gear application. (b) Load Equalization - Transverse Direction Note the horizontal pivot in FIG. 3G. Wheel load equalization (fore and aft) is achieved by the use of primary 220; 320 and secondary 221; 321 truck beams as shown, e.g., in combinations of wheels above 5 (see FIGS. 3E and 3F). Wheel load equalization (transverse) is achieved by introducing a horizontal pivot 100 to each axle (see FIG. 3G). In wheel load equalization a wheel truck for an aircraft landing gear carries a plurality of wheel sets, each set having a left-hand wheel 122; 222; 322 and a right-hand wheel 124; 224; 324. The wheels of any given set are free to rotate about a longitudinal axis 100 so that they will follow transverse ground contours as the aircraft taxis. Each wheel set, as a whole, can also pivot about a transverse axis 123; 223, 225; 323, 325, in order to adaptively follow longitudinal ground contours. The pivotable mounting arrangement of the wheels to the truck enables all wheels to maintain an equal force contact with the ground, so that one wheel will not be overloaded relative to another. ADVANTAGES OF MAIN LANDING GEAR SINGLE WHEEL STEERING 1. MULTIPLE STEERING MODES The arrangement herein before discussed of FIG. 1 can be incorporated for use in multiple wheeled gears as shown in FIGS. 4,5,6,9 and 10. The directional control of each wheel is achieved by electronic control of the hydraulic directional valves of the wheels. Steering arrangements for steering a 4 wheeled truck are shown in FIG. 4A, 4B and 4C while steering a 6 wheeled truck is shown (moving to the right) in sequenced steps of FIGS. 5A, 5B and 5C. Steering arrangements for an 8 wheeled truck are shown in FIGS. 8A, 8B, 8C and 8D. FIG. 9A, Band C shows how the present single wheeled system can be utilized to offset the effect of a side drift landing. The angle of yaw is measured by any means (e.g., radar, laser inertial control) and fed as an electrical signal to hydraulic directional control valves to obtain the required position of all wheels. 2. REDUCED MOVEMENT ARM The reduced movement arm is a result of the reduced distance from the kingpin centerline and tire centerline as compared to distances currently utilized in e.g., a Boeing type 747 body gear (of FIG. 6) where: Steering Torque = FL 2 in Nm(lbs) (approximately) The example for main gear single wheel steering is then as shown in FIG. 7 where: Steering Torque = FL 5 in Nm(lbs) approximately ADVANTAGES OF THE PRESENT MAIN LANDING GEAR SINGLE WHEEL STEERING EMBODIMENTS INCLUDE: 1. The use of the "Ackermann Steering System" on aircraft landing gear, i.e., the method of minimizing the loads normally transferred to the steering actuators. 2. The use of a steering system which permits individual wheel movement in different directions as required. 3. The feature of the steering angle being twice that of the single wheel angular movemenL This is achieved by steering leading and trailing twin axles in opposite directions. (See FIGS. 4C, 5C and 8A, 8B and 8C.) 4. The ability of all wheels on all gears to be directionally controlled for all side drive landing using the main gear steering system. (See FIGS. 9A, and 9D.) 5. Possible positioning of wheels for relieving pivoting loads.

Claims: Landing gear for an aircraft, including a plurality of wheels (22,24) mounted in pairs on at least one truck (20), at least one of said pairs of wheels (22,24) being steerable, characterized in that each steerable wheel (22,24) is connected to the truck (20) by a pivot pin (28) that is substantially vertical when the landing gear is on the ground, the steerable wheels (22,24) of a pair on opposite sides of the truck (20) being connected by a tie rod (30) and a steering actuator (32) being coupled between the truck (20) and one (24) of said pair of steerable wheels (22,24). Landing gear as claimed in claim 1, characterized by a plurality of steerable pairs of wheels (22,24), each said pair of wheels (22,24) being steered independently from the other pairs. Landing gear as claimed in claim 2, characterized in that the steerable pairs of wheels (22,24) can be turned in opposite directions to give minimum turning radius. Landing gear as claimed in claim 2 or 3, characterized in that the steerable pairs of wheels (22,24) can be turned in the same direction to compensate for a side drift landing. Landing gear as claimed in any one of the preceding claims, characterized by means for equalizing loads on the wheels. Landing gear as claimed in claim 5, characterized in that the wheel load equalization means comprise multiple truck beams (220,221;320,321) connected to each other by transverse pivots (225,325), each truck beam (220,221;320,321) carrying at least one pair of wheels (222,224;322,324) such as to achieve fore and aft wheel load equalization. Landing gear as claimed in claim 5 or 6, characterized in that the wheel load equalization means comprise a longitudinal pivot (100) for each pair of wheels (122,124) such as to achieve transverse wheel load equalization.

Inventors: RALPH HARRY C, RALPH, HARRY C.
Applicants: BOEING CO, THE BOEING COMPANY
Application year: 1997
Main classification: B64C25
IPC codes: B64C25