Aeronautics and astronautics – Aircraft structure – Fuselage and body construction
Reexamination Certificate
2001-05-29
2003-08-12
Dinh, Tien (Department: 3644)
Aeronautics and astronautics
Aircraft structure
Fuselage and body construction
C244S131000, C403S170000, C403S180000, C403S217000, C446S124000, C446S126000
Reexamination Certificate
active
06604710
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to the fabrication of truss or frame structures, such as those used in an aircraft fuselage and to structural connectors that can be used not only to connect tubular members forming the structure, but which also serve as jigs for assembling the components of the structure.
2. Description of the Prior Art
There are a number of methods of fabricating an aircraft structure, in particular a fuselage structure suitable for use in a light aircraft. One approach employs a structural frame or truss to support all or substantially all of the loads or forces that must be carried by the fuselage. Another approach, commonly employed on larger or more sophisticated aircraft is to employ a fuselage constructed of thin sheets or webs of sheet metal. The sheets are suitable for resisting shear or tension loads in the plane of the sheets. These sheets must be stiffened by members more capable of carrying compression loads and loads normal to the sheet, or skin or web. Semimonocoque structures employ thin webs, such as the skin or a fuselage, to carry tension and shearing forces and stiffeners to carry compression or normal loads. A semimonocoque fuselage structure typically employs closely spaced rings or bulkheads, which resist loads in transverse planes, while the fuselage shell resists loads in the longitudinal direction. Additional longitudinal structural members, such a stiffeners, stringers or longerons span between bulkheads and transfer loads to the bulkheads.
The simpler trusses or frames commonly employed in light aircraft commonly employ chrome-molybdenum steel tubes. Tubular frame structures formed from welded chrome-molybdenum tubes are the standard structural components used in light and ultralight aircraft. These tubular frame structures or trusses are commonly employed with a fabric or non-load bearing outer surface or external skin. They also require extensive bracing and cross bracing.
Welding is used extensively for steel-tube truss structures, such as fuselages. The most common type of welding consists of heating parts to be joined by means of an oxyacetylene torch and then fusing them together with a welding rod. The tensile strength at the weld can become similar to that of cast metal, and it is more brittle and less able to resist shock and vibration loading than is the original material. Aircraft tube walls are thin and more difficult to weld than other machine and structural members. At one time all aircraft welding was torch welding, but electric arc-welding has also been used. For arc-welding, the welding rod forms an electrode from which current passes in an arc to the parts being joined. The electric arc simultaneously heats the parts and deposits weld metal from the electrode. Heating is more localized than for torch welding, and the strength of the heat-treated parts is not impaired as much by arc-welding.
The strength of conventional welded joints depends largely on the skill of the welder. The stress concentrations can vary and it is customary to design welded joints for aircraft fuselages with a liberal margin of safety. Welded joints should be in shear or compression but design often dictates that tensile loads must be applied to a welded joint. Steel tubes, such as chrome-molybdenum alloy tubes, are usually spliced by prior art fish mount joints as shown in FIG.
17
. These joints are designed so that most of the weld is in shear and so that most of the weld is not confined to one cross section of the tube. If a butt weld in necessary, the weld should be diagonal and not perpendicular to the centerline of the tube, as shown in the prior art weld of FIG.
18
.
Fuselage truss members are often welded as shown in the prior art weld shown in FIG.
19
. In that Figure only the horizontal member is highly stressed. If members other than the horizontal member are stressed, common prior art practice is to insert gusset plates as shown in FIG.
20
. Steel tubes often have walls as thin as 0.035 in. The welder must control the temperature to keep from overheating the thin walls and burning holes in them. It is extremely difficult to weld a thin member to a heavy one, as more heat is required for the heavy member. The thickness ratio of parts being welded should be less that 3:1, and preferably less than 2:1.
Conventional concentric butt welded fuselage joints between tubes in aircraft and fuselage structure may be satisfactory where vibration is not present. However, the fatigue strength of butt welded joints is compromised when subject to reverse bending. Therefore common practice requires that finger plates or insert gussets should be added to joints subject to vibration. Indeed, the standard practice used in fabricating light and ultralight aircraft is to weld gusset plates at welded intersections of tubes in the fuselage and cabin. However, the configuration of the different welded joints in an aircraft fuselage is generally not uniform. This lack of uniformity gives rise to two problems. First that shape of the tubular members at different joints will be different, in part because of the orientation of the tubular members entering that joint, and the shape of the gusset plates will also differ from joint. This means that a large number of different parts are necessary and that jigs are necessary both for the fabrication of different components as well as for the assembly of multiple components at each joint. The integrity of the welded structure is also dependent upon the skill of the welder, and each weld can take a relatively large amount of time to complete.
Another approach to connecting thin-wall hollow tubes to create a lightweight three dimensional truss structure that can be used in aircraft is shown in U.S. Pat. No. 4,624,599. According to the method disclosed in that patent, the ends of coplanar tubes are partially flattened into an elongated flattened oval shape. Portions of the ends of the tubes are cut away so that the oval ends can be partially telescoped to fit in a mutually nesting relationship with partially flattened ends overlapping. Multi-layer sandwich splice plates are located on the interior of the oval end sections and the plates are bolted to the flattened ends of the tubes so that the tubes can be clamped together. An overlying bracket including formed end plates and welded gussets is used to connect tubes extending in different planes. It would appear that each of these joints would require considerable fabrication and assembly. Not only are the tube ends to be deformed into an oval shape, but the ends of the tubes are machined so that the tube ends can partially telescope at a prescribed angle. Each splice plate is also formed from multiple components which must be separately machined and assembled. Not all joints in an aircraft structure, such as a fuselage, have the same configuration, so it would appear to be necessary to separately machine, form and fabricate and assemble different subcomponents at each joint, and fabricate multiple dies for different joint components. As such, this approach would appear to be a rather expensive way to fabricate a light aircraft.
SUMMARY OF THE INVENTION
Structural frames, such as frames forming an aircraft fuselage or cabin, fabricated using these prior art techniques tend to be labor intensive to assembly, especially when gussets must be welded to the tubular members, and when the shape of the components, such as gusset plates, must be different for virtually all nodes of the frame. A large number of different parts are required and the quality and integrity of welds are often dependent on the skill of the welder. Care must also be taken to insure that the load carrying capacity of the tubular members is not diminished by the welds and that appropriate safety margins are not compromised. It is also important that the overall weight of the structure does not become too great. The instant invention, comprising a method of assembling and welding a structural frame, such as an aircraft fuselage, and components employed i
Ohmer Richard Edward
Ohmer Timothy John
Dinh Tien
Pitts Robert W.
RST Aircraft Corporation
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