6604710 Light Aircraft Fuselage and Stru

8/4/2019 6604710 Light Aircraft Fuselage and Stru

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(12) United States PatentOhmer et al.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1US006604710B2(10) Patent No.:(45) Date of Patent:

US 6,604,710 B2Aug. 12, 2003

(54) LIGHT AIRCRAFT FUSELAGE ANDSTRUCTURAL FRAME CONNECTORS

(75) Inventors: Richard Edward Ohmer, Hickory, NC(US); Timothy John Ohmer, Gastonia,NC (US)

(73) Assignee: RST Aircraft Corporation, Hickory,NC (US)

( *) Notice: Subject to any disclaimer, the term of thispatent is extended or adjusted under 35U.S.c. 154(b) by 78 days.

(21) Appl. No.: 09/866,246(22) Filed:(65)

May 29, 2001Prior Publication Data

US 2002/0179772 A1 Dec. 5, 2002Int. CI? . ... ... ... ... ..... ... ... .... B64C 1/08U.S. CI. 244/119; 244/131; 403/170;

403/180; 403/217; 446/124; 446/126Field of Search 244/119, 131,

244/117 R, 120, 123; 403/169, 170, 171,172, 217, 219, 180; 52/653.1, 653.2, 655.1;

446/124, 126

(51)(52)(58)

(56) References CitedU.S. PATENT DOCUMENTS

1,198,263 A1,588,268 A1,751,957 A *1,852,208 A1,880,481 A1,881,296 A2,149,476 A2,149,844 A2,367,750 A2,709,318 A2,818,226 A

9/19166/19263/19304/193210/193210/19323/1939* 3/1939* 1/19453/1955* 12/1957

PajeauRocheTowleHugginsRagsdalePotezTetzlaffGeorgeBerkow et al.BenjaminHiller et al.

2,976,968 A 3/1961 Fentiman3,048,109 A 8/1962 Feemster3,445,129 A * 5/1969 Penote3,632,147 A 1/1972 Finger4,259,821 A 4/1981 Bush4,479,662 A 10/1984 Defour et al.4,612,070 A 9/1986 Sikka4,624,425 A 11/1986 Austin et al.4,624,599 A 11/1986 Piasecki4,627,149 A 12/1986 Colas4,660,345 A 4/1987 Browning4,728,113 A 3/1988 Thun, Jr.4,776,721 A 10/1988 Lange4,822,199 A 4/1989 Nehls4,930,930 A 6/1990 Coppa5,169,258 A 12/1992 Raynak5,399,043 A 3/1995 Plumeyer5,518,208 A * 5/1996 Roseburg5,549,408 A 8/1996 Lo6,032,430 A 3/2000 Soukup6,126,113 A * 10/2000 Navickas

FOREIGN PATENT DOCUMENTSFR 566256 4/1958* cited by examinerPrimary Examiner-Tien Dinh(74) Attorney, Agent, or Firm-Robert W. Pitts(57) ABSTRACTA structural frame or truss, suitable for use as part of a lightaircraft fuselage includes steel tubular members joined bystructural connectors or connector blocks. The blocksinclude bore holes into which the ends of lateral tubularmembers are inserted, and longitudinal tubes or longeronsare inserted through holes extending between opposed blockfaces. The structural connector blocks not only connect thetubular members, but they serve as jigs for assembling thestructural frame. Tubes can be plug welded to the blocks,and the fuselage outer skin can be attached to the structuralconnectors to form a dual fuselage structure.

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US 6,604,710 B21

LIGHT AIRCRAFT FUSELAGE ANDSTRUCTURAL FRAME CONNECTORSBACKGROUND OF THE INVENTION

1. Field of the InventionThis invention is related to the fabrication of truss orframe structures, such as those used in an aircraft fuselageand to structural connectors that can be used not only toconnect tubular members forming the structure, but whichalso serve as jigs for assembling the components of thestructure.2. Description of the Prior ArtThere are a number of methods of fabricating an aircraft

structure, in particular a fuselage structure suitable for use ina light aircraft. One approach employs a structural frame ortruss to support all or substantially all of the loads or forcesthat must be carried by the fuselage. Another approach,commonly employed on larger or more sophisticated aircraft 20is to employ a fuselage constructed of thin sheets or webs ofsheet metal. The sheets are suitable for resisting shear ortension loads in the plane of the sheets. These sheets must bestiffened by members more capable of carrying compressionloads and loads normal to the sheet, or skin or web. 25Semimonocoque structures employ thin webs, such as theskin or a fuselage, to carry tension and shearing forces andstiffeners to carry compression or normal loads. A semi-monocoque fuselage structure typically employs closelyspaced rings or bulkheads, which resist loads in transverse 30planes, while the fuselage shell resists loads in the longitu-dinal direction. Additional longitudinal structural members,such a stiffeners, stringers or longerons span between bulk-heads and transfer loads to the bulkheads.The simpler trusses or frames commonly employed in 35light aircraft commonly employ chrome-molybdenum steeltubes. Tubular frame structures formed from weldedchrome-molybdenum tubes are the standard structural com-ponents used in light and ultralight aircraft. These tubularframe structures or trusses are commonly employed with afabric 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 weldingconsists of heating parts to be joined by means of an 45oxyacetylene torch and then fusing them together with awelding rod. The tensile strength at the weld can becomesimilar to that of cast metal, and it is more brittle and lessable to resist shock and vibration loading than is the originalmaterial. Aircraft tube walls are thin and more difficult to 50weld than other machine and structural members. At onetime all aircraft welding was torch welding, but electricarc-welding has also been used. For arc-welding, the weld-ing rod forms an electrode from which current passes in anarc to the parts being joined. The electric arc simultaneously 55heats the parts and deposits weld metal from the electrode.Heating is more localized than for torch welding, and thestrength of the heat-treated parts is not impaired as much byarc-welding.The strength of conventional welded joints depends 60

largely on the skill of the welder. The stress concentrationscan vary and it is customary to design welded joints foraircraft fuselages with a liberal margin of safety. Weldedjoints should be in shear or compression but design oftendictates that tensile loads must be applied to a welded joint. 65Steel tubes, such as chrome-molybdenum alloy tubes, areusually spliced by prior art fish mount joints as shown in

2FIG. 17. These joints are designed so that most of the weldis in shear and so that most of the weld is not confined to onecross section of the tube. If a butt weld in necessary, the weldshould be diagonal and not perpendicular to the centerline of

5 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 thehorizontal member is highly stressed. If members other than

10 the horizontal member are stressed, common prior art prac-tice is to insert gusset plates as shown in FIG. 20. Steel tubesoften have walls as thin as 0.035 in. The welder must controlthe temperature to keep from overheating the thin walls andburning holes in them. Itis extremely difficult to weld a thin

15 member to a heavy one, as more heat is required for theheavy member. The thickness ratio of parts being weldedshould 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 besatisfactory where vibration is not present. However, thefatigue strength of butt welded joints is compromised whensubject to reverse bending. Therefore common practicerequires that finger plates or insert gussets should be addedto joints subject to vibration. Indeed, the standard practiceused in fabricating light and ultralight aircraft is to weldgusset plates at welded intersections of tubes in the fuselageand cabin. However, the configuration of the differentwelded joints in an aircraft fuselage is generally not uniform.This lack of uniformity gives rise to two problems. First thatshape of the tubular members at different joints will bedifferent, in part because of the orientation of the tubularmembers entering that joint, and the shape of the gussetplates will also differ from joint. This means that a largenumber of different parts are necessary and that jigs arenecessary both for the fabrication of different components aswell as for the assembly of multiple components at eachjoint. The integrity of the welded structure is also dependentupon the skill of the welder, and each weld can take a

40 relatively large amount of time to complete.Another approach to connecting thin-wall hollow tubes to

create a lightweight three dimensional truss structure thatcan be used in aircraft is shown in U.S. Pat. No. 4,624,599.According to the method disclosed in that patent, the ends ofcoplanar tubes are partially flattened into an elongatedflattened oval shape. Portions of the ends of the tubes are cutaway so that the oval ends can be partially telescoped to fitin a mutually nesting relationship with partially flattenedends overlapping. Multi-layer sandwich splice plates arelocated on the interior of the oval end sections and the platesare bolted to the flattened ends of the tubes so that the tubescan be clamped together. An overlying bracket includingformed end plates and welded gussets is used to connecttubes extending in different planes. Itwould appear that eachof these joints would require considerable fabrication andassembly. Not only are the tube ends to be deformed into anoval shape, but the ends of the tubes are machined so that thetube ends can partially telescope at a prescribed angle. Eachsplice plate is also formed from multiple components whichmust be separately machined and assembled. Not all jointsin an aircraft structure, such as a fuselage, have the sameconfiguration, so it would appear to be necessary to sepa-rately machine, form and fabricate and assemble differentsubcomponents at each joint, and fabricate multiple dies fordifferent joint components. As such, this approach wouldappear to be a rather expensive way to fabricate a lightaircraft.


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US 6,604,710 B23

SUMMARY OF THE INVENTIONStructural frames, such as frames forming an aircraft

fuselage or cabin, fabricated using these prior art techniquestend to be labor intensive to assembly, especially whengussets must be welded to the tubular members, and whenthe shape of the components, such as gusset plates, must bedifferent for virtually all nodes of the frame. A large numberof different parts are required and the quality and integrity ofwelds are often dependent on the skill of the welder. Care 10must also be taken to insure that the load carrying capacityof the tubular members is not diminished by the welds andthat appropriate safety margins are not compromised. It isalso important that the overall weight of the structure doesnot become too great. The instant invention, comprising a 15method of assembling and welding a structural frame, suchas an aircraft fuselage, and components employed in theframe, addresses these problems. This invention also pro-vides a simple and relatively inexpensive means to fabricatea light aircraft structure.A method of assembling an aircraft fuselage, according to

this invention, includes the following steps. Connectorblocks are formed with bore holes extending from edges andthrough holes extending between opposite faces. Ends of afirst set of lateral tubes are inserted into bore holes inconnector blocks. The bore holes in individual connectorblocks are oriented to function as jigs to properly orient thetubes to form a portion of an aircraft fuselage frame. Asecond set of longitudinal tubes are inserted into the throughholes so that the tubes in the second set extend continuously 30through the connector blocks. The tubes in the second setform longitudinal members in the aircraft fuselage. Thethrough holes are oriented to function as jigs to properlyorient longitudinal members relative to each other andrelative to the remainder of the aircraft fuselage.More specifically the method of fabricating an aircraft

fuselage according to this invention includes the step ofassembling a series of triangular bulkheads. Each bulkheadhas three tubular members assembled to connector blocks atthe apices of each triangular bulkhead. Each connector block 40has two bore holes extending into a peripheral edge of theconnector block. Each triangular bulkhead is assembled byinserting opposite ends of each of the three tubular membersinto bore holes in the connector blocks. Longerons areinserted into through holes on the connector blocks so that 45a series of triangular bulkheads are positioned at spacedlongitudinal positions on the longerons to form an internalframe for the aircraft fuselage. An external skin can be addedattached to the connector blocks to form a double fuselagewith both the truss or frame and the outer skin carrying loadsor forces applied to the aircraft fuselage. The tubes can alsobe welded to the connector blocks.The assembly constructed according to these methods

includes tubular members and structural connectors for usein connecting the tubular members to form a structuralframe, such as an aircraft fuselage frame or truss. Thestructural connectors comprise a block having a thicknessgreater than an external diameter of tubular members to beconnected by the corresponding structural connector. Atleast one bore hole extends into the block from a peripheral 60edge of the block. The peripheral edge extends betweenopposite faces of the block. The bore hole has an innerdiameter sufficient for insertion of an end of a tubularmember into the bore hole. A through hole extends betweenthe opposite faces of the block. The through hole has a 65diameter sufficient to permit a second one of the tubularmembers to pass through the block. The block connects at

4least two tubular members to form a portion of the structuralframe or fuselage. The tubes can be plug welded to thestructural block, and the bore holes and through holesfunction as jigging means for properly assembling the

5 tubular members.These structural connectors can also include a transversehole extending from one face of the block and intersecting

a bore hole, that extends inwardly beyond the transversehole. The transverse hole permits an assembler to determineif the end of the tubular section is fully inserted into the borehole. The transverse hole can also provide an opening forplug welding the structural connector to the tubes.An aircraft frame using these structural connector blocks

and constructed according to this method has a plurality oftubular sections connected by a plurality of connector blocksand an external skin. The tubular sections comprise bothlongerons and lateral tubular sections. The connector blocksconnect a plurality of lateral tubular sections to each long-eron at multiple longitudinal positions on each longeron.

20 The external skin is attached to the connector blocks so thatloads, applied to the aircraft frame, are carried by both thetubular sections and the external skin. Lateral tubular sec-tions or members form bulkheads in which the tubularmembers are joined together at the ends thereof by the

25 connecting blocks located at apices of each bulkhead. Theconnecting blocks also connecting the bulkheads to thelongerons.

BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a three dimensional view of a light aircraft of the

type that employs a fuselage structure in accordance withthis invention.FIG. 2 is a view of the tubular frame forming the cabin35 section and the fuselage of the aircraft according to this

invention.FIG.3 is a view of the fuselage frame including a forward

bulkhead subassembly of the aircraft shown in FIGS. 1 and2.FIG. 4 is a view of a forward face of a representative

structural connector used to connect tubular sections in thefuselage frame structure shown in FIGS. 2 and 3.FIG. 5 is a edge view of the structural connector shown

in FIG. 4.FIG. 6 is a front view of a structural connector used to

connect and inclined longeron with lateral tubular membersforming part of a tubular bulkhead structure in the fuselagestructure of FIGS. 2 and 3.FIG. 7 is a view of the rear face of the structural connector

50 shown in FIG. 6.FIG. 8 is a view of a triangular frame using the structural

connectors shown in FIGS. 4-7. This triangular frame formsa part of one of the bulkhead subassemblies in the fuselage

55 structure shown in FIGS. 2 and 3.FIG. 9 is a view of the front bulkhead subassembly shown

in FIG. 3.FIGS. lOA and lOB are views of the fuselage showing the

manner in which the external skin is affixed to the structuralframe to form a dual fuselage structure.FIGS. 11-16 are views of structural connectors employed

in assembling the cabin structure shown in FIG. 2. The sizeof these structural connectors is such that they are not shownin FIG. 2, but the location of each of the connectors shownin FIGS. 11-16 is indicated on FIG. 2.FIG. 17 is a view of a prior art fish mount welded joint

that can be used on an aircraft fuselage.


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US 6,604,710 B25

FIG. 18 is a view of a prior art nutt weld that can be usedon an aircraft fuselage.FIG. 19 is a view of a prior art fuselage truss weld.FIG. 20 is a view of a prior art fuselage gusseted truss

weld.DETAILED DESCRIPTION OF IREPREFERRED EMBODIMENT

A light aircraft 2, of the type shown in FIG. 1, can employa welded structure or frame 10 in the aircraft fuselage 4. Thisframe includes chromium molybdenum steel longitudinaltubular members 12 and lateral tubular members 14 that areconnected by structural connectors 20 located intermediatethe ends of the longitudinal tubular members 12 and atintersecting ends 16 of lateral tubular members 14. At leastone of these longitudinal tubular members extends into thetail section or empennage 6. The load bearing structure ofthe fuselage 4 includes both this structural frame or truss andthe external sheet metal skin 46 that is riveted to thestructural connectors or connector blocks 20 to form afuselage assembly 10. Other tubular members 52 and struc-tural connectors 50 form the frame or truss for the aircraftcabin 8, as shown in FIG. 2. The fuselage structural con-nectors 20 and the cabin structural connectors 50 not onlyserve to interconnect the tubular members and to attach theaircraft skin, but as will be subsequently discussed in greaterdetail, these structural connectors 20,50 also serve as jigs tosimplify the assembly of the aircraft structure.The longitudinal tubular members 12 and the lateral

tubular members 14 in the preferred embodiment are chro-mium molybdenum steel tubes of varying diameters andthicknesses, but would include tubes having a diameter of0.500 inch and a wall thickness of 0.058 inch. Steel tubes ofthis type are commonly used in light aircraft construction.The structural connectors 20, 50 are formed from chromiummolybdenum steel bar stock. The longitudinal tubular mem-bers 12 and the lateral tubular members 14 are attached andwelded to the structural connectors 20, and the cabin tubularmembers 52 are welded to the cabin structural members 50.In the preferred embodiment of this invention, plug weldsare employed. A plug weld is a circular fusion weld, involv-ing partial melting of the base or parent metal, made in holesin the structural connectors 20, 50. The plug welds in thepreferred embodiment of this invention are made byTungsten-inert gas welding or TIG welding, where an arcplasma from a nonconsumable tungsten electrode radiatesheat onto the work surface. A weld puddle is created in aprotective atmosphere provided by a flow of inert shieldinggas. Heat must then travel by conduction from this puddle tomelt the desired depth of weld. In the preferred of thisinvention, a filler material is added to complete the weld ofthe tubular members to the structural connectors.Although the dimensions of the individual structural

connectors or connector blocks 20 employed in the fuselageframe structure 12 differ depending upon the point in theframe in which they are employed, the differences need notbe discussed in detail. These differences in the structuralconnectors 20 are primarily in the location, orientation anddimensions of holes or openings in the connector blocks 20to properly receive and align the longitudinal tubular mem-bers 12 and the lateral tubular members 14 along the taperedfuselage assembly.In the preferred embodiment, as shown in FIGS. 4--7, each

of the fuselage structural connectors 20 comprises a cylin-drical member having a substantially circular cross section.In the preferred embodiment, a cylindrical peripheral edge

622, having a thickness of 0.750 inch, extends betweenopposite faces 24, but connectors having a different thick-ness could also be employed. The solid structural connectors20, formed from steel bar stock, include bore holes 26 drilled

5 into the peripheral edge 22, and drilled through holes 30extending between opposite faces 24. Each bore hole 26 hasa closed end 28. Transverse or plug holes 32 are drilled fromone of the faces 24 to intersect a corresponding bore hole 26.Each transverse hole intersects the corresponding bore hole

10 26 between the closed bore hole end 28 and the peripheraledge 22 through which that bore hole extends. Transverse orplug openings 34 extend from an adjacent portion of theperipheral edge 22 and intersect the through holes 30. Thetransverse opening 34 can be drilled holes or slots extending

15 along the peripheral edge 22 between opposite faces 24.The closed end bore holes 26 in structural connectors 20

have an inner diameter sufficient for insertion of the end ofa lateral tubular member or tube 14. A relatively tight fit ispreferred. When fully inserted into a bore hole 26, the lateral

20 tubular member end 16 extends beyond the intersection ofthe transverse hole 32 with the bore hole 26. The transverseholes 32 thus permit observation to determine if the lateraltubular members 14 are fully inserted into the bore holes 26.The transverse holes 32 also serve as plug holes in which a

25 plug weld can be formed between the lateral tubular mem-bers 14 and the structural connectors 20. In the preferredembodiments of this fuselage structural connector 20, theaxis of each bore hole 26 extends perpendicular to the axisof revolution of the cylindrical structural connector block

30 20, although this perpendicular orientation can be altered inother configurations.The through holes 30 have an inner diameter sufficient to

permit insertion of longitudinal members 12. A relativelytight fit is preferred. The axes of the through holes 30 will35 typically extend at an acute angle relative to the axis ofrevolution of the cylindrical connector block 20. In anaircraft fuselage 4, the longitudinal members 12 will not bemutually parallel because the cross section of the fuselage 4is smaller adjacent the tail section 6 than adjacent to the

40 cabin 8. In some cases, the through holes 30 will extend ata compound angle relative to the faces 24 of the connectorblock 20. In other words, the axis of a through hole 30 mayextend at an acute angle relative to each of three orthogonalaxes in which the axis of revolution of the cylindrical block

45 20 is one of those axes. The through holes 30 will then serveto orient the longitudinal tubular members 12 that fit tightlyin the corresponding through hole 30. The mutual orienta-tion of different longitudinal members 12 will thus bemaintained by structural connectors 20 that are intercon-

50 nected by lateral tubular members 14. The transverse open-ings 34 intersecting the through holes 32 form plug holes inwhich a plug weld can be formed. However, if the tolerancesof the through holes 30 are sufficiently accurate, the longi-tudinal tubular members 12 can be held in proper position

55 without any welds.The fuselage structural connector 20 of the preferred

embodiment are used to form a triangular section or subas-sembly 40 that includes three structural connectors 20A,20B and 20C located at the apices of three lateral tubular

60 members 14 forming the sides of the triangular section, asshown in FIG. 8. Each of these structural connectors 20A,20B and 20C has two bore holes 26, the axes of whichextend at an angle forming the angles at the three corners ofthe triangular section 40. The triangular section 40 can be in

65 the form of an equilateral triangle or in the form of a moregeneral triangle, depending upon the desired local geometryof the fuselage 4.


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US 6,604,710 B27

Each of the three structural connectors 20A, 20B and 20Calso includes one through hole 30 that is located between thecenter or centroid 36 of the block and an adjacent portion ofthe peripheral edge 22. In the preferred embodiment thethrough hole 30 is substantially aligned with and is inter-sected by the axes of the bore holes 26 located in the sameblock 20.The triangular subassembly 40 is fabricated by inserting

lateral tubular members 14 into the bore holes 26. If theclearance is sufficiently tight, as desired, the tubular mem-bers 14 can be simultaneously inserted into the bore holes 26of the three structural connectors 20 to form the triangularsection. The lateral tubular members 14 should primarily beloaded in compression since they will form a tubular framein a bulkhead assembly 42 in the fuselage 4. If the tolerances 15are sufficiently tight, the structural connectors 20 and thelateral tubular members 14 in this triangular configurationwill be able to carry loads placed on this subassemblywithout requiring any additional fastening or securingmeans. However, the structural connectors 20 include trans- 20verse or plug weld holes 32 that provide space for a plugweld between each structural connector 20 and the lateraltubular members 14 connected thereto. Plug welds at thesepoints do secure the lateral tubular members 14 in place, butthese welds should not be subjected to significant loads.Each structural connector 20 in each triangular subassem-

bly also includes a through hole 30 that will receive alongitudinal tubular member or longeron 12. The longerons12 extend continuously through the holes 30 and are notconnected or welded to the structural connectors 20 at the 30ends of the longerons 12. In the preferred embodiment, thelongerons or longitudinal tubular members 12 extend fromthe aircraft tail section or empennage 6 to the cabin 8. Thesecontinuous members 12 can thus carry tensile loads alongtheir complete length. If the through holes 30 are drilled to 35sufficiently tight tolerances the longerons 12 will fit snuglyin the structural connectors 20. The longerons 12 will not beparallel in the preferred embodiment of this fuselage frameassembly 10. As shown in FIG. lOAthe topmost longeron 12will be formed so that it will continue to form the leading 40edge structural member for the vertical stabilizer in the tailsection 6. In view of the relative shapes and relative orien-tation of the three longerons 12 where they pass througheach structural connector 20, the axes of the three throughholes 30 in each of the three structural connectors 20 will not 45typically be parallel. As mentioned earlier, the through holes30 is some structural connectors 20 will not be parallel to theaxis of revolution of the cylindrical connector 20, nor willthese through holes 30 extend perpendicular to the faces 24through which they extend. Because of the unique orienta- 50tion of the through holes 30 and the shape of the longerons12 in the entire fuselage frame assembly 10, the triangularbulkhead subassemblies 40 can typically be positioned atonly one location within the frame assembly 10. Thus theentire frame assembly 10 can be assembled with the struc-tural connectors 20 serving not only as a means for con-necting the tubular components 12 and 14, but also as a jigto assembly all components in place. To assemble thevarious components in place, the lateral tubular members 14are first attached to structural connectors 20 to form trian-gular bulkhead subassemblies or sections 40, and the lon-gitudinal tubular members or longerons 12 are then insertedthrough the holes 30 until the lateral subassemblies 40 arepositioned at their proper locations. Itshould be understoodthat the triangular subassemblies 40 can be positioned on the 65longitudinal tubular members 12 either before or after thelateral tubular members 14 are plug welded to the structural

8connectors 20. In either case, the longitudinal tubular mem-bers 12 can also be plug welded to the structural connectors20 through the transverse openings 34 extending through theperipheral edge 22 to intersect with a longitudinally oriented

5 hole 30 through which the longeron 12 extends.In the preferred embodiment of this invention, the plugwelds are formed by TIG welding. Since the structural

connectors 20 are fabricated from solid chromium molyb-denum steel bar stock, it is possible to preheat each of the

10 three weld locations in each block 20 by simply heating theblock. The heat generated by the TIG electrode is sufficientto heat the entire block 20 so that each weld site can beadequately preheated. In the preferred method of assemblingthe fuselage frame 10, all of the welds are made after theframe has been assembled using the structural connectors 20as jigs. It then becomes a simple matter to make the threewelds to two lateral members 14 and a single longitudinalmember 12 in each structural connector 20. The entire frameassembly 10 can then be welded by sequentially formingplug welds in all of the structural connectors 20 during onecontinuous operation.The structural connectors 20 serve not only to connect the

tubular members 12 and 14 to form the frame 10, but alsoserve to attach the aircraft or fuselage outer skin 46 to this

25 frame 10. When the sheet metal skin 46 is attached to theframe 10, the truss frame 10 and the outer sheet metal skin46 both form a double fuselage load bearing structure, sincethe sheet metal skin can support loads in the plane of the skin46. Bulkhead panels 44 are first attached to the triangularframe subassemblies 40. These bulkhead panels 44 gener-ally extend laterally relative to the fuselage 4. The panels 44are parallel to the lateral tubular members 14 and transverserelative to the longitudinal tubular members 12. Panels 44are open in the center to provide space for cables, hydrauliclines or other conventional equipment extending through thefuselage 4 to the tail section 6. Each panel 44 can comprisea single sheet metal panel or a series of sections that areconnected together, either by the structural connectors 20 orby separate fastening means. The panels or panel subassem-blies 44 are, however, attached directly to the structuralconnectors 20. In the preferred embodiment, the panels 44are secured to the structural connectors 20 by standardaircraft rivets 38 or AN535 drive screws that have beensecured to the structural connectors 20 by conventionalmeans. See FIG. 8. In the preferred embodiment, the panels44 have a peripheral lip that extends generally longitudinallyand includes rivets for attaching the panels forming theexterior skin 46 to the panels 44. In this way the panels areconnected to the structural connectors 20. Stringers 48 canalso be attached to strengthen the exterior skin 46.The double fuselage assembly fabricated in this manner

can be attached to a cabin assembly that also includesstructural connectors 50 joining tubular members 52. Thetubular members 52 used to form the cabin frame are

55 oriented in much the same manner as in a cabin frame in acomparable aircraft. Representative cabin structural connec-tors 50, which are similar to fuselage structural connectors20, are shown in FIGS. 11-16. The location of six individualcabin structural connectors 50A-50F is shown in FIG. 2.

60 These cabin structural members would also be machinedfrom solid steel, preferably chromium molybdenum steelhaving the same composition as the fuselage structuralconnectors 20. Cabin structural connectors 50A-50F do notgenerally have a cylindrical configuration. They are insteadmachined to fit their specific application. These cabin struc-tural connectors include bore holes 54, through holes 56 andtransverse holes 58, which serve the same purpose as those


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9US 6,604,710 B2

10used in the fuselage connectors 20. In other words they areused both as fasteners and as jigs for assembling the cabinframe. Of particular significance is cabin structural connec-tor SOE that forms the main connection point between thetopmost longeron 12 and the cabin assembly. By employing 5structural connectors 20 and 50, the entire aircraft can beeasily assembled and welded in an operation the bothreduces assembly time and complexity and simplifies weld-ing by eliminating gusset plates and butt welds.The structure depicted in the representative embodiment

is primarily intended for use in the construction of lightaircraft, but could be employed in the fabrication of otherframes. Structural connectors of the type depicted herein canbe used in an aircraft structure either in conjunction with aload bearing external skin or with a non-load bearing fabric 15skin. These structural connectors can also be used to replacesome, but not necessarily all of the joint connections in anaircraft structure. The aircraft structure is also not limited tothe precise configuration depicted herein. For example, thecross section of the fuselage could differ from that repre- 20sen ted by the preferred embodiment. The shape of thebulkhead panels could be changed to conform to a differentfuselage cross section. The embodiment of this inventionshown in the drawings and described herein is therefore onlyrepresentative of numerous other configurations and equiva- 25lent structures that would employ the invention defined bythe following claims.We claim:1. A structural connector for use in connecting tubular

sections of a structural frame, the structural connector com- 30prising:a block having opposed planar faces and a peripheral edgeextending between the two opposed faces;

at least one bore hole extending inwardly from the periph- 35eral edge, the bore hole comprising means for receivingan end of a tubular section; and

a transverse hole extending from one face of the block andintersecting the bore hole, the bore hole extendinginwardly beyond the transverse hole;

the transverse hole comprising means for determining ifthe end of the tubular section is fully inserted into thebore hole.

2. The structural connector of claim 1 wherein the blockincludes a through hole extending between opposite faces of 45the block, the through hole extending along a longitudinalaxis transverse to a lateral axis of the bore hole.3. The structural connector of claim 2 wherein a secondtransverse hole extends between the peripheral edge and the

through hole.4. The structural connector of claim 3 wherein each

transverse hole comprises an opening means for weldingtubular members crossing the transverse hole to the block.5. The structural connector of claim 1 wherein the trans-verse hole has a size sufficient to permit formation of a plug 55weld with a tubular section positioned in an intersecting borehole.6. The structural connector of claim 1 wherein the bore

holes and transverse holes have the same diameter.7. The structural connector of claim 1 wherein a through

hole extending between opposite faces of the block andalong a longitudinal axis transverse to a lateral axis of thebore hole, and wherein the through hole is offset relative tothe centroid of the structural connector.8. The structural connector of claim 1 wherein two bore

holes extend into the peripheral edge of the block, axes ofthe two holes intersecting to form an acute angle.

9. The structural connector of claim 1 including rivetsextending from one face of the block.10. The structural connector of claim 1 wherein the blockincludes a through hole extending between opposite faces ofthe block, and an opening in the peripheral edge of the blockintersecting the through hole.11. An assembly comprising tubular members and astructural connector for use in connecting the tubular mem-bers to form a structural frame, the structural connector

10 comprising:a block having a thickness greater than an externaldiameter of tubular members to be connected by thestructural connector;at least one bore hole extending into the block from aperipheral edge of the block, the peripheral edgeextending between opposite planar faces of the block,the bore hole having an inner diameter sufficient forinsertion of an end of a tubular member into the borehole; anda through hole extending between the opposite faces ofthe block, the through hole having a diameter sufficientto permit a second one of the tubular members to passthrough the block;

the block comprising means for welding at least twotubular members to form a portion of the structuralframe.12. The assembly of claim 11 wherein each bore hole hasa closed end spaced from the peripheral edge of the block.13. The assembly of claim 11 wherein the through holecomprises a hole drilled at a compound angle.14. The assembly of claim 11 wherein two bore holesextend inwardly from the peripheral edge, the two bore holesbeing positioned such that two tubular members positionedin the two bore holes extend at an acute angle relative to eachother.15. The assembly of claim 14wherein centerlines of eachof the two bore holes intersect the through hole and thethrough hole is located between the centroid of the block andthe peripheral edge of the bock.16. The assembly of claim 11 wherein rivets are posi-tioned on one face of the block.17. The assembly of claim 11 wherein the block com-prises a cylindrical block.18. The assembly of claim 11 wherein the through hole isoffset relative to the centroid of the block.19. The assembly of claim 11 wherein the tubular mem-bers are welded to the block.20. The assembly of claim 11 wherein the frame com-prises at least one triangular section having structural con-nectors at each apex of the triangular section, with lateraltubular members extending between the structural connec-

50 tors and with longitudinal tubular members extending trans-versely relative to and through structural connectors at eachapex of the triangular section.21. An aircraft fuselage comprising an internal frame andan external skin covering the internal frame, the framecomprising a plurality of longerons wherein each connectingblock includes at least one bore hole for receiving a tubularmember, and a tranverse hole intersecting the bore hole, thecorresponding tubular member extending into the bore holepast the transverse hole and a plurality of bulkheads sup-porting the longerons at longitudinally spaced positions,

60 each bulkhead comprising tubular members joined togetherat the ends thereof by connecting blocks located at apices ofeach bulkhead, the connecting blocks also connecting thebulkheads to the longerons.22. The aircraft fuselage of claim 21 wherein each bulk-

65 head comprises a triangular structure including three tubularmembers joined at adjacent ends thereof by three connectingblocks.

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23. The aircraft fuselage of claim 21 wherein the tubularmembers comprising the bulkhead are welded to the con-necting blocks.24. The aircraft fuselage of claim 23 wherein the tubular

members are plug welded to the connecting blocks. 525. The aircraft fuselage of claim 21 wherein the long-erons comprise tubular members.26. The aircraft fuselage of claim 21 wherein the external

skin is attached to the connecting blocks.27. The aircraft fuselage of claim 26 wherein the external 10

skin is riveted to the connecting blocks.28. The aircraft fuselage of claim 21 wherein the long-

erons are welded to the connecting blocks.29. An aircraft frame comprising a plurality of tubular

sections connected by a plurality of connector blocks and an 15external skin, the tubular sections including longerons andlateral tubular sections, the connector blocks connecting aplurality of lateral tubular sections to each longeron atmultiple longitudinal positions on each longeron, the exter-nal skin being attached to the connector blocks so that loads 20applied to the aircraft frame are carried by both the tubularsections and the external skin.30. A method of assembling an aircraft fuselage compris-

ing the steps of:forming bore holes and forming through holes extending 25between opposite planar faces, in connector blocks; and

12inserting the ends of a first set of tubes into bore holes inconnector blocks, the bore holes in individual connec-tor blocks being oriented to function as jigs to properlyorient the tubes to form a portion of an aircraft fuselageframe;

inserting a second set of tubes into the through holes sothat the tubes in the second set extend continuouslythrough the connector blocks, the tubes in the secondset forming longitudinal members in the aircraftfuselage, the through holes being oriented to functionas jigs to properly orient longitudinal members relativeto each other and relative to the remainder of theaircraft fuselage.

31. The method of claim 30 comprising the additional stepof welding at least a portion of the tubes, in the first set, toconnector blocks located at the ends thereof.32. The method of claim 31 wherein the tubes in the first

set are plug welded to connector blocks.33. The method of claim 30 wherein the aircraft fuselagecomprises a cabin section and a tail boom section, the first

set of tubes and connector blocks joining tubes in the first setbeing positioned in both the cabin section and the tail boomsection of the fuselage.

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6604710 Light Aircraft Fuselage and Stru

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