R. Scholler Pober Sport

STEEL TUBE FUSELAGESFactors for Economy in Fabrication and Assembly

P articularly adapted to the massproduction of the smaller types

of commercial and military aircraftis the welded steel tube type offuselage. This form of constructionpermits fabricating the fuselage, thebasic structural element of the air-plane, in simple, accessible jigs, andmakes it possible to apply the fuse-lage skin or covering as the last stepin the assembly of the airplane. Localconcentrations of work in the assem-bly line are avoided, thus eliminatingone of the major problems of massproduction, which must be predicatedupon a smooth flow of work beingapplied to the airplane as it passesthrough the various stages of fabri-cation and assembly.

The purely abstract considerationsinvolved in the design of welded steeltube structures are identical to thosecommon to other phases of structural

engineering. The stress analysis meth-ods applicable to steel tube fuselagesare but modifications of the estab-lished procedure used for analyzingbridge trusses and similar structures.After the directions and magnitudesof the loads involved are known, andthe required margins of safety estab-lished, the determination of tube sizesand joint requirements become com-paratively simple matters. The tubescan be joined by welding with eitherthe electric arc or gas. The processdiffers from other applications ofwelding only in that greater skillis required in working with thin tubegages to maintain required accuracyin alignment.

When laying out a welded steeltube fuselage it is of prime import-ance that the designer have a prac-tical knowledge of all the factors af-fecting the economical production of

the structure. These factors begin toinfluence the layout of the basicstructure at the time when it issimply a geometrical concept of linesand points, and continue to shapethe design throughout to the deter-mination of major attachment fit-tings, selection of the method ofcovering the tube structure, and fin-ally of detail fittings and attachmentpoints required for controls andequipment. If these factors are neg-lected anywhere in the progress ofthe design, beginning with the geo-metric layout of the basic structureand concluding with the design of thelast attachment fitting, the funda-mental economy of welded steel tubestructures will be sacrificed.

In the design of a welded steel tubefuselage the first step is the deter-mination of the fuselage form tobe used. The basic fuselage structure

SPORT AVIATION 27

Front cockpitrudder pedal' ~support

Rear cockpitr--mstrument

t ' panel supports

Windshieldsupportstructure

Tube truss connecting upperr--longerons to semi-monocoque\ attachment fitting

Semi-mon-ocoqueattachmentfittings

\

Rear cock pH- rudder pedalsupport

Front cockpit tabcontrol support ~

I Wing attach-_ment fittings '- - -Rear cockpit tab control support

may consist of two main forms: theall-steel-tube structure wherein thecomplete primary structure is awelded steel tube truss, or the so-called "composite" structure madeby joining together a welded steeltube forward portion which includesthe cockpits, and an aluminum-alloysemi-monocoque aft structure.

Some designers favor the all-steel-tube design, believing that a singlefabrication process throughout for thebasic structure facilitates ^productionby simplifying tooling, minimizingthe factory equipment required, andeliminating the fittings necessary forattachments of the semi-monocoquesection. This form is also desirablebecause access panels extending thefull length of the fuselage are re-movable thus providing ample roomfor installation operations during theairplane assembly. The panels canalso be removed for inspection andservice.

Another group of designers believethat complete access to the extremeaft portion of the fuselage is rarelyrequired. This group prefers the com-posite type in order to take advant-age of its lighter weight, higher rateof production in factories having fa-cilities for sheet metal construction,and the economy gained by eliminat-ing the necessity of fairing side pan-els and their supports to the aft por-tion of the fuselage.

Regardless of the type selected,however, there are certain factorsinvolving the basic geometry of thetube structure layout which must not

Pober Sport28 DECEMBER 1971

be neglected if economical produc-tion is to be achieved.

Consider the all-steel-tube struc-ture first. Because of the need forcockpit spaces, landing gear and low-er wing attachment points, the fourlongeron type structure is almost al-ways used.

The four longeron tube type ofstructure inherently provides the de-signer with a choice of any two ofthe four sides as major sub-assembliesof both the fore and aft sections. Thepractice of dividing the structure intofore and aft sections (to be laterjoined by welding) is universally fol-lowed, partly for production break-down reasons and also to save weightby reducing tube sizes in the rearsection. In cabin type airplanes it isalso necessary to build an irregularlyshaped front section because of pas-senger compartment space require-ments, with the result that the struc-ture is divided aft of the cabin topermit economical weight distribu-tion, and to simplify tooling, as wellas to facilitate final alignment of thetube structure.

Naturally, they include two longe-

rons and the tubes connecting same,a choice must be made to build eitherthe top and bottom frames or theside frames, as sub-assemblies. Theforward section bottom frame isusually well braced and may be de-pended upon to hold its shape andresist distortion through the majorassembly. The top frame, however, isusually poorly adapted to retainingits shape as a sub-assembly panel,because of the necessity of providingclear unobstructed bays for cockpitsin open cockpit airplanes. At theopen bays deflection of the framemay occur after removal of the framefrom the sub-assembly jig.

After a consideration of these fac-tors it is logical to design the sideframes as sub-assemblies, consistingof the upper and lower longerons, in-ter-connecting tubes and fittings. Afurther advantage is obtained in thatthe sides are usually symmetricallybraced so that welding distortion andshrinkage are practically the samefor both sides when consistent weld-ing procedures are followed. Withproper guidance and indexing theside frames, being well braced, may

be depended upon to determine ac-curately the final structure assembly,thus eliminating the need of an elab-orate f inal assembly jig.

The choice of sub-assembly framesto form the aft section, however, maybe the top and bottom frames ratherthan the side frames. The decisiondepends largely upon the type and lo-cation of empennage and tail-wheelattachment fittings. In any event, therigidity of any side of the aft sec-tion is usually sufficient to maintainits shape and permit sub-assemblyframe fabrication.

The Pratt truss is shown in all ofthe fuselage layouts illustrated in thisarticle as this type of truss offersthe advantages of normal verticallines to work from during draftingand tooling layout. However, the War-ren truss will usually consist of few-er tubes, and for this reason may belighter for a given fuselage size. Thetype of truss used will actually de-pend upon the required attachmentpoints as governed by factors ex-traneous to the tube geometry, suchas placement of the wing to obtainthe correct airplane balance.

Geometry of All-Steel Tube FuselagesIn the line drawings of tube layouts in Figs. 1 to 6 are

shown logical development of the geometry of an all-steel-tube fuselage design. The sequence of these figures isbased upon the design's desirability from the standpointsof fabrication and tooling, beginning with Fig. 1 as thtmost desirable. All illustrations are for a fuselage suitablefor a two-place, tandem, open-cockpit airplane, but the

principles discussed are applicable to all types and there-fore detail treatment of the cabin-type airplane has beenomitted purposely. Note that in all designs the object isto accomplish the same result, namely to retain all framesin true flat planes whenever possible especially theside frame.

^ .-Station cross tubeK • Forward section\ Braces in these cockpit

bays in bottom pane/on/y,-

Aft section! . Spliced lap

we/a/

*~-f~ronf side frame''

/Upper forward longeronst-Upper forward frame - -

PLAN VIEW

" Cross tube /'*#ear side frame----''

Upper aft longerons^~ Upper aft frame - -

*~-Lower forward frame- --'''-Lower forwardlongerons SIDE ELEVATION

FrontElevation

~--Side tubes t o bevertical andparallel

*-Cross tubes tobe horizontal

er forward longerons >~~--Lower aft frame------"

FIG.l

TypicalIntermediate

Stationin Forward

Section Typicalin aft

StationSection

All-steel-tube structure that isideally adapted for economicalmass production. From the planview in Fig. 1, it can be seenthat both the upper and the low-er longerons of the forward sec-tion are perfectly straight in allplanes, and are also parallel toand equidistant from the center-line of the airplane for the en-tire length of this section. Theupper and lower longerons ofthe aft section lie in verticalplanes, with the bend in thelongerons at a position adjacentto the welded junction of theforward and aft sections. Alllongerons are straight lines ex-cepting where the bends occurat the junction just forward ofthe splice. Also note that thesebends are at the same stationin both the plan view and theside elevation, and because ofthis fact the necessary toolingand production operations con-nected with this fuselage canbe readily accomplished.

SPORT AVIATION 29

PLAN VIEW

FrontView

TypicalIntermediate

Station inForwardSection

SIDE ELEVATION

FI6.2

Typical Stationin Aft Section

Upper longerons in the side ele-vation, Fig. 2, are straight andparallel to centerline of thrust.In the lower longerons one bendis at the same station as thebend in the plan view, anotherbend is at an intermediate sta-tion further forward. If possible,one section of the forward low-er longerons should be parallelto the centerline of thrust. Inthe plan view longerons arestraight, equidistant from andalso parallel to the airplane'scenterline. Upper and lowerlongerons of aft section lie ina vertical plane. All longeronswith the exception of the lowerforward are straight betweenjunction points. All of the sideframes are in one plane.

SIDE ELEVATION

FrontView

TypicalIntermediateStation in .Forward Typical StationSection «n Aft Section

FIG.3

Fuselage in which the upper andthe lower longerons are straightin the plan view of the frontsection, shown in Fig. 3, withthe exception of the one bendat the station forward of thejunction of the forward and theaft sections. The aft portions ofthe forward longerons falls on astraight line with the aft sectionlongerons. The front and therear sections have vertical sides.In the side elevation the upperlongerons are perfectly straightfor the length of the fuselage.They are also parallel to thecenterline of the thrust. Onebend is made in the lower long-erons, it is located at the samestation at which the bend occursin the plan view.

Bend-

TypicajIntermediateStation inForward

FIG.4

rorwwrc* -.. . . p, . -<i»r+ion Typical StationSection £HAft Secfion

Layout of fuselage similar tothat in Fig. 3, but not quite aswell adapted to production isshown in Fig. 4. Although sidesare vertical, they taper in to-ward the rear, starting at thefront station. The welded junc-tion of fore and aft sections isat the identical station for alllongerons. This type of con-struction introduces some diffi-culties. Drafting becomes com-plicated since tubes will be en-countered in many layouts thatare not in plane of the paper,and must be projected to obtaintrue views. The same difficultywill be encountered by tool de-signers when laying out assem-bly jigs.

30 DECEMBER 1971

- Bottom et,

PLAN VIEW

TypicalIntermediateStation in

ForwardSection

FIG.5

Typical Stationin Rear

Upper longerons in plan view inthe forward section, Fig. 5, arestraight and parallel to center-line of airplane; lower longer-ons are also straight but notparallel with upper ones, beingtapered in toward the aft endso that side frames are in a"twisted" plane that complicatesall layout and tooling work. Thesame condition exists with re-gard to longerons in the aftsection. In side view upper long-erons are perfectly straight, andparallel to line of thrust andthus present the only truly un-broken plane in the fuselage,but unfortunately one that can-not be built up as a sub-assem-bly because of the unbracedcockpit openings, precludingthe necessary rigidity. A designof this nature should be avoidedbecause of the high tooling andproduction expense involved.

Geometry of Composite-Type Tubular FuselagesComposite-type fuselages shown in Figs. 6 and 7, are

advanced developments wherein the steel-tube structureterminates immediately aft of the rear cockpit, and theaft portion of the fuselage is formed by an aluminum-alloy semi-monocoque structure. A composite type fuse-lage may employ any one of the basic steel tube layouts

shown in Figs. 1 to 5, although it is quite obvious thatit is still important to use a simple tube structure. Themajor additional consideration lies in the method ofattachment used between steel tube structure and semi-monocoque structure.

Steel-tube section-^

PLAN VIEW

Built-up -tripod tube s ̂ -.^

-Semi-monocoque section

-•Upper attachment point'Stringer

Benc/-'FIG.6 SIDE ELEVATION Skin-'

Front View erf

'King orbulKhecno/

Cutouts in^butkhead

to clearbv/bang/e\stringers

Steel-Tube Section JVP'9al Intermediate ^-_£_-^Station in Steel-Tube Typical Bulkhead Station

section in Semi-Monocoque Section

Three-point attachment designfor connecting a steel tube trussstructure to a semi-monocoquesection similar to the type thatis illustrated in Fig. 6 is con-sidered by airplane designers tobe the most practical, since thismethod of construction permitsplacing the attachment fittingsin favorable position such thatmuch of the torsion and thebending resistance which is in-herent in the semi-monocoquetype of structure can be util-ized to good advantage, whichis to say, that these attachmentpoints are placed as wide apartas possible in the structure thusobtaining a particularly favor-able result, the achievement ofstructural strength with goodweight economy. The benefitsderived from this three-pointmethod of attaching a semi-monocoque structure, however,are offset somewhat by a slightdisadvantage in that a separatesub-assembly is required for thetriangular tube truss which con-nects the longerons to the up-per semi-monocoque fitting.

SPORT AVIATION 31

Front View Typical IntermediateStation in Steel-Tube

SectionTypical Bulkhead Stationin Semi-Monocoque Section

Alternative method of semi-monocoque attachment shownin Fig. 7, involving four attach-ment fittings formed by term-nations of longerons, avoidsnecessity of a separate tube sub-assembly for upper fitting. How-ever, this may not be as satis-factory from a structural stand-point, as the material of thesemi-monocoque section is notalways as evenly loaded as withthree-point attachment, and itwould probably be necessary touse a slightly heavier gage sheetfor the skin covering. To com-prehend this factor, it shouldbe understood that the semi-monocoque structure is rarely,if ever, a round section, butrather a sort of flattened el-liptical section having a smallerminor axis at its upper end tomeet the requirements of aero-dynamic form, particularly withthe smaller two-place tandemtype of airplane wherein thefuselage must be deep and nar-row. If the semi-monocoquestructure could be designed as atrue circular section, then thefour point attachment wouldhave definite structural advant-ages.

32 DECEMBER 1971Acro Sport