US2387219A - Aircraft structure - Google Patents

Description

Uct. 16, 1945. B. N. WALLIS AIRCRAFT STRUCTURE 2 Sheets-Sheet l Filed June 17, 1942 B. N. WALLIS AIRCRAFT STRUCTURE 0ct. "l6, 1945.

Filed June 17, 1942 2 Sheets-Sheet 2 Patented Oct. 16, 1945 AIRCRAFT STRUCTURE Barnes Neville Wallis, Weybridge, England, as- "sighor to Vickers-Arm'strongs' Limited, Westminster, London, England Application June 17, 1942, Serial No. 447,406 1 I In Great Britain August 8, 1941 6 Claims.

The invention has reference to the construction of stressed skin structuresfor aircraft fuselages and Wings. It has heretofore beenrthe custom to divide the surface of the outer plating into a series of relatively small panels by riveting or welding the plating to closely pitched longitudinal spars or stringers and transverse ribs or intercostal members in order to enable the relatively thin plating to develop a high proportion of the inherentshear strength whichit is capable of developing in bulk form. The attachment of the plating to the frames at frequentintervals serves to stabilise the plating against the formation of wrinkles which tend to occur in thin plate under the action of shear stress, but this form of construction is disadvantageous in that the multiplicity of riveted joints involves a large amount of skilled labour, while the use of a large number of closely pitched longitudinal members results in these members being correspondingly thin in cross-section with consequent instability and reduction of the compressive strength which the material is capable of developing in bulk form, as a result of which the overall weight of the structure is appreciably increased.

In the case of an aircraft fuselage (or a restricted portion thereof) which is supercharged to compensate for the low external atmospheric pressure prevailing at high altitudes, there is the added disadvantage that local bulging of the skin is caused by the high pressure diiference within and without the structure; such bulging not only impairs the aerodynamic qualities of the fuselage, but also originates stresses in the rivets at the points of attachment of the plating to the structural members. thereby starting the joints, producing air-leaks and weakening the structure.

Alternative constructional methods have been proposed, consisting in attaching the plating either to the longitudinal members only, or to the transverse members cnlv. but the structural difliculties have not been remedied, because in each case the plating is still free to bulge, i. e. either fore and aft between the longitudinal members, or transversely between the ribs, so that the tensile leading upon the rivets is accentuated rather than relieved and the bulges in the plating become disproportionately more pronounced by the reduction in the number of points of its attachment to the skeletal structure. The first alternative of attaching only to the longitudinal members does not havethe effect of reducing the number of transverse members as these are required in any case to support the slender longitudinals. The second alternative of attachment to transverse members only is particularly disadvantageous in the. case of pressure cabins, since it, gives rise to an ,eX- aggerated form of the worst kind of bulging, when the furrows are atfright angles to the air stream. In neither case is the total amount of riveting required very greatly reduced.

The object of the present invention is to provide an ,improved method of construction, where by (a), the longitudinal members may be reduced in number to the, minimum, to enable them to be made of thick and robust cross-sectional proportions and consequentlly improve their ability to withstand compressiveloading, whilst also permitting a corresponding reduction in the riveting of plating to longitudinals, (b) the riveting of the transverse members to the plating (previously considered necessary), may be entirely'eliminated, and (c) in-a.pressure cabin both forms, of bulging will be precluded. This method improves the structural 'efficiency by achieving a considerable reduction 'in overall weight, and uses proportionately less labour ,during construction. Afulther important ad'vanf tage possessed by the invention lies in the elimination of the bulging of the plating in thecase of 'the upper surface of wingswhere the pres sure distribution is below the static pressure of the atmosphere in the locality. g

In a fuselage or wing structure in accordance with the invention, the skin plating is attached to longitudinal members between which it is stretched over shaped ribs in such manner that it is subjected to a circumferentialtensild stress running in the direction of the planes containing the 'ribs, andso that said ribs also serve to support, the plating against inwardcollapse by strong centrifugal pressure against the plate, there being however no attachment of the plating'to said ribs. v f 7 j The imposition of centrifugal pressure upon the plating in this manner enables it to withstand a high degree of shear stress, and thereby to resist the shear and torsionalloads to which the structure is subjected in use. In the ap'plication of the invention to a non-pressure cabin or fuselage, or to a wing, the plating remains tightly held against the supporting ribs, but in a pres- I sure cabin theabsence of any attachmentofthe platin tosaidribs permits the skin together with the longitudinal members to which it is at,-

tached, .to breathe by free cylindrical expansion shear stress increases as the internal pressure rises in relation to the external atmospheric pressure, so that when such dilatation occurs the inward support of the ribs is no longer required, although the ribs are ready to resume their duty of supporting the plating against inward collapse when the internal and external air pressures are equalised; p

The invention :is hereinafter more fully described with reference to the accompanying drawings, which illustrate byway of example the application of the invention to the construction of a pressure cabin for use in an aircraft intended for flight at high altitudes; In said drawings,

3 are diagrammatic views showing the relative dispositions of the component sections of the fuselage during the process of their assembly, 1. e.

respectively as seen before and after the longeron channel-membersare. bolted together. (In Fig. 2 extent tol which the sections ,have reverted from the forminwhichitheyr were held duringsthe riveting of the ribs toithe channel-members, and whichthey resumewhen boltedtogether as shown inILig. '3, vhasbeen-rexaggerated forthe purpose of illustration). Fig.4 is a detail view depicting the manner in which the ribs are .locat'edrlaterally in .relation to theskin plating, andrFig. 5 is a fragmentary perspectiveillustration of ajig. suitable for. the purposeof imposing circumferential stress ,inthe plating whilst clamping the ribs .and channelmembersin position= for their attachment.

The structureofwth fpressure cabini1lustrated Lin-the drawings "issdivided longitudinally intosthree-segmentsh, .B: and. G of equal arc dimension,- and therpresentsdescription is confiiiedrt'o astructure Of thiS kind, although it will be: understood that the. inventimr isnot i so restrictedinitsapplication. I

.Theilongerons.larecomposed-of pairs of channel-section members .-6, i, respectively attached to the opposingedges ofsadjacent segments of the fuselage; the individualsmembers ofeachvpair be ingbolted together-duringthe assembly of the separately constructed segments, so that they combine :to form" completeelongerons. Alternativeliangle-bars .may employed :lieu of channel-section members, if desired. The stiffening-ribs, whichare spaced along thelength of thesfuselage, are eachconstituted by three individual arcuate segments '7 fabricated from Lesect'ion-barsirolled to the appropriate curvature and=provided alongtheedge of the outer flange with a cushioning-or anti-chafingflpad or head 8 of vulcanized fibre; india-rubberor other suitable non-metallicmaterial. Itwillbeunderstood'that the-number of: the arcuate segments which constit-ute the stiffening-ribs. at eac'h cross-sectional pitch will correspond with the numbenof longitudinalsections which-are to compose the-structure. .Beforeproceedingto a:descripti0n of the erection of; thefuselage, there follows anv explanation of.our-.preferredimethod ofwmanufactmeh flone of thecomponenhsections thereof. The plating of arsectionalunitisi'first formed from #8.. sheet -9 which; after it-has been wrought-to the requisite cylindrical or part-spherical curvavture, is riveted at 9l .along each =ofrits longitudinal borderstoone the flanges: I of one of: said channel-membersei, so "that the plating lies upon the .outs'idefaceof said flange, thefree edge of which is directed towards the middle-oftheplate.

AJongitudinal reinforcing Plate Illis preferably interposed between the flange 6] of the channelmember 6 and the underside of the plating 9 along the boundary of the latter, in order to assist in distributing the stresses to be transmitted between th plating and the longerons. The said reinforcing plates I!) may have tongues I01 extending inwardly wherever a segmental tiffen- 'then supported upon a jig comprising two members having surfaces which, as in the present case of a fuselage to be erected from three component sections of equal arc dimension, are relatively inclined-at an angle of One of the two members of the jig is indicated at J in Fig. 5, anditwill'be seen that the outwardly directed back-0f the web of one of the channel-members 6 is constrained to lie flush against the inclined surface J l of said jig member J by means of temporary servicerbolts ll. Said bolts H are passed through pads 52 which are riveted between the flanges of. the channel-members. The other channelem'ember 6 of the sectionis bolted insimilar fashion to the other jig member (not shown). It will be understood that the:re1ative inclination of. the surfaces J of the jig members J will .be varied to suit the case of a three-section fuselage of which the sections are not all of the same arcrdimension, or. to suit fuselages or wings composed of a difierentnumber of sectionalunits. r Thesegmental stiffening-ribs! required for the unitunder construction, being provided at their extremities with gusset-plates H and 12, are assembled-tin parallel arrangement beneath-the platingi}, with their ends suitably disposed in relation to the flanges of the channel-members 6,.and withethe parts assembledin this conditionseachirib l is subjected to an endwiseforce so.as-.topress it outwards to the required degree. The Tmeans employed toimposesaid force upon the 'rib comprises; inassociation. with 143E611 jig, anIsbutment-bar vl? detachably fitted transversely through aehole "l3 -drilled the web of the-rib near its extremitm andtwo cranked:levers--Q,Q whicharesarranged so that they bear-with their toes Q upon the pads 52 as closely-as possible to the underside of the outer flange-5i of the channel-member -6; whilsttheir otherrendsare pressed upwardly againstrsaid bar Pfby operation ofset-screws R, R. The eifcctof "-tighteningthe set-screws 1% against the face -of'th-e jig member J isthusto impose astrong outward pressure uponrthe arouate rib-'1', through the -medium of thesabntment bars P, whilst the reaction of the outward-pressing .forceserves to intensify the pressure of the web faces of the -channel-members 6 against the inclined jig-surfaces'd The outward motion of the ribs i is resisted by the plating S attached between the members'ii; and'a tension is thus set up in the plating 9 by the radial outward pressure of the ribs '1', the 'degree of suchrtension being readily determinedby adjustment of. thema-gnitude of the-endwise force impressed upon-the ribs?! by the 'set-screws"R. In this. condition'ofthe parts; with the plating 9. stressed at the appropriate circumferential tension,.holes aredrilledin thechannel-members'fi toregister with holes-1e already drilled in the gussetsliiand 12, :and'theg-ussets are then'rrveted to the members 6, which latter are thereby securely attached to the ribs 1, before the setscrewsR arereleased.

, c When .theunitsection' is-removedfrolnthe jig, it reverts partially-from the arcuate cross-sectional form in which it has been held by the pres-- sure of the levers Q against the bars P, the tension in the plating Qbeing reduced by a slight straightening of the ribs 1 until a balance of forces is attained.

Three sectional units A, B and C constructed in this manner are then assembled with the outer faces of the webs of their channel-members 5 in juxtaposition. It will be evident that, owing to the aforesaid reversion in shape of the units when released from the jig, the outer faces of the channel-members 6 are no longer inclined at an angle at which two such members can make flush contact to constitute a single longeron; rather is the disposition of said channel-members 6 relative to one another, substantially as shown in Fig. 2, in which the faces of the members 6 gape to a greater or less degree. The requisite condition is however restored by bolting said pairs of channelmembers 6 together by-means of bolts l2 situated near to the outer edge of the web of the longeron, said bolts l2 being tightened until the opposing faces of the channel-members 6 are uniformly forced into contact when the fuselage re-assumes a circular cross-section as depicted in Fig. 3; in this manner the circumferential tension, to which the plating 9 was subjected in the jig, is re-imposed in the complete structure,

The plating 9, being now pressed strongly against the rib members I, is prevented from buckling inwards should the application of shear or torsional loads tend to make it do so by passing the limit of stability of the platein the free state. By this means the thin plating is enabled to develop the same degree of shear stress as ifit were actually attached to the rib members by rivets or bolts, but it will be understood that no mechanical connection between the plating and the segmental ribs does in fact exist, so that the plating is not held against cylindrical expansion if and when the pressure of the internal atmosphere of the fuselage is raised above that of the surrounding air.

Thus, should the internal pressure of the fuselage rise to such a degree that the resultant hoop stress in the plating is equal to or greater than that imposed during construction by a method such as that hereinbefore described, the plating is free to expand to a greater size than the internal skeletal structure, by pure cylindrical dilatation, and no bulges are formed such as those which would result from a mechanical attachment of the plating and the ribs or stringers. In order to locate the ribs 1 in their correct positions in relation to the longitudinal axis of the fuselage, pairs of cleats or the like I 3 may be provided, which are fixed upon the inner face of the plating 9 and between which the ribs 1 are free to slide when cylindrical expansion of the skin occurs.

The joints between the adjoining faces of the channel-members of the longerons are rendered air-tight by the interposition of strip rubber or other suitable jointingmaterial H! before they are bolted together.

The method of construction hereinbefore de scribed is equally suited to the construction of non-pressure cab-ins and fuselages and wings, the customary ovoid or elliptical cross-section being achieved by means of suitably curved internal stiffening-ribs, or by suitably disposing the 1ongerons.

An aircraft wing manufactured in accordance with the invention is conveniently formed in two component sections, each of which is so disposed along its'leading and'trailing edgesas-to form 'a half-longitudinal member, one section forming the upper part of the wing and the other section the lower part thereof, and the arrangement beingsuch that when the corresponding edges of the sections are joined, they combine to compose the leading and trailing longitudinal spars of the wing. As in the method of construction previously described, the act of bolting said halfmembers together imparts the requisite trans-1 verse tensile stress to the skin plating which is stretched between thespars over the arcuate internal ribs. Additional main spars may he intro-. duced into the wing, in which case they may be utilised to maintain the correct loc'ation'of the ribs and thereby permit theaforesaid cleats to be dispensed with. J It will be evident that the method of con- 'struction herein described possesses the additional advantage that it is possible to complete the attachment of all fittings and equipment 611 the interior of a sectional unit before it is assembled for erection with the other component units, the only operations remaining to'be carried out after assembly being the connection of pipe lines, electric leads and the like; In'this manner it is possible to avoid the wastage of time occupied in inserting such fittings and equipment which would otherwise occur by reason of the restricted space within the structure and the con sequent difficulty of access to its interior and the limited number of persons who can be em-' ployed within the structure at the same time.

What I claim as my invention and desire to secure by Letters Patent is: ,1

1. The method of making and'assembling stressed skin structures for aircraft or the like, which comprises warping a'sheet of skin plating to an appropriate curvature, anchoring the sheet at its opposite edges respectively to two longitudinal elements of the skeletal portion of said structure, applying transverse skeletal elements at spaced intervals against the inner surface of the skin with their ends abutting said longitudinal elements at intervals therealong while maintaining said skin in contact with but free from connection with said elements, stretchin said skin over said transverse elements without attachment thereto, and securing the ends of said transverse elements to their respective adjacent longitudinal elements while the skin is stretched.

2. The method of making and assembling hollow substantially tubular stressed skin structures for aircraft or the like which comprises forming longitudinal segmental sections of said tubular structure by anchoring two longitudinal skeletal elements, having a substantial radial dimension, in the approximate spaced apart positions they will occupy in the completed structure, warping a sheet of skin plating to the desired curvature of said section, and securing it along its opposite longitudinal edges to the radially outward edges of said longitudinal elements, applying a series of longitudinal spaced curved ribs against the inner surface of said sheet without attachment thereto, with the ends of said ribs disposed adjacent said longitudinal elements, applying endwise compression to said ribs whereby they are caused to press radially outwardly against said sheet, and securing the ends of said ribs while thus stressed to the longitudinal elements, releasing the stress on the ribs, removing said longitudinal elements from their anchored position, whereupon they will be sprung to a somewhat non-radial position as the tension in the attached plating and the compression: in the adjacent ribs are, i balanced; thenysecuring the-segmental sections: together edgeto edge, drawing the, longitudinal elements together-and-gjoining them firmly, whereby they again assume zit-substantially: radial position: and the previous tension in, the plating is reimposed.

3; A pressure cabin for highaltitude aircraft or-the-like, of substantially circular cross section, comprising circumferentially spaced longerons, 'a plurality of arcuate ribs spaced apart and extending ,betweensaid longrons, skin plating attached to said longeronsand applied. over said ribs. in initial and normal contact therewith but notiattached thereto, said skin plating being in aeondition of circumferential tensile stress and the ribs ina: corresponding-condition of endwisecompression, whereby they exert strong radial outward pressure against the-plating, and-guide means on theplating to locate the ribs laterally with-respect :thereto but to permit cylindrical dilatation of the skin independently of the .ribs.

4. A stressedskinstnicture for an aircraft fuselage;:wing, or like walled structure comprising, azhollow, continuous-walled construction constitutedby, a iplurality' fr' unitary segmental prefabricated sections and including, .in combination,

- peripherally spaced longitudinally extending I bracingimembers; each comprisinga pair of abutendwise. compression substantially along the chords of thearcs they represent, whereby they exert a continuous -pressure radially outwardly against the sheet which is therefore in a condition of corresponding tension, the firm attachment of the marginal longitudinal elements of adjoining sections serving to maintainthem in substantiallyradial positions and maintaining the stress in the ribs andsheet;

5. The structure set forth in claim' 4 in which there is additionally provided guide means on'the plating to locate the ribs laterally with respect thereto but to permit relative :movement of the plating over the ribsin a circumferentialdirection;

6. An aircraft'wingiof hollow construction comprising-two outwardly bowed complementary sections respectively forming the upper and lower surfaces'of the wing, said sections being joined at the leading and trailing edges, each section comprising a. longitudinal .element'adjacent each of said edges, a plurality of spaced arcuate transversely extending ribssecured between said longitudinal elements of the section, saidribs being in a state of endwise compression, substantially along the chords of the arcs which they represent, a curved sheet of skin plating secured marginally to the radially outward edges of the said longitudinal elements of the section and being normally in firm'contact' with the outer surfaces of said arcuate ribs whereby said ribs exert a continuous pressure radially outwardly against the sheet whichis thereforein =a condition of corresponding tension, and means for rigidly securing together, in abutting relationship, the adjoining longitudinal elements of the two sections at the leading and trailing edges thereof to compose the wing-spars at the leading and trailing edges of the wing.

BARNESNEVILLE WALLIS.

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