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Aviation History
1946
1946 - 2037.PDF
OCTOBER IOTH, 1946 FLIGHT 395 Repair of Stressed - Skin Air frames The Importance of Stress Transfer : Danger of Potential Differences with Dissimilar Metals By JOHN T. - • IT is probably true to say that efficient repair of air-craft is as difficult as efficient design. Both arehighly skilled operations requiring considerable know- ledge and experience if the many problems which arise areto be overcome. The engineer in charge of repairs must have as sound a knowledge of the principles of design andconstruction as the designer has if he is to be really effi- cient. If he has not, his repairs may not be strong enoughand will probably be heavier and more involved than necessary. The simplicity or difficulty of a particular repair dependsmore upon the position of the damage than upon the extent. For example, a small crack in a main plane sparboom close to the root joint may be much more trouble- some than, say, an extensively damaged skin panel in therear fuselage. Hence, slight damage in a highly stressed region may call for a major repair. Regions of high stress cannot be de-fined in detail to cover all aircraft, since, obviously, the individual designgoverns the stresses present in any one component. It is possible, however,to name those components of the airframe which are heavily loaded under various conditions, and it is also pos-sible to indicate certain features of design which cause high stresses to develop and which must, therefore, beregarded- as danger points when assessing damage. These points are shown in Fig. 1. Chief of the vital components on most aircraft are thespars, particularly the booms. Repairs here may be critical. The loading on the spars increases as the wingroot is approached, and generally the stresses increase also, reaching a maximum at the wing root. The centre section, embracing both main planes and fuse-lage, is often regarded as the foundation of the airframe and, because it is required to transmit the flight loads fromthe fuselage to the wings, it is, in all cases, heavily loaded. It is interesting to note that the centre section acts as the , AILERON SPAR HENSHAW, A.F.R.Ae.S. HATCH FRAMINGS WING RIBS SUPPORTING ENGINE AND UNDERCARRIAGE REAR FIN POST FRONT FIN POST RUDDER DOOR OPENING MAIN PLANE SPARS FORMERSSUPPORTING MAIN PLANE SWRS ^LONGERONS BEAM AND root of four cantilever beams, namely, the front and rearfuselage sections and the port and starboard main planes, with the result that the centre section is heavily loadedon all sides. The principle of stressed-skin design requires continuityof the stringers and skin if the strength is to be satisfac- tory. When holes are cut in the skin, this continuity isdestroyed and difficulties arise. Unfortunately, cut-outs for hatches, doorways, access holes, etc., are essential, butthe designer overcomes any weakness by fitting stiffening frames around the openings. The purpose of these framesis to divert the load in the skin and stringers and carry it round the opening, thus avoiding any appreciable loss instrength. It will be apparent that the frames carrying these diverted loads are likely to be heavily loaded and, forthis reason, damaged framings should be treated carefully. Damaged longerons also require_/ careful treatment, especially when affected in the region of the cen1 re sec-tion, and no risks should be taken with doubtful repairs. Stringers, too,in spite of their comparatively small size, are vitally important members. In repair the originalcontinuity must be retained and the stringer securely riveted to the skin. Otherwise the value of the combina-tion of skin and stringer working together as a single unit is lost. Heavily loaded wing ribs or fuselage formers which havespecial duties are also likely to be highly stressed. In this class we can include ribs supporting fuel tanks, under-carriages and engine mountings; fuselage formers support- ing main plane or tailplane spars or tailwheel beams; orany formers or ribs carrying similar heavy, concentrated loads. Other formers and ribs are usually only lightlyloaded. The undercarriage and engine mountings are, of course,very heavily loaded, and it is common practice to forbid any structural repairs to either of these components. Con-cession may, however, be granted to repair certain of the less critical members, which the designers are prepared toaccept with a slightly lower strength, but, in the absence of official instructions, repairs to these components shouldnot be attempted. Other components to which repairs are often forbiddenare the control surface spars. As a precaution against flutter, the ailerons, elevators and rudders must be torsion-ally stiff. This requirement is always fulfilled in the original design, but, owing to the difficulty of makingrepairs as stiff as the original spar, repairs to these parts are often forbidden. If, therefore, such repairs are per^mitted, particular attention should be paid to torsional stiffness. Strength of Repair Fig. 1. Regions of high stress. SUPPORTING TAIL WHEEL UNDERCARRIAGE It is true ^ that in many cases a repair 100 per cent asstrong as the original member is not possible, owing to the necessity of drilling rivet 9oi bolt holes in both themember and the patch plates, but maximum strength should always be aimed at. With this end in view the proposed repairshould be considered with three points in mind: — (1) Has the patch plate or insertion piecebeen allowed sufficient overlap to form an efficient attachment? Length of attach-ment is very desirable, as it enables the load to spread gradually into the sound structure. (2) Does the repair enable the load to passdirectly from one side of the joint to the other? In a built-up member all the parts FORMERS SUPPORTING TAIL PLANE SPARS
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