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Aviation History
1946
1946 - 2038.PDF
FLIGHT OCTOBER IOTH, 1946 REPAIR OF STRESSED-SKIN AIRFRAMES must share tHe load and, if one of them is cut, the load it carries must find another path. In so doing it may overload an adjacent member. Discontinuity of repair members within the length of the repair should, there- fore, be avoided, (3) Considering any cross-section through the repair, is the strength at that section as nearly as possible that of the original member? Where the same or equi- valent material is used, the cross- sectional area at all points in the repair should be at least equal to that of the sound member. If these three requirements are ful- filled, and if the patch plate or inser- tion piece is similar in design to the member which it repairs, the struc- tural arrangement should be satis- factory. Reinforcing plates and insertion pieces should have embodied in them any method of stiffening which is pre- sent in the sound member. For example, a member shaped as that in Fig. 2A, if repaired with a patch or sleeve shaped as shown in Fig. 2B, would not be as strong as the original member. If the patch is to be as stiff as the sound member it must be shaped to fit as shown in Fig. 2C. When a former is buckled it may be permissible to dress it back to shape and, if no cracks are then present, to accept it as repaired without reinforcement. This prac- tice is not without its dangers, however. Duralumin, in its hardened state, does not react kindly to working with- out heat treatment, and it is likely that the beating of a buckled former will damage the already strained crystalline structure of the metal and destroy or further impair its properties. For this reason any heavily- loaded member which may be damaged should have rein- forcing plates fitted after being dressed back to shape. Skin Plating Repairs The skin plating on a stressed-skin airframe is, of course, a vital member. On its strength, and the effi- ciency of its combination with the stringers, the strength of the airframe mainly depends. Skin plating repairs are, therefore, very important and, even though they are not usually as involved as repairs to the internal structure, their planning requires care. 2. Sleeve shaping. FORMER IF A LARGE PATCH PLATE IS NECESSARY, A LIGHTENING HOLE MAY BE CUT AS SHOWN FORMER HOLE CUT IN SKIN AROUND DAMAGE Fig. 3. Surface patching. Fig. 4. Flush patching. Patch repairs to the skin fall into two main classes: surface patches/ and flush patches. Figs. 3 and 4> respectively, show the essentials of each class of repair. The adequate riveting, made possible by ample overlap" of the patch, will be noticed in each case. This is" essential if the repair is to be efficient, as tM load in the.' skin must travel across the patch plate and can do so only if the attach- ; ment is strong enough to pass the load from the sound skin into the repair plate. If the damage is adjacent to formers or stringers, it is advisable to make the patch large enough to pick up the adjacent members. This ensures a more rigid attachment and makes a stronger repair. It will be noticed from an inspec- tion of the skin laps of any stressed- skin airframe that additional rivets are always fitted along the joints.: Close-pitched or double-row riveting is found at the joints, while the re- maining ri^ts in the panel are more widely spaced. These extra rivets are fitted at the joints between the panels to enable the load in the skin to pass from one panel to" the next, and they are the only means by which this trans- ference can take place. Riveting at lap joints must, therefore, always be adequate. • Practical Considerations In the actual repair there are several features which, ••, although w€li known, are important enough to call for more than a passing reference. /'• First, there is the question of material. Where pos- •; sible, repair parts should be made from material of the ; same specification as that of the components being repaired. If this material is not available, it may be per- missible to use material to an equivalent specification, in which case the alternatives quoted in the official repair handbook should be used. The use of steel for the repair of light-alloy components should be resorted to only on special instructions. Generally speaking, it is an undesirable practice as the physical properties of steel prevent its working efficiently with the light alloy to which it is fixed. Such repairs are, however, sometimes called for by the designers of the aircraft in positions which render more efficient methods impossible and, in these cases, the official instructions should be closely followed. Another difficulty is often intro- duced by the use of an alternative material, namely an excessive poten- tial difference, which increases con- siderably the danger of electrolytic corrosion. As will be seen from the table of potential values, this is par- ticularly liable to occur when steel is used to repair light-alloy components. Where dissimilar metals must be used, a potential difference greater than 0.25 volts between the metals in contact should be avoided if possible, and, as an additional precaution, the contacting surfaces should be liber- ally coated with suitable jointing compound before assembly. The dangers of introducing mag- netic materials into the region of a compass should also be remembered/ No magnetic material is alowed to be ' fitted within 18 inches of a compass and where repairs are required within r
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