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Aviation History
1928
1928 - 0667.PDF
JULY 19, 1928 55 THE AIRCRAFT ENGINEER SUPPLEMENT to FLIGHT :05» FQg Hot-D1M<J » BOTTOM HALVES Of 3-91' 3 91 II A BASE. 9. I IB 10. shops and drawing office can often save much unnecessary labour. For example, suppose a spar has been designed having overall dimensions 4 ins. by 2 ins., and suppose that when the first sample or test specimen of spar is made up, the overall dimensions are 3|f ins. by 2T'S in., then if the spar produced withstands its specified test load, and it is known that spars of these dimensions and having the same uniform material can always be produced, it is obviously easier and cheaper to modify the design than to remake the tools to produce the original spar section to the calculated dimensions. It is true that minor difficulties will appear in the early stages of this work. One of the most common is for the edges of a formed strip to buckle as they emerge from a die or pair of rolls or to take a sinuous form : this is nearly always due to the strip being wrongly fed into the tool, causing a stretching of the material at the edges. By altering the method of lead-in this trouble can usually be overcome. As illustrating this, if we have a rectangular piece of paper held so that one end is in the form of a circle, the other end being left flat, and then an attempt is made to deform the sheet so that the end curve will lie in a plane at right angles to the centre line of the paper, then the long edges will tend to tear. If this were steel strip, and the die gap was of similar shape to the curved end of the paper, then the strip would have to be fed in so that the edges could follow the natural curve indicated by the paper, otherwise the edges would stretch and finally wave. The reader may think that the forming of section from strip becomes easier with softer material: within limits, '49" 12. 13A 13B •5 15 16. 14. 614c
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