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Aviation History
1942
1942 - 0096.PDF
36 F L I G HT • JANUARY 8TH, 1942 AIRCRAFT SECTIONS (CONTINUED) The following is a summary of the available alloys : Extruded material to DTD 259, proof stress n tons per sq. in., limiting value of bjt is 10 ; Sheets suitable for welding to DTD 120A, proof stress 7-9 tons per sq. in., limiting value of bjt is 12 ; Sheets suitable for welding to-DTD 118, proof stress 6-8 tons per sq. in., limiting value of bjt is 13. The limiting values are shown graphically in Fig. 6, the value of L/< being taken as 100. Strength-Weight Comparison In conclusion, it is informative to compare the strength weight ratios and the limiting bit values for extruded, drawn or folded sections, and sheet and strip material. an alloy steel of say 50 tons per sq. in. proof stress, and an aluminium alloy of say just below 18 tons per sq. in. proof stress. An exceedingly useful quality of the magnesium alloys is that on account of the relatively low proof stress, thicker sections are required and the liability to instability reduced. The increased weight entailed by the thicker section is offset by the lower specific gravity of the material. Material, extruded, drawn, folded ,* sheet and strip Allov Steels ,, Aluminium Alloys .. ., .. Magnesium Alloys ' .. ., ... Proof Stress tons per sq. in. 80 60 40 26/28 21 18/19 13/15 n - 8 7 Strength Weight Ratio 10.2 7-7 51 9.8 7.6 6.7. 5-o 6.0 4.4 3-8 Limiting bit Value 8 10 12 8 9 10 12 10.5 12 135 •^--~4- M IU~ T_ T The above figures will enable comparisons to be made. It is interesting to notice that a comparison of the high strength alloy steels and the aluminium alloys shows that the strength weight ratios and limiting breadth to thickness values run somewhat parallel. Thus an 80- ton steel has approximately the same strength weight ratio and limiting bjt value as the highest strength aluminium alloy. Similarly, an 11-ton magnesium alloy will compare with Fig. 7. Symmetrical and asymmetricai " pi "-sections. Earlier in this paper reference was made to the search for high strength light, alloys, and it will be seen that there is still scope for an ultra light alloy which would enable thicker sections to be used with increased elastic stabilit, Application to Modern Aircraft Sections The swift development of aircraft has created a vast number of sections, many of which require individual analysis to determine the load-carrying capacity. One very common and useful section to which the above method of analysis can be applied is the double tee section (some times called the " pi " section, on account of its resemblance to the Greek letter w). Fig. 7 gives two forms of this section. „ /' Thus to prevent instability the ratio of bit must not exceed the limiting .valul(i,!i'"g1iVe,iv*iiio\,e Jor the particular allo,y in use. Other non-standard secjmAns which can be S0 tested are the unequal channel, Qlflique tee, cruciform, .zed, and what might be termed yrne multi-flat sections, i.e. anj sdction in which tlin omstaoning legs or flanges occur* wR£L FV,REEK?ELIVUR.Y IN J?E VNiTED STATES : African paratroops in action. This time it was only a demonstration, but they have been well framed for -the real thing." The exit in the D!C.2S is far back and must result in a^ood deal of tail-heaviness with the troops concentrated near the door.
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