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Aviation History
1940
1940 - 0364.PDF
FEBRUARY 8, 1940 A good example of the single-spar wing (above). The wheels of the Spitfire leave the leading portion uncut. On the right is the front part of the outer wing of the Flamingo. The wing of the Hampden is also of single-spar type, but has a girder spar. The false front spar serves mainly as a support for the leading-edge unit. The single-spar wing of the Heinkel 112 (below) is bad design because the under- carriage cuts into the leading-edge shell. spar, and aileron and flap hinges are carried on short ribs from a relatively stiff rear spar. Openings may be made locally between the spars, especially near the root, without greatly reducing the strength and stiffness, as the two spars can take the tor- sion in differential bending. The front and rear walls of the box, in the case of the two-spar type, are at about 15 per cent, and 65 per cent, of the wing chord; in the true box type they are much closer together. Thus in the latter the stress-bearing part of the structure is more concentrated and it is not so easy to make open- ings. This type of wing is slightly more difficult to build than the single-spar type, as the use of two main spars necessitates a wing remaining longer in an erection jig. Its other advantages, however, make this a relatively unimportant point, as the simpler type of rear portion of the wing is easier to fit. Machines using this type of wing are the Blenheim, Skua, Whitley, Short G Class and Heinkel 111. The Blenheim wing is of the two-spar type; it is in- teresting in view of the fact that the spar flanges are of steel. The efficiency of this type of wing is shown by the very low structure weight of the machine. The Skua wing is also of the two-spar type and is a good example of how openings can be made between spars to take wheels, etc. In the Whitley wing we have a true box type, with heavily reinforced top and bottom skins made ot corru- gated sheet. The nosing and trailing edges are added to this, and all loads are taken by the box unit. The Short G and Empire Class boats have a similar type of wing, but the spar units are of the girder type, with extruded T-section flanges. This type of wing does not easily permit of making openings in the spar portion, and all equipment has to be stowed fore or aft of it, except tanks, which can be specially designed to ht inside it.
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