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Aviation History
1940
1940 - 0376.PDF
FEBRUARY 8, 1940 (a) (b) (c) Fig- 3- weight to head resistance from perhaps 4:1 to 100:1. They have brought rigidity into the front rank with strength, and brought H. volans from his technologic- ally comfortable habitat in a virtually incompressible fluid to the threshold of the velocity of sound in air. They have rendered necessary, first, a smooth and con- tinuously curved form for the structure, with the corre- sponding necessity for containing within that form many subsidiary organs and parts formerly external to the wings and body, notwithstanding the fact that some of them, like the undercarriage, must be tem- porarily capable of extension outside the form, and that others,"like bombs, must be capable of being discharged inflight. Finally, and as the result of this, they have re- quired the lowest possible area of "wetted surface" consistent with other requirements. In short, they have brought about the aeroplane of the form we know to-day typified in Fig. 1. Stabilised Form We appear to be passing through a phase of stabilised form, so that few marked variations from this form are to be seen or, so far as I know, to be expected in the near future, except such variations as " high wing" as distinct from the more general "'low wing " illustrated, which do not seriously affect this discussion. It may be taken as a matter of observation that the wing organ has a section typified in Fig. 2, and that it is subjected along its span to forces and couples which may be conventionally indicated as in the same figure. The forces are not necessarily of the sign indicated by the figure, and vary very greatly along the span from the plane of symmetry to the tip. A hollow shell, represented by the thick line, is, in principle, capable of resisting these forces and couples, but such a struc- ture is not practicable to-day. With the current inten- sities of loading, and the materials and processes avail- able, it is difficult to realise in such a shell (1) stability of the shell, (2) effective graduation of quantity of material along the span, (3) access to the interior. Other types of structure are indicated in Fig. 3. The function of the shell is to provide the smooth form required, and to resist the forces and couples im- posed by the air pressure. In 3 (a) the shell is helped to resist bending and shear arising from forces along the y-y axis by the introduction of a single spar. In 3 (b) it is similarly assisted by two spars; there is addi- tional resistance to the couples from differential bend- ing. In 3 (c) the couples are restrained by the boxes formed at (d) and (e), permitting the temporary re- moval ot the lower shell plating between the spar flanges, as indicated by the dotted line. The spars fulfil the further function of stabilising the shell in some degree, and the spar flanges are in their turn stabilised by the shell and web systems; for this reason their efficiency in compression may be as high as the properties of their material will permit. The cross- sectional area of the flanges may be more conveniently varied along the span than that of the shell, which is frequently limited by other considerations. The arrangements 3 (b) and 3 (c) are both commonly found together in the lifting organs of aeroplanes, and I believe they represent the arrangement which to-day is generally found most convenient. Many other arrange- ments are possible, but I know of no evidence that they offer outstanding and general advantages. If they did, it would be reasonable to .expect a marked trend in some other direction than the arrangement described. The structure of the wing requires to be able to resist the air forces without such deformation as would cause damage, e.g., plastic deformation not exceeding the order of 1/1,000 under the greatest forces likely to be met with in flight. There must also be a considerable reserve of strength during the phase of plastic deforma- tion, at least 33£ per cent, in excess of probable flight loads. Concentration of Material The wing structure must also have adequate rigidity, notably in torsion, and for this reason a high polar moment of inertia is desirable. Since the external form is limited by aerodynamic considerations, the highest pos- sible effective concentration of material at the form boundary is desirable. The shell is subject to air pres- sures, the resultants of which are included in the forces and couples illustrated. The shell may require stiffen- ing to avoid distortion under these forces (e.g., "oval- ling") and to resist waving under shear or compression with consequent loss of rigidity (e.g., reduction of apparent elastic modulus) and instability. By the use of a secondary system of ribs and stringers the necessary shell support may be obtained. This will be most effective when the loss of rigidity in compression is just sufficient so to adjust the stress distribution between spar flange and shell that each will contribute the apparent stresses which they are capable of bearing. With such an arrangement, the shear rigidity may be raised some 50 per cent, higher than with a tension diagonal system, the ribs fulfilling the function of stretcher bars between the spars, supports for the stringers (as laterally loaded struts) and diaphragms to maintain the form. Many devices for local stabilisation by fixation, or virtual fixation, are available; one only can be mentioned. The pressures on the shell (particu- larly the curved portion) serve to stabilise it. The external pressures are determined by aerodynamic con- siderations, but the internal pressure may, in favourable circumstances, be controlled by seals and appropriately placed apertures, and even by "artificial" inflation. For convenience the wing must usually be made to take to pieces, or even fold. Considered solely as a fly- ing organ, the characteristics which have no value in flight could be abandoned with advantage. The adverse part played by joints in structural economy is often underestimated. Important changes in the wing structure described appear to me likely to arise only from external causes. SD3.T1 2 Permissible reduction of -~.fr—, with or without in- w creased wing loading, would follow reduction of import- ance of induced drag (e.g., increased power for climb) with or without increase in stalling speed or higher maxi- mum lift devices. This increase in loading may be accompanied by the necessity for greater strength. For example, raising the
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