Materials Used in Construction - Aeroplane or Airplane

MATERIALS USED IN CONSTRUCTION - AEROPLANE or AIRPLANE.

Composite Structures.

The materials which have been most 'commonly used hitherto for the main structural members of aeroplanes are wood and steel, with a fabric covering for the wings. The wood used was chiefly spruce of the highest grade, and in many aeroplanes the whole of the main structure, with the exception of ties and joints, was of this material. Ties and the joints or fittings by which they were attached to the other mem bers were generally of steel. Steel tubes were used in some parts, either, as in the undercarriage, on account of their robustness, or, as in the control mechanism, because of their efficiency in transmitting torsion.

The arrangement of the members of a conventional aeroplane structure of this composite character will be seen in fig. 9 of AERONAUTICS. The cross section of the wing spars has the I section used in general structural engineering, though the thickness of the web and flanges is much larger in proportion to the overall dimen sions of the section than is usual in steel I girders.

The wings of the aeroplane shown in fig. 9 are in biplane form with external bracing. Composite construction has also been ap plied to wings with no external bracing, generally, but not invaria bly, monoplanes.

In aeroplane construction the primary requirement is light ness, and a modern composite aeroplane structure is on the whole nearly as light as one of any other type of construction so far developed, for the same strength. In so far as it embodies wood composite construction suffers from the following disadvantages : (I) Wood of a suitable grade cannot be freely obtained. (2) When exposed to the atmosphere, particularly to large changes in temperature and humidity, wood deteriorates more rapidly than steel or other metals. (3) Wood is not a reliable material. Its external appearance is often misleading as an indication of its internal condition. (4) In aeroplane structures glue and wood screws cannot be entirely avoided. Both are sources of weak ness and uncertainty. (5) When a wooden structure is involved in an accident, many of its members break completely and splinter, and the structure disintegrates, whereas metal members often merely bend, and an all-metal structure generally preserves much of its original shape. The passengers in a wooden aeroplane are thus exposed to greater danger in case of an accident. (6) Wood has no inherent resistance to fire.

These disadvantages of wood have been the chief incentive to the development of all-metal construction for aeroplanes.

All-metal Structures.

The development of all-metal struc tures was originally mainly confined to replacing each of the wooden members of the composite structure described above by a metal member, using either steel or duralumin. In such a struc ture the load to be borne by each member is of a fairly simple character and can be estimated fairly closely. Hence a compari son of the merits of a composite and an all-metal structure on these lines can be reduced to a comparison of typical members.

However, recent tendency is toward Monocoque or Semi Monocoque construction with the entire outer surface of the aeroplane of the metal stressed skin construction. Recent develop ments in the production of large, thin, uniform sheets of alum inium alloys, and in the art of welding and riveting have made desirable the Monocoque type of construction in which the cover ing of the fuselage furnishes the strength. In the Semi-Mono coque type part of the stress is borne by the stressed skin and part by reinforcement.

The metal used in fuselage and wing construction of this type is usually an aluminium alloy. Alclad, a very thin sheet aluminium alloy which has been coated with pure aluminium, is very satis factory in that the necessity for painting is obviated. It also gives excellent results when used about salt water where salt corrosion usually gives trouble.

The change from wood construction to all-metal has been pro nounced during the past 15 years. In modern all-metal aeroplanes aluminium alloys represent from 8o to 85% of the total weight of the structure.

Physical Properties of Materials.

The disadvantages of wood as a material for the structure of an aeroplane, outlined above, give an idea of what should be the characteristics of a satis factory material. There is the usual difficulty, common to most problems of engineering design, of assessing the relative im portance of each of these characteristics. In fact, their relative positions in the scale change with time and circumstances. But, owing to the low weight of the composite structures characteristic of modern aeroplanes, the technical problem is simplified into that of finding means of making, at the worst equally light, and preferably lighter structures from materials that do not share with wood the disadvantages mentioned. Table I. (p. 258) gives the strength and specific gravity of various materials which have been, or are likely to be, used for the main structural members of aircraft. They fall into three classes : wood, steel and light non-ferrous metals. The comparatively short life of wood has been quoted as one of its main disadvantages. Both steel and aluminium alloys need protection against corrosion. Of the former spot-welded stainless steel appears to have great promise. Of the latter alclad has proven highly satisfactory. Although it would be misleading to suggest that all the problems of protec tion of metal structures have been solved, the experience gained suggests that effective protection can be given against all the certain. Shapes for members (see fig. 13) have been devised in which the greatest stress at failure is some 6o to 70% of the ultimate tensile strength of the materials (see Table I., column headed "Effective Strength in Compression"), and there is reason to suppose that this is not far from the best that can be achieved. Some improvement may be anticipated from an extension of these results to still higher grades of steel, or from the development of a light alloy stronger than duralumin.

The ruling principle in the shapes of cross section which have achieved these results with steel is corrugation of the thin sheet ordinary causes of corrosion.

Struts and Beams of Thin Metal.

It is customary to re gard the single figure obtained by dividing the strength by the specific gravity as a figure of merit on which the material can be judged. Such figures are not given in Table I. as they are not, in fact, a reliable criterion for all types of structural members. The types of members responsible for the larger part of the weight of the structure of an aeroplane are two, struts and beams, the latter very often having to serve also as struts. Such members have essentially a variation of stress over their cross section, so that when the most heavily stressed point reaches the limiting stress of the material (this may be taken to mark the failure of the member as a whole), the majority of the material is under a lower stress. Hence the density of the material divided by its limiting stress is not in general proportional to the weight of the member.

In general, for a given area of cross section and a given limiting stress, the more the material can be spread out, e.g., in the form of a circle, the greater will be the strength of the strut or beam, but the thinner will the material become. It appears, however, that a limit is reached for steel when the thickness is about the radius, beyond which an increase in the radius causes a de crease in strength. The member will then fail owing to local buck ling of the material, at a load which corresponds to a maximum stress in the material, as calculated in the conventional way, which is below the limiting stress. The design of metal struc tures for aircraft hinges round the discovery of shapes of cross section which will enable the limiting stress of the material to be reached, and will be convenient for manufacture. The members must also be robust to ensure that ordinary handling does not damage them, a serious problem with thin sheets or tubes.

Limiting Stress.

The limiting stress in question is probably what is termed the yield point in compression, the stress at which plastic strain begins, for materials such as mild steel. For high tensile steel and light alloys (in practice the aluminium alloy known as duralumin is the one most used), there is no definite yield point and the limiting stress is accordingly somewhat un material used. The theory of the collapse of corrugated material awaits development, design being at present largely empirical. But the general nature of the strengthening effect of corrugation is well understood. A flat sheet of thin metal buckles readily under compressive stresses. Corrugation, in any direction which does not make too great an angle with the direction of the stress, stiffens it against such buckling. It greatly reduces the influence of the inevitable local irregularities in actual materials, and gives robustness to what would otherwise be a very easily damaged mem ber. It is essential in struts and beams of thin metal, to corrugate longitudinally. Transverse corrugations have proved ineffective.

Steel strip of thickness as low as in. has been used in aeroplane spars and ribs. The material is either cold-rolled me dium carbon steel, whose essential properties have been improved by a process known as blueing (heating to some 350°C.), but is otherwise not heat treated, or a nickel chromium alloy steel, hardened and tempered. Hitherto the strip has been formed into the final shape cold and without any subsequent treatment. This restricts the sections which can be produced* and the materials which can be used owing to the need for ductility in order to avoid cracks. Progress has been made in the direction of forming the shapes while the material is in an annealed state and heat treating subsequently. This method will probably supersede "hard" drawing and rolling.

For spars of the size shown in fig. 13, it has been found that the low specific gravity of duralumin makes it possible to use material of such a thickness that corrugation is not always necessary. The resulting shapes resemble those used in bridge construction, though theiattice girder seen in fig. 13 d is found in practice to be heavier than the simpler shape in e. It is probable that, for the size of spar required in the largest present-day aeroplanes of the type under discussion, it is more economical of weight to use a spar composed of a few parts (such as a, c and e) than to build it up from many pieces. On the other hand for larger beams, such as are used in airships, a lattice construction is lighter. Probably the lightest all-metal construction for a wing structure using spars and ribs, etc., combines both steel and duralumin, the former for the main members (spars and struts) and the latter for the sub sidiary ribs and edges. With the methods outlined above it is now possible to make an all-metal aeroplane as light as, and lighter than, the corresponding composite machine.

BIBLIOGRAPHY.-G.

P. Thomson, Applied Aerodynamics (1919) ; Bibliography.-G. P. Thomson, Applied Aerodynamics (1919) ; A. J. Pippard and J. L. Pritchard, Aeroplane Structures (1919) ; L. Bairstow, Applied Aerodynamics (1920) ; F. T. Hill, Aeroplane Con struction (1920) ; H. Glauert, Aerofoil Theory (1926) ; H. Harper, Steel Construction of Aircraft; F. T. Jane, All the World's Aircraft (published annually), and the Proceedings of the Royal Aeronautical Society (monthly). See also the publications of the Aeronautical Research Committee (Great Britain) and the National Advisory Committee for Aeronautics (U.S.A.), particularly in reference to §I (A.R.C.) §2 (N.A.C.A.) 116; §4 (A.R.C.) 300 (N.A.C.A.) 221; §5 (A.R.C.) 676; §7 (A.R.C.) 678, 740, 747, 836; §§9 and io (A.R.C.) 476, 673, 776 (N.A.C.A.) 203.