To deal effectively in the course of a single lecture with the subject of light alloys for aircraft is, to-day, practically an impossibility. Actually, the various alloys of aluminium that are put forward for consideration by the aeronautical engineer do not vary amongst themselves to such an enormous extent as to make one or other of over-riding interest, and much the same might be said of the alloys of magnesium. Despite this position, so much of interest is occurring in both fields, and so many alloys are being put forward having some characteristic or another of real interest, that the aircraft engineer, and in fact any other engineer, could readily be excused if he felt somewhat nonplussed when he surveyed the multitude of materials that are offered for his employment. I am relieved, therefore, that your Council decided that this lecture should give special prominence to the alloys of magnesium. I am assuming that by this they mean those alloys which consist of magnesium in a great preponderating proportion, but I feel that the time is appropriate also to make some reference to those alloys which consist principally of aluminium, but which possess definitely useful properties in consequence of the presence in them of certain relatively small proportions of magnesium. I propose, however, first to consider the alloys rich in magnesium, and will endeavour to provide some data of a rather fundamental type which may be of definite service to the aeronautical engineer when he is engaged in comparing the relative importance and value of the magnesium alloys made available to him in commerce.

The chemist regards magnesium as a highly reactive metal for such reasons as the inflammability of its powder or foil in air, its active displacement of hydrogen gas from many aqueous chloride solutions and its position near the reactive end of the electrode-potential series. All these suggest that the metal would be unsuitable for constructional engineering. Yet engineers use alloys, rich in magnesium, up to 98 per cent. of the metal, for an increasing number of services, although the alloying elements do not, as a rule, greatly cut down, and may even increase, the corrosion rate. Their industrial use is possible because the liability to corrode, reckoned over a reasonably prolonged period, is not a definite property of a metal such as conductivity which is subject only to relatively small changes with alteration of environment, but is highly specific to metal-liquid and metal-gas systems. Moreover, these systems may undergo important changes with time owing to the intervention of films of corrosion products, and the rate of attack may be governed by the physical characteristics of these films which will vary with the adjacent liquid and gases. Thus in stagnant caustic alkalies magnesium may be reckoned as almost incorrodible because of the intervention of a film of hydroxide of the self-healing type which, in these conditions, is highly impervious to magnesium ions; but in the presence of alkali chlorides the corrosion product is physically different and rapid corrosion occurs. Many dilute acids attack magnesium rapidly but hydrofluoric acid scarcely at all, no doubt owing to the formation of a protective film of fluoride.

The method generally used in England for calculating the aerodynamic characteristics of wings is that suggested by Glauert (1), in which the circulation across the span of the wing is represented by a Fourier series. On account of the form of the series, certain difficulties arise which make the calculation very tedious when more than about four points along the semi-span are considered.