Head Resistance of Strut Sockets.
A point calling for comment is the apparent oversight or neglect of the amount of head resistance offered by the average strut fitting, although great care is taken to ensure the strut and wing sections being of correct form. It seems probable that some difference must occur, especially at the high speeds now prevalent, between the air flow across the plane and that which meets the strut terminal. Anyway, some discontinuity of flow exists, and whether or no the aggregate resistance of all the fittings is of any great moment provides matter for discussion. It is quite possible to fair off any irregularities in air-flow due to the strut connections by the attachment of sheet-aluminium fairings, which could be beaten, pressed, or spun with little difficulty. Although examples of this practice are very little in evidence, the writer inclines to the belief that the additional weight would be negligible compared with the ensuing reduction in head resistance.
The foregoing examples cannot be said to constitute the latest practice, nor is it possible under present conditions to give such details, but sufficient has been said to indicate the progress and trend of design.
CHAPTER VII.
WING-TRUSSING SYSTEMS.
Although the trussing of aeroplanes is carried out along certain well-defined lines, there are occasional divergences from the orthodox. The differences now existing are not nearly so great as those of former days, this being explained by the fact that the progress of any science or industry tends towards uniformity of method, while practical experience eliminates the undesirable systems. This does not necessarily mean that the present methods in vogue are incapable of improvement, but merely denotes their suitability for present requirements.
The Pratt Truss.
Fig. 56.—The Pratt truss.
The basis of all modern trussing systems, with modifications, is the Pratt truss ([Fig. 56]), familiar in bridge-building circles, the basic principle of which is that the compression members are disposed vertically, and while of minimum length are most favourably placed for obtaining the maximum efficiency. There are other types of trusses used in structural engineering, as, for instance, the Howe truss, in which the compression members are arranged diagonally, and the Warren lattice-type girder; but for various reasons these are not applicable to the needs of aeronautical engineering. But a brief consideration of the chief features of the Pratt or box-girder system of trussing will suffice to illustrate its great advantages for aircraft work, particularly for machines exceeding a certain span; and it is this limiting span to which a monoplane can safely and efficiently be built which is largely responsible for its present spell of unpopularity.