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.