In no branch of structural engineering is there a rigorous definition of the term “secondary stress.”

If the primary stresses in a structure are determined, that is, the stresses in the members due to an external load system, assuming that all the members are joined together by perfect pin- or ball-joints, then the secondary stresses are in general taken to be the additional stresses due to the rigidity of the actual joints used in practice.

In such a highly redundant structure as an airship hull the labour involved, in determining even the primary stresses, precludes the use of the normal methods of stressing. It is usual to make use of generalised methods which give approximate results. These generalised methods imply that the external loads are applied to the structure in a certain distribution. Though this is rarely achieved, yet the results obtained are in most cases sufficiently accurate if suitable bracing is supplied to redistribute the external loads over the cross-section; the effect of the initial wrong distribution being then merely local.

An investigation of channel wall interference with the object of verifying experimentally the accuracy of the results given by the theory developed by Prandtl and extended by Glauert. Tests were made, with cylinders and aerofoils, using the “ mirror ” method, for some ten different channel sizes. The results show :

1. 1. The presence of the channel walls results in an increase in the drag of a symmetrical body, such as a cylinder.

2. 2. Agreement with the theoretial corrections for incidence and drag for channels of reasonable size.

3. 3. The extent and nature of the wall interference effects on the aerodynamic characteristics of aerofoils in much restricted streams, beyond the limits of applicability of the theory.

From time to time attempts have been made to produce a dynamometer capable of measuring the power output of an aero-engine during flight at various atitudes. The type that has proved the most suitable up to date in flight tests is the Bendemann hub acting on the hydraulic principle which was originally designed in Germany. The principle has recently been tested in America with moderately satisfactory results, and experiments with a modified form of the apparatus are at present being conducted at the Royal Aircraft Establishment. The Bendemann hub and its accessories are somewhat complicated and are difficult to instal in a machine and to maintain in a serviceable condition, also the apparatus will not work properly at altitudes above 20,000 feet owing to increased viscosity of the working fluid at low temperatures.

The apparatus described below is simpler in construction, lighter and more robust than the Bendemann hub, and should be capable of giving equally good results irrespective of altitude; moreover, the work of fitting it to a Service machine would not be great and would not necessitate the employment of specially skilled mechanics.