Although columns built almost 5,000 years ago still stand in Egypt, the first theoretical calculation of the load-carrying capacity of columns was published by Euler only 209 years ago. The classical theory established in this publication is still the foundation of all stability analyses and is likely to remain so in the future. It was derived for perfectly elastic columns and for specific end conditions. These end conditions are not realised in the ordinary column test nor do they correspond exactly to the conditions prevailing in an aeroplane wing when it is subjected to a gust or to the shock of landing. They are not the same as the end conditions of a column in a building when it collapses during a tornado or an earthquake.

The presence of foam or unduly aerated oil in the lubricating-oil systems of aircraft power plant can be very troublesome—more so in reciprocating engines than in gas turbines, since the rates of circulation are greater and the viscosity grade of the oil is higher. Consequently in this paper, which surveys present knowledge about oil foaming, reciprocating engines are the main consideration.

In the conventional aircraft “dry-sump” oil system, a simplified diagram of which is shown in Fig. 1, oil is pumped from a supply tank to the bearings, and so on, by a so-called “pressure” pump. This oil then drains down into the sump and is picked up by “scavenge” pumps which return it to the supply tank. There are usually at least two scavenge pumps so that the oil is cleared away irrespective of the attitude of the aircraft.

Two design conditions for an axial flow compressor stage are proposed and examined. These are, the constant reaction condition (incorporating I “ radial equilibrium ”), and the condition that the Mach number at inlet to the rotor shall be invariant with radius. In addition, the combination of these two properties in one stage is considered. It is found, with further assumptions regarding the nature of the flow, that a forced vortex type of flow will satisfy both design specifications. The forced vortex solutions for the various cases are presented, and for constant Mach number at inlet to the rotor, more general solutions are given.

In view of the recent notes in this Journal by Professors Jacobs and Duncan, it may be of interest to give some additional references to investigations of the flexural centre of beams, which otherwise might be overlooked, and to add some remarks which seem to have escaped the attention of previous writers.

Although the earliest investigation seems to have been published by Griffith and Taylor (Ref. 4 of Duncan's paper), Eggenschwyler gave an independent discussion of the problem for narrow beams. Considerable progress was made by Weber in two papers, published in 1924 and 1926. Similar results were obtained by Schwalbe, who was apparently unaware of Weber's previous investigations.

A general expression is obtained for the potential of sound or aerodynamic disturbance from a source moving with variable vector velocity, based on linear theory. Knowing the space time curve for the source, the Doppler frequency corrections and intensities can be determined. If in any part of its trajectory the aircraft exceeds the velocity of sound, the number of sonic bangs and the times of occurrence can be calculated. A numerical example is given. The following conclusions are deduced: (i) If at any point of its trajectory, the component of the aircraft velocity in the direction of the observer is equal to the velocity of sound, a sonic bang is propagated to the observer with the velocity of sound. This has been previously postulated by C. H. E. Warren. (ii) A bang as determined by conclusion (i) comprises two components and in certain circumstances the components in some of the bangs may be acoustically separable, thus giving rise to double bangs.