Schlieren photographs of a small scale approximately two-dimensional air jet in air above the critical pressure have shown the existence of an instability displaying an anti-symmetric pattern, and the associated sound field having several distinguishing characteristics, has been photographed. An elementary theory based upon the hypothesis that the acoustic energy originates from the interaction of the stream disturbances with the shock waves of the flow, and connecting the stream disturbances with the radiated sound, is suggested. This is shown to be consistent with the physical dimensions of the phenomenon and predicts a sound field in reasonable agreement with that observed after certain simplifying assumptions have been made.

An analysis has been made which suggests that, with the types of pressure distribution most usual in practice and free stream Mach numbers up to 10, no serious errors would be introduced into the calculation of the laminar separation point by the assumption that σ, the Prandtl number, and ω, the index of variation of viscosity with absolute temperature, are equal to unity. (Typical actual values of σ and ω for air are 0.72 and 0.89 respectively).

The problem of the buckling of flat oblique plates (i.e. in the shape of parallelograms) with all edges clamped under a uniform system of stress is considered. An energy method of calculating the critical stresses is developed and applied to the particular case of a clamped oblique plate under uniform compression parallel to two of the sides. Numerical values for the buckling stress coefficient are obtained for plates having acute angles of 60° and 45°, and side ratios varying between 3/5 and 5/3.

The one-dimensional equations governing the mixing of compressible flows in a channel of constant cross-sectional area are derived and are used to investigate the running of an induction-type high-speed wind tunnel. While in practice it is usually difficult to achieve working section Mach numbers greater than about 1.8 to 20 in this type of tunnel, this simple theory gives no obvious reason for this and indicates that higher Mach numbers should be attainable.

It is also shown that an increase in efficiency is predicted, particularly at the higher supersonic Mach numbers, if the injected stream enters the mixing section at the same Mach number as that of the working section main stream.

The strip-theory equations of flow through a ducted fan were the basis of a method of designing a fan for a given performance. The merit of the method was that it amounted to an explicit solution of the design problem, no iteration being necessary.

In the inverse problem, of determining the flow past a given fan in conditions other than those of the design, two relations—the motor or tunnel characteristics, for example—will be given between the four quantities P, Ω, η, u (the motor power and angular velocity, the fan efficiency and the air speed at the fan). The problem is to determine the values of these four quantities together with details of the flow past any blade element.

This paper shows that to a high degree of accuracy the performance of a ducted fan can be summed up by saying that P/Ω3 and η are both functions of Ω/u only, functions which depend only on the fan shape. Solution by graphical means is therefore almost instantaneous.

The fan-tunnel combination is shown to be governed by extremely simple equations which amount to saying that the tunnel power factor is a function of Ω/u only which, again, can be calculated once for all for a given fan and tunnel.

The resistance of various screens in a tube is reduced to the drag of a single piece of wire by assuming that the average velocity in the screen itself rather than the free stream velocity is the significant factor. Almost all the available tests on screens or gauzes are brought into line with a single experimental function similar to the drag of a cylinder plotted against Reynolds number.

This note discusses changes in the state of a gas flowing in a duct of constant area. In the past, changes, such as normal shock waves, combustion and other phenomena, which are defined by the equations of energy, conservation of momentum and mass flow, have each been treated on their merits. In this note a method is developed whereby all phenomena governed by these three equations can be solved by a single general method. The method rests on the derivation of a parameter which is unaltered in value by the change, in all cases where the total temperature is constant. A shock wave is an example of such a discontinuity. In problems of heat addition or extraction, the parameter changes its value only because of the change in total temperature. The change in total temperature may be calculated from the known quantity of heat added or extracted.

The parameters derived are useful in showing how problems of this type should be attacked analytically. With oblique waves it is easy to derive a relation between the normal velocities before and after the wave, and it is probable that this relationship has not been published before.

The paper describes in detail an older method than Relaxation of approximating to the solution of equations of the Laplace and Poisson type. The corresponding fourth order equations are discussed briefly, and a method of dealing with certain non-linear equations is indicated. A description is also given of the propagation of errors in the fields due to various causes.

The Laplace transform of the lift distribution on an oscillating rectangular wing in a supersonic flow is obtained by separating the linearised equation for the velocity potential in elliptic (cylindrical) co-ordinates. The results for the case of no spanwise distortion are expanded in ascending powers of the aspect ratio in order to compare with the slender body theory, and the longitudinal stability derivatives are calculated. It is found that at either supersonic or transonic speeds single-degree-offreedom instability in pitch is impossible insofar as the fourth power of the aspect ratio is neglected.