When hovercraft were first introduced the lift or thrust augmentation ratio was often used as a measure of their effectiveness. Although this ratio has been used less with subsequent development, it continues to be quoted in some literature. It is defined as the ratio of the hovercraft lift to the lift that would be obtained if the curtain jet were used to produce direct lift by jet reaction. An analytical expression is determined in outline.

In his paper, Mahalingam describes a simple receptance approach to the displaced frequency method, and shows how (in the simple case where the added mass affects the kinetic energy of one co-ordinate only) this can be applied in the determination of corrected generalised masses. A method had been propounded previously by the present author, and this accomplished the generalised mass corrections by a procedure related to the Collar-Jahn Method for the correction of an approximate set of eigenvectors.

At first sight, the methods of refs. 1 and 2 appear very different—the first being based on dynamic flexibility relations and the second on dynamic stiffness relations. Indeed, the final forms of the equations for the corrected generalised inertias obtained from the two approaches appear radically different, and it is by no means obvious that the two methods will, in general, produce identical (or nearly identical) results.

This note considers the buckling of a circular lenticular plate subjected to a temperature which varies parabolically in its plane. The novel feature in the analysis is the determination of the stiffening effect due to the prevention of deflections, but not slopes, along a diameter.

Mr. Lyall's remarks may be answered from the evidence already present in refs 1 and 2 if these are considered objectively. They establish that the ratio of the shear stresses as given by Preston tubes is correct. Evidence on this point is given by the value obtained for rmt from the Preston tube measurement of shear stress ratio and its agreement with the correlation of rmt as a function of radius ratio given by Leung, Kays and Reynolds.

In a recent technical note Rajaratnam has pointed out the possibility of using the pressure difference measured between two static pressure holes of different diameter as a means of determining skin friction. The device exploits the fact that for any hole there is an error in measurement of static pressure which depends both on the hole diameter and on local skin friction, as Shaw has demonstrated. Consequently the difference in pressure measured between two adjacent holes of different diameter will be a function of the skin friction. In this note the device will be called a “static hole pair”. The larger hole will be referred to as the “test” hole, and the smaller hole as the “reference” hole.

Analytical studies involving the pressure on inclined surfaces in hypersonic flow are made difficult by the lack of an explicit and simple relationship connecting the pressure p with the flow deflection angle 8. The various existing relationships are either too complicated or valid only over a very limited range.

In their pioneering paper published in 1917 Nayler and Frazer showed that the process of vortex formation behind a circular cylinder can have significant three-dimensional features even when care is taken to create two-dimensional flow. Hama produced photographs which show initially straight vortex filaments becoming curved some distance behind the cylinder, with consequent three-dimensional flow. In their experiments on boundary layers Hama et al found the formation and development of “vortex loops” from such curved vortex filaments to be an essential feature preceding transition from laminar to turbulent flow, and they stated that the same would be true for transition in wakes and jets.