Before discussing the noise associated with supersonic flight, it is pertinent to restate the sources of noise at subsonic speeds.

As Lilley has pointed out, noise at subsonic speeds is due to turbulence, such as occurs in jets, wakes, boundary layers, and regions of separation, and to vortices and regions of shear flow. In all these types of flow viscosity plays a predominant part, and therefore at subsonic speeds the noise of an aircraft depends very much upon the viscosity of the fluid: in an inviscid fluid, an aircraft would make no noise, because, at subsonic speeds, the pressure changes induced by its motion would be too gradual to be audible.

The association of a gas turbine with a piston engine is by no means a recent conception. As early as 1924 a Napier Lion Series V engine was equipped with a turbine in the manner shown in Fig. 1, the exhaust gases being fed into the turbine to provide all the power to drive the engine supercharger.

At a later date during the 1939-1945 War, many engines were installed in operational aircraft fitted with “turbo-chargers” of the type shown in Fig. 2, in which an exhaust activated turbine drives a subsidiary blower boosting the air intake of the normal engine supercharger.

From the earliest days of flying, gusts have been a problem for aircraft. In the very early flights aircraft were flown only when it was nearly dead calm and few gusts were met. Later on and, in fact, almost up to the Second World War, flying was greatly restricted by the ground and air conditions at the aerodrome. This meant that there had to be fairly good visibility and that the grass aerodrome must not be unduly softened by water. Since the war, aircraft have been flown with much greater regularity.

One of the many problems facing the aeronautical engineer is to make a vehicle, the reliability of which is tolerable, from an assembly of bits and pieces which, individually, are more or less unreliable. The practical answer has been to duplicate, triplicate, and generally to multiply the items about which doubt is felt.

This approach to reliability (and hence to safety) finds its way into codes of airworthiness, either as straightforward demands for duplication, or as requirements that no single failure should cause trouble.

On the whole these intuitive assessments of the probability of failure, and the design consequences in terms of duplication, seem to have been sound. It is, however, as well to examine the problems numerically, if we can. Possibly no very clear and decisive answers will emerge, but a little light may be shed on the paths we seek to follow.

In view of recent adverse publicity I must begin this paper by stating that anything I say must not be taken to imply any criticism of the Aircraft Industry. Any conclusions I draw, probably from insufficient evidence, must be regarded as purely intentional!

It has been difficult to decide how to prepare this paper. Should I confine myself to technological matters or should I range wide over the Industry's past, present and future problems, and run the risk of displaying my ignorance? I decided to range wide.

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.

It is now the usual practice in Great Britain to estimate the “ safe ” life of an aircraft wing from fatigue tests which are carried out on specimens representing the more highly stressed portions of the tension boom or booms. Such tests are designed to produce a tensile load in the boom equal to that produced in steady flight, and superimposed on this, a small fluctuating load corresponding to that which would be produced by an up gust immediately followed by a down gust of the same magnitude.

In the analytical presentation of curves the main requirements perhaps are those of accuracy and simplicity and the problem confronting the engineer is the efficient combination of the two. A further important consideration is that of the range over which the empirical formula (or formulae in some cases) is to be effective, for clearly the greater this range is, the more useful will be the formula.

It is obviously desirable that any equation that is chosen should be as close a fit as possible to the practical curve. It must not be sensitive to small parametric differences and it should be possible to determine these parameters with a fair degree of precision.

It is only in very recent years that the helicopter has begun to find a place in the pattern of Aviation. Although its development has not been so intensive nor so widespread as that of the more conventional fixed wing aircraft it has remained a positive and progressive force running on a parallel course.

The idea of the screw, as a method of propulsion or lifting, is not new, yet at the beginning of the 20th century its use was still restricted to propelling ships. It is therefore not surprising to find that the development of an early helicopter was carried on in a shipyard.

The Torsion-Bending Constant of open sections is of great importance in the calculation of buckling loads under compression of struts failing in the torsional or torsional-flexural mode. It involves computation of the warping of the cross section and misunderstandings arising out of terminology and notation often mislead the users of the formulae. The purpose of this note is to clarify the terminology without going into proofs as well as presenting the equations required using a consistent notation.

The issue of the Royal Aeronautical Society Structures Data Sheets 02.01.28 to 37* prompts the publication of a short survey of the theory on which these sheets are based and of the manner in which they may be applied. To save the need to refer to original sources the theory is developed from first principles; but for brevity the application is illustrated only by one simple example. Other more elaborate applications are illustrated by the example data sheets themselves. A recent paper by Chilver covers the same ground from the mathematical point of view and the purpose of the present paper is to illustrate the practical application of the method to familiar problems in the computation of initial buckling stress for sheet-stiffener combinations.

In cases of noise nuisance, measurement of the noise has several uses. It helps in deciding the degree of noise reduction which is desirable; it may serve to identify the most important source or sources of the noise where this is in doubt; and it is of assistance in devising methods for reduction of the noise. For the first purpose a single figure to characterise the noise would be preferable, but for the others some form of noise analysis is needed.

To the sufferer from the noise the most important characteristic is the annoyance it causes, or its nuisance value. This may be assessed very differently by different individuals, and consequently it is not very amenable to measurement.

The available methods of braking a modern aeroplane with its high landing speed are not entirely satisfactory. This difficulty in braking is particularly noticeable with pure jet aircraft. Owing to the absence of propellers, reversible pitch propellers are no longer a solution to this problem and the under-carriage can be simplified so that it occupies a smaller fraction of the all-up weight of the aeroplane.

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.

Carving from the solid is a manufacturing process dating back many thousands of years. Thomas Moore in his sculpture “Recumbent Figure” (Fig. 1) has instanced an example to aircraft engineers of integral construction and, has even gone to the trouble of providing discreet lightening holes.

Art throughout the ages has always looked to the future and while we are still busily applauding Shakespeare and Beethoven we are often left wondering with Moore and Walton.

Aircraft engineering is very much an art but Moore takes his integral construction for granted while aircraft designers are only just beginning to wonder.

In two recent notes Jacobs and Duncan considered the St. Venant torsion and flexure problems for beams of aerofoil cross section and have given methods for determining the centre of shear once the torsion function for the section has been found. The purpose of the present note is to give results for the location of the centre of shear for a limited range of cross sections bounded by two circular arcs for which the torsion function has long been known. The calculations shed some light on the relative influence of camber and thickness on the location of the centre of shear and are compared with the well known results for thin circular arc sections illustrated in the Structural Data Sheet 00.06.04.