In applying the numerical correction method to the case where two natural frequencies of the system under test are equal, it is necessary to ensure that the matrix

is diagonal in order to eliminate singularities. (In this equation δA is the displaced mass matrix; is the partitioned matrix containing the two modal columns appropriate to the equal frequencies and β is a transformation matrix (2 x 2), chosen to ensure that δαb is diagonal.)

First let us look at the history of our engines’ reliability. Fig. 1 shows the reliability in terms of unscheduled removals per 1000 hours and in-flight shut-downs for four Rolls-Royce civil engines since the time of their introduction into service.

It is significant that the general trend is similar and particularly, that after the initial years settling down period, the level of reliability defined in this way has stayed almost constant, despite considerable increases in overhaul life and, for the Dart, great increases in power output as later types of engine were produced.

The Early experimental work at the Royal Aircraft Factory, South Farnborough (as it was called then) has been described by a number of people. There was no organisation in the First World War, as there was in the Second, for the use of scientific man power. The gravitation of scientists to the Royal Aircraft Factory can, I imagine, only be explained by the attraction of working in an unexplored field. The results of the impact of this remarkable group on the future of aeronautics in this country were enormous.

My own relatively late appearance at Farnborough, towards the end of 1916, was the result of a series of accidents.

In a recent paper, Gibbings has made an interesting comparison of the length of two-dimensional contractions based on three criteria, namely:

1. (1) the rapidity with which the boundary slope approaches its zero value;

2. (2) the rapidity with which the centre-line velocity approaches its asymptotic value; and

3. (3) the degree of uniformity of the velocity expressed as a percentage of its mean value.

In 1954, Preston introduced a new concept of skin friction measurement in turbulent flows by using a surface pitot tube. Since then considerable controversy has risen in the use of Preston tubes for skin friction measurements. However, Head and Rechenberg investigated Preston tubes extensively and confirmed the validity of the Preston tube method. Recently Rajaratnam has given a theoretical calibration curve for the Preston tube using logarithmic velocity distribution for the boundary layer velocity profile.

A solution of the problem of the initial flow in the entrance region of a pipe has been given by Atkinson and Goldstein. In this note the authors compute this solution in greater detail, and compare their results with those of Atkinson and Goldstein. The range of the validity of the solution is shown to be smaller than originally thought.

Atkinson and Goldstein considered the equations for the steady flow of an incompressible viscous fluid into a straight circular pipe of radius a. The flow is supposed to have axial symmetry, u being the velocity component parallel to the axis x, and v the radial velocity. The, distance from the axis is denoted by r.

Special reliability clauses have been introduced into a small minority of our contracts for military aircraft. This does not imply that reliability is otherwise ignored. Where operational requirements call for certain standards of reliability and where these are translated into equipment specifications or test schedules, the contract covers them in the same way as any other performance requirements. This has been particularly the case in contracts for aircraft engines.

The Royal Aeronautical Society had already celebrated its first fifty years when, in January 1917, the infant Testing Squadron of the Royal Flying Corps took up residence at Martlesham Heath in Suffolk, where it grew up to become the Aeroplane and Armament Experimental Establishment. It was at Martlesham that A & AEE built its reputation for thoroughness and accuracy in the assessment of aircraft and their equipment, a reputation which is still enjoyed today by the Establishment at Boscombe Down, its home since 1939. To see the origins of A & AEE, however, we must go back to the early months of the First World War, to the Central Flying School at Upavon on the northern edge of Salisbury Plain.

The linear buckling of circular cylindrical shells is considered with particular attention to the cantilever shell subjected to either a pure bending moment (M) or transverse load (P)—see Fig. 1. It is believed that the conclusions reached have wider application to more general loading cases.

The note by W. T. F. Lau upon the effective “length” of two-dimensional contracting ducts has raised further points of interest.

Lau's paper presents three different criteria which can control a contraction length. Of these three, the second and third, that respectively are based upon the approach of the centre-line velocity to its asymptotic value and upon the approach of the flow to uniformity, were compared in ref. 1. The first criterion that is based upon the approach of the boundary slope to zero is not in general identical with the second.

Three of the tenets of a Contracts Manager are (a) that he should not be seen, (b) he only speaks if spoken to, and (c) he ensures that every answer he gives is capable of being interpreted in three different ways. You can imagine, then, how embarrassing it is for me to make what is supposed to be a forthright statement to an audience which I imagine contains a number of customers and potential customers.

I suppose one of the most ulcer-making things that can happen to an airline management is to see a piece of brand new and expensive aircraft machinery sitting on the tarmac unserviceable. With the increasing cost of civil aircraft, profits can only be made by keeping them in the air at a high degree of utilisation.

The planar librational motion of a dumb-bell satellite has been studied by several authors. In most cases the analysis has been restricted to circular or nearly circular orbits. In an elliptic orbit, the variation of the angular velocity of the satellite around the central body together with the change in the local gravity gradient provides the satellite with a mechanism for exchanging energy between the librational and orbital modes. In general, this imposes a limit on the orbit eccentricity for which stable (non-tumbling) librational motion is possible. This note investigates the bounds that must be placed on a disturbance applied to a slender gravity gradient stabilised satellite so that it will librate and not tumble. The determination of the stability boundary as presented here was arrived at by the numerical solution of the exact equation of motion, obtained using an IBM 7040 computer.