The Ninth Louis Blériot Lecture was given in Paris on 7th March 1956 under the auspices of the Association Française des Ingénieurs et Techniciens de l'Aéronautiques, by Mr. E. T. Jones, C.B., O.B.E., M.Eng., F.R.Ae.S., Director-General Technical Development (Air), Ministry of Supply, and President-Elect of the Royal Aeronautical Society.

The Lecture was attended by the President, Mr. N. E. Rowe, C.B.E., B.Sc., F.C.G.I., F.I.A.S., F.R.Ae.S., and several members of Council of the Royal Aeronautical Society, by the Secretary, Dr. A. M. Ballantyne, T.D., B.Sc., Hon.F.C.A.I., A.F.I.A.S., A.F.R.Ae.S.; by three former British Blériot Lecturers, Air Commodore F. R. Banks, C.B., O.B.E., F.R.Ae.S., Mr. H. Knowler, F.R.Ae.S., and Mr. R. Hafner, F.R.Ae.S.; and by members of the Society and the British Aircraft Industry and by a large and distinguished French audience. M. Jules Jarry, President of A.F.I.T.A., presided at the meeting and welcomed the guests before introducing Mr. Rowe and later, Mr. Jones. Mr. Jones spoke for a few minutes in French and then his paper was read by M. Maurice Roy, Director of O.N.E.R.A., and himself a Blériot Lecturer. After the lecture a film was shown of the Rolls-Royce “ Flying Bedstead.”

The Lecture was followed by a Reception and cocktail party and then by a dinner attended by 150 distinguished members and guests at the Aero Club de France. Among the guests were Madame Louis Blériot and M. Henry Laforest, French Secretary of State for Air. Following the dinner, speeches were made by M. Jarry and M. Laforest, and by Air Commodore Banks, deputising for Mr. Rowe who had had to return to England, and by Mr. E. T. Jones.

For centuries man has watched with envy the easy flight of birds and has tried to simulate their basic form, structural principles and methods of control. Often too much enthusiasm and too little technical knowledge has ended in disaster, and the most successful people were those who relied upon models for their initial data.

The Wright Brothers were among the first to apply an engineering approach to the problems concerning the magnitude of loads occurring on wing and control surfaces. To this end they constructed a wind tunnel as long ago as 1901 and succeeded in measuring actual forces on model aeroplanes, and in comparing them with systematic glider experiments. They continued to develop their research along these lines to achieve the first power flight in December 1903.

Three of the primary considerations involved in the design of an aeroplane are safety, weight and serviceaability. The structure weight being a major item in the design which can influence its efficiency as a flying machine, the considerations of safety and serviceability have always to be studied with respect to their effect on the weight of the structure. The history of the development of aircraft structural design indicates clearly the tremendous amout of work which has been devoted to the development of higher strength materials and more accurate stressing and testing methods, always with the aim of ensuring adequate safety and reliability consistent with efficiency of design, construction, and operation.

As the treatment of the different subjects must be brief and general this paper can only deal with the complex subject of aircraft fatigue in very broad terms, attempting to trace the development of the subject over the past ten years or so, to present some of the problems and the methods being adopted to solve them and to emphasise the very wide extent to which fatigue affects aircraft design today.

The fatigue of metals was recognised as a separate form of failure as long ago as 1854 and is now the subject of a very extensive literature. Mechanical engineers have long been familiar with fatigue as a criterion of design and are apt to be critical and even amused at the aeronautical engineer's recently found interest.

The part played by strength testing in establishing structural integrity and efficiency is well known to aeronautical engineers. In principle, the great advances made in structural theory and stressing methods should lessen the pressure on strength testing facilities. In practice, things do not seem to work out this way. For this there appear to be two reasons. The first is the continuous change in aircraft structural design to meet new or more exacting operational roles. The second is the demand for still lighter structures without prejudice to safety. Strength testing, therefore, is as necessary as ever it was if the best results are to be obtained.

For a great many years structural strength testing meant just static strength testing. This is no longer true, since for many types of aircraft fatigue testing has become at least as important as static testing.

The Society's All-Day Discussion on Aircraft Design Philosophy was extremely well supported and at one stage there were more than 350 members and visitors present. As is inevitable on these occasions there was insufficient time available for all those who wished to make a contribution to do so, even though the four lecturers had reduced their contributions to a bare minimum by presenting summarised versions of their papers.

With such an all-embracing title as Aircraft Design Philosophy it was obvious that some main channel of exploration would have to be chosen; that which evolved during the planning of this event was structural safety, as will be apparent from the discussion. The preparation of a summary of discussions presents a number of problems.

A simple and convenient “ endurance chart” for the presentation of basic fatigue data is described, where static failure is regarded as simply fatigue failure in one cycle of loading.

This chart shows the effects of mean stress, residual stress and pre-tension, and displays several interesting features of fatigue data. It is easily constructed directly from fatigue test results, and is very well suited to the calculation of stress concentration effects, including “ plastic relief ” effects.

With the aid of such charts the relative fatigue strengths and structural efficiencies of different materials may be readily compared, and it is clearly shown that in certain circumstances a “ high-strength” alloy may in fact have considerably less fatigue strength than a “ medium-strength ” alloy at a given life, or conversely a lower life at a given stress level.

It is demonstrated that the fatigue life of a component subjected to combined mean stress and alternating stress may be found only with a knowledge of the complete fatigue characteristics of the material (such as are presented in the endurance chart); and that attempts to estimate fatigue life from spot-value comparisons, such as the fatigue strength at an arbitrary given endurance, can be dangerously inaccurate. A chart of the form described thus forms a valuable step towards the accurate calculation of fatigue life.

The method is illustrated by the provision of endurance charts for 14S-T (L.65), 75S-T, and D.T.D.683; and with experimental results for notched components of L.65 and D.T.D.683.

An example illustrates the application of the chart to the calculation of the fatigue life for the case of two independent systems of loading acting on a pressure cabin with circular window cut-outs.

It is usual when plotting characteristic diagrams for unsteady flow fields to assume a constant ratio of specific heats. In the general theory of characteristics developed by Guderley the variable specific heats are introduced into the calculation of the characteristic constants. In the following method the variation in the ratio of the specific heats is included directly into the slope of the state characteristics. The position diagram may then be plotted directly by the usual techniques.

An approximate method is shown for the estimation of the velocity and pressure distribution over the surface of circular annular aerofoils with thick symmetrical sections, at zero incidence.

Figure 1 shows an annular aerofoil or profiled ring obtained when an aerofoil with symmetrical section rotates about an axis.

From the theory of wing sections it is known that the aerodynamic effect of a symmetrical aerofoil section, at zero angle of attack, may be approximately replaced by the effect of sources and sinks distributed along the chord of the section.

A presentation of the solution of linear equations with complex coefficients is given, in which current checks can be introduced to obviate inaccuracies and thus engender confidence in use. The method is based on the elementary concept of elimination of each unknown in turn, and details only of its application are given since the theory demands lengthy exposition. Evaluation of determinants is also illustrated.