The results are presented of an investigation of a particular type of baffle for the production of symmetrical velocity profiles having high ratios of maximum to mean velocity in ducted incompressible flow. Two similar families of profiles are obtained depending on whether a short (12 diameters) or a long (48 diameters) entry length is used before the baffle. The highest value of the maximum to mean velocity ratio obtained is 1·42 and the pressure loss coefficients associated with the use of the baffle are given together with an indication of the effect of Reynolds number.

Even from the early days of 1860, when your countryman Wenham experimented with gliders, the development of aerodynamics has been a systematic search for facts. The well–directed efforts of Wilbur and Orville Wright to search out the fundamentals of flight gave a profound impetus to contemporary and later experimenting, and libraries have been filled with the orderly research of aeronautical science. The development of manufacturing practice, however, has been dictated by the immediate needs of the moment, largely without the benefit of methodical study.

I trust I shall not be suspected of pleading a case either for the art of manufacturing, or for aeronautical science, as in the early days it was not unusual for me to find myself designing with one hand, while, in a manner of speaking, manufacturing with the other.

This paper describes an investigation conducted at the request of the Aeronautical Research Committee, who made a grant to one of us for the purpose. It is submitted by permission as an essay to compete for the R.38 Memorial Prize.

No essential alteration has been made in the form of presentation adopted in our preliminary reports, but to meet the needs of the general reader we have added the introductory paragraphs which follow. These explain briefly the nature of the problem which we have set ourselves, the range of our solution, and the bearing of our results on practical questions which confront the designer of a rigid airship hull.

Some liberties have been taken with the contents of the lecture in relation to the title. Firstly, it is proposed to examine safety in general, for only by this means can airworthiness be seen in its true perspective. Secondly, some trends in airworthiness and inter-related operational matters will be discussed. Finally, crashworthiness is dealt with at some length.

Of the many kinds of fatigue of materials, this paper deals only with “The fatigue of ductile metals by the imposition of cyclic stresses.” It is the purpose of the paper to show that: —

1. (i) Metals have certain very human attributes.

2. (ii) They have both memory and temperament.

3. (iii) They remember with increasing resentment the stresses which have been imposed upon them.

4. (iv) They are extremely sensitive as to the appropriate moment at which to break.

The question arises of why a piece of any ductile metal, other than a ferrous one, free from any abrupt change of section, when subjected to cyclic stresses of any magnitude, however small, ultimately breaks although the maximum stress imposed is one which, if static, it could have borne indefinitely. The answer is—mainly because of its metallographic structure.

Consider the growth of the boundary layer over an aerofoil placed at high incidence, the potential flow pressure distribution of which is shown in Fig. 1. In the region AB, the flow will normally remain laminar because of the large favourable gradient, but after reaching the peak B, instability will begin. Depending on the Reynold's number and the actual adverse gradient, the boundary layer will do one of three things:

1. 1. It will separate from the surface while still remaining laminar;

2. 2. It will become turbulent (transition) and will remain on the surface, or;

3. 3. It will become turbulent and will separate from the surface a little later.

This Paper was originally written at the request of L'Association Francaise des Ingenieurs et Techniciens de L'Aeronautique (AFITA) and was presented at a meeting in Paris on 30th April, 1946. The Service Technique of the French Air Force were also interested.

The Paper reproduced here represents the entire lecture, except for those parts which were concerned purely with the French picture.