When invited to give this lecture in a building overlooking London's river, my thoughts wandered back to many pleasant hours spent in watching Nature's graceful handiwork—the common seagull, so lamiliar to those of us who still have “time to stand and stare” in these hustling days of speed arid yet more speed. Wheeling with arched pinions and webbed feet retracted backwards into auxiliary tail organs, these effortless soarers are a vision of to-morrow. What aeronautical engineer, released for a moment from the narrow confines of the drawing board and the workshop, could wish for more profitable day-dreaming than a quiet half-hour spent on London Bridge (shall we say?), admiring the cunning perfection of natural flight? And who could wish for better inspiration for the subject of this paper than a glimpse now and again at Nature's own elegant solution of the undercarriage problem?

Already wireless is playing an extremely important part in connection with the smooth and safe operation of the commercial aviation services of to-day and it is evident that the present-day trend is to place more and more reliance upon wireless aids for the achievements of that regularity of service in all weathers, which is looked upon as necessary for the ultimate success of truly commercial aviation.

It seems therefore desirable to review at the present juncture the functioning of wireless communication and direction finding systems as used by the commercial air services; to consider some of the problems with which designers and operators are faced, and to bring to light the limitations and possibilities of this medium of communication and direction finding, both in respect to present and future requirements.

Cooling problems have recently been occupying a considerable portion of the time of the Aerodynamics Department at the Royal Aircraft Establishment. The work had hitherto been of a theoretical nature combined with tests in the small tunnels. The new 24ft. tunnel has provided us with a very valuable piece of apparatus to deal with the problem. The work is proceeding and is still incomplete, but we are beginning to have a clear idea of the principles involved. In this lecture I propose to describe briefly the tunnel and its working and the work we have been doing in it. The lecture is given by permission of the Director of Scientific Research, Air Ministry, but the views expressed represent only my own opinions.

Airscrew development has proceeded along four main lines :–

1. (1) Aerodynamic design.

2. (2) Materials.

3. (3) Adaptation to engine and aircraft.

4. (4) Fans for cooling and ventilating purposes.

Section (4) I have omitted. It may form the subject of a paper elsewhere. Section (1) I have barely outlined. It is of great interest, but I have concentrated on Sections (2) and (3) as being of immediate practical concern.

In speaking to you to-night on the Economics of Air Line Operation, I propose to speak primarily from the viewpoint of the operator of an internal air line. I do not mean, by that, that I shall confine my remarks strictly to the various problems encountered by my own particular company, nor do I mean that any opinions which I may express will, of necessity, be of interest only to operators of internal services. On the other hand, I do not intend to venture into the realms of very long distance air routes, such as Empire routes, many of the problems connected with which have already been dealt with in considerable detail by Mr. Woods Humphrey in his interesting series of articles in The Times in May of this year. Such small experience as I have had so far with international flying has convinced me that the political difficulties, which are undoubtedly encountered by many of the national air transport companies, must force upon them policies other than those dictated by strictly economic requirements, and in consequence, must render a true comparison between their operation and purely European flying almost impossible.

This is the first time I have been given the honour of reading a paper before the Royal Aeronautical Society, and I propose to give you a brief review of the progress made in commercial air pilotage and some of the problems the presentday captain of aircraft has to tackle on every day all-the-year-round flying in Europe and on the Empire routes to Africa, India and Australia.

It is perhaps desirable at the outset to see how commercial flying differs from military and private flying.

In the first place commercial routes are fixed and aircraft operate to schedule with many ground aids, compared with R.A.F. operations which of necessity have to be carried out without the assistance available on regular organised routes.

In commercial flying, formation and any flying involving steep turns and sudden manoeuvres must be avoided. Passengers’ comfort must be studied. Flights are of longer duration, and the commercial pilot flies many more hours during the year than does a military pilot.

As time passes more and more knowledge of the flow of fluids past bodies accumulates and of this increase some becomes applied to the problems of the day. Boundary layer theory is being applied at the present time to the problem of the degree of polish which should be given to a wing in order to reduce its profile drag to a minimum. Tests in the compressed air tunnel at the National Physical Laboratory and in flight at Cambridge and Farnborough have recently been directed to this point and give quantitative assurance of the correctness of theory. In what follows, a survey is made of a group of theorems relating to the resistance of various bodies such as aerofoils and flat plates and more generally to streamline forms. The theorems are partly physical and partly mathematical and approximations are numerous and of very different degrees of validity.

When the Council of the Society did me the honour to invite me to deliver the Wilbur Wright Lecture I found myself in a dilemma. Many of those whose lives are concerned with engineering or with scientific research must feel, as I do, that after a certain stage they cease to be quite the real thing. The realities of the workshop give place to a daily round of files, reports and estimates, and the laboratory knows one only as a visitor.

When a man has entered upon this ink-stained path and is invited to give a public lecture, there are only two things he can do, except refuse. He must either plagiarise or generalise. Of the two, on grounds of morality, I prefer the latter; but it has the disadvantage common to most moral alternatives, it is much the more difficult. For myself, too, there is a further complication, because the field in which I might feel qualified to generalise would be on some subject relating to engines and their development.

1. I wish in the first place to express my warmest thanks to the Council of the Royal Aeronautical Society for the honour they have done me in inviting me to read this paper on alloy structures. I have no claim whatever to be considered a metallurgist or an expert in any way on the technical uses to which alloys are put. I only venture to give this lecture because nowadays a physicist has a finger in most pies. The discoveries of physics concern all sciences interested in the structure and properties of matter. Since ultimately the properties of matter are dependent upon its atomic structure, and this is the field in which physics has made such rapid recent advances, its findings have become of especial importance.

It is, I suppose, known to most of those present in this lecture hall that the Royal Aeronautical Society was founded in 1866–to be exact the first preliminary meeting was held, under the chairmanship of the Duke of Argyll, on January 12th, though the first meeting of members did not take place until the 27th of June following. It is, therefore, within a few days of 70 years ago that the Society came into being. That is a full and honourable age, and with it goes the distinction–in which we are entitled to take some pride–of being the oldest Aeronautical Society in the world. The anniversary may not arouse the enthusiasm usually associated with the more youthful glamour of a silver or a diamond jubilee. On the other hand it denotes another decade of progress in the growth of aeronautical science, and of even more marked developments in the application of that science, on which grounds alone it deserves passing notice.

Flying, in common with all means of transport, is affected by adverse weather conditions, but the necessity of aeroplanes to maintain flying speed introduces a major difficulty of its own. The older forms of transport are able, in the last resort, to evade their difficulties by coming to a dead stop. An aeroplane must, literally, fly in the face of its difficulties. It must fly blind in clouds and perhaps land in fog. Over and above this, flight under certain meteorological conditions introduces a danger unique to aircraft. Ice may deposit at all leading edges and grow to windward, at critical regions of the relative airflow, in shapes which increase drag and seriously decrease lift. The accumulated ice adds to the weight. Unsymmetrical ice deposits on the airscrew blades cause dangerous engine vibrations which can only be kept in check, if at all, by throttling back at the expense of thrust. Venturis and pressure head orifices become blocked with ice, rendering the instruments they serve useless. External controls may become jammed. In short, many adverse factors to prevent flight may be brought into play simultaneously by the mere fact that particular meteorological conditions have been encountered.

This is a subject which during recent years has aroused considerable interest in all industries and particularly in the aircraft industry and a large amount of investigation work has been carried out. In some industries, both light and heavy, the growth of welding as a “ production tool ” has been phenomenal, and the same growth is at the present time occurring in the structural steel work and shipbuilding industries. If one views this change which is taking place it will be observed that the change is entirely at the expense of riveting and bolting, and before proceeding I will roughly summarise the general disadvantages of these two processes.

The strengths of the metals at present available to industry are of especial importance to the aeronautical engineer who is also in a position to appreciate the need for greatly improved materials, the absence of which often places restriction on much needed developments. Although the materials of the future may become available by the somewhat fortuitous development methods at present employed, it is undeniable that greatly accelerated developments would result if a correct understanding was obtained of the fundamental characteristics of the cohesion and fracture of metals, of which the former belongs to the field of the atomic physicist.

It has been found possible, for the first time, to show that failure under static and fatigue stressing is associated with changes in the crystalline structure which are identical. These changes are (1) a dislocation of the initially perfect grains into large components which vary in orientation from that of the internal grain by amounts up to about 2°,(2) the formation of “crystallites,” approximately 10-4 to 10-5 cm. in size, whose orientation varies widely from that of the original grains, and (3) the presence of severe internal stresses in the crystallites. At fracture, whatever the type of applied stressing, the whole of the specimen behaves to the X-ray beam as a medium of crystallites showing marked lattice distortion and oriented completely at random. X-ray diffraction methods are shown to distinguish clearly between the effects of the application of safe and unsafe ranges of stress; the first method that has been successful in this respect.

In order to show the relationship between the new work described and previous work dealing with the use of X-rays in studying the deformation characteristics of metals, a preliminary section of the paper deals with cold-rolling and drawing. A survey is also presented of the present position regarding strength and atomic structure, together with references to various theories regarding the imperfections of crystals as encountered in practice. An introductory section describes briefly the atomic structure of metals, as revealed by X-rays.

The present 2nd Series of the “Rendiconti sperimentali del Laboratorio di Aeronautica del Reale Istituto Superiore di Ingegneria di Torino” follows on the 1st Series published in April, 1934. It includes:—

1. (1) Description of a new six-forces balance.

2. (2) Statement and discussion of the results of the tests on :—

   1. (a) a 600 mm. wind tunnel with open jet and return flow, with the object of deter- mining the influence on the steadiness and distribution of velocity of the stream in a cross-section of the jet of various details of design (dimensions of the cells of the honeycomb, cascades, etc.) and the diffusion of the portion of the airstream undisturbed by the air carried along by the jet owing to the effects of viscosity.

   2. (b) on fifteen models of wings.

   3. (c) on three motor car models.

   4. (d) on an airscrew model mounted at various lateral angles to the relative asymptotic wind (airscrew in a side wind).

   5. (e) on three models of tail plane in the presence of an airscrew.

The last-mentioned tests on tail planes are part of a series of tests still being carried out with the financial assistance of the Directorate of Research and Experiments of the Air Ministry at the suggestion of General Crocco, with the object of determining the influence of the working airscrew on the aerodynamic characteristics of tail surfaces.

With the growing popularity of the stressed skin or monocoque fuselage it is thought that the following notes may be of interest. There are many advantages in using stressed skin construction, but one reason against it in the past has been the difficulty in accurately calculating the strength of such structures. Sufficient information has now been accumulated, however, mainly from American sources, to enable the designer to obtain a fair idea of the strength of stressed skin structures, although the stresses occurring under any system of loads cannot be calculated at present with the same certainty as for the conventional braced structure. In the braced frame fuselage the loads are taken by struts and tie rods in tension and compression, but in the stressed skin fuselage they are taken as bending and shear loads in the skin. Once the ultimate tension and compression stresses are known for the material used, the strength of the struts and tie-rods for any system of braced structures is easily calculated. In the stressed skin structure, however, the ultimate stresses for the material are never realised over the whole structure, as failure invariably occurs through local buckling of the skin. The question is further complicated by the fact that the stress at which local failure will occur in sections built of thin plate varies with the design of the section, and two sections having the same area and same moment of inertia would not necessarily develop the same stress in bending.

These notes are an attempt to examine what may be achieved in the way of performance, with existing materials and knowledge of aerodynamic engineering.

No claim is made for novelty, either in the data used or in the methods of working from such data to obtain the results arrived at.

Only the simplest engineering mathematics are employed so these results cannot claim academic accuracy, in fact they are most probably considerably inaccurate. But though inaccurate absolutely, the results should be accurate enough relatively for comparative purposes.

Full details of the methods employed are given in the various Appendices. This appears advisable, as it should make quite clear how the results are obtained and leaves it open for anyone to work out further examples by the same methods or to modify the methods themselves.

The object of the present report is to make a general survey of the simple formulas and methods of calculation for the determination of the strength of thin-walled structures (plate and shell structures) which are increasing in importance in aeroplane construction, and to facilitate the study of original papers by adding a detailed list of relevant literature. an essential characteristic of the sheet metal covered structures in question is, that the metal skin in addition to the stiffening elements participates in the transmission of force. structures in which the metal sheet serves merely as a covering but which is not loaded in accordance with its strength, do not come within the scope of the present considerations. non-stiffened and stiffened plates and shells, mostly of very small wall-thickness (about 0.5 to 1.2 mm.) may be regarded as structural elements in plate and shell structures (especially shell fuselages, “ monocoques ” and wings).

The investigation of the strength of a rigid-jointed frame is a problem of considerable practical importance. The precise effects of the secondary stresses arising from the rigidity of the joints have in the last half century been the subject of much speculation. But the calculation of these secondary stresses is extremely tedious by any existing method and in consequence they have rarely been ascertained with any degree of accuracy. The first real attack on the problem was made by Manderla in his Munich Polytechnic prize essay–“ The Calculation of Secondary Stresses which Occur in Simple Trusses ”–published in 1880 in Allgenieine Bautzeitung.