PROLEGOMENA

1.1 Some introductory observations. The aerodynamic properties of aircraft have received much attention from the earliest days of flying, and there is a vast number of papers and also books in which the findings so far have been recorded (see e. g. H Schlichting & E Truckenbrodt (1959) and (1969)). What will be attempted here is something different: not to give yet another account of what we know about the aerodynamics of aircraft we know, but to deal with the question of how aircraft should be designed aerodynamically. Thus we shall concern ourselves only as far as is necessary with the question: "What are the properties of an aircraft of given shape?" Instead, we shall concen­trate on the question: "What shape should an aircraft have to give certain desirable properties?" In this way, we can discuss not only the design of existing types of aircraft but also possible future improvements of existing types and the development of entirely new types. Such a first attempt at this subject must necessarily present a personal view. Some, as yet hypo­thetical, types of aircraft are included also because the author is confident that the time will come when they will be needed and fly.

Another aspect of the approach adopted here should be made clear from the beginning: there will be no ready-made set of recipes for how the design of aircraft is done. Rather than to provide a repertoire of specialised present – day techniques which may rapidly date, the aim is to explain the basic fluid – motion phenomena and aerodynamic concepts which may be of more permanent value in a wide field. Thus we want to deal with conceptual frameworks of aircraft design and again, in this way, we can try to look forward into the future.

Some peculiar features of aeronautical research work and some specialised methods will become apparent, which have, perhaps, been evolved earlier or on a broader basis than in other fields. The resolution of the main tasks into an extraordinarily fine net of partial problems is one of these peculiarities. Another is the widespread use of abstractions and simulations, of models and analogies. The concept of a model may be taken quite literally, as in experi­mental work. But it may be understood to include also models of thought and mathematical models of physical occurrences, and these have probably a wider and deeper meaning and importance; usually, it is only through their use and through experiments in the mind that reasonable designs and actual experi­ments can be undertaken and carried out and those cycles of conjectures and refutations initiated and continued, which characterise all research work.

The method of enquiry and hence the presentation adopted here attempts to be generally in keeping with what is called the hypothetico-deductive method of Kant and others, as it has been defined, analysed and advocated recently by К R Popper (1934), (1963), and (1972) and by P В Medawar (1969). According to this, the generative or creative act is the formation of a hypothesis or conjecture. This process is neither logical nor illogical – it is outside logic. It does not rely on "facts", but allows for imaginative preconceptions, intuition, and even luck. But once a hypothesis has been formulated, it can be exposed to criticism, usually by experimentation. By logical deductions, inferences and conclusions can be drawn and predictions made. If the predictions are borne out, we may extend a certain degree of confidence to the hypothesis. "It is the daring, risky hypothesis, the hypothesis that might easily not be true, that gives a special confidence if it stands up to criti­cal examination" (Medawar).

What Medawar claims for biologists is also true for aerodynamicists: we work very close to the "frontier between bewilderment and understanding". Thus the view that an aerodynamicist is a man of facts and not of fancies, and that he is primarily a critic and a skeptic, is incomplete, to say the least.

To "stick to the facts", or to expose errors of fact, is not our main occupa­tion; and "to prove that pigs cannot fly is not to devise a machine that does so". The aerodynamic design of aircraft requires, more than anything else, creative imagination and initiative in speculations and conjectures followed by persistently thinking up comprehensive experiments which provide really searching tests of the design concept applied, to establish the confidence needed before we can let an aircraft take to the air.

In aerodynamics, the experimental tools are primarily windtunnels but also research aircraft and computers, and we must not forget that we can also perform experiments in the mind. But these experiments need not range over all conceivable observables – they can be confined to those which have a bearing on the concept under investigation.

For these reasons, we shall concern ourselves a great deal with hypotheses, premises, abstractions3 simplifying assumptions, and design concepts. It seems more important to understand these than to absorb theorems and infer­ences that can be deduced from them, or to be able to manipulate "facts". We shall see that many of the concepts in current use are really personal views of the matter, put forward by some individual scientist or some school of scientists or engineers and then more generally adopted (and quite often mis­takenly treated by some as though they were "laws of Nature" which permit "exact solutions" to be obtained if only the computers were big enough). On the other hand, we must realise that this treatment of simplifying matters and of concentrating on what are thought to be the fundamental concepts and overriding relations falls short of what needs to be done on the actual job of designing an aircraft project, in many ways. We must remember at all times that the actual design of aircraft is much more complex. Nevertheless, it should be a good preparation to have thought about the overall concepts and the coherence of the whole process. The actual way to carry out the work is best learnt on the job, anyway.

The subject of the aerodynamic design of aircraft will be seen to be largely in a fluid state and very much alive. Very little has settled down to a permanently "frozen state". In fact, the reader may be left at the end with the impression that the design of aircraft is as much an art as a science and. that the technology applied is still far from mature and well-established.

Such an impression probably corresponds to the real situation. It would be a dangerous fallacy to pretend that our knowledge of the design of aircraft is nearing its peak and reaching the "ultimate", that nearly everything worth knowing is known already, and that there is not much more to come (see e. g.

D KUchemann (1975)). On the contrary, we shall find that aviation, and air­craft design in particular, is only just growing up and that the main work still remains to be done.

One further general feature is necessarily associated with our subject and our presentation of its everything that will be said has a firm aim in mind, namely, the design of aircraft, and we shall concern ourselves almost exclusively

with matters which can usefully be applied to this purpose. This approach should go well together with the hypothetico-deductive method of enquiry we want to adopt. Even the concept of conjectures would seem to imply that we have an aim in mind; it is difficult to see how there could be completely aimless conjectures. To have an aim also implies that we want to move forward towards it. Hypothetical reasoning is the kind of argument which starts new ideas and brings us forward, and a clearly defined aim may help to straighten out our efforts and to set a train of thoughts in motion. This does not mean to say that we only need to state a demand and that it will he fulfilled, given sufficient funds. Such business manipulations are not our concern here. We want to work in the realistic world of scientific discovery and technologi­cal developments. On the other hand, it is not an easy matter to define specific aims which we may reasonably set ourselves. However, an attempt will be made because we think we have reached a stage in the development where we can foresee some long-term prospects and recognise at least some of the long­term aims. We may get some rough idea of what is still to come, if we set our sights high enough. Thus one purpose of this book is to give throughout an outline of what the main problems are and what remains to be done, again as a personal view.

It should help our purpose to set the scene, as it were, and to take an overall view of aviation as a whole before we go into details. To obtain a balanced overall view, we must try to be reasonably clear about what kind of strategy we want to adopt. We must consider not only the technical prospects but also the motivation and the purpose of our work. We must concern our­selves not only with the technical side but also with the social aspects. We want our problems and our work to be significant and worthwhile.

What do we mean by worthwhile? As far as the scientific problems and aspects are concerned, two criteria are sometimes put forward:

1 they should be intellectually challenging;

2 they should lead to results that can explain or predict physical phenomena. The first criterion would already be sufficient for the "pure scientist", but not for us. We need and we have to meet both. We shall see that problems in fluid mechanics will be prominent to satisfy the second criterion. Fluid mechanics is at the heart of the aerodynamic design of aircraft. However, we must also qualify the second criterion: there will be no firm and unquestion­ably true results, no infallible statements. We hold with К R Popper (1963) that we cannot ever provide positive proof – we can only disprove beyond doubt and we can refute. Thus we shall be concerned with conjectures and refuta­tions. We shall use results as long as they have not been refuted.

On the other hand, we are faced with engineering and technological problems, and these should also be worth our while. It may be argued that aircraft are among the most beautiful things that man can create. To fly seems to have been man’s dream from the earliest recorded days. There have always been "scientists" who wanted to find out how flying was done, and there have always been "engineers" who wanted to create the tools to do it with. When we get on to discussing aerodynamic problems, we must have the aim to go far enough to provide concepts and tools for engineers to be able to design aircraft. We must not stop half way at some interesting theory or at some large body of experimental data. We must go further and know what these mean in terms of their usefulness and applicability when designing aircraft. Thus we hold with Georg Christoph Lichtenberg (1742-1799) that "knowledge does not mean all the things we happen to know but only those we have thought about enough to know how they hang together and how they can be applied usefully". What we

want to do, in particular, is to apply basic aerodynamic concepts to engineer­ing situations.

Lastly, we must take a wider view and look at the social aspects. Is our work significant and worthwhile with regard to human society and the way we live? What is the social motivation of aviation? Like everybody else, we have a social responsibility to look at our actions in terms of what they mean to all the others. This implies that we know something about the aims of society. We are not likely to get much help here from what happen to be the opinions and movements of the day, and so we may turn to what we know about the nature of man: in ecology, by studying peoples and institutions in relation to their environment, and in ethology, by studying the behaviour of man and his natural make-up. Both are young sciences and we cannot expect to get very clear and complete statements. Nevertheless, we can get some useful pointers and indications even now. We shall find that the technical prospects and the social aims may be quite compatible. In fact, aviation may well be needed to help to achieve some of the social aims.