CONCLUSIONS AND OUTLOOK

We began this introduction to the aerodynamic design of aircraft with an overall assessment of the aeronautical scene, its technical prospects, and of its possible future social implications and motivations. This was a personal view, and some results were anticipated, which have only been des­cribed in more detail later on. We can now review very briefly the main conclusions to be drawn from the results obtained and how far they have taken us towards the aims set at the beginning.

We have discussed design procedures in terms of various types of aircraft, and we distinguish between these solely on the basis of the different types of flow involved: the possible forms of aircraft were grouped together according to three types of flow which are, at this stage, known to be stable and eontvoVlable and, therefore, suitable for engineering applications. No excursion has been made in the discussions here into any other and less orderly flow regimes; nor was there any need for this: it is possible to construct a whole spectrum of major types of aircraft, which covers a network of global transport operations, on the basis of these flows alone. It is hoped that sufficiently convincing explanations have been given of why the types of aircraft can be classified according to their aerodynamic shape, as illustrated in Fig. 9.1, where speed and range are linked so that the flying time is much the same for all.

Fig. 9.1 The spectrum of aircraft

Whereas the aerodynamic aspects alone allow us to define the types of aircraft and to describe the evolution of the aerodynamic shape, a more complete and unified treatment makes it necessary to consider also the non-aerodynamic aspects. A general method is applied for this purpose and this framework provides a good representation of the overriding trends in the aircraft characteristics, at least in those cases where it can be checked against the properties of actual aircraft. The main conclusion which can be drawn from these investigations is that the types of aircraft differ from one another not only in their aerodynamics but also in most other aspects. Thus different types of fuel go together with different types of aircraft and, in general, each of the latter requires a particular mode of propulsion and type of engine. Again, each is likely to be associated with the use of particular materials and methods of construction, and the modes of operation may also differ. As a consequence, the design procedures generally differ from one type to another, and entirely different types of layout are obtained. These profound and widespread differences between the types of aircraft require equally radical changes in the outlook and the attitude of research workers and designers. In this respect, some psychological difficulties must be overcome because the immense success of the traditional classical layout and method of construction has imbued us all with the notion that every aircraft should have a fuselage for providing the volume needed, separate wings for providing the lift, and separate engines for providing propulsion. We can see now that this concept applies only to classical and swept aircraft but not to slender wings and not to waveriders.

Classical and swept aircraft are designed according to Cayley’s concept and the means for providing volume, lift, propulsion, and controls are separate and largely independent of one another. The most significant feature of the results obtained for classical and swept aircraft is that the practical requirements are in complete accord with the Kutta-Joukowski type of flow and the assumptions of classical aerofoil theory. Sweep is primarily a means for reducing the flying time for a given range. Swept aircraft are eminently suited for flight over a network of short and medium ranges at high-subsonic and low-supersonic speeds. At higher supersonic speeds and longer ranges, the swept aircraft is likely to be superseded by the supersonic slender wing. The type of flow is characterised by vortex-sheet separations from the aero­dynamically sharp leading edges. A flight Mach number of around 2 is a good choice for a transatlantic range. An advantage is to be gained by going to larger sizes, when the means for providing volume and lift will be more and more integrated.

For the special purpose of flying large numbers of passengers or goods over very short ranges, the results suggest that the slender-wing type of aircraft offers a natural solution in the form of an allwing aerobus for subsonic speeds. The layout of this as yet hypothetical aircraft may be much more compact than the corresponding classical layout: its size may be smaller and its weight less, for a given payload. The relatively large passenger cabin performs a useful function as part of the lifting surface.

The waverider type of aircraft to fly at hypersonic speeds over very long ranges up to global distances is as yet entirely hypothetical. It is regarded as a genuine aircraft and not as an orbital transport or spacecraft. The wave – rider is the only type of aircraft among those discussed here where the type of flow changes during flight. At high speeds, the waverider is a fully – integrated non-slender propulsive lifting body with shockwaves contained between the sharp leading edges; at lower speeds, the flow is like that over

Conclusions and Outlook

a slender wing. The changes are expected to be gradual and smooth. The aero­dynamics are such that they allow the use of high-energy fuels with large volume requirements.

It should be noted that we have concentrated on the aerodynamics of what we consider to be the major types of aircraft and that we did not discuss the many existing and possible future derivatives of these. Thus jet-lift air­craft and rotorcraft for vertical take-off and landing have not been dis­cussed. Also, attention has been paid mainly to transport aircraft and not so much to special problems of military aircraft. But it is expected that many of the results discussed here can be read across to these special appli­cations, once they have been fully understood.

Another conclusion that should have become apparent is that the problems which are facing us grow in number and complexity (some discussion of these general trends may be found in AGARD AR-60 (1972)). This is partly a con­sequence of the fact that the requirements for performance, handling quali­ties, manoeuvrability, agility, and, above all, safety are much greater and more stringent than ever before. This is as it should be in a technology which is alive. In view of the very large funds which every worthwhile aero­space project demands, it becomes also increasingly important to ensure that the design aims will be achieved. Consequently, much more information, and much more accurate information, must now be provided before and during every stage in the design of an aircraft to give sufficient confidence for a suc­cessful outcome. Also, the lead time needed for results to be obtained and applied successfully is getting longer and longer. For the same reasons, the penalties for failing to meet the more exacting requirements and specifica­tions are now much higher, and design deficiencies which are discovered late in flight tests can seldom be remedied because of the cost and delays involved.

It is fortunate that the tools required to carry out this work, such as wind – tunnels and computers, have also become much more efficient and powerful and can be expected to continue to be improved. Looking back at the many prob­lems we have discussed, we become acutely aware of the need to use every tool at our disposal to the full so as to obtain the information required. How­ever, as scientists we wish to understand things, and as engineers we wish to alter things and make new ones. In both these undertakings, the acquisi­tion of data needs to be accompanied by the growth of conceptual frameworks, which can account for the data we already have and show us where more are needed. Above all, it is such conceptual frameworks which enable us to formulate intelligent ways of modifying and controlling our part of human endeavours. Ideas and concepts come out of the mind, not out of computers or windtwmels. If there is one overriding purpose throughout these notes above all others, it is to demonstrate the continued need for conceptual frameworks and for understanding the physics of airflows in any work on aerodynamic design.

It is also hoped that the reader will have come to the conclusion that aero­dynamic design is very much alive and that it would be a grave mistake to believe that we have already almost reached the ‘ultimate’. We have by no means reached the aims we can rightly set ourselves. It should have been made clear that major advances are still to come. In a very large number of instances, we have come to a point where we had to state that research had not been completed and had not been brought to any engineering application. Many new ideas and concepts have been described, which have never been taken up. There are probably more open questions than answers in these pages. The existence of so many open and loose ends may be interpreted in part as an indication that we suffer from the effects of vagaries of fashion and of rambling and capricious research policies and personal interests. These notes should prove convincingly that more steadiness and consistency in aero­dynamic research would be beneficial to advances in our knowledge and its application in aircraft design (see also J Seddon (1973)).

Altogether, we can look forward to the most promising technical developments in aviation, and in aerodynamic design in particular, with considerable improvements in existing types of aircraft to be achieved and major new types to be developed. A whole spectrum of aircraft appears before us, with a global network of routes where distances are measured in terms of hours.

This can have beneficial repercussions on the way we want to live, and avia­tion can play a vital part in the task of making man more in control of, and in harmony with, his environment. Never before have the technical and social prospects in aviation been so varied and promising. There is a very long way to go.