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Aviation History
1944
1944 - 1032.PDF
53& FLIGHT MAY ISTH, 1944. Research Concluding Instalment of SevenflL-J&idght Brothers Lecture to the Institute of^tne Aeronautical Sciences By W. S. f ARREIS^ S Concluded from page 507, May lltif Issue) Stability and Control Up to this point I have said nothing of theVcontribution of^ research to the production of stable and controllable airc I am glad to say that the time is now long past WrTen lack of stability is regarded by anyone as a virtue in an aircraft. In fact, it is unquestionably a most serious defect, whatever the duty of the aircraft. But it has always been. difficult to define the necessary or desirable margins of stability and the associated general stability and control characteristics. The designer must, however, have the requirements expressed in terms that can be reflected in his layout, both as a whole and in detail. He must be able to judge fairly accurately how the changes inevitable as a design develops will react on the stability and control, and he must have at his disposal means of dealing economically with the consequences both of the variation of load distribution resulting from operational. con ditions and of the changes involved in the development of the aircraft. There is a good deal about the stability and control of air craft in which there has been little apparent change over the period covered by the examples I have taken. I believe; how ever, that this is simply because the desirable general character istics were attained by about 1918. Since then our main problems have been, first, to preserve them substantially un changed in spite of the profound changes in the form of air craft, and, secondly, to enable the same man to control much larger and much faster aircraft. The foundations of stability' and control theory were laid, and well laid, long ago. Much labour has been spent on expand ing it to embrace new developments, such as structural dis tortion and on the analysis of the controlled and uncontrolled motion of aircraft. A vast amount of experimental evidence has been accumulated. Much of this, however, is related rather to specific problans than to the systematic development of an understanding of the matter. There is room here for a whole sale improvement, particularly by an attack on a wider front in flight. I am not among those who criticise our record here on the grounds that we did not undertake enough basic work at the time when the airplane, as we now know it, first crystal lised. I regret that circumstances made it impossible to give thi9 work high priority. Had we been able to do so, we might have avoided many troubles and saved much labour. But I do not believe that, on the balance, we would have reached our objective—usable aircraft—more quickly. We relied on our past experience, on our ability to improvise, and—most signifi cant of all—on our conviction that the theory available was soundly founded on experimental evidence. We discovered, by the attacks we were forced to make on troubles as they arose, much more about stability and control than most of us believed there was to learn. Thus, and f believe only thus, could we have advanced at the rate we did. It is an excellent example of the interworking of research and application. Control Balance In the field of control balance we have made tremendous advances in the face of difficulties that are sometimes hardly appreciated. The 1917 bombers operated at speeds—80 to 100 m.p.h.—at which the pilot could provide the forces neces sary for control with little or no aerodynamic balance. Take the 0/400 ailerons. The maximum hinge moment required was probably equivalent to a force on the pilot's hand of the order of 50 lb., with ailerons on which the aerodynamic balance was probably no better than one-half. In the Lancaster the same movement of surfaces of about the same size is required at 300 in.p.h., requiring nine times the forces. The pilot is no stronger, so the aerodynamic balance must reduce the hinge moment to say one-eighteenth of that of unbalanced ailerons. This is a difficult requirement, but it has been met. Suppose we put up the weight at the same wing loading to too,000 lb., one and one-half times that of the Lancaster. The linear dimensions will rise in the ratio 1.5V2 and the hinge moment at the same speed in the ratio r.53/2. The aero- dynamia^balance must therefore reduce the hinge moment in the raffo T • 1 30 similar argument leads to a figure of 1/400 if the weight is increased to 500,000 lb. We can certainly achieve 1/30 and possibly 1/400 in ideal conditions. But it is doubtful whether this is a wise policy, since we can hardly expect to define or to maintain the shapes of surfaces sufficiently closely. Power- operated controls have been avoided so far, but it is unwise to assume that we can neglect them indefinitely. There seems to be no good reason to be doubtful of our ability to make them reliable. Power Plant Developments ,1 do not propose to extend this survey to the two other mautJW factors that have contributed to the changes we have seen in aircraft—the power plant and the airscrew. I have already quoted some figures that show how remarkably the recipro cating engine has advanced. I have also said that there have since been further advances, which, however, serve rather to emphasise the comparisons I have made than to invalidate them. This is because there have been accompanying changes in weight and other characteristics that leave the main con clusions substantially unaffected. Our debt to the engineers who, aided by research, have achieved these results is immense. To the constant-speed airscrew the performance of aircraft must also acknowledge a great debt. But the flying mas is even more grateful for what it has provided—almost complete freedom from his chief anxiety, namely, the liability to misuse his engine. We now look forward confidently to new methods of propulsion for aircraft. But I believe the airscrew has a long and useful future before it and one in which research will play an outstanding part. The Task of Research I trust that this short survey has gone some way to show why I am convinced that the research worker and the engineer must work together if we are to make significant progress. In his James Forrest lecture to the Institution of Civil Engineers in England, Dr. Southwell said that "aeronautical engineer ing is ordinary engineering made more difficult." If that was true in 1930, as I believe it was, it is more than ever true now.. We can see clear prospects of great advances in aircraft in size, in performance, and in safety. The curve of improvement against time shows no real signs of flattening out. But we shall need all our ingenuity to avoid or to overcome the barriers which we can see ahead. I think the engineer has made good use of the outstanding contributions of research for aeronautics. If at times he has appeared slow to appreciate the significance of new develop ments, he has a good excuse in his preoccupation with producing something on which we can rely. This is a sufficiently serious responsibility and one that he has borne with credit. But it is this very preoccupation that emphasises the need for employ ing as part of an engineering organisation men competent to detect those advances in knowledge which are potentially valu able and to work out the technique of applying them. The research worker himself is not blameless in this respect. We can call to mind the case of Mendel, the significance of whose work in genetics was not recognised until he had been dead many years. His case is an example of discovery not appreciated because it is too far in advance of the general state of development of the science. Dr. Lanchester's books " Aero- donetics" and "Aerodynamics" contained much which may perhaps be regarded in the same way. The instances I have mentioned may, of course, be regarded as classic examples of the difficulty of disseminating know ledge. As the volume of knowledge increases, this difficulty grows. But the research worker has, in my view, a part to play in " putting across " the results of research. It is reason able to ask that he should put his results in such a form that they can.be used. To those who feel that this is hardly worthy of so much of their time and attention, as it fcrtainly demands if it is to be well done, I would commend the "example OF
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