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Aviation History
1919
1919 - 0956.PDF
Analyses of Weights.—As in all forms of science, progress is very largely dependent on the ability to measure accurately the various items and factors. No genius can make up for the neglect of this essential. Once all the factors, whether they be weights, sizes or performances, are accurately mea sured, it is not so difficult to decide in which direction to proceed. A large amount of attention was given to the careful analysis of the weights of all available aeroplanes, good or bad, and tables Were prepared of which samples are given in Figs. 4, 5 and 6. By a study and comparison of these results in conjunction with the strength of the various aeroplanes, much knowledge of the possibilities of construction was obtained. The interesting fact was arrived at—that for a range of well- designed practical war types, the structural percentage re mains roughly constant for aeroplanes of total weight varying from 1,000 to 30,000 lbs. From a theoretical point of view, this is somewhat sur prising, because, as is well known to engineers, the law of dimensions lays down that area increases as the square and weight as the cube of the dimensions, and that this will there fore put a limit on size. In fact, a very eminent aerodynamic theorist, working on these lines several years ago, put the limit of the weight of an aeroplane at about 10,000 lbs. There is very little doubt now that aeroplanes of 100,000 lbs. are a practical proposition. Some of the reasons for this apparent theoretical discrepancy may be of interest. One is, that the larger the aeroplane, the more sober is the method of progression. No one wants to loop or do vertical banks on a big passenger aeroplane, and therefore it is not necessary to maintain as high a strength factor on the big type as on the small. As far as actual fly ing stresses are concerned, it would be possible for an aero plane with a load factor of only 1$ to be flown without collapse even on a windy day. This statement is only used for illustra tion, and must not be taken to suggest that a load factor of 1J would produce a practical aeroplane ; it would probably be too weak to stand landing. Another reason for the dis crepancy is, that the bigger the aeroplane the more detailed can be the design work, and it becomes possible to use material in a more efficient way. It is not considered probable that aeroplanes made of wood will increase to a size representing a weight of much more than 40,000 lbs., but by the use of high grade steel and duralumin, it will certainly be possible to go far beyond this limit. Influence of Tunnel Experiments.—At one time the small scale work carried out in the wind tunnel was regarded as of little practical value ; now, judging by the results, I do not think that it would be too much to say that the work which was put into tunnel research, when this work at the National Physical Laboratory was under the direction of Mr. Leonard Bairstow, before as well as during the War, was the real basis of the technical success which we undoubtedly attained, particularly in the aerodynamic field. The data from such work is more useful to the designer from a comparative aspect than for the absolute values obtained, but without their help it is only too easy to stray off into blind alleys leading no where except to disappointment. Some designers have undoubtedly a wonderful facility in guessing the next step to take, but they are too few and far between for responsible authorities to rely solely upon them. In any case such men always work best if there is a solid background of research knowledge behind them, from which they draw sometimes perhaps unconsciously. In 1914 no private firms had a wind tunnel of their own or went in seriously for research. Now there are four or five first-class installations in constant use by the designers of the manufac turing firms. As instance of more important results there now exists fairly exact knowledge of the best wing sections, strut shapes, propeller blades and body resistances. Stability and controllability.'—There are two means of obtain ing stability in an aeroplane : first, by means of an automatic device, such as a gyroscope ; and second, by such a disposition of the surfaces of the aeroplane that the machine has inherent stability. Very little success has been obtained from the first method, but the second is now very largely employed. Questions relating to stability and controllability are intimately connected, but in one sense they are distinct. Thus it is possible to have an unstable aeroplane which is readily controllable and very popular with pilots. Stability may be considered under three heads :— Longitudinal stability ; Lateral stability ; and Directional stability. The first, longitudinal stability, is obtained by means of the tail plane, and the size of this for any particular type determines within limits the degree of stability. The funda- JULY 17, 1919 mental point, however, is the position of the centre of gravity of the aeroplane relative to the main planes. If this is too far aft no tail plane can be found to give stability. The farther forward the centre of gravity, the smaller is the tail plane required. The problems of lateral and directional stability are very closely connected, and must be considered together. Lateral stability is obtained by giving the main planes a dihedral angle, and directional stability is obtained by a proper regard to the dimensions and dispositions of the fins and rudders. The area of the fin and rudder required is a function of the dihedral angle. The outbreak of war found us in a very favourable position with regard to the development of an inherently stable aeroplane. The importance of this feature from a military point of view had been fully realised, and special efforts had been made to produce a stable and at the same time con trollable aeroplane. That these efforts were successful was largely due to the late E. T. Busk, of the Royal Aircraft Factory, as it was then called, and the " B.E.2C " aeroplane, which embodied the results of his work, had been |fully tested and demonstrated to possess complete adherent stability prior to the outbreak of war. Unfortunately, Mr. Busk was killed in a flying accident shortly after his experi ments had been brought to a successful issue. However, his full scale research had been carried so far that the principles underlying the design of this machine could be applied to any other design of aeroplane. Subsequently nearly all machines were designed for inherent stability, except such types as were considered to be more suitable for their specific work if a certain degree of stability were sacrificed for very quick manoeuvrability. An impression was prevalent, at any rate during the first two or three years of the War, that a stable aeroplane must necessarily be very heavy on its controls, and since quick manoeuvrability jwas an essential for fighting scouts, the aim of designers, encouraged by fighing pilots, was to obtain the maximum controllability and quick ness of handling, irrespective of stability. It was gradually realised that this was a mistaken view, and that the comparatively poor manoeuvrability of some of the earlier stable machines was due, not to the fact of these machines being stable, but to the particular design of the controlling surfaces. A very great amount of research into the conditions governing stability and controllability was carried out both from the theoretical and experimental stand point, and this revealed enough data to enable aeroplanes to be designed combining inherent stability with good manoeuvrability, and as a result the prejudice against stability in small fighting aeroplanes quickly disappeared. A good example of the advantage of this is afforded by Capt. Ball's wonderful return to our lines on his stable " S.E.5 " machine after his controls had been almost com pletely shot away. The aeroplane practically flew itself back, and with only half of his normal elevator control he was able to make a safe landing on the aerodrome. Such a feat would have been out of the question on an unstable machine. At the end of the War we were calling for aeroplanes with neutral stability for fighting work, i.e., aeroplanes which followed the pilot's mind and hand in whatever attitude they were put. For bombing and long-distance work stability is a very important asset, as it relieves the pilot of fatigue, and facilitates the maintenance of his course and his sighting for bomb dropping. The disadvantages of not putting a sufficient amount of study into these features are demonstrated by German practice. Although a few of their small aeroplanes were fairly manoeuvrable, their larger ones did not compare favourably with our own when tested under the same conditions, and in the case of their very large bombing aeroplanes we had evidence that the pilot's difficulties on a long flight were very considerable, and it is probably not too much to say that the heavy proportion of German bombers which were crashed on returning from their raids was largely due to lack of stability and controllability. Monoplane, biplane or triplane.—During the War all these types have been experimented with, and, in fact, have been built on a production scale, and it cannot be definitely stated that one type is more suitable than another without knowing the exact purpose for which the aeroplane is required. Broadly, the comparative advantages of the three types are as follows :<—In comparison with the biplane, the monoplane is 5 per cent, more efficient as a weight carrier per square foot, and can be made to afford a better fighting view. On the other hand, it is weaker for the same weight of structure, and is less manoeuvrable for equal total weight. Similarly the triplane, comparing it to the biplane, is 5 per cent, less efficient, but is more manoeuvrable, and affords opportunities 958
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