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Aviation History
1909
1909 - 0107.PDF
FEBRUARY 20, 1909. In the course of a few remarks on gliding flight which Professor Bryan made in the course of a Friday evening discourse at the Royal Institution in 1901, it seemed to me evident that he had a greater grasp of the mathematical side of the problem of aerial navigation than had been previously evidenced, and, at my request, he wrote the remarkable mathematical discourse on the subject which was read before the Aeronautical Society of Great Britain on December 3rd, 1903. The remarks of Professor Bryan as to the distinction between equilibrium and stability—a distinction not very generally appre- ciated—may perhaps with advantage be here quoted :— " We say that the motion of a flying machine is steady when the resultant velocity is constant in direction and magnitude, and when the angle of the machine to the horizontal is constant. If this motion is slightly disturbed, the machine may either return after a time to the original motion, or it may take up a new and altogether different mode of motion. In the first case, the steady motion is said to be stable, and the second unstable. "It is evidently necessary for steady motion of any kind that there should be equilibrium—i.e., that there should be no forces acting on the machine (apart from accidental disturbances) which tend to vary the motion, and hence it follows that the number of modes of steady motion of which a mrchine is capable is, in general, limited, and that when an unstable, steady motion is disturbed, the new mode of motion taken up is entirely different from the old. " It is necessary to distinguish carefully between equilibrium and stability, as the two are very often confused together. Equilibrium is necessary to secure the existence of a mode of steady motion, but is not sufficient to ensure the stability of the motion. " The question of the stability of a rigid body moving under the action of any forces has been solved by Routh. In order to apply his results to the stability of flying machines, it is necessary to know the moment of inertia of the machine about its centre of gravity, the resistance of the air on the supporting surfaces as a function of the velocity and angle of incidence, and also the point of application of this force, i.e., the centre of pressure for different angles of incidence. If these are known for the surfaces con- stituting any machine, then the problem of its stability for small oscillations can be completely solved. Unfortunately, our know- ledge of these points is very unsatisfactory. Several valuable series of experiments have been made to determine the resistance on planes, but there is still some doubt as to the position of the centre of pressure at small angles of incidence, especially for oblong planes, and very little indeed is known as to the movement of the centre of pressure on concave surfaces. Until experiments are made on this point it will be impossible to solve the problem of stability for machines supported on concave surfaces." The last words of Professor Bryan emphasise the necessity of laboratory research, as well as continuing cur experiments in the open. Regarding experiments as to the movement of the centre of pressure on concave surfaces, it may be hoped that when the Brothers Wright publish the full results of their own laboratory researches, light on this subject will be forthcoming. The photographs of the paths of aerial gliders taken by Professor Bryan and Mr. W. E. Williams are suggestive of the utility of further photographic research on a larger scale. These photographs were taken by attaching magnesium wire to small gliders, consisting of square planes and pairs of square planes, and allowing them to descend in front of a camera in a dark room with the wire burning. By placing a rotating wheel in front of the camera, a dotted instead of a continuous track was obtained, enabling the velocities at different points to be compared. When the path is nearly straight two sets of oscillations are observed. If either of these oscillations increases as the glider descends, the glider will be longitudinally unstable. With regard to the equilibrium and stability problem, we have not yet got quite beyond the utility of observations with gliding models in the open. There is much yet that might be learnt as to the behaviour of various forms of sustaining surfaces. An instance of very successful and instructive glides (with models) was afforded on the occasion of the kite display, at Sunningdale, in 1907, the experimenter being Mr. Jose Weiss. His demonstration of the possibility of the maintenance of balance for a considerable distance, with a model launched from a hill-top, was one that should encourage himself and others in further research into the difficult problems of soaring flight. He exhibited three model gliders, having wing areas of 3-6, 8'4, and 12'8 sq. ft., with total weights of i\ lbs. and 15 lbs. respectively, the lead ballast in each case representing about two-thirds of the total weight. When launched from the highest hillock available the best glides obtained were some 200 yards in length, with drops from 30 to 50 ft. The small model, raised some 200 ft. by a large kite, and released from that height, righted itself instantly in each case, and gave some very fine glides, the longest being about 600 yards. Some further com- parative tests of this description might prove useful. Professor Bryan has suggested that model flying machines might advantageously be fitted with instruments to register stability. In connection with aerial navigation, a line of research, the importance of which cannot well be overestimated, are those investiga- tions which deal with the motions of the medium of travel. Thanks to the indefatigable efforts of Dr. William Napier Shaw, the investi- gation of the upper air is forming a feature of the work of the Meteorological Office, and most important results have been obtained. Such investigations are all essential for the progress of meteorology ; but they are equally important for the advance of aerial navigation, and their continuance and extension is worthy of the heartiest national support. It has been said that the ideal flying machine will be attained by a system of automatic stability. Since Mr. Brennan showed how a train could travel on a mono-rail and keep its stability by the appli- cation of the gyroscope, a new hope has arisen that the gyroscopic principle may be so applied to flying machines as to render them automatically stable. Simple experiments with the ordinary gyro- scopic top shows us that rotary motion can annul the effects of forces other than gravity. Though we are yet in a stage of experimental flight, and much has to be learnt in theory and practice before it can be adapted to the requirements of daily life, still even in its partially developed state the aeroplane may prove to be a potent factor of war. Under the cogent force of necessity the slenderest threads may have a power that in peace and prosperity would never be accorded them. I will forebear the discussion of the much vexed question as to whether, when frontiers are obliterated and war made hideously terrible by the flying machine, there will come the end of strife. But at any rate we may hope that the common paths of the air that will unite nations will remove many prejudices and prolong the blest hours of peace. ® ® ® ® ... . .__.... CORRESPONDENCE. %* The name and address of the writer (not necessarily for publication) MUST in all cases accompany letters intended for insertion, or containing queries. AERONAUTICAL TERMINOLOGY. To the Editor of FLIGHT. SIR,—You are shortly, as per your issue of January 30th, going to place before your readers a glossary of terms applicable to aeronautical science. The immediate necessity of doing so must be apparent to all ; for in a short period words frequently used by the Press will have a tendency to become fixed, and it will be of great advantage, therefore, that such be very carefully and scientifically thought out by those competent to do so. I venture to suggest that you first submit your list to the Council of the Aero- nautical Society of Great Britain, and also to the Committee of the Aero Club (the two paramount associations in this country), indicating to both bodies the necessity for dispatch in making any suggested alterations they propose ; this could, with a little energy, combined with early notices to those concerned, be readily accom- plished at one meeting. Then, what you deemed advisable of such revision, could be published in your excellent periodical, on the understanding that terse criticisms are invited from your numerous readers, after which the whole would become permanent. A most important factor in the arrangement of this glossary- appears to be, that, as far as possible, the language should be concise and quite distinctive, preferably words of one or two syllables, as indicated in your introductory article of January 2nd, for in these pushful go-ahead times, any lengthy forms of speech are bound to be cut down by popular usage into possibly objec- tionable non-euphonious sounds. The Aeronautical Society, due to its scientific trend, would be prone to suggest polysyllabic expressions derived from the usual ancient sources, but this would be counteracted by the sportive tendencies of the Aero Club into a usable up-to-date form. The wrong method to base this important subject is painfully evident in the deplorable verbose hash produced by the " Permanent International Aeronautical Commission " in 1903 vide Aeronautical, fournal, Vol. VII, No. 26). However, that being for International intercourse may be left for awhile, this glossary is presumably British. A few items from present aeronautical literature may be com- mented on as examples :— " Aeroplane," as used by " the man in the street," due, perhaps, in the first instance, to the aforesaid Permanent I. A. Commission, and also to its misapplication by the Press generally, but more par- ticularly to the insistence of the Daily Mail, to which popular paper we aeronautical students and sportsmen owe so much otherwise, is a most detestably objectionable and unscientific term, which, of course, should only be used to denote a flat surface. Instead, for a IG>9
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