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Aviation History
1915
1915 - 0965.PDF
herself could provide a suitable prime mover in the force ot gravity. It is with the name of Lilienthal that the introduction of the art of gliding is invariably associated, on account of the very wide influence of his practical work,* but chronological priority appears to rest with Prof. J. J. Montgomery of California, who made gliding experiments in 1884^ The practice of gliding, that is to say the use of wings as a toboggan for sailing downhill through the air, brought the pilot into direct realisation of the difficulty of balance and direction, and this led, by degrees, to the evolution of a moderately safe machine. Many lives were lost J in the process, but the thought of danger availed little against the enthusiasm of those who were determined to learn to fly, and the advice of the mathematician who advised delay, pending the symbolical solution of the problem, was laughed to scorn. Nevertheless, the scientific aspect is already assuming a very considerable importance in modern aeroplane design, and this is largely owing to the very painstaking labours of Prof. Bryan and Mr. Harper, who have worked out a basic method for the treatment of stability problems in general,§ and to the late Edward T. Busk,I! who was responsible for the stability calculations of the aeroplanes designed at the Royal Aircraft Factory. As might be expected, the process scarcely represents mathematics in its most elementary form, and hardly lends itself to any brief explanation. An aeroplane in flight has six degrees of freedom. It can move longitudinally, sideways, or vertically : it can also rotate about any one of these axes of direct motion. A partial rotation about the transverse axis is called " pitching " ; if about the longitudinal axis, it is called " rolling " ; and if about the vertical axis, it is called " yawing." The situation is further complicated from the fact that rolling will produce yawing and vice versa—the general stability of the machine is thus a question of some complexity. When a machine pitches, the oscillation will die out of its own accord if the tail is in proper relationship to the wings. In general longitudinal stability depends on the existence of a dihedral angleli between the wings and the tail. Thus in the elementary case of flat surfaces, the wings must be set at a steeper angle of incidence than the tail plane to produce longitudinal stability. In Bryan's mathematical treatment of these problems, the existence of stability, or otherwise, is shown qualitatively by the solution of an equation being a positive or negative quantity. In a method of treatment given in Lanchester's " Aerodynamics," the criterion appears in the answer being greater or less than unity. Quantitatively, the problem resolves itself into ascertaining the decrease in the successive maximum ordinates of the oscillation graph. In describing the organs of control, it has been explained that the elevator forms an extension of the tail. The value of the tail as a stabilising organ is thus a variable quantity, so long as the elevator is under the pilot's control. Skilfully handled, the elevator will augment the natural stability of the machine—if used otherwise, it may promote dangerous consequences. The same may, of course, be said of each organ of control in turn. If some disturbance causes an aeroplane to roll, it will immediately yaw also : for directly the wings become canted over to one side, the air pressure upon them possesses a lateral component that pushes the machine off its former course. It is by this means that most aeroplanes are steered ; that is to say, an intentional roll, or bank, is established by warping the wings. As the machine moves diagonally under the influence of the above lateral force, opposing pressures will be generated on any vertical surfaces that the machine may possess, and these pressures will tend to restore the initial balance, or augment the roll, according as the balance of vertical fin area lies above or below the centre of gravity. So far as lateral stability is concerned, therefore, the fundamental problem resolves itself into a consideration of the effects produced by various dispositions of vertical fins, and on these lines it has been worked out by Prof. Bryan. In the actual design of a practical aeioplane, it is undesirable to introduce fins merely as stabilisers, and the endeavour is always so to dispose and proportion the * Lilienthal was a student of aviation from boyhood, but only commenced his gliding experiments in 1881. His work inspired Pilcher in England, and Chanute, Herring and the Wrights in America. For a summary of his work, see "Aviation," Chap. XI. . . ,. ,,,..., r t For an account of Prof. Montgomery's work, see Lougheed s Vehicles 01 the Air,'' p. 138. , . ,.. X Lilienthal and Pilcher were both killed by accidents to their gliders. !> See Bryan's "Stability in Aviation." „ _ , . , . 1 Although his name was not widely known, Mr. E. T. Busk deserves to be remembered by future students of aviation for his pioneer work in problems ot stability. He was also a brilliant pilot. During the early part of the war, he was flying at Farnborough when his machine caught fire and he was burned to death. % The dihedral must open upwards V-fashion. essential parts of the machine as to produce the desired fin effect. This, as may be imagined, calls for an uncommon combination of mathematics, practical experience, and sound engineering sense on the part of the designer, to say nothing of an infinite capacity for taking pains. Moreover, the study of meteorology in the form of wind gusts is equally essential, for it is the wind gust that disturbs the balance of the machine. Having so briefly indicated the mere nature of the stability problem, and having no space for its further perusal, there remains no alter native but to refer in similarly laconic terms to one or two interest ing phenomena observed in this field of research. For example, there is the fin effect of the propeller** and ihe influence of torque on the lateral balance of the machine. This latter force may be balanced by a fin or by a permanent warp, or a spring may be attached to the warp lever to give the same effect. Mention was made in a recent paragraph of the need for a study of wind gusts. On some machines a gust will cause the wings to warp automatically and thus to some extent " spill " the wind. At first sight, this might seem to be a most desirable attribute, but, as is usual in such cases, there are two sides to the question.ft The automatic warping, itself, results from the disposition of the front and back wing spars and the travel of the centre of pressure on the wing when the relative wind changes its trend. In wings constructed to warp, the rear spar is hinged to the body, so that the other extremity can be raised or lowered, and the wing twists about the axis of the fixed front spar when warping takes place. Altering the position of the front spar serves to modify, or to eliminate, the self- warping tendency. It is the inherent instabilityXZ (see Fig. 9) of the cambered wing considered as an aerofoil in isolated flight that is at the root of most of the trouble with aeroplane stability. A flat plate is inherently /S> n 15 /3 )/ 9 7 S 3 / 0 -I 1,.' 70NB <CIDi Q — — 1 1 1 \ i — - - B^ — 0 •/ -2 -3 -4 S -6 • _ CHORD. LEADING £OC£- •8 -9 -10 Fig. 9.—Graph Illustrating the movement of centre of pressure along the chord of a wing as the angle of incidence is changed. This graph is for a particular wing section only. It will vary considerably with the profile of the section and particularly with the position of the maximum camber. In general, however, all cambered wing sections are inherently unstable, because the centre of pressure moves towards the trailing edge, as the angle of incidence becomes finer, and thus a disturbance tending to reduce the angle is augmented by the consequences of that reduction. With a flat plate, on the contrary, the centre of pressure approaches the leading edge when the angle of incidence decreases, and thus tends to counteract a disturbance and to produce natural stability in the system. ** See "The Technical Report of the Advisory Committee," Vol. iaic-13. tt For comments on the automatic warp, nee " The Technical Report of the Advisory Committee," Vol. 1912-13, p. 251. XX See " Aviation," Chapter VII. In " The Technical Report of the Advisory Committee," Vol. 1912-13, p. 10J, it is pointed out that the centre of pressure moves suddenly to the rear near the wing tip. 929
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