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Aviation History
1914
1914 - 0524.PDF
[/ycFifj in more ways than one. Any movement of the tail plane or " elevator," for example, by altering the attitude of the main aero foil causes the machine to become self-supporting at a lower or higher velocity, that is to say, alters its natural velocity, and we thus may represent such a change in the manner indicated in Fig. 3. Here a machine is presumed flying at a certain velocity corresponding to the height H„, its elevator at the point a is altered to correspond to a lower flight velocity corresponding to a height, h„ ; this is equivalent to altering the scale of the chart at that point, and the subsequent path of the machine is represented by the phugoid curve a />. This path may undergo damping, either due to the inherent stability of the flight-path or due to the intervention of the pilot, as shown by the line a c. In the case of a model of unstable flight-path with no intervention from the pilot, the flight-path becomes one of augmented amplitude, a d. When a machine is fitted with an elevator (or adjustable tail- plane) of large surface, it is possible for the pilot to take such entire charge of his machine that he appears to be designing his own flight- path curves rather than modifying or damping the natural curves of the equation. It is quite true that this is one way to fly; it is, in fact, the old Wright method of flying, the original Wright machines MAY 15, 1914. flight-path (Fig. 4) ; likewise a gust or disturbance acting on a model of this kind in flight, may be sufficient to invert the flight- path and determine its downfall. From our present point of view, regarding the air as the "permanent way," the position is as though the model, or machine, were continually crossing a number of facing points arranged, not quite as on a railway, but in a vertical sense (Fig. 5), so that the machine is always in danger of being switched off on to an inverted flight-path, a a, if an aerial distur bance of the right kind and sufficient magnitude and duration happen to be encountered.* In my opinion the soundest way to avoid danger from this cause is experiment in a wind channel with scale models, both of the aerofoil and of the machine as a whole, prepared from the working drawings. According to the evidence that has up to the present been col lected the lift diagram for any machine passes without break of con tinuity from positive to negative values, and the angle of inclination is a single valued function of the pressure reaction a a, Figs. 6 and 7. The pitching moment in some cases is a curve of similar character, b b, Fig. 6, in other cases it is of the form b b, Figs. "]a and jb, the latter of which represents the case of the ballasted * - -1 ft \ B -1 it/ /a. . / \A \t ft fa *>/ / w*p. * h A > X ANCLE Or PLANK TO WIND Fig. 6. Figf. 7a. Fig. 7 b. having been furnished with a ront elevator carrying little or no load. That type of machine, however, may be regarded as a thing of the past. The Wright machine could be "piled up" by inattention or want of skill at any moment, and if once its flight velocity fell below a certain value, either from want of attention on the part of the pilot, or from a wind gust from abaft, or other cause, the pilot was definitely unable lo restore his normal flight condition ; it if for this reason that the Wright type of machine has been abandoned.* 2. Catastrophic instability.—Before entirely quitting this branch of the subject, attention will be directed to a point first raised by me within the last twelve months under the title of Catastrophic Instability. It is a curious fact that, although the author and other investigators had been studying the question of stability by various methods for some twenty years, more or less, and such items as longitudinal stability, lateral stability, and a form known as asymmetric or " rotative" stability, have been " catalogued " and investigated, both theoretically and experimentally, a form of instability which may in practice be far more serious and deadly, has until quite recently escaped notice. There are certain types of flight model, of which the ordinary " ballasted plane" is an example, in which the flight-path is ambiguous. In the case of the ballasted planet the position is quite simple ; this type of model is symmetrical, it has no " upside down " ; if launched at its correct flight-velocity to travel on flight-path No. I (Fig. 1) it is equally capable of travelling on an alternative flight-path intermediate to those numbered 11 and 12, the only determining factor being whether at the moment of launching the pressure reaction is in an upward or downward direction. A very slight want of skill in launching one of these ballasted planes gives at once the inverted ' Practically the whole of the distinctive features of the early Wright machine have disappeared to-day ; for example, the tailless machine is a thing of the past, nearly every modern machine is fitted with a tail plane. The forward elevator is obsolete or nearly so The twin propeller has given place to the single propeller in almost every case. The gear-driven propeller also has been abandoned. The vertical four-cvlinder or motor-car type of engine has proved itself inadequate. The exposed position of the pilot, engine, &c, has gone, never to return. The Wright method ot launching on runners and alighting on skids also is a thing of the past. The biplane construction and the fore-and-aft vertical surface have to some extent survived, but these features were in no wise new when adopted by the Wright Brothers. The wing warping and vertical rudder (neither feature in itself new), operated by the Wright Brothers from one control lever in common, are nowadays operated from two entirely separate controls. t A rectangular plate of mica, conven'ently 0^003 inch thick, 8 inches by = inches, ballasted at the centre of the leading edge. Compare Aerial Flight, vol. i, P- 33< J vol. ii, p. 4. plane. In Fig. 6 the model may be considered as catastrophically stable, but in Figs, "ja and jb there is instability ; there are three positions, or attitudes, of the machine, at which the pitching moment is zero, the two outer two,/j and fa, defining respectively the stable positions of normal and upside-down flight, and /. mark ing the critical angle of unstable equilibrium when the machine passes from one state to the other. In Table II are givfn results of some experiments recently carried out with a model machine at the N. P. L. These were not directed TABLE II. ng e o pt c . latching moment. Chord as datum. fa - 14 +o'0264 -12 +0-0152 -10 +0-0063 Angle of stable equilibrium upside down. - 8 -0-001* - 6 -0-0059 . - 4 -0-0049 - 2 -0-0014 Critical angle ot catastrophic change of flight-path. O + o'0030 2 o•0070 4 +O'0I02 6 + 0-02IO 8 +0-0218 10 +0-0127 12 +0-0072 14 +0-C026 Angle of stable equilibrium right way up. 16 -0-0043 18 -0-017 to the point in question, but incidentally serve as an apt illustration, and roughly form the basis of the plotting. Fig. la. In the experimental figures as tabulated, the evidence of catas trophic instability is seen in the column headed pitching moment ; * The disastrous nature of this sudden inversion of the flight-path may be gauged from the fact that it represents in effect a complete reversal of gravity, the machine is accelerated downwards with a force comparable to that previously giving it support, and any loose tools, instruments, or fitments, including the pilot himself, are liable to be jettisoned by the machine, whose subsequent career is an upside-down flight carried out on its own account. The facts on record relating to the fatal accident to Major Merrick at the Central Flying School (October 3rd, 1913), point strongly to catastrophic instability as the 524
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