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Aviation History
1913
1913 - 0062.PDF
implies a machine without inertia, and, therefore, without mass, which is impossible. It should be noted that the gyroscopic effect of the propeller lends to affect weathercock directional stability and also to react on the longitudinal stability. Alternatively, if there is no spin, then the leeward slip of the machine ultimately acquires the velocity of the real wind. When some spin accompanies the drift, equilibrium is established when the flow of the relative wind is once more axial. TtiE. J)1HEI?RA!_ IN -STI1X. AlB UlH&TA&LC THRUST 2 ©TABUE. THRUST 3 -STABLE. THRUST 4 IMPROVCD 3T*BIUTY fflhJ THRUST riQ.i In any case, equilibrium demands an axial flow of the relative wind, for while the flow is oblique there is a component of lateral drift, inducing acceleration to leeward. Moreover while there is lateral acceleration of the system, there is a virtual relative spin about its vertical axis, and the appropriate consequences will follow. In a system having compass directional stability, the ultimate leeward drift velocity is that of the lateral component of the wind. In the presence of real winds, therefore, the natural undirected course of such a machine is liable to be very wide of the mark. Even if the principle confers the power of recovering equilibrium after dis turbances during the windy periods, there thus still remains the necessity of steering a curved course in a calm and the instability of positive wing tips in that manoeuvre have already served as subject matter for the preceding article. It now remains, therefore, to consider what power of inherent recovery of balance may be possessed by wings with positive tips, if the system to which they belong likewise possesses compass directional stability. In this connection it is interesting to recall the elementary experi ment of the ballasted flat plate. A small sheet of mica, or stiff note paper, suitably loaded on its leading edge with a split shot, a drop of sealing wax, or the like, will maintain its balance when gliding in still air. This it does by virtue of symmetry of form, its "compass" directional stability and its longitudinal " weathercock "stability. If canted, it immediately proceeds in an oblique direction, its longitudinal axis remaining parallel with its original position. There is a sideways component to this motion, and in the sideways component lies the reason for the recovery of lateral balance, which ensues after the model has flown a little way on its oblique path. Considering the sideways motion separately, the plate represents an inclined plane in respect thereto, and as it is well known from experimental evidence that the centre of pressure on such a system is nearer the leading edge than the trailing edge, it follows that the leading edge will tend to rise, for the e.g. is central. The leading edge in respect to the sideways motion is the lower wing tip of the canted plate. Equilibrium is thus automatically restored by the sideways motion so long as the compass directional stability is maintained. If the flat plate were fitted with a vertical tail fin, such as a neutral rudder, and were thereby possessed of inherent weathercock directional stability, it would capsize instead of righting itself. When canted, it would begin to move sideways, as before, but its longitudinal axis would swing in harmony with the obliquity of its motion, which would accelerate the relative velocity of the higher wing and so augment the cant. The effect being cumulative, the system would thus ultimately capsize in a spiral volpiqutox sideslip. By the same reasoning, a vertical fin in front ot the e.g. to neutralise the effect of a vertical tail fin, naturally suggests itself as a means of obtaining " compass " directional stability. Also, a fin above the c. g. seems reasonable as a means of enhanc ing the rapidity of recovery from the disturbance. It is equally conceivable that the effect of these three fins could be secured by two fins of appropriate area and position. Or, that " projected " areas of the whole or portions of the wings might be regarded as virtually fins, and so serve the purpose of such. From the work of Prof. Bryan, wio has investigated this aspect of the subject mathematically, it appears that the combination of a vertical tail fin (situated more or less in the position ordinarily occupied by a rudder) with a vertical fin in a rather elevated position, and in front of the e.g. (Fig. 2D), is likely to give the best results. These two fins, if properly arranged, confer the power of automatic recovery of lateral balance, and they maintain the compass directional stability of the system. It would seem from Mr. E. H. Harper's remarks at the Aeronautical Society recently, that the elevated fin in a forward position can be virtually created by the use of up-turned wing tips suitably arranged. Wings set at a dihedral angle to each other are also, by the same reasoning, equivalent to the combination of level wings with a vertical fin above the shoulder. In order, however, that the system should be stable, it is necessary that the e.g. of the system should be behind the wings. When thus situated, the virtual fin represented by the dihedral is again p'aced in an elevated forward position. The accompanying set of diagrams (Fig. i), showing the dihedral in still air, are based on remarks contained in Mr. E. H. Harper's recent paper before the Aeronautical Society, and they sum up, I believe, the situation there presented of the dihedral in still air. •Centre of Pressure <*- X .---T -^L D MAO** FicH.fi i It will be observed that Diagrams 2 and 3 show stable conditions without a tail fin. This is due to the fact that there is a particular region above the e.g. in which a single fin has some stabilising value. It implies, of course, the absence of any weathercock directional stability in the tail. Another set of diagrams (Fig. 2) reproduced from Mr. E. H. Harper's paper, and originally published, with the exception of D, in Prof. Bryan's Book on Stability, shows various combinations of fins that have been investigated.
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