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Aviation History
1915
1915 - 0182.PDF
whole being that proper to the motion of its fellow : it will be demonstrated that it is the reaction due to the continuous produc tion of this vortex pair which is the eventual source of sustentalion in flight. S 3. Independent Evidences of the Trailing Vortex Pair.— Before dealing with the whole consequences which follow from the established ex stencc of this vortex pair, certain comments will not be out of place as to the independent evidences, such as they are, for the existence of these vortices, and some of the indica tions that the designer of the aerofoil may gather by taking them into account. Firstly, it is not difficult by passing an inclined p'ane through air charged with smoke to see with one's own eyes the vortex motion produced. Secondly, the extent of the "wash" left behind by the aerofoil, and acting on the tail member behind it, can be roughly computed on the basis of this residuary vortex motion (Appendix I). Thirdly, as stated in the author's " Aerodynamics" (p. 174, Fig. 80), if an inclined plans be moved under water in the vicinity of the surface, indications of vortex motion are found in the dimples which can be seen in the vicinity of the wing terminations. To the designer, the immediate importance of considering the function of the aerofoil in producing these trailing vortices lies in his realisation of the need for the pressure diminution towards the aerofoil extremities ; manifestly, if the pressure is carried too uni formly into the terminal regions, the rotational core of the vortices will be concentrated, and the energy left behind in the vortices themselves becomes disproportionately great; that is to say, the motion will tend to become a strong local vortex at the wing extremities containing great energy, instead of being more diffuse and consequently more economical. Probably, there is yet con siderable work to be done in the direction of improvement from this point of view. J: 4. The Cyclic Component in the Motion around the Aerofoil.— We will now pass on to consider a very important consequence of the vortex theory of sustentation. It may be taken that it is impossible that two vortices of opposite hand should be attached, or attach themselves, to anv material body without the vortex motion extending continuously around the body from one to the olher. From the strict standpoint of hydrodynamic theory, we know that before cyclic motion can be regarded as possible, the region involved must be doubly or multiply connected, and we know that in the case of an aerofoil (under actual conditions in three dimensions) this condition is not complied with. But we also know perfectly well that in the real fluid the conditions of double connectivity can be and are simulated, in so far as the central bar to connectivity is concerned, by a filament or column of fluid in rotation. In some cases this filament or column forms a com plete loop as in the ordinary smoke ring : in other cases its ex tremities attach to some boundary snrface, such as the earth or sea in the case of a whirlwind or waterspout, or to the surface of the water (from beneath) in the case of the half vortex hoops which are generated by a stroke of a paddle in water. The greatest difficulty that presents itself is the case of vortices trailing away indefinitely into the distant air (or water) without apparently any direct boundary connection, as in the case of the lateral vortices we have been considering ; probably it will be found when th? ® ® theory of the real fluid is sufficiently understood that this condi tion is dynamically possible, and is in some way related to a rate of decay or degeneration; in any case, it is definitely a condition which we know exists if only from optical demonstration, hence even if we cannot for the time being satisfy ourselves as to the theoretical aspect of the subject, we must at least be prepared to swallow it as a fact. (Appendix II.) Thus each of the lateral trailing vortices has one of its extremities dying away in the distant air astern, where its motion is undergoing decay, and its other extremity, its forward end, attached to the aerofoil, where it is continuously being re newed ; thus, the aerofoil forms, as it were, a bridge connecting two vortices which (viewed from their points of attachment) are of opposite hand. Now it is quite clear when we view the aerofoil an d vortex system as a whole, that the interruption in the simple connectivity of the region introduced by the rotation in the vortex cores is maintained between them by this bridge, i.e., by the aerofoil itself, and moreover that the vortex trunks on the two flanks are really part of a single system whose cyclic component surrounds the aerofoil in at least the same strength as it exists at the region of attachment, Fig. 6. The validity of this view can scarcely be disputed ; the sudden termination of the vortices at the Fig. 6. wing extremities would be quite inconsistent with hydrodynamic theory. Beyond this, all the analogies which are known to exist between electro-magnetic and hydrodynamic theory point in the same direction ; a line of magnetic force, for instance, pissing through a particle of iron is not and cannot be regarded as two separate lines terminating on the surface of the iron and unconnected —they are pirt of a continuous unit. Or again, the continuity of lines of electrical stress, which results in the law founded on Faraday's "ice-pail" experiment. Thus, the author regards the two trailed vortices as a definite proof of the existence of a cyclic component of equal strength in the motion surrounding the aerofjil itself. (To be continued.) ® ® AIRCRAFT AND THE, WAR. IN the "wireless" news sent out from Berlin on the 3rd inst. there was the following :— " At Peronne, a French flying machine descended on account of a defect in the motor. The occupants were taken prisoners." In a message from the Athens correspondent of the Messaggero on the 3rd inst. dealing with the bombard ment by the French of the Bulair lines, it was stated :— "The two Turkish aeroplanes which were sighted quickly disappeared when pursued by the Allies'airmen." In a message from the Turkish headquarters, received in Amsterdam on the 3rd inst., it was stated :— " Our aviators successfully bombarded the enemy ships at Ivak." Other reports received from Athens state that the bombardment on the 3rd inst. took place in a fog which rendered it impossible for the aviators to ascertain what damage had been done. Writing to the Daily Telegraph on the 4th inst. from Basle, Mr. A. Beaumont said :— "Two days ago the weather cleared up in many parts of the Vosges, and renewed activity has been observed on the part of the belligerents on both sides. No less than six aeroplanes could be seen at one time flying over the German lines in the valley of the Large, between the town of Pfetterhausen and Altkirch, and the guns on both sides kept up an uninterupted fire all day " A German captive balloon appeared for a moment, but immediately afterwards two French aeroplanes were circling over it, and the balloon was rapidly taken down." It was reported that the. L 8 was wrecked on the 4th inst. at 11 p.m. near Tirlemont, and the following details were obtained by a correspondent of the Dutch Telegraaf:— " Two Zeppelins passed over Tirlemont, and it was clear that the engines of one were not working satisfactorily, as a series of loud reports could be heard. At 3 a.m. L 8 descended rapidly in a field near Wommerson, smashing seven poplar trees. The airship was badly damaged and one of the gondolas was driven into the ground to a depth of 6ft., while the frame was broken in the middle. As the ballonets fore and aft were still inflated the airship took the form of a V. Several of the crew were killed and were buried the same morning near the scene of the accident. The airship became 182
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