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Aviation History
1940
1940 - 2236.PDF
io8 AUGUST 8, 1940 VISIBLE VORTICES (CONTINUED) cortices or "smoke trails" seen recently are consequentlymerely elongated Cirrus clouds. Some readers may recall the school laboratory experiment ofcooling a saturated solution of some salt in a glass beaker which on being lightly tapped immediately crystallised. It isthe disturbance and, I suggest, the noise created by the passage of the aircraft which is responsible for the formation ofthe Cirrus cloud. In fact, one of the investigators into the aircraft de-icing problem suggested the generation of a power-ful sound wave directed ahead of the aeroplane as a means of causing the supercooled moisture to freeze before reaching thewing leading-edge and thereby prevent freezing and ice accre- tion on the wing itself, although this original solution of theproblem unfortunately proved to be sounder in theory than in application. A point in support of the foregoing explanation is the factthat the diameter of these cloud trails is obviously many times greater than the wing span of the aircraft responsible forthem. London, S.W.i. A. R. OGSTON, F.Inst.Pet. ON the afternoon of July 26 the atmospheric conditions overthis village were becoming suitable for the formation of vortices,by aeroplanes, and one or two short ones were visible. The weather looked thundery with very big cumulous clouds and fleecier ones between them, and I have quite often noticed vortex streamers on similar days. The vortices were similar to the diagram on page 51 of Flight for July 18. About 1 a.m. this morning, the 27th, we had a number of enemy machines over us at great height and long vortices (many miles) were easily visible to the naked eye, and the evolutions of the machines could be followed. The streamers were lighted up by the moon, but the search- lights also played on them, and one or two operators quickly tumbled to the idea of moving their beams until they cut across a streamer and then following it along quickly until the machine causing it was caught up and held in the beam. The weather conditions were still thundery and lightning was playing now and again. I thought these particulars might interest you. Blandford, Dorset. G. LEE EVANS. TZLIGHT has given us some interesting observations about •L the visible vortices. This phenomenon is well known to the Polish pilots of the PZL 11 fighter plane When pulling out from a dive, or making a steep turn at a high speed, two white vortices streaming from the ailerons and wing-tips were very often distinctly visible, and Polish fighters used to say that the fight was going on " with foam streaming from both planes." I' had a chance to observe a very interesting phenomenon of similar nature, which I would like to communicate to Flight readers. On the afternoon of March 7th, 1940, "somewhere on the English coast," the sky was covered with a very thin and very low layer of stratus. This layer was transparent and its presence was evident by a shadow thrown on its surface by a Cumulus tun and a very large "halo" ring. This "halo" ring was cut in the diameter by two bright straps of very faint rainbow. Some miles away there was heavy artillery shooting. Before each detonation something like greyish lines appeared on the circumference of the halo ring, travelling with an even and high speed towards the diameter of the halo ring and dis- appearing in the vicinity of the rainbow straps. Their high speed, even propagation and coincidence with the detonation which followed afterwards, were a proof that we had visible acoustic waves under observation. My primitive diagram gives an idea of what this visible propagation of acoustic waves looked like. The visibility of these waves on the '' halo'' ring can only be explained because.of the changed refraction conditions, caused by the pressure wave. The wave travelled everywhere on the sky, but was visible only there, where the refraction took place, i.e., on the "halo" ring. Polish Units, R.A.F. B. S. SUNDAY, July 28, time 5.30 p.m. Across sky from Hartle-pools at angle, say, 45 degrees. Towards Cumberland county there were plainly visible to the naked eye three distinct vortices parallel all the way, coming from out of Cumulus clouds. Remember we are in the St. Swithin's Day period. These clouds are generally from 30 to 45 feet in thickness and are always very close to the earth. Due to the natural (un- known) gravity which exists above them, fast-flying or heavily loaded aircraft always leave these marks, especially in a clear sky. I also disagree with a remark .from a reader airman that people cannot distinguish the difference between an enemy aircraft and one of British manufacture. It has been proved here, without knowledge of class or petrol in use! I may also say that a Topophone was not in use at this time. Wallsend-on-Tyne. -..:; - -. JOHN G. LAWS. "TYURING March or April of fKis year, while serving as a -*--' height-finder in an A.A. battery, one of my instrument numbers and .myself spotted an aircraft making a white visible trail in the sky. So far as I can remember neither he nor I could see the plane with the naked eye although his vision was extremely acute. We laid the instrument on and took some cuts which came consistently to 30,000ft. It was a Hurricane and the black and white marking was plainly visible at mag. 25 in the instrument. From this I should estimate that the plane mentioned on Sunday afternoon, July 7, which I, too, observed from N.W. London, was flying at about the same height. Surely at 30,000ft. the fall in temperature with increase of altitude would be so large that the white trail was ice rather than fog. There must be plenty of other height-finders who have ob- served this phenomenon. They could soon say if they saw two trails, i.e., one from each engine exhaust pipe, which would disprove the interesting vortex theory, since it would indicate that the trail was simply caused by condensation of water vapour in the exhaust gases on the necessary particles of solid matter also from the exhaust gases. Ash Green. A. O. EDWARDS. MAY I suggest another explanation for these white " smoketrails" formed by aircraft flying at considerable heights under certain atmospheric conditions ? At these heights clouds are probably formed largely of ice crystals. The chemist is familiar with the condition kno.wn as "supersaturated solu- tion '' in which a liquid holds in solution a greater quantity of a solid than should be theoretically possible at that solution temperature and pressure. Provided such a supersaturated solution remains undisturl>ed, it will contain liquid, but if a small particle of the solid or even of dust is dropped into, it immediately crystallises out. May it not be possible that under certain conditions of temperature and pressure the at- mosphere may contain water in a supersaturated condition, without such water actually crystallising out as ice crystals ? The passage of an aeroplane through such a position of the atmosphere may provide first the disturbance, particularly in the low-pressure parts of the propeller or wing-tip vortices to initiate the crystallisation and form of trail of ice crystals- or of water particles. Recently three Hurricanes passed over my house at a height of about 14,000ft. All three were form- ing trails, but these were not continuous. At intervals they passed through a patch of air where conditions were unsuitable and so no trail was formed. Two of the trails were double, i.e., wing-tip trails. The other machine was making a single trail—i.e., propeller vortex trail. This machine appeared to be flying slightly lower than the other two. Sussex. " G. M. PAKT,
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