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Aviation History
1940
1940 - 2045.PDF
JULY 18, 1940 VISIBLE VORTICES? White Sky Trails : A Possible Explanation SOME readers may have observed lately what they atfirst thought to be sky-writing, and a member of thestaff of Flight saw a particularly good example on. Sunday afternoon, July 7, over London. The same sort of thing had been seen previously, but this was the best example to date and exhibited some features not observed on other occasions. For the benefit of those who have not seen the phenomenon it consists of a thin line of what looks like white cloud, or perhaps of very white smoke •made by a sky-writing aeroplane. On the first occasion that these were observed (in April), they were criss-crossed about the sky and some of them were quite steep. If done by an aeroplane, it was appar- ently on the glide. The lines lasted for a long time, at least an hour or more, and dissipated only very slowly. The weather was very fine, the time about 4 or 5 p.m., there was no wind on the ground and there appeared to be none at height among the cumulus clouds. The weather of Sunday, July 7, bore a marked resem- blance to that of the other occasion; numerous white cumulus clouds filled the sunny sky. But this time the trail was observed in the making. It was very white, exactly resembling the texture and colour of the clouds, very long, and apparently was on a slight climb. This last may, however, be open to some doubt as the trail was very high. It was so high that the aeroplane making it could not be seen, although the fact that the trail was being made was established beyond doubt—its steady increase of length testified to that- Nor could the sound of the engine be heard. This seems to point to a height of something like 20,000ft. or more. One very noticeable feature was that the trail had a definite helical character, with the pitch of the helix of the same order as its diameter. Apparently the helix was composed of air which had been through the slipstream. The origin of the phenomenon is worthy of some specu- lation. It may, perhaps, be due to the discharge of white smoke from a military aeroplane for some purpose con- nected with the war. In such a case we will, of course, not be told anything about it. But it may be another form of the visible vortex, cases of which have been numerous enough and well enough substantiated to be accepted without doubt. The same member of the staff who saw the trails has also seen the ring around the air- screw disc which is sometimes visible. Each time this was seen it was on a bright but fresh morning, the sort of morn- ing on which the dew sparkles on the grass, and the time was between 7 and 8 a.m. in Australia ; to be exact, at Sketch of a helical vortex sheet behind an airscrew, from Bairstow's "Applied Aerodynamics" by courtesy of Longmans. The shape of the trail seen bore a marked resemblance to this. the Essendon Aerodrome, Melbourne. The machine in question was a Moth, and the ring was seen only during the time of opening up the throttle in the take-off run. The same thing has been photographed and Flight pub- lished a view of a Handley Page Hampden on page 601 in the issue of June 15, 1939. On the same page is a letter stating that the correspondent had seen a wing tip vortex when the Martin Baker fighter was pulled out of the dive into the zoom. This was said to be a brilliant white streamer some 6 or 8 feet long. It was also stated that vortices from wingtips have been visible in high speed dives by American aircraft in the 1936 races and also at one R.A.F. display at Hendon. The explanation which has been given before as a pos- sible reason for visibility of these vortices is that there is condensation of moisture. Such condensation might per- haps be caused in regions of low pressure which may be those parts of the vortex where the velocity is highest. Perhaps there is significance in the fact that it is at the tip of the airscrew (where the blade velocity is greatest) that the visible ring occurs. A fog formed by reduction of pressure can be seen in tunnelling work under the earth when, in order to keep out water, compressed air is sup- plied to the working face. The men, to get out, have to go into a chamber where the pressure is reduced before they can go into atmospheric pressure. During this decompression, the whole chamber may be filled with fog. In the case of the trail behind an aeroplane, the con- densation theory might be correct as there is plenty of water vapour in the products of combustion in the exhaust gas. If the atmospheric conditions are right, the condensa- tion would certainly cause a visible trail. This explanation may not be the right one, but it does seem to fit the facts fairly well. It would be interesting to hear from readers who have any additional facts to add or any alternative explanations to give. • • s..;;•••••• CORRESPONDENCE (Continued from page 50) steady rate. The combined steady actions will trace the top curve of an airfoil, and a million of these (an abstract number) will lorm the wing's cover.' " A wing surface is no more inert than a buzz-saw, Davis declares. It is a mechanical device whipping the tar out of the airstream, smashing all its resistance and converting it into work for the 'plane. That, stripped of elegant technical terms, is ' lift.' To illustrate this, Davis uses the golfer. The golf club rotates around the hand as a centre. Both hand and club ar-e mo\ing—translation and rotation. If the swing is not smooth the ball won't travel far. A weakling with a steady swing will send the ball farther than a muscular gent, with jerky style. " An airfoil is the golf club in opposite action—seizing the hall as it would come to it in a reverse motion picture and killing its resistance. The more smoothly the wing does this with air the faster Lhe 'plane will travel with least effort. A wing in flight, to Davis at least, is nothing more than millions of lever arms grabbing and taking the punch out of the air. " Where is the mystery in an airfoil so readily explained by its creator? Ah, that is the formula which expresses the most perfect relationship between the rotational and transla- tion speeds. With the seemingly endless possibilities involved no less than an Einstein, one would surmise, could arrive at this formula. But Davis calls it a natural conclusion reached once the old ideas of airfoil design are junked." The article goes on to tell of the innumerable and possibly inevitable difficulties Davis encountered in trying to get people interested, how the N.A.C.A. were flabbergasted when they observed for themselves the amazing efficiency' of the Davis foil when compared with the super-foils they had evolved. In discussing foreign interest in the Davis wing, the follow- ing is relevant: "... negotiations with one particular country (reliably reported to be ENGLAND.—ED.), whose name he cannot disclose, because of the international situation, have reached such an advanced stage that before this is published the Davis section may be putting new wings on European aircraft." Apparently, Davis is not enamoured by the Dictators, or anybody who has discarded the democratic ideal. Whether or no we are already building 'planes with this new aerofoil, I thought that I would bring this article to the notice of Flight readers. JOHN W. MORRISON. London, N.W.3.
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