How the aeroplane can write advertisements in the sky and throw out smoke screens
LAYING A SKY TRAIL. Each year at the R.A.F. Display, Hendon, a selected unit gives a demonstration of aerobatics in smoke. In 1935, five pilots of No. 3 (Fighter) Squadron, equipped with “Bulldogs”, carried out such a programme: here we see them rehearsing over the outskirts of London. The parallel lines of coloured smoke show the accuracy attained by the pilots in their stepped-up echelon formation.
THE hanging of curtains of smoke in the sky has frequently proved of great value to naval and military forces. But there is also another purpose served sometimes by the emission of smoke from aeroplanes in flight; this is the purpose of skywriting — the tracing of characters in the sky in such a way that they can be read by people on the ground.
Both forms of smoke emission are interconnected, but we shall deal first with sky-writing as being more especially an aerial development of the use of smoke. Many people who watch an aeroplane writing an advertisement at perhaps 10,000 feet or 15,000 feet believe that the pilot writes much as one writes on paper, forming the characters by eye. Actually, he does nothing of the kind. He must master an entirely special technique and, while writing, he is unable to see what his handiwork looks like. He acts by rule, and every movement of his machine is determined by time and compass bearing. Moreover, he must use mirror writing in order that the people on the ground shall see normal writing.
Invented by Major Jack C. Savage, skywriting has been developed almost entirely for advertising purposes; but the methods of smoke emission have been adopted by the Air Ministry and are employed in almost exactly the same way for certain kinds of smoke screen laying. The aeroplane for skywriting must be fairly fast, or else the beginning of the word, or words, starts to fade away before the end has been done. It is also an advantage if the exhaust pipes of the engine, for a reason which will become clear in a moment, reach well behind the cockpit.
Actually, the S.E.5 is the aeroplane which has been most extensively used for sky-writing. It can reach the operating level quickly, and it has sufficient speed. At first, dark grey smoke was the only kind that could be used, and the introduction of colour presented difficulties.
AIRMAN’S “ICH DIEN”. The manoeuvre known as the “Prince of Wales’ Feathers” forms a graceful addition to any aerobatic pilot’s repertoire, and also provides a thrilling spectacle at the annual R.A.F. Display, as seen here. A “Virginia” heavy bomber is landing over the heads of the crowd in the foreground.
After many experiments, in May, 1924, colour was first introduced successfully, the tint being dark red. After that colour development proceeded, and by the time of the R.A.F. Display of 1933 it had been found possible to give a good rendering in smoke of several of the primary colours.
Major Savage, when he first launched his invention, got together a small band of expert pilots, most of them men who had been flying in the Royal Flying Corps and Royal Naval Air Service in France. Sky-writing was taken up more generally abroad than in Great Britain, and at one time 94 per cent of all skywriting was being done abroad. But it was always done by Major Savage’s methods and by members of his band of trained pilots.
The training is a fairly elaborate business and, when one knows something about it, it leads one to regard the tiny little silver aeroplane which twists and turns in the summer sky, leaving its trail of smoke behind it, in a rather different light. I have said that every stroke is made in reference to the aeroplane’s compass course; but the pilots do not look at the compass — they go by the sun, noticing how it throws shadows of struts and other parts on the wings of their aeroplane.
A word of ten letters may demand a smoke trail ten miles long and take about four minutes to write. During that four minutes the pilot is intensively engaged in probably the most astonishing form of writing ever invented. It is so remarkable that it makes the work of those who chisel characters in stone and of those who paint elaborate oriental characters seem simple by comparison. Let us consider how those early pilots worked out their technique and trained themselves to write, in legible characters, in the sky.
First of all, they practised writing the word they proposed to write in the sky on paper with pen and ink; but writing it backwards. They learnt to write the word backwards with such ease that they could dash it off in that direction more easily than most people can sign their name in the ordinary direction.
ARCS OF SMOKY GRANDEUR. The extraordinary skill — one might almost say virtuosity —of the fully trained R.A.F. pilot is magnificently portrayed in this panorama of smoke-writing in a summer sky. The five Bristol “Bulldogs”, practising for the Hendon Display, seem quite dwarfed by the immensity of their trail as they loop in unison. Writing in smoke requires a technique of split-second accuracy, without which such symmetrical beauty is entirely spoiled.
For the next stage of training the pen and ink, strangely enough, were exchanged for a bicycle. The pilot tied to the bicycle a tin with water or fine sand in it, and some means of stopping and starting the flow, and then wrote the word on the aerodrome tarmac, still going backwards — with the word, not the bicycle.
The final stage was air practice, and that, during the early days of skywriting, took a great deal of time and was often exceedingly difficult to arrange, because it was not desired that the public should make any mistake about what was and what was not the finished article. In order to get the length of the strokes correct the pilot either counts or uses a stop-watch. If the pilot were writing the word “air” he would begin by opening the smoke valves and making a run away from the sun for about 15 seconds to form the first stroke of the letter A. Immelmann turns are usually employed for the purpose of making the line of smoke return quickly upon itself, or close to itself, as in the loop of an and in a letter which has to be left and returned to again, as i. The dot of the i is made by a brief opening of the smoke valves, while the aeroplane continues on the course on which it was running for the body of the letter. The method of producing the smoke is usually by introducing certain chemicals to the exhaust pipe. The exhaust pipe is heavily lagged with some non-conducting material to enable it to retain most of its heat and thus make it a suitable medium for activating the chemical smoke producer.
Before turning to the laying of smoke screens by aeroplane, there is another form of sky-writing invented by Major Savage; this is the searchlight method. In essentials the searchlight method of sky-writing consists in using a special form of projector for throwing images of the letters on clouds. The beam thrown by the searchlight is really a cluster of 300 beams, each originating at a small mirror whose position is, within narrow limits, adjustable. The generation of the light and its transmission are, therefore, separate processes.
THE WORD “PERSIL” COVERS THE SKY for a total distance of two miles, and is begun at the great height of 12,000 feet, where the air is extremely cold and thin. Each of the smaller letters is about on-third of a mile from top to bottom.
According to the inventor’s statements the light available from this searchlight is 3,000,000,000 candle-power, and the loss amounts to only about 5 per cent for reflection and about 2½ per cent for the blank regions between the individual beams of the cluster. The use of this searchlight for aiding anti-aircraft gunners at night was suggested, and the searchlight was tried by the War Office. When used for gunnery, the cluster of beams is made to form, not the letters of a word, but a grid of known size. The movement of the aeroplane across this grid, it is stated, enables the gunners to calculate range and speed.
When sky-writing by searchlight was first proposed there were many protests against its use. The question of prohibiting or regulating sky-writing was examined by Parliament in the period 1923-25, when the Advertisements Regulation Bill was under consideration. The clause in the original draft of this bill which affected sky-writing was deleted at the instance of the Air Ministry, which is an indication that the authorities believed that sky-writing has military as well as commercial value.
Smoke Screens
Sky-writing in smoke does not owe much to the earlier forms of smoke screen production. It was a development of its own. But much of the knowledge collected in sky-writing has since been applied to smoke screen work. Smoke screens have frequently been used by both armies and navies; but, although they have been laid by aircraft, they have not yet been used as a tactical device in aerial battle. The idea of hanging curtains in the sky is old, but it has been enormously developed in method. Historical instances of the tactical employment of smoke screens during the war of 1914-18 occurred during the operations off Ostend in 1917. Smoke screens were used by both sides, and more than once British plans to bombard Ostend were foiled by the laying of smoke screens by the enemy. On the other side, motor launches and coastal motor boats were used to lay smoke screens behind which our ships might move into their positions.
SHUTTING AN ENEMY’S EYES. Smoke screens discharged from destroyers proved to be a valuable defence in naval warfare during the Great War. But smoke defence, like gas attack, has developed immensely since then, and aeroplanes rising from a sea-going carrier can now put up a cloud which in calm weather would completely blind an enemy fleet. This photograph shows three United States Navy planes laying down a smoke screen during a demonstration on Navy Day.
Methods of producing the smoke by ships such as destroyers include that of doing what the aeroplane pilot would describe as running on an over-rich mixture. The ratio of fuel to air burnt in the ship’s furnaces is altered until dense clouds of black and oily smoke pour from the funnels. But in the aeroplane the chemical method, such as that described, is the only method available.
Aeroplanes which are to lay a smoke screen can be furnished with a device of the sky-writing kind for emitting smoke themselves, or they can carry smoke-producing floats or containers which can be dropped in a row along the line on which it is desired to produce the smoke curtain. So far as direct emission of smoke is concerned, the methods used for sky-writing and for smoke screen laying are almost identical. If aeroplanes are to lay an effective smoke screen by direct emission —and not by laying a train of containers — it has been shown by experiments made in America that a large number of machines are needed, and even then success can hardly ever be guaranteed. At sea the conditions are, on the whole, more favourable than on land. Some estimate of the thickness of the smoke emitted by an aeroplane in flight can be gained from watching the aerobatics with smoke which usually form spectacular items in the annual R.A.F. Display.
The reasons that smoke screens have been found impracticable for use in aerial battle are many. Fighting aeroplanes move too quickly, and change height too rapidly, to allow any smoke curtain or smoke cloud to be of much value for concealment. It is the view of the air staffs of most of the world’s leading air forces that smoke screen laying by air may have to be employed for the assistance of the army or the navy, but that it would be of no value for influencing an aerial battle.
THE BUSINESS SIDE OF SMOKE. This photo show clearly the way in which smoke is emitted from an aeroplane engaged in sky-writing. The machine is an S.E.5, a war-time type used for this work for many years by Major Savage, of Skywriting, Ltd., the pioneers of this branch of aviation. The S.E.5 had the necessary speed, manoeuvrability and strength for the job, which entails the continual use of the Immelmann turn and other strenuous aerobatics. The flow of smoke, generated by chemicals in the specially constructed exhaust, is controlled by a single lever in the pilot’s cockpit.
Actually, the S.E.5 is the aeroplane which has been most extensively used for sky-
