Chapter XVII : WIRELESS IN AVIATION

In a previous chapter the various methods of signalling betweenthe ground and the airman aloft have been described. Seeing thatwireless telegraphy has made such enormous strides and hasadvanced to such a degree of perfection, one naturally wouldconclude that it constitutes an ideal system of communicationunder such conditions in military operations.

But this is not the case. Wireless is utilised only to a verylimited extent. This is due to two causes. The one is of atechnical, the other of a strategical character.

The uninitiated, bearing in mind the comparative ease with whichwireless installations may be established at a relatively smallexpense, would not unreasonably think that no seriousdifficulties of a technical character could arise: at least nonewhich would defy solution. But these difficulties exist in twoor three different fields, each of which is peculiarly complexand demands individual treatment.

In the first place, there is the weight of the necessaryinstallation. In the case of the dirigible this may be asecondary consideration, but with the aeroplane it is a matter ofprimary and vital importance. Again, under present conditions,the noise of the motor is apt to render the intelligentdeciphering of messages while aloft a matter of extremedifficulty, especially as these are communicated in code. Theengine noise might be effectively overcome by the use of amuffler such as, is used with automobiles, but then there is thefurther difficulty of vibration.

This problem is being attacked in an ingenious manner. It isproposed to substitute for audible signals visualinterpretations, by the aid of an electric lamp, the fluctuationsin which would correspond to the dots and dashes of the Morsecode. Thus the airman would read his messages by sight insteadof by sound.

This method, however, is quite in its infancy, and althoughattractive in theory and fascinating as a laboratory experimentor when conducted under experimental conditions, it has notproved reliable or effective in aeronautical operations. But atthe same time it indicates a promising line of research anddevelopment.

Then there are the problems of weight and the aerial. So far aspresent knowledge goes, the most satisfactory form of aerial yetexploited is that known as the trailing wire. From 300 to 700feet of wire are coiled upon a reel, and when aloft this wire ispaid out so that it hangs below the aeroplane. As a matter offact,when the machine is travelling at high speed it trailshorizontally astern, but this is immaterial. One investigator,who strongly disapproves of the trailing aerial, has carried outexperiments with a network of wires laid upon and attached to thesurface of the aeroplane's wings. But the trailing wire isgenerally preferred, and certainly up to the present has provedmore satisfactory.

The greatest obstacle, however, is the necessary apparatus. Theaverage aeroplane designed for military duty is already loaded tothe maximum. As a rule it carries the pilot and an observer, andinvariably includes a light arm for defence against an aerialenemy, together with an adequate supply of ammunition, whileunless short sharp flights are to be made, the fuel supplyrepresents an appreciable load. Under these circumstances theitem of weight is a vital consideration. It must be kept withina limit of 100 pounds, and the less the equipment weighs the moresatisfactory it is likely to prove, other things being equal.

The two most successful systems yet exploited are the Dubilierand the Rouget. The former is an American invention, the latteris of French origin. Both have been tested by the BritishMilitary Aeronautical Department, and the French authoritieshave subjected the French system to rigorous trials. Bothsystems, within their limitations, have proved satisfactory.

The outstanding feature of the Dubilier system is the productionof sine waves of musical frequency from continuous current, thusdispensing with the rotary converter. The operating principle isthe obtaining of a series of unidirectional impulses by acondenser discharge, the pulsating currents following one anotherat regular intervals at a frequency of 500 impulses per second,which may be augmented up to 1,000 impulses per second. Thecomplete weight of such an apparatus is 40 pounds; the electricgenerator, which is no larger than the motor used for driving theordinary table ventilating fan, accounts for 16 pounds of thistotal. Under test at sea, upon the deck of a ship, a range of250 miles has been obtained. The British Government carried outa series of experiments with this system, using a small plantweighing about 30 pounds, with which communication was maintainedup to about 20 miles.

In the French system the Reuget transmitter is employed. Theapparatus, including the dynamo, which is extremely small, weighsin all 70 pounds. A small alternator of 200 watts and 100 voltsis coupled direct to the aeroplane motor, a new clutch couplerbeing employed for this purpose. By means of a small transformerthe voltage is raised to 30,000 volts, at which the condenser ischarged. In this instance the musical spark method is employed.

The whole of the high tension wiring is placed within a smallspace so as not to endanger the pilot, while the transformer ishermetically sealed in a box with paraffin. The aerial comprisesa trailing wire 100 feet in length, which, however, can bewound in upon its reel within 15 seconds. This reeled antenna,moreover, is fitted with a safety device whereby the wire can becut adrift in the event of an accident befalling the aeroplaneand necessitating an abrupt descent. With this apparatus theFrench authorities have been able to maintain communication overa distance of 30 miles.

In maintaining ethereal communication with aeroplanes, however, aportable or mobile station upon the ground is requisite, and thisstation must be within the radius of the aerial transmitter, ifmessages are to be received from aloft with any degree ofaccuracy and reliability. Thus it will be recognised that theland station is as important as the aeroplane equipment, anddemands similar consideration.

A wide variety of systems have been employed to meet theseconditions. There is the travelling automobile station, in whichthe installation is mounted upon a motor-car. In this instancethe whole equipment is carried upon a single vehicle, while theantenna is stowed upon the roof and can be raised or loweredwithin a few seconds. If motor traction is unavailable, thenanimal haulage may be employed, but in this instance theinstallation is divided between two vehicles, one carrying thetransmitting and receiving apparatus and the generating plant,the other the fuel supplies and the aerial, together with spareparts.

The motive power is supplied by a small air cooled petrol orgasoline motor developing eight horse-power, and coupled directto a 2-kilo watt alternator. At one end of the shaft of thelatter the disk discharger is mounted, its function being tobreak up the train of waves into groups of waves, so as to imparta musical sound to the note produced in the receiver. A flexiblecable transmits the electric current from the generator to thewagon containing the instruments. The aerial is built up ofmasts carried in sections.

The Germans employ a mobile apparatus which is very similar, butin this instance the mast is telescopic. When closed it occupiesbut little space. By turning the winch handle the mast isextended, and can be carried to any height up to a maximum ofabout 100 feet. The capacity of these mobile stations varieswithin wide limits, the range of the largest and most powerfulinstallations being about 200 miles. The disadvantage of thesesystems, however, is that they are condemned to territories wherethe ground at the utmost is gently undulating, and where thereare roads on which four-wheeled vehicles can travel.

For operation in hilly districts, where only trails are to befound, the Marconi Company, has perfected what may be describedas "pack" and "knapsack" installations respectively. In thefirst named the whole of the installation is mounted upon thebacks of four horses. The first carries the generator set, thesecond the transmitting instruments, the third the receivingequipment, and the fourth the detachable mast and stays.

The generator is carried upon the horse's saddle, and is fittedwith a pair of legs on each side. On one side of the saddle ismounted a small highspeed explosion motor, while on the oppositeside, in axial alignment with the motor, is a small dynamo. Whenit is desired to erect the installation the saddle carrying thisset is removed from the horse's back and placed upon the ground,the legs acting as the support. A length of shaft is thenslipped into sockets at the inner ends of the motor and dynamoshafts respectively, thus coupling them directly, while thecurrent is transmitted through a short length of flexible cableto the instruments. The mast itself is made in lengths of aboutfour feet, which are slipped together in the manner of thesections of a fishing rod, and erected, being supported by meansof wire guys. In this manner an antenna from 40 to 50 feet inheight may be obtained.

The feature of this set is its compactness, the equal division ofthe sections of the installation, and the celerity with which thestation may be set up and dismantled in extremely mountainouscountry such as the Vosges, where it is even difficult for apack-horse to climb to commanding or suitable positions, there isstill another set which has been perfected by the MarconiCompany. This is the "knapsack" set, in which the whole of theinstallation, necessarily light, small, and compact, is dividedamong four men, and carried in the manner of knapsacks upon theirbacks. Although necessarily of limited radius, such aninstallation is adequate for communication within the restrictedrange of air-craft.

Greater difficulties have to be overcome in the mounting of awireless installation upon a dirigible. When the Zeppelin wasfinally accepted by the German Government, the militaryauthorities emphasised the great part which wireless telegraphywas destined to play in connection with such craft. But havethese anticipations been fulfilled? By no means, as a littlereflection will suffice to prove.

In the first place, a wireless outfit is about the most dangerouspiece of equipment which could be carried by such a craft as theZeppelin unless it is exceptionally well protected. As is wellknown the rigidity of this type of airship is dependent upon alarge and complicated network of aluminium, which constitutes theframe. Such a huge mass of metal constitutes an excellentcollector of electricity from the atmosphere; it becomes chargedto the maximum with electricity.

In this manner a formidable contributory source of danger to theairship is formed. In fact, this was the reason why "Z-IV"vanished suddenly in smoke and flame upon falling foul of thebranches of trees during its descent. At the time the Zeppelinwas a highly charged electrical machine or battery as it were,insulated by the surrounding air. Directly the airship touchedthe trees a short circuit was established, and the resultantspark sufficed to fire the gas, which is continuously exudingfrom the gas bags.

After this accident minute calculations were made and it wasascertained that a potential difference of no less than 100,00volts existed between the framework of the dirigible and thetrees. This tension sufficed to produce a spark 4 inches inlength. It is not surprising that the establishment of theelectric equilibrium by contact with the trees, which producedsuch a spark should fire the hydrogen inflation charge. In factthe heat generated was so intense that the aluminium metallicframework was fused. The measurements which were made provedthat the gas was consumed within 15 seconds and the envelopedestroyed within 20 seconds.

As a result of this disaster endeavours were made to persuadeCount Zeppelin to abandon the use of aluminium for the frameworkof his balloon but they were fruitless, a result no doubt due tothe fact that the inventor of the airship of this name has but asuperficial knowledge of the various sciences which bear uponaeronautics, and fully illustrates the truth of the old adagethat "a little learning is a dangerous thing." Count Zeppelincontinues to work upon his original lines, but the danger of hissystem of construction was not lost upon another Germaninvestigator, Professor Schiitte, who forthwith embarked upon theconstruction of another rigid system, similar to that ofZeppelin, at Lanz. In this vessel aluminium was completelyabandoned in favour of a framework of ash and poplar.

The fact that the aluminium constituted a dangerous collector ofelectricity rendered the installation of wireless upon theZeppelin not only perilous but difficult. Very seriousdisturbances of an electrical nature were set up, with the resultthat wireless communication between the travelling dirigible andthe ground below was rendered extremely uncertain. In fact, ithas never yet been possible to communicate over distancesexceeding about 150 miles. Apart from this defect, the danger ofoperating the wireless is obvious, and it is generally believedin technical circles that the majority of the Zeppelin disastersfrom fire have been directly attributable to this, especiallythose disasters which have occurred when the vessel has suddenlyexploded before coming into contact with terrestrialobstructions.

In the later vessels of this type the wireless installation ishoused in a well insulated compartment. This insulation has beencarried, to an extreme degree, which indicates that at last theauthorities have recognised the serious menace that wirelessoffers to the safety of the craft, with the result that everyprotective device to avoid disaster from this cause has beenfreely adopted.

The fact that it is not possible to maintain cornmunication overa distance exceeding some 20 miles is a severe handicap to theprogressive development of wireless telegraphy in this field. Itis a totally inadequate radius when the operations of the presentwar are borne in mind. A round journey of 200, or even moremiles is considered a mere jaunt; it is the long distance flightwhich counts, and which contributes to the value of an airman'sobservations. The general impression is that the fighting lineor zone comprises merely two or three successive stretches oftrenches and other defences, representing a belt five miles or soin width, but this is a fallacy. The fighting zone is at least20 miles in width; that is to say, the occupied territory inwhich vital movements take place represents a distance of 20miles from the foremost line of trenches to the extreme rear,and then comes the secondary zone, which may be a further 10miles or more in depth. Consequently the airman must fly atleast 30 miles in a bee-line to cover the transverse belt of theenemy's field of operations. Upon the German and Russian sidesthis zone is of far greater depth, ranging up to 50 miles or soin width. In these circumstances the difficulties of etherealcommunication 'twixt air and earth may be realised under thepresent limitations of radius from which it is possible totransmit.

But there are reasons still more cogent to explain why wirelesstelegraphy has not been used upon a more extensive scale duringthe present campaign. Wireless communication is not secretive. In other words, its messages may be picked up by friend and foealike with equal facility. True, the messages are sent in code,which may be unintelligible to the enemy. In this event theopponent endeavours to render the communications undecipherableto one and all by what is known as "jambing." That is to say, hesends out an aimless string of signals for the purpose ofconfusing senders and receivers, and this is continued withoutcessation and at a rapid rate. The result is that messagesbecome blurred and undecipherable.

But there is another danger attending the use of wireless uponthe battlefield. The fact that the stations are of limited rangeis well known to the opposing forces, and they are equally wellaware of the fact that aerial craft cannot communicate over longdistances. For instance, A sends his airmen aloft andconversation begins between the clouds and the ground. Presentlythe receivers of B begin to record faint signals. They fluctuatein intensity, but within a few seconds B gathers that anaeroplane is aloft and communicating with its base. By the aidof the field telephone B gets into touch with his whole string ofwireless stations and orders a keen look-out and a listening earto ascertain whether they have heard the same signals. Somereport that the signals are quite distinct and growing louder,while others declare that the signals are growing fainter andintermittent. In this manner B is able to deduce in whichdirection the aeroplane is flying. Thus if those to the eastreport that signals are growing stronger, while the stations onthe west state that they are diminishing, it is obvious that theaeroplane is flying west to east, and vice versa when the westhears more plainly at the expense of the east. If, however, bothshould report that signals are growing stronger, then it isobvious that the aircraft is advancing directly towards them.

It was this ability to deduce direction from the sound of thesignals which led to the location of the Zeppelin which came downat Lun6ville some months previous to the war, and whichthreatened to develop into a diplomatic incident of seriousimportance. The French wireless stations running south-east tonorth-west were vigilant, and the outer station on the north-westside picked up the Zeppelin's conversation. It maintained adiscreet silence, but communicated by telephone to its colleaguesbehind.

Presently No. 2 station came within range, followed by Nos. 3, 4,5, 6, and so on in turn. Thus the track of the Zeppelin wasdogged silently through the air by its wireless conversation aseasily and as positively as if its flight had been followed bythe naked eye. The Zeppelin travellers were quite ignorant ofthis action upon the part of the French and were surprised whenthey were rounded-up to learn that they had been tracked soruthlessly. Every message which the wireless of the Zeppelin hadtransmitted had been received and filed by the French.

Under these circumstances it is doubtful whether wirelesstelegraphy between aircraft and the forces beneath will beadopted extensively during the present campaign. Of course,should some radical improvement be perfected, wherebycommunication may be rendered absolutely secretive, while nointimation is conveyed to the enemy that ethereal conversation isin progress, then the whole situation will be changed, and theremay be remarkable developments.

[ << Back to Aeroplanes and Dirigibles of War]

Chapter XVII : WIRELESS IN AVIATION

Categories