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Aviation History
1938
1938 - 0473.PDF
FEBRUARY 17. I938- FLIGHT. 169 CORRESPONDENCE The Editor does not hold himself responsible for the vL-ws expressed by correspondents. The names and ad tresses of the writers, not necessarily for publication, must in all cases accompany letters. NO FIRE (BRIGADE) WITHOUT SMOKE YOUR correspondent, Mr. A. J. Leland, makes a very goodsuggestion (for smoke-screening a city by chemicals burnton domestic fires), but, unfortunately, it would only work in the winter, when such fires aie burning, and even then not during the hours of nightfall. The simplest of all methods of laying a smoke-screen quickly would not be by smoke-distributing aeroplanes, but by firing smoke-bombs from the ground to explode at 300 feet. Every fire-engine station would be equipped with an extra vehicle which could dash through the streets firing smoke-bombs from a suitably designed gun, and the already trained fire-engine personnel could drive, round their area in a preset route and black it out completely in a very few minutes, repeating the dose on the windward edge of their territory as the wind necessitated. A great advantage of this would be that at night-time the city would be completely concealed from view, and it would not need to extinguish its lights. CIVILIAN. Cambridge. A.C. IN THE AIR. " : 7I N the correspondence columns of your issue of February 3, Mr. F. P. Wills, B.Sc, A.M.I.E.E., advocates the use of 120 or 140 volt A.C. current (360 or 800 cycles) on aircraft, used in conjunction with transformers. The first point that the writer seems to have overlooked is that a battery, though " an old, conventional idea " is a necessity : neither should the battery be regarded as the supply of energy, but essentially as a reserve of energy. The battery is required for supplying energy if the generator fails, for aircraft lighting, and for instruments such as fuel contents gauge, cabin temperature thermometer, etc., when the engines are not running. The writer states that " aircaft motors can be started electrically, very simply, by the inertia starter via an electric motor running at 20,000 r.p.m. This is true, but again a battery is necessary as there would be no supply of A.C. until the engines were running. We will suppose that a battery is incorporated in the A.C. supply, persist in the use of A C. apparatus, and consider the engine starting problem. The alternator supplies A.C. to a rectifier through which the battery is charged; it will probably be necessary to use a transformer also as a 120-240-volt battery is not a very practical proposition. The batten- would have to drive a motor-generator or a rotary convertor in order to supply current to the A.C. motor driving the inertia starter. The case above shows that D.C. is more suitable, using a dynamo to charge the batter ' and the batterj- to operate a D.C. motor direct for starting purposes. The use of a voltage of 120-240 will not be so satisfactory as the writer would seem to think, from considerations other than that of the battery: — (i) Granted there will be a saving in weight of copper, but there will be an increase in the insulation required to give the same margin of safety from fire and shock. As the supply is alternating, and assuming the voltage to be 240, insulation will have to be supplied to give protection from a maximum value of 340 volts. (ii) The use of a higher voltage introduces the possibility of the pilots, crew or passengers getting shocks from the equip-men t they are using: This is impossible on a 24-volt system. (iii) Filament-type lamps operating on the higher voltage would not last so long as there is still considerable vibration on aircraft, military types in particular, and the longer, thinner, and more fragile filaments would break under these conditions. It is extremely doubtful whether all the instruments men- tioned could be "better -worked from A.C. Petrol gauges must he able to indicate without the engines running; therefore they must be D.C.-operated for convenience. Temperature indicators and exhaust-gas analysers (combustion indicators) »oth require sensitive indicators^ and there is no A.C. indica- ting instrument, including the rectifier-operated moving coil, which will give the same degree of sensitivity and economy as does the D.C. moving-coil indicator. An external rectifier would therefore have to be used and tins would certainly not make things any simpler. four correspondent suggests that magneto ignition could wh w?etiSed Wilh if -the suPP1^ were AC- One °! the reasonsy dual magneto ignition is used on aero-engines is from considerations of safety, i.e., if one magneto fails the engine can still run on the remaining serviceable one until such time as it is convenient to lard. If the ignition depended on the aircraft electrical supply this would require to be duplicated to give the same margin of safety. This would entail an increase in cost and also weight, as two alternators, giving the same output as a bigger unit, would weigh more. It is agreed that all gyroscopes would function better from A.C. as they would then always run at the designed speed, and freedom from contacts and small brushes would be a boon in many ways. Air-driven gyroscopes are also apt to give false indications through lack of rotor speed caused by low vacuum or pressure due to "iced-up " Venturis, leaky pumps and relief valves, etc. All things considered, however, there does not seem sufficient reason for changing the main supply from D.C. to A.C. and in any case not to a voltage of more than about 3«- An auxiliary supply of A.C., either from the same generator or a separate one, for a few minor purposes where it is definitely more suitable, would be acceptable. I might add that I am not a designer of aircraft electrical equipment and that I realise that for general purposes A.C. is superior to DC. A.F.R.Ae.S. Norfolk. RADIO FOR THE PRIVATE OWNER. IN Mr. Samuelson's letter in your issue of November 25.replying to my own earlier letter, he suggested that amateur pilots and private owners might make some suggestions in your Correspondence Page on the sort of radio equipment which they would like to see made available. Since then I have waited in vain for some fresh aspect of amateur require- ments to be brought to light and have, very regretfully, come to the conclusion that the manufacturers, in apparently doing nothing about the whole business, are very accurately meet- ing the existing demand. In spite of the evident lack of enthusiasm I still feel con- vinced that most people must at one time or another have felt the need of some means of getting in touch with the world outside their own small cabin or cockpit, and this aspect of the question is, I think, in many ways more important than the cold figures of D/F bearings or the almost equally cold and impersonal reading of the Borough Hill announcer. We have most of us, at one time or another, received weather reports before starting, which have proved, on a closer and more personal investigation, to be more than a trifle astray. Weather forecasting is not as yet an absolutely exact science, and there are occasional days when changes can occur very rapidly indeed. It is in circumstances such as these that I suggest that the means of being able to call up one's destina- tion, or some other useful point near the route, would be of very real comfort, as well as of considerable assistance. Exactly the same thing, of course, applies where sea crossings are concerned. In the United Kingdom there are several hundred of machines in private hands—I hesitate to give an exact figure, since the position has had time to change a little since the last Civil Aviation Report came hot off the presses. Few of these machines are capable of carrying an extra heavy load, but a fairlv large proportion of them should be able, without difficulty, to support a lightweight short-wave transmitting and receiving installation. This being so, and in spite of the lack of interest aroused by Mr. Samuelson's letter, it does seem that a tolerable proportion of the owners of these aircraft might be induced to buy installations, thus enabling the manu- facture of such equipment to be carried out at a reasonable, although hardly a cheap, price. Aircraft screening and bond- ing is, of course, a very definite difficulty, but it has become a comparatively frequent operation on a number of small American aircraft, so that it should not prove impossibly expensive. I am afraid that this very largely repeats what I said before, but I am still of the opinion that, were short-wave two-way communication provided for the private owners, together with some sort of an assurance that the question of short- wave D/F was being actively pursued, such equipment would find a ready demand from the public, limited though that public unfortunately is. JOHN CARR Belfast.
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