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Aviation History
1937
1937 - 1907.PDF
JULY 8, 1937. FLIGHT. 45 checked our position against familiar landmarks from time to time. Capt. Wilcockson had no landmarks, but, in stead, he had short, inter mediate and medium wave two-way radio sets, a rotat ing loop, two compasses, auto matic pilot, a bubble (or artificial horizon) sextant and a chart table. Ship's cap tains have no more, yet it is customary for the Queen Mary to make accurate landfalls. Between Foynes and Box wood the magnetic variation, or difference between the true and magnetic north (lying somewhere in Baffin Land), increases from 16 deg. to about 32 deg. If, in still air, a.pilot made the necessary allow ances, minute by minute, to his compass course, he would arrive at Botwood in due time and with out possible failure. Unfortunately, adjustments must be made for changing wind speeds and direc- Temporary Irish base : A view of the harbour and village of Foynes, on the south side of the Shannon Estuary. tions ; furthermore, the pilot in this case would not be travelling by the shortest route. In fact, however, the difference between the rhumb line, or constant true bear ing course and the great circle, or "straight line" course over this particular route is not really worth worrying about. Very accurate work oil a great circle track, with regular magnetic course alterations to suit, would save a matter of twenty-five miles or less—and Caledonia would arrive just ten minutes earlier at Botwood. Since a rhumb line course carries the machine nearer the steamer tracks the advantages of flying with acad- demic efficiency on the shortest possible route are more than outweighed. While the proximity of shipping may provide a certain amount of moral support, the real advantages of being near to these regular tracks are that radio information may be obtained and, more important, radio bearings may be taken. Unless the boat is very Caledonia's Crew : From left to right they are Mr. T. E. Hobbs (Wireless operator), Mr. T. A. Vallette, seconded from Marconis, First Officer C. H. Bowes and Capt. A. S. Wilcockson, who is in command. badly off course, bearings from Ireland and Newfoundland will be almost reciprocal and a third bearing will be neces sary to obtain a "cut" in order to estimate the machine's position with any real accuracy. Compromise, in this case, provides the best solution in the matter of dead reckoning. By steering on 290 deg. magnetic, Capt. Wilcockson would have allowed too much for variation over the first half of the journey and too little over the second half. The result of flying on this course—suitably modified according to meteoro logical information—is to provide a track somewhere between the rhumb line and great circle tracks. This useful effect is largely limited to this particular part cf the northern hemisphere. During the past six months or more an Air Ministry meteorologist has been travelling to and fro on the mer chant ship Manchester Port, other ships have made regu lar reports, and the met. people have been at work for a long period in the Shannon and Botwood areas. Theoretically, therefore, the knowledge of upper air and surface conditions over the North Atlantic should be suf ficient for a dead reckoning flight to be made with safety and efficiency. Radio Equipment That, however, is only a small part of the story. It will be remembered that the Marconi equipment on all the Empire boats includes medium and short wave two- way sets and a loop aerial for medium-wave direction finding. That in Cambria and Caledonia is a develop ment of this. At both Botwood and on the Shannon there are examples of the recently developed short-wave direction-finding systems (described in Flight this week). During the experimental Atlantic flights it is probable that watch at these two points and on the boat will nor mally be kept on a common wavelength of 900 m. (333 kc/s), while the short-wave equipment will be used mainly for long-range direction finding. The crew of the boat will use their own loop aerial for obtaining bearings from different ships, the radio operators on which willr of course, need to report their own positions. These will be necessary for obtaining regular fixes. In any case, it will be possible for the boat to obtain bearings—and, con sequently, the angle of drift—from the Botwood and Shannon stations. It is, however, very important that the crew should know exactly where they are all the time ; the strength of signals provides only a very rough guide of the distance from the transmitter, but, if the machine was homing on Botwood, the bearings would, of course, be reversed as the machine crossed over the station itself in due time.
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