FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1932
1932 - 1039.PDF
FLIGHT, OCTOBER 13, 1932 AIR NAVIGATION HE first lecture of the session at the Royal Aero nautical Society. given by Capt. Norman Macmillan, M.C., A.F.C., A.F.K.Ae.S., on October 6, was very well attended. Capt. F. Tymms, M.C., took the chair in the absence of Mr. C. R. Fairey, who had been called away owing to the serious illness of a relative. Capt. Tymms, who. is Director of Civil Aviation in India, is home for a short time, and, in introducing the lecturer, said that Capt. Macmillan was so well known to all present that a lengthy introduction would be quite superfluous. On the morning of the day before, Capt. Macmillan was in Oslo, but he had flown home to give them his lecture in person. Capt. Macmillan said that the study of air navigation required both theory and practice. Study was necessary to acquire a knowledge of the theory, while facility and speed could only be gained by hours spent in the air. Theoretical navigation was simple enough. The art lay in being what sailors call weather wise in the actual practice of pilotage, and that was where practice counted. Most civil pilots were, in their profession, most akin to bridge officers of the Mercantile Marine. After referring to the graticule of meridians and parallels, Capt. Macmillan said that as nature had not provided a corresponding graticule on the earth itself, the air pilot-navigator must make shift with such land marks as existed equally upon the map and the landscape. The best natural landmarks were composed of woods, water and hills, while the best artificial landmarks were railways and roads. For flying over mountainous country it would be useful to the pilot if oblique photographs or isometric projection charts could be introduced in the margin of maps. Most pilots who lost their way did so because thev became agitated about the non-arrival of the next land mark as early as they expected. They became impatient, searched their map for another landmark near where they imagined themselves to be, found a good-looking one to right or left, turned off their course to discover it, failed, and were lost. When flying on a compass course a pilot should never lose heart if the clock seemed to be getting the better of the compass. Let him keep on, and if his landmark did not turn up let him search for and find another before he changed course. It was an axiom that a pilot should never alter course unless he knew his position. Concerning long-distance flights, Capt. Macmillan said that the constant-bearing course, although a little longer than the great-circle and rhumb-line course, was the only practicable one in many cases, unless the aircraft carried an expert navigator who had no other duties to attend to. When flying on a constant-bearing course, the pilot needed to consider only one other thing—drift. Drift was really the most important factor in pilot navigation on a long distance flight. The only satisfactory maps were those drawn on the Mercator projection. Any straight line drawn thereon was a rhumb line, which gave the compass course except for magnetic variation and deviation. The scale of longi tude was constant, but the scale of latitude increased out wards from the equator. Since maps on the Mercator projection were based on the Equator, they were not of use for navigation in the vicinity of the Poles. The Mercator projection was the basis of practically all marine charts, and was also the best basis for aeronauti cal maps. It was not available for use near the Poles, but if the Polar routes were opened in the future by means of aircraft, he saw no reason why Polar charts should not be constructed on the Mercator projection, using an arc of a meridian passing through the pole as the unit base in the same way that the Equator was now used. Capt. Macmillan paid a tribute to the navigation regu larly practised by the Fleet Air Arm, the machines of which left the aircraft carrier and made reconnaissance flights. Most likely the machines were required to fly to a certain spot, make observations from there over a given area involving the change of course several times, and at the conclusion of the reconnaissance a return flight to the carrier, which in the interval had probably changed its position, being made along a course known before the air craft left the deck. This was a very different problem from that of straight-line flight, and he thought it was unjust 967 that the very difficult work carried on by the Fleet Air Arm was practically never heard of by the general public. The lives and safety of the crew depended entirely on navigation. There were no landmarks. There were only the indicated speed and height, the course, the track and the clock, the observer's course-setting compass, drift gauges, a course and distance calculator, the chart board, and the wireless operator. Often the visibility on the return journey was very poor, and he had heard of cases when the ship was not seen until the aircraft was almost over her. Aerial navigation of this sort was as intricate a piece of work as any called for on the part of the air navigator. It required exactness, patience, swiftness and, above all, practice. Astronomical Navigation Turning to the question of astronomical navigation, Capt. Macmillan said the altitude of a celestial body was the elevation of the body measured from the angle of the horizon towards the zenith. The difficulty of poor horizon and of height of flight had made astronomical navigation from aircraft more difficult than from sea, added to which the increased speed of aircraft had made the problem of the time required one of the utmost importance. It was desirable, therefore, to employ methods which reduced the work of manipulation to a minimum. The main requirements for astronomical navigation were a sextant with which to measure the altitude of the heavenly body; second, a nautical almanack; third, a reliable chronometer; and fourth, a book of the necessary conversion tables and, possibly, a Bygrave slide-rule for rapid calculations. Astronomical observations were only a check on naviga tion to ensure that the dead-reckoning course was being maintained. All calculations which could be made on the ground before flight should be made. By working out in advance the assumed course, speed and relative positions, together with the observations which might be expected to be required and their calculations, references to books of tables would be eliminated as far as possible when in flight. The best book of tables for rapid astronomical naviga tion which the lecturer had come across was an American publication published by the Hydrographic Office of the U.S. Navy. Its title was H.O. No. 208, Navigation Tables for Mariners and Aviators, by Lt. Com. J. Y. Dreisenstok, of the United States Navy. The book could be purchased by anyone for 75 cents. Only one other book, the Nauti cal Almanack, was required to solve rapidly all navigation problems. The compass was the heart of navigation. It was the duty of every pilot and navigator to know that his com pass was well positioned, so that he could read it easily under all conditions, that it was not interfered with by local attraction in the aircraft, that it was swung at regular periods if used over the same routes, that it was swung after changes in locality, that it was swung after heavy landings, and that any compensation was made with the minimum number of corrector magnets so that the directive force was not rendered unnecessarily sluggish. Drift was the great enemy of the compass. Faults of variation and deviation had been got over by making cor rections to the compass reading to allow for them, and if it were not for meteorological conditions nothing more would be required of the pilot to make a straight-line flight to any part of the earth. Of meteorological enemies, fog was the greatest, for although blind flying instruments had been developed to a stage where the pilot need not fear to fly through fog, yet there were still three main evils to be overcome: the absence of a height indicator which showed the actual height above the territory flown over ; the possibility of ice formation on the wings ; and the difficulty of making harbourage. Drift was the angular relation between the course and the track. When one was flying over country with plenty of landmarks and in conditions of reasonable visibility, it was easy enough to check drift and to compensate for it. Above regular air routes wind strength and direction were measured and reported at regular intervals, and if the pilot was equipped with wireless, he could obtain reports during flight, but above the wilder spaces of the earth's
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
