Steering a Plane

Sun (Auckland), Volume II, Issue 454, 8 September 1928, Page 11

Steering a Plane

HOW BEARINGS ARE TAKEN

Air Trickier Than Sea

LARGER MARGIN OF ERROR

STEERING an airplane through darkness, fog and rain uiust necessarily become a matter of fortune in which the navigator and pilot, having set their course, must leave much to fate. But. it is uearly always possible to rise above the clouds, and it is then upon the skill of the navigator that the success of a flying venture rests.

According to a recently published bulletin of the Daniel Guggenheim Fund for the Promotion of Aeronautics, the fundamental problem of determining an air position by means of the stars or the sun is the same as that which has long been used on the sea. However, since the margin of error at sea is comparatively small and since bearings are far easier to take than in the air, there is a wide difference between “avigation” and navigation. A ship at sea usually takes its bearing once a day, at noon, and following a compass course it can plough the sea for 24 hours with comparatively small error, especially on large liners which have automatically controlled helms. But not so with an airplane. The difference and the difficulty is this, a ship at sea is influenced by the wind to far less extent than is an airplane: its speed is practically constant, whereas that of a plane varies, if only slightly, from one minute to another, depending on the steadiness of the pilot and the fluctuations in the strength of the wind. For example, if a ship is sailing due east at 20 knots an hour, bucking a due north wind its tack will not be north-east, but somewhere between north-east by east and east, the density of the water largely counteracting the rapidity of the drift due to the strength of the north wind. In an airplane on a like course the drift is almost equal to the strength of the wind, and the direction in which an airplane would have to fly would be approximately due north-east in order to make headway due east Error in Air Observations

Fluctuations in the strength of the wind, easily determinable in a ship, are extremely difficult to detect in a plane, and the same can be said of changes of direction. Thus to find an air position with relation to the earth while flying is an exceedingly difficult thing in practice because of the wide margins of error, and these difficulties are made clearer in the Guggenheim bulletin: “The question may rightly be asked as to what degree of accuracy may be expected in using the latest methods of celestial avigation. A definite check on nine lines of position was reported by Lieut. Commander Weems in the navy mail plane making daily trips between San Diego and San Pedro. “The distance covered was 90 miles, the time in the air was 90 minutes, and the average error for the nine lines were 6.6 miles. “The conditions were good on the day these observations were made, and it is therefore believed that an average error of less than 10 miles is as accurate as may be expected. Furthermore, in bumpy air this error may be as high as 30 or 40 miles.” Instruments for Position Finding To find an air position in the daytime four things are necessary; a bubble sextant, an aerochronometer rated to Greenwich apparent time, tables for solving the triangle and a chart with an aircraft plotter (combined protractor and ruler). With these instruments the avigator must first determine the exact second of time. This is a fairly simple matter, the exact sun or star time being obtainable from two aerochronometers, one set to

Greenwich sidereal time and the other to Greenwich apparent time. Next a. line of position is made by observation through the bubble sextant- When the horizon, is visible it is used in preference to the bubble, due allowance being made for altitude; when it is not the bubble forms an artificial horizon, and a slight error in this respect may throw the observation out by several degrees. The lino of position having been observed and plotted, which shows the plane to be flying in a given direction at some unknown point, it is necessary to solve the triange formed by the observer’s zenith, the Pole and the observed body. Special tables and instruments are compiled and made for the solving of this triangle, some of them very complicated. others reasonably simple. Once truly solved, a second line of position is formed and at the intersection of the two lines the exact position of the piano is found, and it is thus possible to correct the compass course. In the daytime the object observed is, of course, the sun: but at night it is a star. A special type of bubble sextant is preferred, an aerochronometer. and star altitude curves. The method of determining the position is exactly the same, except that almost invariably the bubble has to be used for the horizon, which is rarely plainly enough visible at night. Naturally, when there are no heavenly bodies to be seen and no horizon, as when flying through fog or storms, celestial avigation breaks down and planes are dependent on radio direction or the compass, bank and turn indicator and the inclinometer. Instrument flying was used by most of the long-distance ocean airmen. However, with the development of radio it seems a fair observation to say that radio will play an all-important part in avigation and will for all practical purposes displace celestial observation. But that day is not yet. Aviation’s Progress Yet, even with all the of present-day avigation. the giant strides of progress it has made since the war are apparent when it is remembered that a war pilot was dependent on very inferior instruments for finding his way over the German lines in bad weather. It was customary, for example, for a night bomber to fly short distances over the lines in a pitch-black night on a compact: course, the wind direction and strength having been worked out from the meteorological reports. With this information the pilot w T ould fly for a required number of minutes and then •Irop his bombs. It has been stated that the average margin of error over a 90-mile -x>urse by the latest avigation methods was 6.6 miles. The comparison is not a fair one, but it serves to illustrate that over courses of, say, 20 to 30 miles an average error of something like two miles was to be expected. Actually the error was generally not so wide, but it was wide enough—something more than a mile—to make bombing in such conditions utterly futile. It was discovered about the middle of the war that a plane might conceivably fly on a compass course and at the end of the time limit drop a parachute flare. This lit up a considerable area of country and usually a pilot was able to see his objective, more often than not half a mile away, fly to it and drop his bombs on it.