NAVIGATION BY "DEAD RECKONING"
"Dead Reckoning" is the simplest method of navigation; and, under favorable conditions, it gives a high degree of accuracy. A minimum of observation is required, but careful calculation is essential.
The "Dead Reckoning" position of an aëroplane or airship at any time is calculated from its known speed and direction over the surface of the earth or ocean, and its known course as indicated by the magnetic or gyroscopic compass.
To determine the direction of movement of an aëroplane or airship, as apart from the direction in which it is headed, an instrument known as a Drift Indicator, or Drift Bearing Plate, is used.
One form of Drift Indicator consists of a simple dial, with the center cut away and a wire stretched diametrically across it. The outer edge of the dial is divided into degrees, in a similar manner to that of the compass. It is mounted in such a way that an observer can, by looking through the center of the disc, see the ground or ocean below him. The disc is then turned until objects on the ground—or white-caps, icebergs, ships, or other objects visible on the surface of the ocean—are seen to move parallel with the wire, without in any way deviating from it. The angle which the wire then makes with the direction in which the nose of the aëroplane or airship is pointing gives the angle of drift.
The ground speed (or speed over the surface of the earth) of aircraft can be measured by observing the time taken in passing over any fixed or very slowly moving object, while a certain angular distance is described—this being found by suitable sights, attached to the Drift Bearing Plate. From the result, considered in conjunction with the height of the aëroplane or airship, the actual speed over the surface is calculated. This speed will be in the direction shown by the wire of the Drift Bearing Plate.
The ground speed so found will differ nearly always from the air speed, as shown by the air speed meter, because of the effect of the wind. The difference is greater or less according to the wind's relation to the direction in which the aëroplane or airship is headed.
Having found by observation the drift, the ground speed and the air speed, a simple instrument such as the Appleyard Course and Distance Calculator then permits the aërial navigator to discover without difficulty, as on a slide rule, the strength and direction of the wind. Should the actual track of aircraft over the earth's surface not coincide with the desired course, the Course and Distance Calculator, or a similar instrument, can thus be used to calculate, in connection with the wind velocity and direction already found, the direction in which the nose of the craft must be pointed in order to correct the deviation due to drift.
THE LATE CAPT. SIR JOHN ALCOCK JUST BEFORE STARTING
SHIPPING THE FIRST DIRECT TRANSATLANTIC AIR MAIL
Knowing the latitude and longitude of the point of departure, and noting carefully the time that elapses between each separate observation of the ground speed and of the course, the air navigator, with the aid of a specially prepared set of "traverse tables" (as used by mariners), can easily plot on his chart the distance covered and the direction in which it has been covered. Hence the position of the aircraft at any time is either known definitely, or can be forecast with a fair degree of accuracy.
For aërial navigation by means of "Dead Reckoning," frequent observations of ground speed and drift are necessary. If aircraft are cut off by clouds or fog from all possibility of sighting the surface of the earth, grave errors may occur, since in long distance flights the wind's velocity and direction often change without the pilot's knowledge.