Aerial Navigation - Dead Reckoning Navigation

AERIAL NAVIGATION - DEAD RECKONING NAVIGATION. The basic method of navigating an aircraft is known as dead reckoning (D.R.) all other methods, such as wireless direction finding or the determination of the altitude of heavenly bodies, serve only to check the D.R. position. Dead reckoning necessi tates that a record shall be kept of the speed of the aircraft over the ground and the "track," the latter being the actual direction along which the aircraft is passing over the ground. These values may be obtained from a knowledge of (a) the speed and direction of the wind, derived either by observation from the aircraft or from meteorological reports; (b) the true speed through the air, obtained by correcting the air-speed indicator for height; and (c) the compass course.

If a vector triangle be drawn such that one side represents (a) above, and another side represents (b) and (c), then the length of the third side represents the ground speed and its direction the track of the aircraft over the ground. The ground speed may be determined in two ways, first, by graphical solution from the vector triangle; second, by timing the passage of the aeroplane between points on the ground, the distance between which is known.

The angle between the fore and aft line of the aircraft track is called the drift angle ; it may be determined by observing the direction in which an object on the ground appears to move with reference to the fore and aft axis of the aircraft. If the aircraft were flying directly up or down wind, the drift angle would be zero. The true air speed being known, the wind speed and direc tion may also be found, without timing a ground object, by observ ing the drift angle on two different compass courses, preferably separated by a fairly large angle.

Drift Indicator.

A drift instrument is a desirable auxiliary to a compass for accurate aerial navigation. As is well known, an aeroplane may be headed in one direction yet follow a course in another direction. The difference between heading and course is the drift angle, and in order to "make good" the desired course, it is necessary that the pilot (or navigator) know the drift angle so that he may calculate the heading and that the aeroplane be steered accordingly. The only practical way of ascertaining the drift is by observation of the ground, and a drift indicator is used for this purpose.

The Pioneer drift indicator serves as a representative illustra tion of this kind of instrument. In the top of the device is mounted a ground glass inches in diameter. A lens is inserted in the bottom of the funnel-shaped case, the entire instrument being located in the bottom of the plane readily available for a pilot to glance down and through it. As the aeroplane moves over the ground, an image of the ground below is thrown upon the ground glass screen and objects on the ground appear to travel across the screen in the direction of the movement of the aero plane. The screen itself is rotatable and is provided with paral lel sight wires, and an angular scale graduated in degrees from o to 5o on both sides of zero.

To make an observation of drift, it is only necessary to rotate the screen either to the left or right until objects on the ground appear to travel in a line along the screen parallel to the sight wires. The angle of drift is then read directly from the scale. In order to correct the compass course, simply add or subtract the number of degrees of drift.

While flying at night, lighted objects, such as street lights or electric signs, show up on the screen with remarkable clearness. It is, therefore, just as easy to take a drift reading at night as in the daytime, provided of course the ship is passing over ob jects lighted to sufficient brilliancy.

The desiderata of any type of aircraft compass are: (a) that there should be a datum line indicating a known direction; (b) that a restoring force shall be set up if the direction of this line be altered by some agency external to the system; and (c) that the oscillations natural to the system after a disturbance shall be damped. The magnetic compass is particularly suitable for air craft, since these requirements can be provided without prejudic ing lightness and simplicity (see COMPASS).

Altimeters.

The most convenient method of determining the height of an aircraft above the ground is by the use of an altim eter, a form of aneroid barometer (q.v.). All altimeters are, therefore, set at zero when the aircraft leaves the ground, and they are calibrated in accordance with a standard atmosphere, which has been adopted internationally. The standard atmosphere as sumes that, at sea-level, the temperature is 15° C and that the barometric height, reduced to o° C, is 76o mm. of mercury. The decrease of temperature with increase of height is assumed to be 6.5° C per km. Altimeters so calibrated are usually accurate within I %.

Aside from the standard altimeters, there is available a "sen sitive altimeter" calibrated for detailed registration of height and adjustable for barometric corrections. The instrument has two pointers, one of which makes ten revolutions for Io,000 feet range while the other makes one revolution for the entire range, the instrument being read like a clock. It is provided with a barometric correction scale so that corrections may be made for the barometric pressure conditions existing on the field where the pilot intends landing. In this way, with properly adjusted altimeter, the pilot is able to bring his craft down with an ac curate knowledge of the altitude involved.

Compasses for Use in Polar Regions.

The magnetic types of compass depend upon the existence of a horizontal component of the earth's magnetic field; this latter is zero at the earth's magnetic poles and near these poles its values are too small to afford the directive force which a compass must have. The di rective properties of a gyroscopic compass also decrease as the polar regions are approached, being zero at the earth's geograph ical poles. An interesting attempt to solve the problem of obser vation of the sun, has resulted in the Goertz and Bumstead sun compasses. In the Goertz instrument, for example, an image of the sun is projected on a ground glass screen when the aircraft is flying on a given course ; a rotation of the optical system about a vertical axis once in 24 hours provides that the image is main tained on the screen unless a change of course is made. In prac tice, the pilot would set the sun compass to the desired course and turn the aircraft until the sun appeared on the ground glass screen; he would then steer by this sun image.