NAVIGATION IN FOGS AND AT NIGHT - AERIAL NAVIGATION.
The safety of an aeroplane flying near the ground depends largely upon the pilot's ability to maintain, within limits, an even keel, normally not difficult; but in fog, when visual connection with terrestrial and celestial objects is not possible, means should be provided to enable a pilot instantly to recognize any involun tary change of course. It is not at first obvious why the natural gravitational force is not a sufficient indication of the vertical.
When an aircraft is turning, however, it is flying on a curved path and the centrifugal acceleration, which acts at right angles to the direction of flight, needs to be considered. It is naturally not possible for a ball level to distinguish acceleration due to one cause from that due to any other, and the liquid in the level is acted upon, not by the earth's gravitational force alone, but by the resultant of the centrifugal force and the earth's gravitational force. This resultant acts at right angles to the "floor" of the aircraft and the ball consequently seeks, not the true vertical, defined by gravity, but the apparent vertical, defined by the di rection of the resultant of the accelerations. If, therefore, the aircraft executes a banked turn in flight, the ball remains in the centre of the level and does not indicate a change of course. In fog or cloud a pilot may be unaware that his craft is turning and the magnetic compass alone is of little value under such conditions because it will only indicate correctly during straight flight or in very slow turns. The need for an instrument which will indicate that the aircraft is changing course has led to the development of the gyro turn indicator.
This rotation, which is known as precession, is controlled by springs, g g, so that the deflec tion of b c is a measure of the rate of change of course and so that the axle, b c, may return to a horizontal position when the aircraft is again flying on a straight course. The deflection of b c may be indicated to the pilot by any suitable means. The in strument has proved of the greatest use for flying in fog or cloud ; it has been developed in a number of countries, but all types depend upon the principle outlined above and differ chiefly in the methods of indicating the turn. The Pioneer turn indi cator is one type of the instrument.
The Pioneer turn indicator may be operated either by an engine driven suction pump or by means of a venturi tube. Sucking air from the case of the instrument by either of these means causes a stream of air to flow through an intake jet, driving a small gyro at high speed. The balanced gyro, which is the sensitive element of the instrument, runs on specially designed precision ball bearings to which oil is supplied from a reservoir within the gyro. The gyro is mounted in such a way that it responds only to turns about the vertical axis of the aeroplane. The face of the device is mounted in the instrument panel, with the gyro in the case immediately behind the indicator. Tubes run from the case to the venturi mounted on the exterior of the aeroplane or to a suction pump.
Before the directional gyro and the gyro-horizon were intro duced, pilots relied upon the magnetic compass and the rate in struments such as the turn and bank, the air speed and climb in dicators when flying blind, and in rough air it was difficult to fly the craft straight enough to keep the compass from oscillating. If the magnetic compass could be held stable in azimuth, the problem would become simple, and a desired course could be set and maintained by keeping the lubber line opposite the de sired heading. The directional gyro allows this method of course keeping and flying to be employed. It therefore combines the qualities • of a flight control instrument and an instrument for aerial navigation. The directional gyro is an index of turn which neither lags nor oscillates and is unaffected by rough air or mag netic disturbances.
The gyro-horizon was designed to do what no other instrument had ever done—to provide the pilot with an artificial horizon, which would continually show him the flying attitude of the aero plane. The horizon, by means of a miniature aeroplane and a gyro actuated horizon bar, shows at a glance whether the plane is level or whether it is banking, climbing or gliding. It is used by the pilot easily and without any mental effort or interpretation, exactly as he was taught to use the natural horizon in fair weather. In addition to the miniature aeroplane and horizon bar, a pointer on the dial of the gyro-horizon shows the pilot the exact amount of bank in degrees up to and including a ninety-degree or vertical bank.
The gyropilot is not a new invention. Prior to the World War, a Sperry automatic stabilizer was introduced and a gyro rudder control was developed at the Royal Aircraft Establishment, Farn borough, England. The World War, however, so interfered with progress of civil aviation that it was not until the comparatively recent years that the transport business was sufficiently stimu' lated to demand renewed activity in the design of automatic aero plane control.
The modern Sperry gyropilot consists essentially of a control unit which is mounted as an integral part of the instrument board and a servo unit which supplies the power for moving the controls. The gyros within the control unit are identically the same as the separate flight instruments, the directional gyro and the gyro-horizon, and their dials are incorporated on the front of the control unit so that the human pilots can always refer to their indications, whether the aeroplane is being flown by gyro pilot or by manual control. The indications of the sensitive gyro scopic instruments are translated into corrective movements of the aeroplane controls through a system of air relay-operated hy draulic valves which regulate the flow of oil to the hydraulic pistons of the servo unit. The weight of the entire equipment is 6o pounds, and it occupies but little space.
The gyropilot has introduced a new technique in aerial naviga tion. Where formerly it was practically impossible to steer much closer than two degrees by magnetic compass, the gyropilot holds the aeroplane so steady that courses are now being steered as close as half a degree.
After considerable experimentation with the blind-landing sys tem developed by the U. S. Army Air Corps, the Bureau of Air Commerce found that it had a definite field of usefulness in com mercial aircraft operation as a system for making safe approaches to airports under ceiling and visibility conditions which would otherwise prevent an attempt at landing.
The elements of the system on the aeroplane require a radio di rection finder, a radio receiver which flashes a light when actu ated by the signal of a radio marker beacon on the ground, a sensitive altimeter and a directional gyro, together with the usual flight instruments, such as air speed indicator, turn and bank in dicator, rate of climb indicator, and artificial horizon. The radio direction finder also is used both in navigating to the marker beacon and in approaching the landing area.
On the ground there are two radio stations transmitting on different frequencies. One is located about 1,500 feet from the airport, the other about two miles away. Also, at each radio station there is a secondary transmitter acting as a radio marker beacon to flash on the light in the aeroplane's instrument board whenever the craft passes over the transmitter.
The pilot approaches the field by using his radio direction finder tuned to one of the stations, then establishes his course on a line between the two radio stations, flying back and forth over them as many times as necessary to set the course on his directional gyro. On the final approach to the field, using the sensitive altimeter, he descends to Boo feet over the station two miles away, glides on down to about 15o feet over the station near the airport boundary, and then completes the landing ma noeuvre with the aid of his flight instruments and lights on the ground.
A chain of lights is installed for 1,5oo feet on a straight line of approach to the pri4,cipal runway of the airport from the inner radio station. Further, two rows of lights flush with the ground are installed along the runway. These lights provide visual di rection to the airman after he has used his radio for locating the approach to the edge of the airport.
Over a great deal of the airways systems, radio is one of the mainstays of the aids to air navigation. The radio range beacons, by means of their constantly transmitted directional signal, point out the airman's course for him. Information as to weather con ditions comes to him regularly through airways radio stations. Other radio stations show him how to reach his airport. If he has a transmitter as well as a receiver, he can always talk with the ground, and in connection with a direction finder he can uti lize the entertainment programmes of the high powered broad casting stations to assist his navigation.
However, the system of radio air navigation aids is not per fect and Government departments are constantly experimenting and trying to make improvements. In the course of experimental work, radio engineers have found a way of showing the radio range beacon signals visually on the instrument board of an air craft. These signals were formerly ordinarily received through headphones, but by rigging up a converter box and arranging an instrument with two needles, one vertical and the other horizon tal, across its face the airman can see them. When the aircraf t is on its course the two needles cross in the exact centre of the instrument, but when he is off to one side the vertical needle swings in that direction.
Much of the world's air route mileage is lighted for night fly ing, there being approximately 22,000 miles of lighted airways in the United States alone. In this country, airway beacons are established io to 15 miles apart, and intermediate landing fields marked by beacon lights are established where airports are non existent, so as to provide landing fields along the route 3o to 5o miles apart. The U. S. Bureau of Air Commerce standard air way beacon is a 36-inch rotating unit equipped with two doublet lens assemblies and a iio volt, i,000 watt airways incandescent lamp with automatic lamp changer. This unit shows two beams of light separated by an angle of 18o degrees, and when the unit is used without colour screens each beam has a maximum of about 1,250,000 candlepower. The airway beacons are so operated as to show six clear flashes per minute. In each case the clear flashes are supplemented by either red or green auxiliary flashes. Red flashes are used at beacons where landing fields are not available and green flashes are used at the intermediate landing fields. In France, a thousand million candlepower beacon has been installed near Dijon.
Barring bad weather, these beacons greatly simplify night fly ing, and great floodlights and beacons at the airports facilitate night landing. In the United States, for instance, the major air ports have installed Sperry AGA-BBT i,000 mm. floodlights. One of these units illuminates an entire landing field, spread ing a 18o degree, sharply defined blanket of light over the ground. The source of this light is the same high intensity arc lamp which is used in Boo million candlepower anti-aircraft searchlights, and the light is distributed uniformly over the field by a precision ground lens without interfering in any way with the pilot's vi sion. If the arc stops, due to consumption of carbons or for any other reason, an auxiliary, Io,000 watt incandescent lamp is automatically turned on and moved into focus. The incan descent lamp is powerful enough to illuminate a sufficient area for a safe landing, and thus there is no possibility that the field could be left in darkness at the particular time when full illumi nation is most desired.
An airship is much steadier than an aeroplane, and instruments, particularly the magnetic compass, are more reliable. Under suitable conditions, astronomical observation can be taken and reduced without difficulty and the accommodation for wireless equipment is greater than in an aeroplane. It will be seen that the conditions obtainable for airship navigation are substantially those employed in maritime navigation. It is important to note, however, that the navigation of airships over long distances will render the availability of accurate meteorological information of primary importance, since an airship, unlike an aeroplane, can land or "anchor" only on prepared aerodromes or at masts.