Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 28. Turn Indicators (Directional Gyro)

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXVIII - TURN INDICATORS (DIRECTIONAL GYRO)

131. Purpose and use.-The purpose of a turn indicator is to establish a fixed reference for maintaining flight direction. The specific uses for turn indicators are to

a. Supplement the compass in keeping a course.

b. Show magnitude of turn.

c. Aid in compensation and correction when "air swinging compasses."

d. Maintain alinement when making instrument landings,

e. Coordinate navigators' and pilots' problems of directional control.

f. Coordinate bombardiers' and pilots' problems when performing tactical missions,

g. Aid in location of radio beacon stations.

132. Description.-a. There are in general two types of the directional gyro turn indicators, type A (fig, 75) and type B (fig, 76),

FIGURE 75. -Turn indicator, type A.

They are substantially the same except that the latter type has a pilot director feature obtained by the addition of a remote control reference card adjacent to the gyro card. This reference card is rotatable by the bomber and/or the navigator by a flexible shaft and controller.

b. The turn indicator mechanism shown in figures 77 and 78 is basically a horizontal, axis-free gyro provided with an azimuth card and setting device.

The rotor is universally mounted; that is, it is supported in a gimbal ring which is free to turn about an axis on bearings in the vertical ring, The vertical ring is free to turn about the vertical axis. The circular card which is attached to the vertical ring is observed by the pilot through the rectangular opening in the front of the instrument case. The entire assembly is supported and carried on bearings of which the pivots are mounted in the top and bottom plates of the instrument case.

FIGURE 76.-Turn indicator, type B.

c. The caging knob protrudes through the front of the instrument case. It is used to set the gyro and card assembly on the desired heading and to reset it at periodic intervals for course keeping when flying by compass. By pushing the caging knob "in," it engages the synchronizer lever plunger which normally rests in the cone-shaped interior of the pinion and raises the lever pins which slide in the groove of the synchronizer ring. This lifts the synchronizer ring, pushing up the spring plunger and raising the caging arm so that it makes contact with the bottom of the gimbal ring and holds the gyro horizontal as the card is turned to the desired heading. Pulling the caging knob "out" releases the caging mechanism and leaves the gyro horizontal and free.

FIGURE 77. Turn indicator mechanism.

d. The air stream which spins the rotor enters through a screened opening around the bearing pivot on the bottom plate and is divided into two parallel jets; the air from each jet is directed through nozzles onto the buckets in the rim of the rotor at points equidistant from the center. The pressure exerted by the air on the rotor circumference serves to keep it upright (fig. 77). If the rotor tilts, the air from the jet on one side strikes against the rim instead of against the buckets while air from the other jet strikes the side of the buckets causing the rotor to return to its upright position.

e. The instrument case is cast of aluminum with the front and rear plates held to it by means of flathead countersunk machine screws. The case is sealed airtight at all points except at the suction connections and at the air inlet, Two 3/8-inch pipe tapped openings are located on the bottom plate near the rear for insertion of suction connection nipples. Some models are provided with a 3-volt lamp located in the front upper center of the instrument case just above the azimuth card. The connection is made with a plug located in the rear plate of the instrument case. Fluorescent lighting on all recent instruments has replaced the conventional 3-volt lighting system, All types of turn indicators in use are provided with a ball in glass clinometer located on the face of the instrument for banking purposes.

(1) Top View

(2) Side View
FIGURE 78.-Cross section of turn indicator.

133. Operation.-a. The object of the turn indicator is to establish a fixed reference for maintaining flight direction. The gyroscope. which is simply a spinning wheel mounted so its axle can be pointed in any direction, affords the best way of obtaining this reference without excessive size and weight, Relative movement of the air-plane in azimuth is shown on a circular card graduated in degrees.

b. The turn indicator is operated by vacuum at 4-inch Hg, supplied by means of a venturi tube or by an engine-driven vacuum pump. The rotor spinning about a horizontal axis at approximately 12,000 rpm is so mounted as to be free about the three directional axes. The card, graduated in degrees, is attached to the vertical ring in which the rotor and its gimbal ring is mounted. The vertical ring and card are free to turn about the vertical axis and a rectangular opening in the front of the instrument case permits a view of an ample sector of the card. The gyro axle is horizontal in normal operation. When spinning, the gyro obeys a fundamental gyroscopic principle, rigidity. Thus the rotor and gimbal ring and the card which is attached to the vertical ring remain fixed, and the air-plane moves around them.

c. Unlike a compass, the turn indicator has no directive force to return it to a fixed heading. It must be checked occasionally, and if necessary, reset by means of the caging knob underneath the dial.

Pushing this knob in engages the synchronizer pinion with the synchronizer gear. By turning the knob, the card can be set by means of the reference line in the window to correspond with the compass heading or it can be set on zero when measuring amount of turn from any established heading,

d. After setting the card, pull out on the knob and the instrument is in operation thereafter unless it is either upset or recaged. Any degree of bank or pitch exceeding 55° will upset the gyro and in all probability the card will start to spin, consequently the instrument is useless until the airplane is again leveled and the gyro caged and reset. The gyro in the turn indicator should always be caged if acrobatics are to be performed. The sensitive pivots and bearings are less subject to damage and the over-all life of the mechanism before overhaul will be increased.

e. Directional control is accomplished with the directional gyro which is read in the same manner as a vertical card magnetic compass. More accurate control is possible due to the fact that the directional gyro does not swing or oscillate but remains fixed and therefore provides as positive a reference for steering as objects along a clear natural horizon. The directional gyro must be originally set to the magnetic compass and reset at intervals from 15 to 20 minutes to take up for drift of the gyro. The average drift on cardinal headings should not exceed 3° in 15 minutes and the drift on any single heading should not exceed 5° in 15 minutes. Care should be taken when setting or resetting the directional gyro, especially in rough air, to be certain that a correct compass reading is obtained. It must be remembered that even when flying straight in rough air, the magnetic compass will swing to a certain extent. When the magnetic card appears to be still, it is actually at the end of a swing and therefore farther from true magnetic azimuth. If the directional gyro is set to the magnetic compass at one end of a swing and observed a short time later in connection with the compass when the compass is at the opposite end of a swing, it will seem as though there had been an excessive drift of the gyro. To avoid this trouble, the airplane should be held as straight as possible for about a minute by a directional gyro setting, approximately the same as the compass reading, during which time the compass may be observed to determine its average reading during swings, and the directional gyro then properly set. When uncaging the gyro after setting. the caging knob should be pulled straight out.

134. Installation. a. The general points on installation of instruments given in section III are applicable to this instrument, In general, gyroscopic flight instruments are installed as shown in figure 4 and require the installation of an engine vacuum pump. Venturi tubes are installed as an alternate source of suction on single-engine airplanes. The two-engine airplanes, however, are provided with an engine-driven vacuum pump on each engine. In either case, a suction control valve is provided to permit changing from one source of suction to the other when necessary. In those cases where no vacuum pumps are available, it will be necessary to use venturi tubes for the main source of suction. However, due to the insufficient suction in glides and the possibility of the venturi freezing and becoming inoperative, satisfactory operation cannot be expected from a venturi tube as the main source. Venturi tubes are mounted as indicated by the arrow on the name plate, in the slip stream of the propeller as close as possible to the instrument it is to operate, so as to keep the length of connecting tubing to a minimum. The venturi tube must produce a vacuum of not less than 3 inches and not more than 5 inches Hg. at the instrument when the airplane is flying at cruising speed. The ideal vacuum is 4 inches Hg. at cruising speed.

b. In view of the delicate construction of gyroscopic instruments, they are only installed on vibrationproof instrument panels. The turn indicator is fitted with self-locking nuts so that it is only necessary to insert the four attaching screws from the front and tighten them with a screw driver. It is essential that each instrument is properly leveled on installation. With the airplane in its normal flight attitude, place a spirit level across the face of the instrument to see that it is level longitudinally and laterally. If necessary, shims may be added (the same thickness on each side) between the turn indicator and the panel at top or bottom as required. By slightly elongating the holes in the instrument panel, the turn indicator can be tilted one way or the other sufficiently to make it level. Before tightening the screws, be sure that all four corners of the instrument touch the panel surface or shims (if any have been added) in order that the instrument will not be strained when the screws are tightened. The turn indicator is removable without disturbing any of the other instruments and under no circumstances should support attachments be made except by screws through the mounting lugs,

c. Standard Air Corps fittings and tubing are used when installing the instrument, using the most convenient of the two connections on the case and taking care to see that the one not used is kept tight. On types A and B turn indicators, a terminal is provided on each side of the top front of the case for connecting the card lamp incorporated in this instrument to a source of electrical supply. The lamp is connected in parallel with the compass light, using a radio shielded cable. On the late type turn indicators, the lighting wires are self-contained in the instrument and to make connections to the lamp it is only necessary to remove the small brass screw cap from the boss on the rear cover and screw in a shielded electrical connector plug, using a ferrule assembly with 1/4-inch flexible conduit.

d. When the installation is completed, the vacuum must be checked in flight by means of a suction gage. On fast airplanes where excessive suction is created by the venturi tube, a relief valve must be placed in the air line as close to the panel as possible, and adjusted to produce a vacuum equal to 4 inches Hg, at the instrument. For turn indicators which are to be operated by an engine-driven vacuum pump, a relief valve is supplied as part of the pump or as an accessory to the pump in order to regulate the vacuum at the instrument. Air filters are provided with all turn indicators and require occasional cleaning; this is especially true when operating in localities which are exceedingly dusty.

135. Maintenance.-General information on maintenance of instruments given in section II applies to the turn indicators. Some specific information applicable to the maintenance of turn indicators is as follows:

a. Improper operation is usually caused by excessive vibration or improper supply of vacuum. If the turn indicator does not function properly, the vibration of the instrument board and the vacuum supply are checked before removing the instrument from the airplane. If the trouble is found to be due to conditions within the instrument, it should then be removed and replaced with one that is serviceable.

b. The shafts, pivots, and ball bearings of the instrument are lubricated before assembly in the case at the factory or Air Corps repair depots and no further lubrication will be accomplished by service activities with the exception that if the caging knob is hard to pull out or push in and turns excessively hard, add a few drops of instrument oil to the surface of the shaft that passes through the front plate of the instrument case.

c. The frequency of cleaning the air intake filter will depend upon the service conditions. To clean the intake filter on the bottom of the case, the snap ring and screen are lifted out with a scribe. It is not necessary to remove any screws, The filter is cleaned with carbon tetrachloride, dried, and replaced. When cleaning a filter, it should be thoroughly examined and if found defective replaced with a new one.

d. This instrument is equipped with the conventional 3-volt lamp, therefore its removal and replacement is the same as for all instruments equipped with the individual lamps. In this connection, when checking the light for operation, the compass light and turn indicator light are connected together on a separate circuit and controlled by a separate rheostat; therefore, an "on" and "off" operation will be simultaneous.

e. The correct functioning of this instrument. can be checked in position by the use of two test procedures, the coast test and the drift test. The coast test indicates the condit ion of the rotor and gimbal ring bearings while the drift test checks the balance of the assembly and the condition of the vertical ring bearings.

FIGURE 79. Testing installed gyro instruments using a portable vacmuum pump.

(1) In the coast test procedure, the gyro is operated for at least 5 minutes on the proper amount of suction which is then shut off by stopping the engine. After 8 minutes, a check is made to see if the gyro is still running. This can usually be determined by placing the ear close to or on some surface of the instrument case. Any instrument in which the gyro coasts less than 8 minutes should be replaced by a serviceable one.

(2) In the drift test procedure, by operating the engine vacuum pump or by connecting a portable vacuum pump into the suction system, the gyro in the turn indicator is run on the proper suction for 10 minutes on each of the cardinal headings: that is, 0°, 90°, 180°, and 270°. With the airplane in a tail-down position. the card on the gyro should not drift off more than ±4° in the 10-minute interval on any of the headings. Figure 79 shows the portable pump connected through the venturi system; the same results may be obtained by disconnecting the line at the engine vacuum pump and connecting the portable pump there, provided the selector valve in cockpit is set to correspond with the source of entry into the vacuum system.