Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 29. Flight Indicators (Artificial Horizon)

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 XXIX - FLIGHT INDICATORS

136. Purpose and use.-a. As the natural horizon is the reference which a pilot instinctively uses in flight, the gyro horizon indicator (fig. 80) was designed to afford an artificial horizon within the cockpit of the airplane. By means of a miniature airplane and a gyro-actuated horizon bar it provides the pilot with a simulation of what he would normally see outside the airplane.

Figure 80.-Flight indicator with caging knob (front view)

b. Some specific uses of the flight indicator are to

(1) Supplement the pilot's sense of balance so that when flying blind he can keep the airplane properly oriented by visual reference to this instrument.

(2) Show at all times the attitude of the airplane's flight with reference to the real horizon and consequently the ground underneath.

(3) Maintain the proper glide angle during the approach to the runway when making an instrument landing.

(4) Measure the amount of bank when making precision turns.

137. Description.-a. The mechanism of this instrument consists of a small gyro rotor assembly (fig. 81) positioned so that the axle of the rotor is vertical. The plane of spin is horizontal and the movement of the gyro is transmitted to the face of the instrument by a horizon bar which is actuated by a pin protruding from the gyro case through a slot in the gimbal ring as shown in figure 82. The air that spins the gyro is exhausted in four horizontal jets and through four openings spaced equidistant at the bottom of the gyro case. Pendulous vanes hang down over these openings and when the gyro axle is vertical, each covers half of its corresponding angle. The horizon bar is covered with luminous material to make it visible at night and the dial is finished in a satin black. A pointer is attached to the dial to indicate the amount of bank. The banking scale is marked in degrees and is mounted as a fixed part of the cover glass and front plate. Both the banking indicator, pointer, and its scale are also covered with luminous paint. The dial is an integral part of the gimbal mount and follows the precessional movements of the rotor. The image of a small airplane is imposed on the face of the instrument and is adjustable in the vertical plane to suit the eye level of the pilot. It is a fixed image and its attitude is always that of the airplane on which the instrument is mounted.

FIGURE 81.- Flight indicator rotor assembly.

FIGURE 82.- Flight indicator mechanism.

b. The instrument case is an aluminum casting with removable front and rear plates for insertion and removal of the mechanism. Four mounting lugs with self-locking lug inserts are provided for mounting purposes on the panel. These instruments are provided with the caging device operated by the caging knob, shown in figure 80. By pulling out on the knob which protrudes through the opening on the instrument panel at the lower right-hand corner of the instrument and rotating it clockwise, the caging arms in the instrument case cause the rotor and gimbals to assume a straight position in the lateral and

horizontal planes. After turning the knob, the instrument can be locked in the caged position by pushing in on the knob. The lighting of this instrument is consistent with approved illumination of instruments as discussed in section I. All models are provided with tapped openings in the rear of the case for attachment of the suction connections. When the instrument is not installed, 3/8-inch pipe plugs are screwed into the openings to keep out dirt as shown in figure 83. This figure also shows the air filter assembly mounted on the rear of the case.

FIGURE 83.-Flight indicator rear view

138. Operation.-a. The flight indicator is operated by a vacuum of 3.75 to 4.25 inches Hg. supplied by means of a venturi tube or by an engine-driven vacuum pump. When the suction on this instrument is supplied from a venturi tube, 3 to 4 minutes are required after take-off to permit the gyro to attain its necessary speed. The gyro in the case spins at approximately 12,000 rpm about a vertical axis. The path of the air that spins the gyro may be traced by the arrows in the cross section view of the indicator (fig. 84). The gyro obeys a fundamental gyroscopic principle, rigidity in space. An indication from the gyro is picked up and brought around to the face of the instrument in the form of a horizon bar which is actuated by a pin protruding from the gyro case through a slot in the gimbal ring.

FIGURE 84.- Cross section of flight indicator (top view)

b. Any tendency of the gyro to depart from its true position due to acceleration forces or friction is corrected by a pendulous device which constantly maintains the axle in its vertical position. This device is shown in figure 82. Four pendulous vanes A  are suspended from the under side of the gyro housing. Each one of these vanes partially covers one of the four air ports B  that exhaust the air from the gyro compartment. If the gyro departs from its upright position, gravity holds the vanes vertical and one vane closes one port as shown at the left, while the opposite vane completely opens its port as shown at the right. The reaction of the air from this open port moves the gyro in the direction C  back to its normal position. The gyro is held within a fraction of 1° at all times.

c. The flight indicator is air-driven and contains no magnets. It is entirely free therefore from electrical troubles and is unaffected by magnetic disturbances. The flight indicator provides a horizon bar which remains coincident with the natural horizon through all of the usual maneuvers, banks, climbs, glides, and turns. It has no time lag and the pilot can manipulate his controls to bring the airplane to any desired attitude by noting the relation of the miniature airplane with reference to the horizon bar. By following the position of the horizon with reference to the image on the cover glass, the longitudinal and lateral attitude of the airplane relative to the real horizon and ground can be seen.

d. The limits of operation of the flight indicator are 60° pitch and 90° bank. Any time the attitude of the airplane exceeds these limits from the horizontal around either or both the longitudinal or lateral axes, the instrument will be upset and its indications are erroneous. When this occurs, the instrument can be caged which properly levels the mechanism, and when the caging mechanism is withdrawn, the instrument is ready to use again. The whole operation of caging and uncaging requires about 30 seconds.

139. Installation.-a. The general points on installation of instruments given in section III are applicable to this instrument. The following specific information is necessary for the proper initial or replacement installation of flight indicators. A typical vacuum installation containing a flight indicator is shown in figure 4. 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 source of suction. However, due to 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 of suction. The venturi is mounted as described in section XXVIII.

b. In view of the delicate construction of gyroscopic instruments, they are installed only on vibrationproof instrument panels. The flight 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. This instrument 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. It is essential that each instrument is properly leveled after installation. With the airplane flying in calm air in its normal flight attitude, the instrument is adjusted on the panel so that it is exactly level longitudinally and laterally. If the face of the flight indicator is not exactly vertical, the four attaching screws may be removed and shims added (the same thickness on each side) between the flight indicator and the panel at top or bottom holes as required to line up the horizon bar with the indices at each side. By slightly elongating the holes in the instrument panel, the flight indicator can be turned right or left sufficiently to aline the pointer with the index at the top of the dial. If facilities are available, the instrument may be leveled without flying the airplane as shown in figure 85. In this case, the tail of the airplane is raised until it is In normal flying position which is checked with a spirit level to insure that the airplane is level longitudinally and laterally. The instrument may then be adjusted on the panel as described above. A vacuum pump adjusted to supply a vacuum equal to 4 inches Hg. is used to operate the instrument.

FIGURE 85,- Plan for leveling and testing flight indicator installation.

c. 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 is 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 flight indicators which are to be operated by an engine-driven vacuum pump, a relief valve is supplied as a part of the pump or as an accessory to the pump in order to regulate a vacuum at the instrument.

140. Maintenance.-a. The general points on maintenance given in section II are applicable to this instrument. In addition, there are a number of specific points and procedures applicable to flight indicators. These instruments will operate for approximately 1,000 hours under normal operating conditions without cleaning or renewal of oil. However, continued operation in hot climates or under other adverse conditions necessitates more frequent periods of cleaning and lubricating. Since cleaning and lubricating are not functions of service activities, the gyroscopic instruments are not removed for this reason until the airplanes are sent to repair depots for overhaul. Upon overhaul of the airplanes, these instruments are inspected by the depot and given any necessary servicing and reinstalled or replaced with serviceable instruments. However, where gyroscopic instruments are removed from the airplane between overhaul periods because of malfunctioning and found to be unserviceable, they are forwarded immediately to the repair depot where they are repaired or replaced with serviceable instruments.

b. The frequency of cleaning air intake filters will depend upon service conditions. The procedure is first to remove the snap ring and screen at the back of the case. The filter is then washed with carbon tetrochloride, dried, and replaced. When cleaning a filter, it should be thoroughly examined for defects and rejected if any are found.

c. The suction is checked by using the suction gage on the instrument panel and when adjustment is necessary it is made with the suction relief valve in the vacuum system. When the suction is first applied, the horizon bar will oscillate sometimes violently which is a normal reaction. However, all oscillation should cease and the bar settle and hold a steady position after the expiration of 1 1/2 minutes. The banking indicator must always be perpendicular to the horizon bar; that is, if the bar is cocked to left or right, the banking indicator is off of zero the same amount in the same direction.

d. The coast test on a flight indicator can be made any time after the engines have been running for at least 10 minutes so that the instrument has been running for at least 5 minutes at normal speed on 3.75 to 4.25 inches Hg. When the suction is cut off, the gyro must coast for at least 8 minutes. By placing the ear on the cover glass of the instrument, any rotation of the gyro can be detected. Any flight indicator failing on this test is removed and replaced with a serviceable one. In performing ground checks on installed flight indicators, a portable vacuum pump is used and connected into the airplane vacuum system as described in section XXVIII.