Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 27. Bank and Turn Indicators

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 XXVII - BANK AND TURN INDICATORS

126. Purpose and use.-a, The bank and turn indicator is used for controlling the flight of an aircraft under conditions of poor visibility or when for any reason it is desirable to eliminate any yawing or turning.

(1) Front view.

(2) Top view.
FIGURE 71.-Bank and turn indicator

b. Some specific uses for this instrument are to

(1) Enable the pilot to maintain a straight and laterally level flight.

(2) Enable the pilot to make precision turns at predetermined rates.

(3) Coordinate rudder and ailerons when making banked turns.

127. Description.-a, The bank and turn indicator (fig. 71) is a combination of two flight instruments, the bank indicator and the turn indicator. A cross section diagram of the instrument is shown in figure 72. The turn indicator unit is a gyroscopic device which indicates motion about the vertical axis of the airplane. It is composed of a suction-driven gyro rotor located in the rear section of the case on a restraining spring, and a dash pot for damping, The gyro rotor is carefully balanced, and runs on specially designed precision ball bearings to which oil is supplied from a reservoir within the gyro. The bank indicator unit is a simple pendulous device of the inclinometer type, comprising a black glass ball which moves against the damping action of a liquid in a curved glass tube indicating motion about the longitudinal axis. This clinometer assembly is located the front section of the instrument case.

(
1) Front view.                                                        (2)Side view.
FIGURE 72.-Crossection diagram of the bank and turn indicator,

b, The instrument case is of the standard size, using a 2 3/4-inch dial, and has incorporated into the case proper a drain plug, vacuum connections, damping adjustment, sensitivity adjustment, and lubrication opening. The drain hole and screw are located on the bottom of the instrument just behind the mounting flange assembly to facilitate the removal of collections of water and oil from the interior of the case. One vacuum connection is located on the bottom rear, and the other at the back of the case. Both have 1/8-inch internal pipe threads and are provided with pipe plugs so that either may be removed to connect the vacuum line, depending upon convenience of the installation. The damping adjustment screw and lock nut are located on the right-hand side of the case just behind the mounting flange assembly. When this screw is turned "in," the open area of the damping orifice is increased and the damping effects decreased. The sensitivity adjusting screw and lock nut are located on the left-hand side of the case just behind the mounting flange assembly. When this screw is turned "in'". the tension on the centralizing spring is decreased, permitting the "rate of turn" pointer to deflect farther for a given rate of turn. The lubricating spring is located on the right-hand side of the rear section of the instrument case and is provided with a threaded plug and a lead gasket for sealing purposes.

c. The dial is plain, having only the letters "L" and "R," the neutral mark, and an index on each side of the neutral mark. The index indicates a 1-minute turn of 360° when the pointer is coinciding with the index. Fluorescent lighting is used on latest models of this instrument.

128. Operation.-a. The turn indicator may be operated by an engine-driven suction pump, by suction from the engine intake manifold, or by a venturi tube mounted in the slip stream of the airplane. Sucking air from the case causes a stream of air to flow through an intake jet, driving a small gyro wheel at high speed. The gyro rotates about the lateral axis in a frame that is pivoted about the longitudinal axis. When mounted in this way, the gyro responds only to motion around a vertical axis, not being affected by rolling or pitching. The principles of the gyro assembly refer to a system of axes originating in the center of the gyro rotor shown in figure 73. The axis of rotation of the rotor is called "Y." the longitudinal axis of the frame "X," and the axis normal to these two or the vertical axis "Z." The gyro rotates at high speed as indicated by the arrow a. When the airplane is turning, for example, to the left, the gyro assembly is rotated as indicated by the arrow b as the X axis is fixed to the longitudinal axis of the airplane. The reaction of the gyro to this turning influence is an immediate rotation c about the X axis until Z has alined itself with the original position of axis Y or a displacement. of 90°. This movement is due to one of the fundamental principles of gyroscopes known as precession. Precession of a gyroscope is the natural reaction at right angles to an applied torque. In the turn indicator, the rotation of the gyro assembly around axis X acts against the restraining force of a spring and is limited by stops to about 45° each side of the vertical. This spring serves to balance the gyroscopic reaction during a turn and to return the assembly to its vertical neutral position as soon as straight flight, is returned. The action of the gyro assembly is damped by a dash pot. An opening provided to the interior of the cylinder is controlled by a screw valve adgryo assembly and hand approximately proportional to the rate of gyro, spring, and damping mechanism produces a displacement of the gyro assembly and hand approximately proportional to the rate of turn of the aircraft. When centered, the hand shows that the airplane is flying straight disregarding drift, pitch, and bank,.When the hand is off center, it indicates that the airplane is turning in the direction shown by the hand. The amount the hand is off center is proportional to the approximate rate of turn.

FIGURE 73.- Gyro rotor, bank and turn indicator.

b. The bank indicator is a simple form of pendulum which indicates motion about the longitudinal axis of the airplane and is used to control the lateral attitude of the airplane in straight flight or turns. While the airplane is making a perfectly banked turn, the ball will remain in the center position due to centrifugal force. The correct bank is indicated for any turn but no indication is given of the amount of bank. In either straight flight or turn the centered ball indicates proper lateral attitude. If the ball moves in the direction of the turn, it indicates that the airplane is slipping toward the inside of the turn due to overbanking. If the ball moves in the opposite direction, it indicates that the airplane is skidding toward the outside of the turn due to underbanking.

FIGURE 74.-Indicator readings for several conditions of bank and turn.

c. The indications of these two instruments on the one dial serve to show at all times the rate of turn and the lateral attitude of the air-plane in straight flight or turns. Figure 74 shows some examples of the bank and turn indicator readings.

129. Installation.-a. All general points on installation of air-craft instruments are applicable to this instrument. The bank and turn indicator, grouped with other flight instruments, is mounted so that the dial is vertical when the airplane is in normal flight position and the ball of the bank indicator is in the center position. Before a new instrument or one which has been received from stock is in-stalled, approximately six drops of a mixture of one-third compass liquid and two-thirds gyro instrument oil are added to the rotor pivot.

b. Installation is made in accordance with various airplane and instrument board drawings. When connecting the venturi tube to the indicator, the tubing should be run as straight as possible avoiding bends of small radius and may be connected to the indicator at either the lower or rear connection. The connection not used is plugged with the 1/8-inch pipe plug provided. When installed on a vibration-absorbing instrument board, a suitable length of flexible tubing (approximately 10 inches) is used. Under normal flight conditions, a suction of 1,90 inches of mercury is desired inside the case of the instrument, It is necessary to check the amount of suction after the installation is completed by connecting a suction gage to the connection which is not in use, care being taken to replace the pipe plug after the test has been made.

c. Due to varying temperatures and individual instrument characteristics, the vacuum of l.90 inches of mercury will not always produce the desired sensitivity. The vacuum should be set to produce the required deflection wherever practical during a flight test, Adjustment of the restricting needle valve may be made by reaching under the instrument board. Increasing the vacuum (screwing the valve stem out) increases the sensitivity and amount of deflection, and decreasing the vacuum (screwing the valve in) decreases the sensitivity and amount of deflection.

130. Maintenance.-a. The general points on inspection of instruments given in section II are applicable to this instrument. Service maintenance and inspection of this instrument also includes the following tests:

(1) Static balance test.-The pointer must zero when the rotor is not spinning, +1/64 inch for any position of instrument.

(2) Dynamic balance test.-When the instrument is stationary in normal position and the gyro is operated under a suction of 26 inches of water, the pointer should stand at the zero mark ± 1/64 inch.

(3) Starting friction test.-With the instrument in normal position, stationary, and not being subject to vibration, a suction of not more than 5 inches of water should cause the gyro to rotate.

b. Lubricant is added through a lubricating opening marked "oil" on the right side of the case after the instrument has been removed from the airplane, A fine wire (approximately 0,025-inch diameter) should be used to guide the oil into the hole in the pivot. Any excess of spilled oil may be a source of unsatisfactory operation, For operating conditions in which atmospheric temperatures are above freezing (0° C. or +32° F,). add eight drops of gyro instrument oil. For operating conditions in which atmospheric temperatures are be-low freezing (0° C. or +32° F,), add eight drops of a mixture of one-third compass liquid and two-thirds gyro instrument oil.

c. In testing and adjusting the suction, a source of suction gage is connected to the system ; on the old types at the cross in which the adjustable restriction is located, and on the new types at the plug located in the back of the instrument case. The line leading to the instrument from the suction system should have a suction of 3.75 to 4.25 inch Hg. By means of the adjustable restriction, the suction on the instrument is adjusted to 1,9 inch Hg. as closely as possible. It should never be less than 1,80 nor more than 2.05 inch Hg,

d. The screen or jet may be cleaned by first removing it from the instrument, washing it in a mixture of 50--50 carbon tetrachloride and naphtha, and drying it thoroughly. When reinserting, a wrench is not necessary; finger tight is sufficient. Any condensate and excess oil may be removed by unscrewing the drain plug. All plugs and connections must be tight in order to prevent excessive air consumption.