Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 20. Selsyn Instruments

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 XX - SELSYN INSTRUMENTS

91. Purpose and use.-Selsyn instruments are used as position indicators to show the position of wing flaps and landing wheels.

92. Description, a. The Selsyn instruments (fig. 45) consist of the indicator and the transmitter. The system is designed to operate on direct current obtained from the standard battery generator systems.

(1) Indicator.                                                            (2) Transmitter.
FIGURE 45.-Selsyn position indicator and transmitter.

b. The indicator is placed in a standard 2 3/4-inch shielded instrument mounting case which is provided with a 3-volt lamp for illumination, the receptacle being molded integral with the instrument case. It contains four indicating elements, one for the indication of the flap position and one for each of the three landing wheels. The connection to the elements is brought out to the back of the case through a 12-prong connector. Each element consists of a laminated ring of ferromagnetic material on which three windings are equally spaced in a delta connection. A polarized armature is supported in the center of this ring by two bearings and is free to rotate. It is surrounded by a copper damping ring to prevent excessive oscillations. An aluminum disk, carrying the figure of a wing flap or wheel, is fastened to each of the four armature shafts so that the figure is visible through an opening in the indicator dial. On the dial there is an outline impression of a small airplane. The disks make or break the lines which form the outline so that the pilot is provided at all times with the picture of the actual position of the flaps and landing wheels. The disk at the top of the dial shows the position of the flaps and a scale graduated from 0° to 45° shows the degree of flap position. The two center disks marked "left" and "right" show the position of each of the main landing wheels. If the disks are at the top of the arced opening in the dial, the outline on the miniature airplane is complete and this indicates that the wheels are fully retracted. When the wheels are down in landing position. the disks are at the bottom of the arced opening and the outline of the miniature airplane will be broken. In the same manner the disk located at the bottom of the dial shows the position of the tail wheel.

c. The transmitter unit is housed in an aluminum housing and consists of a resistance wound on a circular form having an arc of 240°. Two spring contacts attached to the shaft and secured together at an angle of approximately 120°, operate on this resistance. Through these contacts the current supply for energizing the units is transmitted from the battery generator system. The transmitter shaft is connected by an adjustable linkage to the wheels or flaps.

93. Operation.-A switch control is located in the cockpit for operating the Selsyn indicators. As shown in figure 46, the three equally spaced taps on the transmitter resistance winding are connected to the windings in the indicator in a series parallel circuit. For one given position of the contact arms, the current in each of the windings of the indicator will have a definite magnitude and direction. These currents each set up their independent magnetic fields to form a resultant magnetic field which has a definite magnitude and direction. The polarized armature of the indicator alines itself with this magnetic field. The pointer dial is thereby controlled by the angular position of the transmitter shaft which in turn is controlled by the position of the wheels or flaps. Instruments designed for 12-volt systems will operate accurately on a voltage of not less than 10 or more than 15 volts.

94. Installation.-a. The Selsyn position indicator is located on and attached to the airplane instrument panel in a conventional manner.

b. The four transmitters are mounted on suitable brackets and located on the airplane so that wheel or wing flaps can be linked to a bell crank and the bell crank fastened to the transmitter shaft. The link and bell crank should be adjusted so that the transmitter shaft will rotate approximately 60° for the change from the extreme up-and-down position of the wing flaps or wheels.

c. Connections from the power source through the transmitter to the indicator are made as in figure 47. Common conduit connectors are used for connecting the indicator to the transmitter and a standard connector plug is used to connect the lamp circuit. Indicators and transmitters are not interchangeable and the indications are not affected by the length of the leads. Correct polarity is of the utmost importance. The direction of indication will be reversed if the polarity, of the transmitter contacts are reversed, if two of the leads between the transmitter and indicator are interchanged, or if the direction of rotation of the transmitter shaft is reversed. The wiring diagram should be followed closely to avoid this possibility.

FIGURE 46.-Operation diagram of Selsyn transmitter and indicator.

95. Maintenance. The general points on maintenance of instruments given in section II are applicable to this instrument. If in checking the continuity of the battery circuit it is found open or intermittent, the probable cause is an accumulation of dirt under one or both of the spring contacts of the rheostat in the transmitter. The contacts are cleaned by first removing the transmitter from the bracket and then removing the two nuts and felt gasket from the shaft and the four screws which hold the end plate to the case. The rheostat can then be carefully removed as a unit attached to the end plate by connecting wires and the contacts can be cleaned with a fine abrasive paper and the windings with a small brush. In assembling the rheostat to the case, care must be taken to have the dowel pin in the case enter the dowel hole in the rheostat to locate the rheostat and prevent it from turning. Care must be taken that neither the leads nor the windings are damaged, and that the lead to the contact arm does not meet with interference as the rheostat is turned.

FIGURE 47.-Installation diagram of Selsyn wheel and flap position indicator