Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-165 Gen. Batt. N S N N S SW N Reverse current coil Main contractor Permanent magnet Temporary magnet Differential coil Pivot Differential relay contacts Voltage relay contacts Temporary magnet Voltage relay coil Permanent magnet In overdrive, an automobile engine operates at about the same rpm at 60 mph as it does in conventional drive at 49 mph. In aircraft, this principle is applied in the same manner. The constant speed drive enables the alternator to produce the same frequency at slightly above engine idle rpm as it would at takeoff or cruising rpm. With the input speed to the drive set at 6,000 rpm, the output speed is the same. This is known as straight drive and might be compared to an automobile in high gear. However, when the input speed is greater than 6,000 rpm, it must be reduced to provide an output of 6,000 rpm. This is called underdrive, which is comparable to an automobile in low gear. Thus, the large input, caused by high engine rpm, is reduced to give the desired alternator speed.
As a result of this control by the constant speed drive, the frequency output of the generator varies from 420 cps at no load to 400 cps under full load. This, in brief, is the function of the constant speed drive assembly. Before discussing the various units and circuits, the overall operation of the transmission should be discussed as follows. Hydraulic Transmission The transmission is mounted between the generator and the aircraft engine. Its name denotes that hydraulic oil is used, although some transmissions may use engine oil. Refer to the cutaway view of such a transmission in Figure 12-334. The input shaft D is driven from the drive shaft on the accessory section of the engine. The output drive F, on the opposite end of the transmission, engages the drive shaft of the generator.
The input shaft is geared to the rotating cylinder block gear, which it drives, as well as to the makeup and scavenger gear pumps E. The makeup (charge) pump delivers oil (300 psi) to the pump and motor cylinder block, to the governor system, and to the pressurized case, whereas the scavenger pump returns the oil to the external reservoir. The rotating cylinder assembly B consists of the pump and motor cylinder blocks, which are bolted to opposite sides of a port plate. The two other major parts are the motor wobbler A and the pump wobbler C. The governor system is the unit at the top of the left side in Figure 12-334.
The cylinder assembly has two primary units. The block assembly of one of the units, the pump, contains 14 cylinders, each of which has a piston and pushrod. Charge pressure from the makeup pump is applied to each piston in order to force it outward against the pushrod. It, in turn, is pushed against the pump wobble plate. If the plate remained as shown in Figure 12-335A, each of the 14 cylinders would have equal pressure, and all pistons would be in the same relative position in their respective cylinders. But with the plate tilted, the top portion moves outward and the lower portion inward. [Figure 12-335B] As a result, more oil enters the interior of the upper cylinder, but oil is forced from the cylinder of the bottom piston.
If the pump block were rotated while the plate remained stationary, the top piston would be forced inward because of the angle of the plate. This action would cause the oil confined within the cylinder to be subjected to increased pressure great 12-166 enough to force it into the motor cylinder block assembly. Before explaining what the high-pressure oil in the motor unit does, it is necessary to know something about this part of the rotating cylinder block assembly. The motor block assembly has 16 cylinders, each with its piston and pushrod. These are constantly receiving charge pressure of 300 psi.
The position of the piston depends upon the point at which each pushrod touches the motor wobble plate. These rods cause the wobble plate to rotate by the pressure they exert against its sloping surface. The piston and pushrod of the motor are pushed outward as oil is forced through the motor valve plate from the pump cylinder. The pushrods are forced against the motor wobble plate, which is free to rotate but cannot change the angle at which it is set. Since the pushrods cannot move sideways, the pressure exerted against the motor wobble plate’s sloping face causes it to rotate. In the actual transmission, there is an adjustable wobble plate.
The control cylinder assembly determines the tilt of the pump wobble plate. For example, it is set at an angle, which causes the motor cylinders to turn the motor wobble plate faster than the motor assembly if the transmission is in overdrive. The greater pressure in the pump and motor cylinders produces the result described. With the transmission in underdrive, the angle is arranged so there is a reduction in pumping action. The subsequent slippage between the pushrods and motor wobble plate reduces the output speed of the transmission. When the pump wobble plate is not at an angle, the pumping action is at a minimum and the transmission has what is known as hydraulic lock. For this condition, the input and output speed is about the same, and the transmission is considered to be in straight drive.
To prevent the oil temperature from becoming excessively high within the cylinder block, the makeup pressure pump forces oil through the center of this block and the pressure relief valve. From this valve, the oil flows into the bottom of the transmission case. A scavenger pump removes the oil from the transmission case and circulates it through the oil cooler and filters before returning it to the reservoir. At the start of the cycle, oil is drawn from the reservoir, passed through a filter, and forced into the cylinder block by the makeup pressure pump. The clutch, located in the output gear and clutch assembly, is an overrunning one way, sprag-type device. Its purpose is to ratchet if the alternator becomes motorized; otherwise, the alternator might turn the engine. Furthermore, the clutch provides a positive connection when the transmission is driving the alternator.
There is another unit of the drive that must be discussed—the governor system. The governor system, which consists of a hydraulic cylinder with a piston, is electrically controlled. Its duty is to regulate oil pressure flowing to the control cylinder assembly. [Figure 12-336] The center of the system’s hydraulic cylinder is slotted so the arm of the pump wobble plate can be connected to the piston. As oil pressure moves the piston, the pump wobble plate is placed in either overspeed, underspeed, or straight drive. govern the speed of the transmission. First, the main points of the complete electrical control circuit are discussed. [Figures 12-337 and 12-338] For simplification, two portions, the overspeed circuit and the load division circuit, are considered as individual circuits.
Note, in Figure 12-337 , that the circuit has a valve and solenoid assembly (O) and a control cylinder (E), and that it contains such units as the tachometer generator (D), the rectifier (C), and adjustable resistor (B), rheostat (A), and the control coil (Q).