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Section 1.20 - Automatic Hydraulic Transmissions

Examples of where automatic hydraulic transmission may be employed will be discussed.

Let us say we want to operate the radar on a jet aircraft. Suppose the radar itself is being operated by an alternator and the alternator frequency (number of cycles per second) depends on the RPM of the engine running the alternator. The RPM of the engine may change due to operational requirements, while we want a steady number of cycles coming out of the alternator to run the radar.
Let us take a hypothetical case of a turbojet aircraft. At take-off, the jet is "revving" at 12,000 RPM, at cruising altitude the jet runs at 6,000 RPM and when landing the jet runs at 3,000 RPM. Let’s say we want the alternator to work constantly at 6,000 cycles. This is what we do.

As the speed of the turbine increases because of take-off, the alternator cycles also increase. Sensing the increase, the constant speed drive "tells" the yoke on the variable displacement pump of the automatic hydraulic transmission (see section 1.19 - Hydraulic motors) to decrease the pump angle, thereby decreasing the volumetric output of the pump. This, in turn, will decrease the speed of the hydraulic motor that runs the alternator, therefore, decreasing the cycle speed of the alternator.
As the turbine decreases its rotational speed when the aircraft reaches its cruising altitude, the constant speed drive senses the decrease and it "tells" the yoke on the variable displacement pump to increase the pump angle to offset the decrease in speed of the jet engine. The increased pump angle will increase the volumetric displacement of the pump as well as the speed of the hydraulic motor that runs the alternator, thereby increasing the cycle speed of the alternator.
At landing, the engine speed decreases further so that the aircraft can lose lift and land. Again, the constant speed drive, sensing the decrease in engine speed, "tells" the yoke on the variable displacement pump to increase angle to offset the decrease in engine speed. The increase in pump angle increases volumetric displacement, causing an increase in speed of the hydraulic motor that runs the alternator and, in turn, causes the cycle speed of the alternator to increase.

The constant speed sensing device could be connected to a pump stroke control. The pump stroke control replaces the yoke as the means of increasing or decreasing the angle of the variable displacement pump.

The hydraulic motor and variable displacement pump can be used separately, as is shown on the diagram below. The pump can be connected to the engine and the motor to the alternator. As one pump decreases its angle, the motor is made to increases its angle, increasing the torque generated to the alternator. As one pump increases its angle, the motor is made to decrease its angle, decreasing the torque and increasing the RPM to the alternator.

With this set-up, we can get any variation of torque and speed (RPM).

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Updated: February 17, 1999