By Chris Grosenick Top: A Boeing 757 No. 1 engine pylon hydraulic pressure and thrust reverser modules. Middle and bottom: Boeing 757 left system hydraulic service center. Hydraulic/pneumatic...

Figure 3. Force, Area, Pressure Relationship Figure 4. Laminar and Turbulent Flow

The interaction between pressure and fluid flow (volume) is sometimes confusing; what makes actuators move, pressure or flow? The answer is both depending on the context of the question, and whether force or movement is the issue. Aircraft hydraulic systems are usually described by pressure, psi, and volume in gallons per minute, gpm. Hydraulic pumps are rated at a certain psi and gpm output, for example, 3,000 psi at 25 gpm. This means the pump will produce 25 gpm of flow at 3,000 psi. As pressure increases to 3,000 psi, flow decreases to zero with no flow demand in the system. Thus, the pressure-flow relationship is an inverse one. Pressure is controlled in a hydraulic system by a pressure-sensitive regulating device such as a pump compensator, pressure regulator valve, pressure-operated relief valve or a pressure switch (electrically operated pump).

Given a pressurized, static, closed hydraulic system, closed meaning no bleed-through or bypass built into any system components, fluid flow is zero and pressure is regulated maximum. Operating the landing gear on jacks, the force to overcome is gravity, and with the gear selector positioned to "up", fluid is ported to the gear actuators. There now exists a demand for fluid flow by the gear actuators prompted by opening the gear selector valve, so flow increases and pressure decreases. As the pressure drops, the regulating device in the system senses this drop and positions the pumping element to increase fluid flow. As this volume of fluid flowing into the system increases, it "crowds" fluid moving already, and resistance (pressure) builds. This delicate balance shifts back and forth many times throughout the entire travel of the actuator stroke, until the actuator movement, and thus fluid flow, stops. Fluid flow moves actuators, and pressure works to overcome gravity and airloads. Both properties must exist for the system to do any meaningful work. What about when something operates and the pressure does not drop? Hydraulic systems are designed taking into account volume requirements for all components, and then using a pressure source that can operate those components at the system rated pressure with some margin for safety and operating efficiency. For example, our 3,000-psi, 25-gpm pump mentioned above could be used in a system that has a volumetric demand of up to 25 gpm, and still maintain 3,000 psi. It's important to remember that pressure will drop past the 25-gpm point, despite the pressure regulator device.

Figure 5. General gas law relationships

Gases are considered fluids in a strict physical sense. They are also highly compressible, which leads to another set of laws to describe their behavior. Pascal's and Bernoulli's laws hold true for gases, but because they are more compressible, several more specific gas laws are used. Charles' Law and Boyles' Law deal with the relationship between temperature, pressure, and volume of a gas, and their respective laws can be summarized in the General Gas Law. These laws state three fundamental properties of gases. With the temperature remaining constant, pressure varies inversely with volume (A). As the temperature of a confined gas increases, the pressure of that confined gas increases as well, the temperature-pressure relationship being directly proportional (B). With the pressure held constant, the volume of a gas increases in direct proportion to the temperature (C).

This article only touches the surface of the physical principles relating to aircraft hydraulic and pneumatic systems. Hydraulic systems are presently the predominant workhorses of the aircraft structure, and knowing and understanding the basis of their operation is crucial for the effective troubleshooting and maintenance of these systems.

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