How Do Aircraft Hydraulic Systems Work?
Aircraft design has been in a state of constant evolution, with many historical and present endeavors seeking means to improve everything from flight capability and performance to the safety of operations. During the 1930s and 40s, aircraft engineering faced a massive shift as the size, speed, and complexity of many airframes quickly led to an issue of rudders, ailerons, and other critical flight controls being less feasible to manage with simple mechanical linkages and pilot strength alone. As engineers sought means to alleviate such issues, the industry rapidly witnessed the adoption and rise of hydraulic systems, which were able to provide the level of power steering and force necessary to make management nearly effortless for pilots. To help you understand how hydraulics achieve this level of support, we will take you through each of the core components found in a typical aircraft and explain how they drive reliability in operations.
Hydraulic Fluid
Before discussing hydraulic systems themselves, covering the basics of fluids as the medium for energy transmission is a must. Generally speaking, a majority of modern hydraulic setups use synthetic phosphate, ester-based fluids, a result of their high flash points and ample fire resistance. Additionally, chemical additives are commonly incorporated as well to prevent any fluid from foaming in the thin air of high altitudes while simultaneously mitigating the risk of oxidation or thickening when moving between cold and hot zones.
With hydraulic fluids being incompressible, any input of force results in a predictable output that is devoid of the delays or power losses typically associated with pneumatics. Moreover, these fluids double as a high-grade lubricant for the moving parts of a system, reducing undesirable friction and wear.
Pumps
Driven by mechanical rotation from an aircraft’s engines, an Auxiliary Power Unit (APU), or a dedicated motor, pumps are a staple of hydraulic systems with their role in displacing fluids and forcing them toward control valves and actuators based on various inputs. Usually, this is accomplished through a series of reciprocating pistons that capture a fixed volume of fluid, ensuring proper pressure for functionality. While the pump is what provides the constant flow of fluid that is needed for operations, it is important to grasp that operating pressure is largely generated by the resistance caused by the interaction of fluids coming into contact with control valves and actuators.
For a majority of modern aircraft designs, variable-displacement pumps that automatically adjust their output flow based on real-time demand prove to be very popular. For instance, when an aircraft is in a steady cruise state, and no controls are moving, a pump like this will move less fluid to avoid imposing heat buildup or unnecessary mechanical load on engines.
Reservoirs
As the central storage and conditioning hub for hydraulic fluid, the reservoir of any system goes beyond the simple role of being a reliable container, instead acting as a displacement buffer. This results in preventing lines from running dry or over-pressurizing as actuators extend and retract, where the reservoir stores surplus fluid during retraction and provides extra volume during extension. Furthermore, internal baffles are commonplace in these assemblies to allow any trapped bubbles in a fluid to rise to the surface before being recycled, as such air pockets can otherwise have negative effects on pressure output. As an added benefit, this space accommodates the natural thermal expansion of the fluid as it heats up during operation, ensuring return lines will not become over-pressurized.
Control Valves
While briefly touched upon before, control valves are mechanical or electromechanical assemblies that direct fluid toward appropriate components by blocking or unblocking ports of a network. Since an aircraft’s hydraulic system generally handles the management of diverse tasks like landing gear extension, braking, and surface adjustments independently, a standard system will leverage dozens of specialized valves to achieve specific needs. For professionals, the most notable categories to familiarize yourself with include:
Directional Control Valves: As the most basic type, directional control valves are responsible for porting fluid to one side of an actuator while simultaneously opening a return path on the opposite end.
- Pressure Control Valves: These valves act as safety limiters and regulators, with pressure-relief variations in particular proving popular with their ability to safeguard systems by venting any fluid back to the reservoir when maximum PSI levels are reached.
- Flow Control Valves: Serving to control the volume of fluid entering a conduit, flow control valves ensure that heavy structures like trailing-edge flaps move at a steady pace under aerodynamic loads to prevent the risk of slamming into position or deploying unevenly.
- Check Valves: Check valves allow fluid to move in only one set direction, preventing backflow that could cause a loss of pressure if a pump fails or a line is compromised.
- Priority and Selector Valves: These valves prioritize fluid flow to essential systems like landing gear when control signals are relayed, temporarily restricting fluids from performing less important actions.
Hydraulic Lines and Hoses
From rigid tubing and flexible hoses to other various forms of hydraulic lines, aircraft will leverage a range of reliable components to establish a network for transporting fluids between pumps and actuators. With the grand expectations placed on aviation systems, these lines are expected to handle pressures upward of 3,000 psi or more, all while being able to reliably maintain a clear path for power transmission. Because of this, many lines and hoses are constructed from high-strength materials like stainless steel or titanium in fixed areas. However, in locations where network elements may need to tolerate some movement, flexible synthetic rubber hoses reinforced with stainless steel wire braiding are employed. When shopping for such solutions, it can be useful to narrow searches to the listings that are FSC 4720 Hose and Flexible Tubing parts.
Actuators
As the final major elements of hydraulic systems that we will cover, actuators prove essential in their role in converting pressure exerted by fluids into mechanical movement that can be harnessed for work. To suit different motion needs, two main types of actuators find common use in aircraft design.
- Linear Actuators: These actuators produce straight-line movement with a piston-and-cylinder design, serving operations like deploying landing gear, extending spoilers, or adjusting flight control surfaces.
- Hydraulic Motors: For tasks requiring continuous or high-torque turning energy, such as leading-edge slats, flap tracks, or cargo winches, hydraulic motors serve to convert fluid flow into the level of rotational energy necessary for carrying out work.
Aerospace Buying: Your One-Stop Shop for Hydraulic System Parts
If you are seeking any of the hydraulic system elements we discussed or others, always be sure to preserve the exact engineering that keeps them reliable by exclusively sourcing approved components and assemblies from trusted distributors. Aerospace Buying, an ASAP Semiconductor-owned and operated website, is your perfect solution with our curated selection of products for these needs and beyond. Whether you are specifically seeking out hoses and tubing, Rosemount Aerospace Inc Parts, or general items for avionics systems, our curated catalogs and convenient search tools will let you easily narrow down what you require from our extensive inventory. We encourage you to learn more about our top-notch offerings and competitive procurement solutions by checking out our website or getting directly in touch with our experts!
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