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 systems are used in modern aircraft because, for the weight, they efficiently transmit power to subsystems that require large operating forces to remote locations on the airframe. Research is currently underway to replace hydraulic systems with lightweight powerful electrical systems, but hydraulic power will be the primary force used in aircraft until electrical technology matures.
Two laws fundamentally describe the ways in which hydraulic systems accomplish work. For this discussion, "work" is defined as the movement of control surfaces, cargo doors, landing gear, and the other hydraulically operated systems on an aircraft. Also, the term fluid describes both liquids (oils) and gases. Pascal's Law states that fluid flow is equal and undiminished in all directions within the confines of a container (Figure 1). The "container" in an aircraft is the system's tubing, valves, actuators, reservoirs, pumps, and other components. Bernoulli's Law states that as the velocity of a fluid increases, its pressure decreases (Figure 2). This law describes the pressure-flow relationship and the operation of an aircraft wing.
Output force in a hydraulic system is calculated by multiplying system pressure by cylinder piston area. In Figure 3, the 10 psi produced in the left hand cylinder is multiplied by the 20-in. piston area in the right hand cylinder to generate 200 pounds of force on the piston rod. The 10 psi is generated by applying 20 pounds of force on the left hand cylinder piston and spreading that force over the 2-in. piston area (force ÷ area). On the working cylinder; F = P x A, and on the work producing cylinder; P = F ÷ A (where P is pressure, F is applied or generated force and A is piston area).
There are several types of fluid flow within a system: laminar and turbulent. Flow types are important because turbulent flow generates heat and decreases the system's ability to perform efficient work. Hydraulic systems incorporate as much laminar flow as possible by the design of tubing runs, and the internal construction of components.
The main difference between liquids and gases is that gases are highly compressible. Liquids are theoretically uncompressible, but they compress a very small amount because entrained air is trapped in the fluid due to the effect of atmospheric pressure. When fluid is pressurized, the air is forced into solution and becomes difficult to detect, but these microscopic pockets of air cause wear and erosion within the hydraulic system, mainly in the pump. When the pressure is relieved in a hydraulic system, air forced into solution bubbles out and causes air pockets in the system. This is similar to opening a soda bottle and having the gas escape out the partially opened top (along with half the soda). A venting reservoir on engine shutdown or pump cut-off is a good indication that there is too much air in the hydraulic system. At high altitude, fluid will foam in an unpressurized reservoir, and this causes problems with supplied pressure and system operation, since hydraulic systems are designed to work with fluid and not foam. High performance aircraft have pressurized reservoirs for this reason, and this is why it is important to bleed hydraulic systems after a component is changed.
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