Basic Hydraulics — Page 52, Lesson 74

Let’s start with the big picture. Hydraulics is the science of how liquids behave under different conditions. In an aircraft, the hydraulic system is the way we transmit power to operate large and remote components — things we could not move satisfactorily by any other means. We send power through an incompressible fluid, along pipelines, into actuators. That gives us the muscle for landing gear, flaps, flight controls, wheel brakes, windshield wipers — anything that needs high power, accurate control, and rapid response. Now, the first key idea is hydrostatic pressure. Look at Figure 2.1 — for an open container, the pressure exerted by the fluid depends only on the height of the fluid. That’s a crucial point: different containers of different shapes and sizes will give the same pressure, as long as they hold the same height of fluid. The shape doesn’t matter; only the height does. Next, Pascal’s Law. Pascal was a 17th-century mathematician, and he stated: if a force is applied to a liquid in a confined space, that force will be felt equally in all directions. That’s the foundation of everything we do with hydraulics — the pressure spreads uniformly through the fluid. Now, let’s look at Figure 2.2, because this is where people often get confused. The force we use when a hydraulic system operates is caused by pressure — but that force is not delivered by the hydraulic pump. Pressure is created only when we try to compress the fluid. So if you pump oil through an open-ended tube, there is no pressure. But block the end so the oil cannot escape, and pressure builds up at once. The rule is simple: without some form of restriction, there can be no pressure. Let me give you the three fundamental relationships. Force equals pressure times area. Pressure equals force per unit area — that is, force divided by area. And force is the total load available. So pressure is what we generate, and force is what we get out. That brings us to Bramah’s Press. Joseph Bramah, who lived from 1749 to 1814, invented a hydraulic press and observed two facts. First: the smaller the area under load, the greater the pressure generated. Second: the larger the area under pressure, the greater the load available. Let’s work the numbers from Figure 2.3. Suppose we apply a force of 1000 newtons to piston A, whose area is 0.002 square metres. That produces a pressure of 500 kilopascals in the fluid. Now piston B has an area of 0.004 square metres. It will support a load of 0.004 square metres times 500 kilopascals — which is 2000 newtons. In other words, F equals P times A. So a small force on a small piston creates pressure, and that same pressure acting on a larger piston gives us a bigger force. That’s the whole principle of the hydraulic press — and of every hydraulic actuator on the aircraft.