Downloaded from https://www.everyspec.com FM 1-509 technical manual. Also, read the instruction plate affixed to the individual unit or reservoir, and check the color of the fluid contained in the system. CAUTION EVEN THOUGH VEGETABLE-BASE FLUID IS NOT AUTHORIZED IN ARMY AIRCRAFT, IT MIGHT BY CHANCE GET INTO THE MAINTENANCE SHOP. NEVER USE THIS FLUID WHEN WORKING WITH HYDRAULIC SYSTEMS. IT DETERIORATES THE SYNTHETIC RUBBER SEALS USED IN SYSTEMS DESIGNED FOR PETROLEUM- OR SYNTHETIC-BASE FLUIDS. VEGETABLE-BASE FLUID IS READILY DISTINGUISHED BY ITS BLUISH COLOR. Resistance. Resistance to liquid flow is a factor that must be dealt with in hydraulic systems.
Resistance results partly from the viscosity (or thickness) of the liquid itself. The greater the viscosity, the greater the resistance. Resistance also occurs as liquids flow through tubing. This movement creates a shearing action between the wall of the tube and the liquid. This action results in a turbulent flow (a nonsmooth whirling flow) along the walls. Turbulence working against the smooth flow of a liquid causes resistance, which causes the temperature to rise; this, in turn, causes a loss of energy. When engineers design an aircraft hydraulic system, they have to consider energy loss. Accordingly, they design the tubing and other units to reduce the resistance as much as possible.
Power pumps (like the power-steering pump in a car) are made large enough to take care of the aircraft’s operational needs, plus the energy loss. No resistance occurs when the liquid is not moving in the system. When it starts to move, resistance begins and increases as the speed of flow increases. For this reason, an orifice plate or restriction is sometimes installed in a line to limit the rate of flow. For example, when a landing gear is being extended, it tends to drop with great force. A restrictor installed in the hydraulic return line (the up line, in this case) decreases the speed of fluid flow.
This decrease reduces the speed when the landing gear comes down and prevents possible structural damage. Computing Force, Area, and Pressure. Hydraulic systems are designed to take advantage of liquid characteristics as they relate to force, area, and pressure. These terms, as used in hydraulics, are defined as follows: Force is the amount of push or pull applied to an object. For example, the force applied to a piston head is the energy applied to the total area of the piston head. In this manual, force is measured in pounds. Area is the measurement of a surface. In the aircraft hydraulic system, the areas of piston heads are a main concern.
If these areas are known, the amount of force needed can be computed to start a mechanism moving. In this manual, area is measured in square inches. 1-3