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Chapter IX Hydraulic and Pneumatic Power Systems The word hydraulics is based on the Greek word for water, and originally meant the study of physical behavior of water at rest and in motion. Today, the meaning has been expanded to include the physical behavior of all liquids including hydraulic fluids. A. Aircraft Hydraulic Systems Hydraulic systems are not new to aviation. Some early aircraft used hydraulic brake systems. As aircraft became more sophisticated, newer systems with greater complexity were developed. to the container shown in figure 9-1 will have the same reading. 2. The Hydrostatic Paradox The pressure produced by a column of liquid is directly proportional to its density and the height of the column, and in no way depends upon the shape of the container or the amount of liquid the container holds. For example, 1 cu. in. of water weighs 0.036 lb. A tube that is 231" tall with a cross section of 1 sq. in. will hold 1 gal. of water (1 gal. = 231 cu. in.). If the tube is Although some aircraft manufacturers make greater use of hydraulic systems than others, the hydraulic system of the average modern aircraft performs many functions. Among the units commonly operated by hydraulic systems are landing gear, wing flaps, speed and wheel brakes, and flight control surfaces. Hydraulic systems have many advantages as a power source for operating various aircraft units. They combine the advantages of light weight, ease of installation, simplification of inspection, and minimum maintenance requirements. Hydraulic operations are almost 100% efficient, with only a negligible loss due to fluid friction. Aircraft hydraulic systems belong to that branch of physics concerned with fluid power/mechanics. They do their work by moving fluid, and the fluid they use is incompressible. Pneumatic systems work in much the same way, obeying many of the same laws, but the fluid they use (air) is compressible. Figure 9-1. Pressure exerted on a fluid in an enclosed container is transmitted equally and undiminished to all parts of the container and acts at right angles to the enclosing walls. To better understand how a hydraulic system accomplishes its task, a brief review of the physics involved is necessary. 1. Pascal's Law This is the basic law we use when we think of transmitting power by a hydraulic system. The French mathematician Blaise Pascal observed that any increase in the pressure on a confined liquid was transmitted equally and undiminished to all parts of the container, and acts at right angles to the enclosing walls of the container. This means simply that if we have an enclosed vessel full of liquid, and we apply a force to a piston in the vessel to raise the pressure, this increase in pressure will be the same anywhere in the system. Each of the gauges attached Figure 9-2. The pressure exerted by a column of liquid is determined by the height of the column and is independent of its volume. 121 Aircraft Technical Book Company http://www.ACTechbooks.com (800) 780-4115 (970) 887-2207 standing straight up, the 1 gal. of water will exert a pressure of 8.32 PSI at the bottom of the tube. If the tube were 231" high and had an area of 100 sq. in., it would hold 100 gal. of water, but the pressure at the bottom would still be 8.32 PSI. The force exerted by the column of water is, of course, equal to the pressure acting on each square inch times the number of square inches, or 832 lbs. It makes no difference as to the shape or size of the vessel that contains the liquid; it is the height of the column that is the critical factor. In figure 9-3, the pressure (P) read by the gauges will be the same in all four instances, since the height (H) is the same. Naturally, all of the vessels must be filled with the same liquid. 3. Relationship Between Pressure, Force, and Area Pressure is a measure of the amount of force that acts on a unit of area. In most American hydraulic systems, pressure is measured in pounds per square inch (PSI). The relationship between force, pressure, and area may be expressed by the formula: Force = Pressure x Area This may be visualized by looking at figure 9-4. The bottom half represents the product of the area in square inches and the pressure in PSI. This gives us the amount of force in pounds, which is represented by the top half of the circle. In order to find pressure, divide the force by the area: Pressure =Force Area In order to find the area, divide the force by the pressure: Area = Force Pressure 4. Relationship Between Area, Distance, and Volume Another relationship in hydraulics is between the area of the piston, the distance it moves, and the volume of the fluid displaced. We can visualize this relationship in figure 9-5. One half of the circle represents the volume in cubic inches, and the other half of the circle the area in square inches and the distance the piston moves in inches. Distance is also known as stroke. The relationship between volume, area, and distance may be expressed by the formula: Volume = Area x Distance To find the area divide the volume by the distance: Area = Volume Distance To find the distance divide the volume by the area: Distance = Volume Area Figure 9-3. Neither the shape nor the volume of a container affects the pressure. FORCE AREA x PRESSURE (A) AREA FORCE / PRESSURE (B) Figure 9-4. Relationship between area, pressure, and force. 122 Aircraft Technical Book Company http://www.ACTechbooks.com (800) 780-4115 (970) 887-2207 PRESSURE FORCE / AREA (C) V VOLUME A AREA DISTANCE AREA = VOLUME / DISTANCE (B) VOLUME = AREA x DISTANCE DISTANCE = VOLUME / AREA (C) Figure 9-5. Relationship between volume, area, and distance. 5. Mechanical Advantage in a Hydraulic System A hydraulic system has two major advantages over other types of mechanical systems. One is the ease with which force can be transmitted over large distances and into and out of sealed compartments. The other is the mechanical advantage made possible by varying the size of pistons. moves down 1 in. spreads out under all 20 sq. in. of the large piston, and will move up only 1/2o". This may be expressed as: A (small) x D (small) = A (large) x D (large) 1 x 1 = 20 x 1/2o 1=1 In figure 9-6, we see the way mechanical advantage is achieved in a hydraulic system. If we have a piston whose area is 1 sq. in. pressing down with a force of 1 lb., it will produce a pressure of 1 PSI, and for every inch it moves, will displace 1 cu. in. of fluid. All hydraulic systems are essentially the same, whatever their function. Regardless of application, each hydraulic system has a minimum number of components, and some type of hydraulic fluid. If the cylinder containing this piston is connected to one having a piston with an area of 20 sq. in., every square inch will be acted on by the same 1 PSI pressure, and a force of 20 lbs. will be produced. The 1 cu. in. of fluid displaced when the small piston While we may not normally think of fluid as being a component, the fluid used in aircraft hydraulic systems is most important. This fluid must flow with a minimum of opposition, and be incompressible. It must have good lubricating properties to prevent wear in the pump and valves. It must inhibit corrosion and not chemically attack seals used in the system. And it must not foam in operation, because air carried into the components will give them a spongy action. F 1# n W = 20# llllllll Manufacturers of hydraulic devices specify the type of fluid best suited for use with their equipment. Working conditions, service, temperatures, pressures, possibilities of corrosion, and other conditions must be considered. Some of the characteristics that must be considered when selecting a satisfactory fluid for a particular system are discussed in the following paragraphs. D 1 INCH D = 1/20 INCH B. Hydraulic Fluid -1 AREA . 1 SO. INCH AREA 20 SO. INCH 1. Viscosity Figure 9-6. The product of the force times the area of the large piston is equal to the product of the weight times the area of the small piston. One of the most important properties of any hydraulic fluid is its viscosity. Viscosity is a measure of internal resistance to flow. A liquid such as gasoline flows easily (has a low viscosity) while a liquid such as tar flows slowly (has a high 123 Aircraft Technical Book Company http://www.ACTechbooks.com (800) 780-4115 (970) 887-2207 viscosity). Viscosity increases as temperature decreases. The viscosity of a liquid is measured with a viscosimeter. There are several types, but the instrument most often used is the Saybolt universal viscosimeter (figure 9-7). This instrument measures the number of seconds it takes for a fixed quantity of liquid (60 cc) to flow through a small orifice of standard length and diameter at a specific temperature. This time of flow is measured in seconds, and the viscosity reading expressed as SSU (seconds, Saybolt universal). 2. Chemical Stability Chemical stability is another property which is important in selecting a hydraulic fluid. It is the ability of the liquid to resist oxidation and deterioration for long periods. Mostl liquids tend to undergo unfavorable chemical changes during severe operating conditions. This is the case when a system operates for a considerable period of time at high temperatures. Excessive temperatures have an adverse effect on the life of a liquid. The temperature of the liquid in the reservoir of an operating hydraulic system does not always represent a true state of operating conditions. Localized hot spots occur on bearings, gear teeth, or at the point where liquid under pressure is forced through a small orifice. Continuous passage of a liquid through these points may produce local temperatures high enough to carbonize or sludge the liquid, yet the liquid in the reservoir may not indicate an excessively high temperature. Liquids with a high viscosity have a greater resistance to heat than light HEATING UNIT THERMOMETER LIQUID BATH or low viscosity liquids which have been derived from the same source. Fortunately, there is a wide choice of liquids available for use within the viscosity range required of hydraulic systems. Liquids may break down if exposed to air, water, salt, or other impurities, especially if in constant motion or subject to heat. Some metals, such as zinc, lead, brass, and copper have an undesirable chemical reaction on certain liquids. These chemical processes result in the formation of sludge, gums, carbon or other deposits which clog openings, cause valves and pistons to stick or leak, and give poor lubrication to moving parts. As soon as small amounts of sludge or other deposits are formed, their rate of formation generally increases. As they are formed, certain changes in the physical and chemical properties of the liquid take place. The liquid usually becomes darker in color, higher in viscosity, and acids are formed. Flash Point Flash point is the temperature at which a substance gives off vapor in sufficient quantity to ignite momentarily (flash) when a flame is applied. A high flash point is desirable for hydraulic fluids because it indicates a good resistance to combustion and a low degree of evaporation at normal temperatures. Fire Point Fire point is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame. Like flash point, a high fire point is required of desirable hydraulic fluids. 5. Types of Hydraulic Fluid To assure proper system operation and to avoid damage to nonmetallic components of the hydraulic system, the correct fluid must be used. OIL When adding fluid to a system, use the type specified in the aircraft manufacturer's maintenance manual or on the instruction plate affixed to the reservoir or unit being serviced. There are three types of hydraulic fluids currently being used in civil aircraft. a. Vegetable-base Fluid n \D. . • Ar 4/ CORK CONTAINER 60 c.c. Figure 9-7. Saybolt viscosimeter. RESERVOIR MIL-H-7644 fluid has been used in the past when hydraulic system requirements were not as severe as they are today. This fluid is essentially castor oil and alcohol. Although it is similar to automotive brake fluid it is not interchangeable. This fluid is used primarily in older type aircraft. It is dyed blue for identification. Natural rubber seals are used with vegetable base fluid. If this system is contaminated with petroleum base or phosphate ester base fluids, 124 Aircraft Technical Book Company http://www.ACTechbooks.com (800) 780-4115 (970) 887-2207 the seals will swell, break down and block the system. The system may be flushed with alcohol. This type of fluid is flammable. Mineral-base Fluid MIL-H-5606 is the most widely used hydraulic fluid in general aviation aircraft today. It is basically a kerosene-type petroleum product, having good lubricating properties and additives to inhibit foaming and prevent corrosion. It is quite stable chemically and has very little viscosity change with temperature. MIL-H-5606 fluid is dyed red for identification, and systems using this fluid may be flushed with naphtha, varsol, or Stoddard solvent. Neoprene seals and hoses may be used with MIL-H5606 fluid. This type of fluid is flammable. Synthetic Fluid Non-petroleum base hydraulic fluids were introduced in 1948 to provide a fire-resistant hydraulic fluid for use in high performance piston engine and turbine powered aircraft. The most commonly used fluid of this type is MIL-H-8446 or, Skydrol ® (a registered trade name of the Monsanto Chemical Co.). This fluid is colored light purple, is slightly heavier than water, and has a wide range of operating temperatures from around -65°F to over 225°F for sustained operation. Currently there are two grades of Skydrol in use, Skydrol 500B4, and Skydrol LD. Skydrol LD has a lower density and offers some advantage in jumbo jet transport aircraft where weight is a prime factor. Skydrol is not without its problems however, as it is quite susceptible to contamination by water from the atmosphere and must be kept tightly sealed. When servicing a system using Skydrol, be extremely careful to use only seals and hoses having the proper part number. Skydrol systems may be flushed out with trichlorethylene. Intermixing of Fluids Due to the difference in composition, vegetable base, petroleum base and phosphate ester base fluids will not mix. Neither are the type of seals for any one fluid usable with or tolerant of any of the other fluids. Should an aircraft hydraulic system be serviced with the wrong type of fluid, immediately drain and flush the system and maintain the seals according to the manufacturer's specifications. Compatibility with Aircraft Materials Aircraft hydraulic systems designed for Skydrol fluids should be virtually trouble-free if properly serviced. Skydrol does not appreciably affect common aircraft metals as long as the fluid is kept free of contamination. Due to the phosphate ester base of synthetic hydraulic fluids, thermoplastic resins, including vinyl compositions, nitrocellulose lacquers, oil base paints, linoleum and asphalt may be softened chemically by these fluids. Skydrol will attack polyvinyl chloride, and must not be allowed to drip on to electrical wiring, as it will break down the insulation. However, this chemical reaction usually requires longer than just momentary exposure; and spills that are wiped up with soap and water do not harm most of these materials. Skydrol is compatible with natural fibers and with a number of synthetics, including nylon and polyester, which are used extensively in many aircraft. Petroleum oil hydraulic seals of neoprene or Buna-N are not compatible with Skydrol and must be replaced with seals of butyl rubber or ethylenepropylene elastomers for units that are intended for use in systems utilizing phosphate ester base hydraulic fluid. These seals are readily available from suppliers. 8. Health and Handling Skydrol fluid does not present any particular health hazard in its recommended use. Skydrol has a very low order of toxicity when taken orally or applied to the skin in liquid form. It causes pain on contact with eye tissue, but animal studies and human experience indicate that it causes no permanent damage. First aid treatment for eye contact includes flushing the eyes immediately with large volumes of water and the application of an anesthetic eye solution. If pain persists, the individual should be referred to a physician. In mist or fog form, Skydrol is quite irritating to nasal or respiratory passages and generally produces coughing and sneezing. Such irritation does not persist following cessation of exposure. Silicone ointments, rubber gloves, and careful washing procedures should be utilized to avoid excessive repeated contact with Skydrol in order to avoid solvent effect on skin. C. Basic Hydraulic Systems A hydraulic system is much like an electrical system. It must have a source of power, a means of transmitting this power, and finally some type of device to use the power. 1. Open Hydraulic Systems The most basic form of an open hydraulic system is that used by hydroelectric power plants. Large dams block streams of water to form lakes that store 125 Aircraft Technical Book Company http://www.ACTechbooks.com (800) 780-4115 (970) 887-2207