Chapter IX Hydraulic and Pneumatic Power Systems

The word is based on the Greek word for to the container shown in figure 9-1 will have the , and originally meant the study of physical same reading. behavior of water at rest and in motion. Today, the 2. The Hydrostatic Paradox meaning has been expanded to include the physical behavior of all liquids including hydraulic fluids. The pressure produced by a column of liquid is directly proportional to its density and the height of the A. Hydraulic Systems column, and in no way depends upon the shape of the container or the amount of liquid the container holds. Hydraulic systems are not new to aviation. Some For example, 1 cu. in. of water weighs 0.036 lb. A tube early aircraft used hydraulic systems. As that is 231" tall with a cross section of 1 sq. in. will aircraft became more sophisticated, newer systems hold 1 gal. of water (1 gal. = 231 cu. in.). If the tube is with greater complexity were developed. 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 com- monly operated by hydraulic systems are , wing flaps, speed and , and . 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 mini- mum maintenance requirements. Hydraulic opera- tions are almost 100% efficient, with only a negligible loss due to fluid friction. Figure 9-1. Pressure exerted on a fluid in an enclosed Aircraft hydraulic systems belong to that branch container is transmitted equally and of physics concerned with /mechanics. undiminished to all parts of the container They do their work by moving fluid, and the fluid and acts at right angles to the enclosing walls. 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. 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 enclos- ing walls of the container. This means simply that if we have an enclosed vessel full of liquid, and we apply a force to a in the vessel to raise the Figure 9-2. The pressure exerted by a column of liquid pressure, this increase in pressure will be the same is determined by the height of the column anywhere in the system. Each of the gauges attached 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 This may be visualized by looking at figure 9-4. pressure of 8.32 PSI at the bottom of the tube. The bottom half represents the product of the area If the tube were 231" high and had an area of 100 in square inches and the pressure in PSI. This gives sq. in., it would hold 100 gal. of water, but the us the amount of force in pounds, which is repre- pressure at the bottom would still be 8.32 PSI. The sented by the top half of the circle. force exerted by the column of water is, of course, In order to find pressure, divide the force by the equal to the pressure acting on each square inch area: times the number of square inches, or 832 lbs. Pressure =Force It makes no difference as to the shape or size of Area the vessel that contains the liquid; it is the height of the column that is the critical factor. In figure 9-3, In order to find the area, divide the force by the the pressure (P) read by the gauges will be the same pressure: in all four instances, since the height (H) is the same. Area = Force Naturally, all of the vessels must be filled with the Pressure same liquid. 4. Relationship Between Area, Distance, 3. Relationship Between Pressure, Force, and Volume and Area Another relationship in hydraulics is between the Pressure is a measure of the amount of force that area of the piston, the distance it moves, and the acts on a unit of area. In most American hydraulic volume of the fluid displaced. We can visualize this systems, pressure is measured in pounds per square relationship in figure 9-5. One half of the circle inch (PSI). represents the volume in cubic inches, and the other The relationship between force, pressure, and half of the circle the area in square inches and the area may be expressed by the formula: distance the piston moves in inches. Distance is also Force = Pressure x Area known as . The relationship between volume, area, and dis- tance may be expressed by the formula: Volume = Area x Distance

To find the area divide the volume by the distance: Volume Area = Distance

To find the distance divide the volume by the area: Volume Distance = Area

Figure 9-3. Neither the shape nor the volume of a container affects the pressure.

PRESSURE FORCE / AREA FORCE AREA x PRESSURE AREA FORCE / PRESSURE (A) (B) (C)

Figure 9-4. Relationship between area, pressure, and force.

122

Aircraft Technical Book Company http://www.ACTechbooks.com (800) 780-4115 (970) 887-2207 V VOLUME

A AREA DISTANCE

VOLUME = AREA x DISTANCE AREA = VOLUME / DISTANCE DISTANCE = VOLUME / AREA (B) (C)

Figure 9-5. Relationship between volume, area, and distance.

5. Mechanical Advantage in a Hydraulic moves down 1 in. spreads out under all 20 sq. in. of System the large piston, and will move up only 1/2o". A hydraulic system has two major advantages over This may be expressed as: other types of mechanical systems. One is the ease with which force can be transmitted over large dis- A (small) x D (small) = A (large) x D (large) tances and into and out of sealed compartments. The 1 x 1 = 20 x 1/2o other is the mechanical advantage made possible by varying the size of . 1 = 1 In figure 9-6, we see the way mechanical ad- All hydraulic systems are essentially the same, vantage is achieved in a hydraulic system. If we have whatever their function. Regardless of application, a piston whose area is 1 sq. in. pressing down with each hydraulic system has a minimum number of a force of 1 lb., it will produce a pressure of 1 PSI, components, and some type of hydraulic fluid. and for every inch it moves, will displace 1 cu. in. of fluid. B. Hydraulic Fluid If the containing this piston is connected While we may not normally think of fluid as being a to one having a piston with an area of 20 sq. in., component, the fluid used in aircraft hydraulic sys- every square inch will be acted on by the same 1 PSI tems is most important. This fluid must flow with a pressure, and a force of 20 lbs. will be produced. The minimum of opposition, and be incompressible. It 1 cu. in. of fluid displaced when the small piston must have good lubricating properties to prevent wear in the and valves. It must inhibit cor- rosion and not chemically attack seals used in the F 1# system. And it must not foam in operation, because n W = 20# air carried into the components will give them a llllllll spongy action. Manufacturers of hydraulic devices specify the D 1 INCH type of fluid best suited for use with their equipment. D = 1/20 INCH -1 Working conditions, service, temperatures, pres- sures, possibilities of corrosion, and other condi- AREA tions must be considered. Some of the . 1 SO. INCH characteristics that must be considered when select- ing a satisfactory fluid for a particular system are AREA 20 SO. INCH discussed in the following paragraphs. 1. One of the most important properties of any Figure 9-6. The product of the force times the area of hydraulic fluid is its viscosity. Viscosity is a the large piston is equal to the product of measure of internal resistance to flow. A liquid the weight times the area of the small such as gasoline flows easily (has a low viscosity) piston. while a liquid such as tar flows slowly (has a high

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viscosity). Viscosity increases as temperature or low viscosity liquids which have been derived from decreases. the same source. Fortunately, there is a wide choice The viscosity of a liquid is measured with a vis- of liquids available for use within the viscosity range cosimeter. There are several types, but the instrument required of hydraulic systems. most often used is the Saybolt universal viscosimeter Liquids may break down if exposed to air, water, (figure 9-7). This instrument measures the number of salt, or other impurities, especially if in constant seconds it takes for a fixed quantity of liquid (60 cc) to motion or subject to heat. Some metals, such as zinc, flow through a small orifice of standard length and lead, brass, and copper have an undesirable chemi- diameter at a specific temperature. This time of flow is cal reaction on certain liquids. measured in seconds, and the viscosity reading ex- These chemical processes result in the formation pressed as SSU (seconds, Saybolt universal). of sludge, gums, carbon or other deposits which clog 2. Chemical Stability openings, cause valves and pistons to stick or leak, and give poor to moving parts. As soon Chemical stability is another property which is impor- as small amounts of sludge or other deposits are tant in selecting a hydraulic fluid. It is the ability of the formed, their rate of formation generally increases. liquid to resist oxidation and deterioration for long As they are formed, certain changes in the physical periods. Mostl liquids tend to undergo unfavorable and chemical properties of the liquid take place. The chemical changes during severe operating conditions. liquid usually becomes darker in color, higher in This is the case when a system operates for a consid- viscosity, and acids are formed. erable period of time at high temperatures. Excessive temperatures have an adverse effect on Flash Point the life of a liquid. The temperature of the liquid in Flash point is the temperature at which a substance the reservoir of an operating hydraulic system does gives off vapor in sufficient quantity to ignite not always represent a true state of operating con- momentarily (flash) when a flame is applied. A high ditions. Localized hot spots occur on bearings, gear flash point is desirable for hydraulic fluids because teeth, or at the point where liquid under pressure is it indicates a good resistance to combustion and a forced through a small orifice. Continuous passage low degree of evaporation at normal temperatures. of a liquid through these points may produce local Fire Point temperatures high enough to carbonize or sludge the liquid, yet the liquid in the reservoir may not indicate Fire point is the temperature at which a substance an excessively high temperature. Liquids with a high gives off vapor in sufficient quantity to ignite and viscosity have a greater resistance to heat than light continue to burn when exposed to a spark or flame. Like flash point, a high fire point is required of desirable hydraulic fluids. HEATING UNIT LIQUID THERMOMETER 5. Types of Hydraulic Fluid BATH To assure proper system operation and to avoid damage to nonmetallic components of the hydraulic system, the correct fluid must be used. 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 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 n \D. . • 4/ Ar and alcohol. Although it is similar to automotive it is not interchangeable. This fluid is CONTAINER CORK used primarily in older type aircraft. It is dyed blue 60 RESERVOIR c.c. for identification. Natural rubber seals are used with vegetable base fluid. If this system is contaminated Figure 9-7. Saybolt viscosimeter. 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 sys- Due to the phosphate ester base of synthetic tem. The system may be flushed with alcohol. This hydraulic fluids, thermoplastic resins, including type of fluid is flammable. vinyl compositions, nitrocellulose lacquers, oil base Mineral-base Fluid paints, linoleum and asphalt may be softened chemically by these fluids. Skydrol will attack MIL-H-5606 is the most widely used hydraulic fluid polyvinyl chloride, and must not be allowed to drip in general aviation aircraft today. It is basically a on to electrical wiring, as it will break down the kerosene-type petroleum product, having good insulation. However, this chemical reaction usually lubricating properties and additives to inhibit foam- requires longer than just momentary exposure; and ing and prevent corrosion. It is quite stable chemi- spills that are wiped up with soap and water do not cally and has very little viscosity change with harm most of these materials. temperature. MIL-H-5606 fluid is dyed red for iden- tification, and systems using this fluid may be Skydrol is compatible with natural fibers and with flushed with naphtha, varsol, or Stoddard solvent. a number of synthetics, including nylon and Neoprene seals and hoses may be used with MIL-H- polyester, which are used extensively in many 5606 fluid. This type of fluid is flammable. aircraft. Synthetic Fluid Petroleum oil hydraulic seals of neoprene or Buna-N are not compatible with Skydrol and must Non-petroleum base hydraulic fluids were intro- be replaced with seals of butyl rubber or ethylene- duced in 1948 to provide a fire-resistant hydraulic propylene elastomers for units that are intended for fluid for use in high performance piston engine and use in systems utilizing phosphate ester base turbine powered aircraft. hydraulic fluid. These seals are readily available The most commonly used fluid of this type is from suppliers. MIL-H-8446 or, Skydrol ® (a registered trade name of the Monsanto Chemical Co.). This fluid is colored 8. Health and Handling light purple, is slightly heavier than water, and has Skydrol fluid does not present any particular health a wide range of operating temperatures from around hazard in its recommended use. Skydrol has a very -65°F to over 225°F for sustained operation. Cur- low order of toxicity when taken orally or applied to rently there are two grades of Skydrol in use, Skydrol the skin in liquid form. It causes pain on contact 500B4, and Skydrol LD. Skydrol LD has a lower with eye tissue, but animal studies and human density and offers some advantage in jumbo jet experience indicate that it causes no permanent transport aircraft where weight is a prime factor. damage. First aid treatment for eye contact includes Skydrol is not without its problems however, as it flushing the eyes immediately with large volumes of is quite susceptible to contamination by water from water and the application of an anesthetic eye solu- the atmosphere and must be kept tightly sealed. tion. If pain persists, the individual should be When servicing a system using Skydrol, be extreme- referred to a physician. ly careful to use only seals and hoses having the In mist or fog form, Skydrol is quite irritating to proper part number. Skydrol systems may be nasal or respiratory passages and generally flushed out with trichlorethylene. produces coughing and sneezing. Such irritation Intermixing of Fluids does not persist following cessation of exposure. Silicone ointments, rubber gloves, and careful Due to the difference in composition, vegetable base, washing procedures should be utilized to avoid ex- petroleum base and phosphate ester base fluids will cessive repeated contact with Skydrol in order to not mix. Neither are the type of seals for any one fluid avoid solvent effect on skin. 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 C. Basic Hydraulic Systems the system and maintain the seals according to the A hydraulic system is much like an electrical system. manufacturer's specifications. It must have a source of power, a means of transmit- ting this power, and finally some type of device to Compatibility with Aircraft Materials use the power. Aircraft hydraulic systems designed for Skydrol fluids should be virtually trouble-free if properly 1. Open Hydraulic Systems serviced. Skydrol does not appreciably affect com- The most basic form of an open hydraulic system is mon aircraft metals as long as the fluid is kept free that used by hydroelectric power plants. Large dams of contamination. block streams of water to form lakes that store

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