AE624 Aircraft Systems - Hydraulics PDF

AE624(18) AIRCRAFT SYSTEMS TOPIC 04 HYDRAULIC SYSTEMS by: Engr. Venice D. Portes, AE  INTENDED LEARNING OUTCOMES Explain the principles of hydraulic actuation, including how hydraulic power is converted into mechanical work to operate aircraft control surfaces, landing g...

AE624(18) AIRCRAFT SYSTEMS TOPIC 04 HYDRAULIC SYSTEMS by: Engr. Venice D. Portes, AE  INTENDED LEARNING OUTCOMES Explain the principles of hydraulic actuation, including how hydraulic power is converted into mechanical work to operate aircraft control surfaces, landing gear, and other systems. Identify and compare different types of hydraulic fluids used in aviation, such as mineral-based, synthetic, and phosphate ester fluids, and discuss their properties, applications, and safety considerations. Analyze the relationship between fluid pressure, temperature, and flow rate in hydraulic systems, and understand how these factors affect system performance and efficiency.  TOPIC 04 OUTLINE OF SUB-TOPICS Introduction to the System of Units Hydraulics, Pascal’s Law, and Bramah’s Press Hydraulic System Loads, Fluids and Seals Hydraulic System Types and Components  SYSTEM OF UNITS A bit of review from your Physics class and some units you will usually encounter in the Engineering discipline... AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  SYSTEM OF UNITS AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  SYSTEM OF UNITS Image Source: Faires & Simmang, Thermodynamics 6th Edition, Appendix B38, pp. 631-632 AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  What is hydraulics? Hydraulics is the science relating to the behaviour of liquids under various conditions and in aircraft, the hydraulic system provides a means of operating large and remote components that it would not be possible to operate satisfactorily by other means. © Britannica, Hydraulic Piston System “Aircraft systems provide a means of power transmission through the medium of hydraulics i.e. transmission of power through an incompressible fluid via pipelines and actuators.” AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pascal’s Law Blaise Pascal (1623-1662) was a 17th century mathematician who stated that: “If a force is applied to a liquid in a confined space, then this force will be felt equally in all directions”. © Getty Images, Art Portrait of Blaise Pascal AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pascal’s Law The force employed when a hydraulic system is operated is caused by “Pressure”. Hydraulic pressure is created only when an attempt is made to compress fluids, therefore, if a flow of oil is pumped through an open-ended tube there will be no pressure, but, if the end of the tube is blocked and the oil cannot escape, pressure will at once build up. “Without some form of restriction there can be no pressure.” © Oxford Aviation Academy AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pascal’s Law A pump is required to deliver a flow of fluid into the system and some form of restriction is required to obtain pressure. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press This principle was discovered by Joseph Bramah (1749 - 1814) who invented a hydraulic press and, in doing so, observed two facts: the smaller the area under load, the greater the pressure generated. the larger the area under pressure, the greater will be the load available. © Institution of Mechanical Engineers (UK) AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press © Oxford Aviation Academy, The Bramah press AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press The WORK DONE by a machine = FORCE applied x DISTANCE moved Then if piston “A” is moved through a distance of 0.6m, and since work done in the system must be constant,(assuming no frictional losses), then: FORCE x DISTANCE (piston A) = FORCE x DISTANCE (piston B) 1000 x 0.6 = 2000 x the distance moved by piston ‘B’ so the distance moved by piston ‘B’ = 0.3 m (1000 x 0.6 = 600) = (2000 x 0.3 = 600 Joules) AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press Example: A hydraulic system is used to lift a 2000-kg vehicle in an auto garage. If the vehicle sits on a piston of area 0.5 square meter, and a force is applied to a piston of area 0.03 square meters, what is the minimum force that must be applied to lift the vehicle? AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press Solution: We imagine the problem like the illustration shown:: AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press Thus, for a given fluid pressure, the force produced can be varied by adjusting the piston area and the resultant linear motion will vary in inverse proportion to the area. This would constitute a Passive Hydraulic System where a force is applied to a piston (piston A or 1) only when it is desired to move the load (piston B or 2). Thereby only generating pressure when it is required rather than generating and maintaining pressure all of the time and only using it when something needs to be moved. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Bramah’s Press A good example of this would be a light aircraft braking system which has a master cylinder to generate the pressure when the brake pedal is pressed, and a slave cylinder to ‘do the work’ of moving a piston and applying the brakes. © Oxford Aviation Academy, A typical light aircraft braking system (only one wheel shown). AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  System Introduction The average modern aircraft utilizes hydraulic systems to operate several systems. Wing flaps Landing gear Wheel brakes Speed brakes AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  System Introduction Hydraulic systems made their appearance on aircraft in the early 1930s when the retractable undercarriage was introduced. © Dietz, Douglas C-47 Skytrain Hydraulic power was seen as an efficient means of transferring power from small low energy movements in the cockpit to high energy demands in the aircraft. Hydraulic systems now have an important role to play in all modern aircraft, both military and civil. © NRK, C-47 retractable landing gear animation AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  System Introduction The introduction of powered flying controls was an obvious application for hydraulic power by which the pilot was able to move the control surfaces with every increasing speeds and demands for maneuverability. This application brought hydraulics in the area of safety critical systems in which single failures could not be allowed to hazard the aircraft. © NASA, Airplane Yaw AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic System Loads © Wiley, Aircraft Systems AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid The working fluid will be considered as a physical medium for transmitting power, and the conditions under which it is expected to work. The efficiency of a hydraulic system is governed by the resistance to motion encountered by the fluid and, for all practical purposes, hydraulic fluids are considered to be incompressible except at high pressures. 27.6 MN/m² and above (276.7 bar or 4,300 psi). AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid If a container with a certain volume of liquid has a pressure of 34.6 MN/m² (346 bar) applied it can be seen that its reduction in volume is small as against a similar air container. Liquid is compressed by only 1% of its original volume, and 99% remains. Air is compressed by 99% of its original volume and 1% (1/300) remains. It should be noted that the pressure in both fluids will be felt equally in all directions. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid The ideal properties of a hydraulic fluid are: be relatively incompressible, i.e. up to be non-flammable. 27.6 MN/m² (276 Bar), so ensuring instantaneous operation. be resistant to evaporation. have good lubricating properties for metal and rubber. be chemically inert. have good viscosity with a high boiling point (helps prevent vapour locking and have freedom from sludging cavitation) and low freezing point e.g. and foaming. temperature range +80°C to -70°C. have good storage properties. have a flash point above 100°C. be non-corrosive. be reasonably priced & readily available. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Some characteristics that must be considered: Viscosity Chemical Stability Flash Point Fire Point AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Viscosity is the internal resistance to flow. Gasoline flows easily (has a low viscosity) Tar flows slowly (has a high viscosity) Gasoline A satisfactory liquid for a hydraulic system must have enough body to give a good seal at pumps, valves and pistons, but it must not be so thick that it offers excessive resistance to flow. The average hydraulic liquid has a low viscosity. Tar AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  The two most common causes of oil darkening are thermal stress Hydraulic Fluid and oxidation. Chemical Stability is the ability of the liquid to resist oxidation and deterioration for long periods. Excessive temperatures have a great effect on the life of a liquid. Liquids may break down if exposed to air, water, salt, or other impurities. Oxidation is the chemical union of oil and oxygen. This is one of the primary sources for decreasing the stability of hydraulic oil. (Left) Hydraulic oil in clean, new state and (Right) oxidized state. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Flash Point is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily when a flame is applied. High flash point is desirable for hydraulic fluids because it provides good resistance to combustion and a low degree of evaporation at normal temperatures. 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. High fire point is required of desirable hydraulic fluids. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid ic Fluid: aul pes of Hydr Ty ed fl uid b le -b a s Veg e ta lu id - b a s ed f M ine ra l t ic flu id Syn th e AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Types of Hydraulic Fluid: Vegetable-based fluid Most often made of castor oil and alcohol Effective but also known to cause some sludge and corrosion Typically only used in older aircraft and is being phases out of use Dyed blue for identification MIL-H-7644 is an example AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Types of Hydraulic Fluid: Mineral-based fluid MIL-H-5606 is the most widely used hydraulic fluid in general aviation aircraft. Kerosene-type petroleum product. Dyed red for identification AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Types of Hydraulic Fluid: Synthetic-based fluid Non-petroleum base hydraulic fluid for use in high performance piston engine and turbine powered aircraft. Most commonly used fluid of this type is Skydrol. Dyed purple for identification. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Fluid Hydraulic fluids should be handled with care as they have a deleterious effect on skin, paintwork, sealing compounds, rubber materials, perspex etc., and they should never be mixed. It is of major importance that only the specified hydraulic oil or its approved alternative is used in a hydraulic system. The colouring of the fluids assists in their identification and also assists in finding hydraulic leaks but the specification can only be confirmed by: consulting the aircraft manual only using fluid from sealed containers or the appropriate replenishment rig. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Pipelines Friction between pistons and cylinders, piston rods and bearings cannot be completely eliminated, it can only be lessened by good design and workmanship. The friction between the walls of the pipes and the fluid depends upon: velocity of the fluid in the pipes. length, bore and the internal finish of the pipes. number of bends. viscosity of the oil. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Seals Static seals, gaskets and packing are used in many locations, and these effect a seal by being squeezed between two surfaces. Dynamic seals, fitted between sliding surfaces, may be of many different shapes, depending on their use and on the fluid pressures involved. “U” and “V” ring seals are effective in one direction only. “O” rings and square section seals are often used where pressure is applied in either direction. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  O-ring Hydraulic Seals U-ring V-ring (cutaway) Square ring AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Seals Dynamic seals require lubrication to remain effective, and wetting of the bearing surface, or a slight seepage from the seals, is normally acceptable. Where high pressures are used, an “O” ring is normally fitted with a stiff backing ring, which retains the shape of the seal and prevents it from being squeezed between the two moving surfaces. © SKF, O-rings and back up rings AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Seals Seals are made in a variety of materials, depending on the type of fluid with which they are to be used. If a seal of an incorrect material is used in a system, the sealing quality will be seriously degraded, and this may lead to failure of the component. Seals are easily damaged by grit, and a wiper ring is often installed on actuators to prevent any grit that may be deposited on the piston rod from contaminating the seals. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Seals The choice of an aircraft’s hydraulic fluid is influenced by the materials used for glands, seals, rings, seats etc There are two in common use. D.T.D. 585 - a refined mineral based oil (Petroleum). Colour - red. Used with synthetic rubber seals (Neoprene). Note: DTD 585 is an obsolete specification. DEF STAN 91-48 replaces D.T.D 585 as the British specification. Other specifications are H515 NATO, OM15 Joint Service, MIL-H-5606F U.S., all for super clean grades. SKYDROL - a phosphate ester based oil. Colour - Type 500A purple, Type 700 green. Used with synthetic rubber seals (Butyl). Is fire resistant and less prone to cavitation because of its higher boiling point. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Basic Hydraulic Systems Open Hydraulic System About Us Having fluid flow, but no pressure in the system when the actuating mechanisms are idle. The pump circulates the fluid from the reservoir, through the selector valves, and back to the reservoir. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Basic Hydraulic Systems Closed Hydraulic System About Us This system differs from the open-center system in that the selector or directional control valves are arranged in parallel and not in series. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Basic Hydraulic Systems Closed Hydraulic System About Us An advantage of the open-center system over the closed-center system is that the continuous pressurization of the system is eliminated. Since the pressure is built up gradually after the selector valve is moved to an operating position, there is very little shock from pressure surges. This action provides a smoother operation of the actuating mechanisms. The operation of an open-center system is slower than the closed-center system, in which the pressure is available the moment the selector valve is positioned. Since most aircraft applications require instantaneous operation, closed-center systems are the most widely used. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Basic Hydraulic Systems Hydraulic Power Pack System About Us A hydraulic power pack is a small unit that consists of an electric pump, filters, reservoir, valves, and pressure relief valve. The advantage of the power pack is that there is no need for a centralized hydraulic power supply system and long stretches of hydraulic lines, which reduces weight. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Basic components of a simple hydraulic system About Us For a complete hydraulic system, it must contain the ff.: A source of energy – engine, A mechanism for hydraulic oil auxiliary power unit or ram air cooling turbine A means of exercising demand – A reservoir actuators, motors, pumps A filter to maintain clean hydraulic A means of storing energy such as fluid an accumulator A multiple redundant distribution system – pipes, valves, shut-off cocks Pressure and temperature sensors AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Schematic of a Simple Hydraulic System © Wiley, Aircraft Systems AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  The primary source of power on an aircraft is the engine, and the hydraulic pump is connected to the engine gearbox. The pump causes a flow of fluid at a certain pressure, through stainless steel pipes to various actuating devices. A reservoir ensures that sufficient fluid is available under all conditions of demand. In practice most aircraft contain multiple pumps and connections of pipes to ensure that single failures and leaks do not deplete the whole system of power. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pumps Draw oil from the reservoir and deliver a supply of fluid to the system. Pumps may be: hand operated engine driven electric motor driven pneumatically (air turbine motor) (ATM) ram air turbine (HYDRAT or RAT) hydraulically (Hyd. motor driving a hyd. pump) known as a Power Transfer Unit or PTU. In most cases the ATM, RAT or PTU is used to provide an alternate supply as part of the redundancy provision for the safe operation of the aircraft. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pumps Hand Pumps may be the only source of power in a small, light hydraulic system, but in larger aircraft are employed: to allow ground servicing to take place without the need for engine running. so that lines and joints can be pressure tested. so that cargo doors etc., can be © Wiley, Aircraft Systems operated without power. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pumps Single-action hand pump draws fluid into the pump on one stroke and pumps that fluid out on the next stroke. Double-action hand pumps produce fluid flow and pressure on each stroke of the handle. Rotary hand pumps produce continuous output while the handle is in motion. AIRBUS 320 LANDING GEAR EXTENSION HANDLE AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pumps Power-driven pumps Constant displacement pumps moves a specific volume of fluid each time its shaft turns. Gear pump Gerotor pump Variable displacement pumps does not move a constant amount of fluid each revolution, but only the amount the system will accept. Vane pump Piston pump Centrifugal pump AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Pumps © Oxford Aviation Academy, Examples of hydraulic pump technology. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  © Oxford Aviation Academy, Examples of hydraulic pump technology. Pumps AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  © Oxford Aviation Academy, Working principle of a piston pump. Pumps AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Actuation Reservoir Store the fluid necessary for the operation of hydraulic systems. Aircraft that operate at altitudes where there is not enough air pressure to assure a positive feed of fluid to the pump have hydraulic reservoirs pressurized. © Wiley, Aircraft Systems AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  © Wiley, Aircraft Systems Hydraulic Actuation Actuator Cylindrical tube and piston assemblies that utilize hydraulic power to generate DOUBLE-ACTING BALANCED LINEAR linear, rotary, or oscillatory motion. (FLIGHT CONTROLS) SINGLE-ACTING LINEAR (BRAKES) DOUBLE-ACTING UNBALANCED LINEAR (LANDING GEAR) AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Actuation The philosophy on larger civil aircraft is where each control surface is split into two or more independent parts. Each part has its own control actuator, each of which is connected to a different hydraulic system. The majority of actuators remain in a quiescent state, either fully extended or fully closed. Actuators are essentially two state devices. © Wiley, Aircraft Systems AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  The actuator can be commanded to one or other of its states by a mechanical or electrical demand. About Us This demand moves a valve that allows the hydraulic fluid at pressure to enter the actuator and move the ram in either direction. A mechanical system can be commanded by direct rod, lever or cable connection from a pilot control lever to the actuator. An electrical system can be connected by means of a solenoid or motor that is operated by a pilot or by a computer output. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Valves Selector valve Used to direct the flow of hydraulic fluid to or from a component and achieve the desired operation. Sequence valve A non-passing valve used to control a sequence of operations. When a preset pressure is reached, it will allow flow rather than sending it back to the tank. Priority valve Gives priority to the critical hydraulic subsystems over noncritical systems when system pressure is low. Pressure relief valve Used to control or limit the pressure in a system. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Accumulator A hydraulic accumulator is used for one of two purposes: either to add volume to the system at a very fast rate or to absorb shock. All accumulators consist of a high strength container divided by some form of movable partition into sections, or compartments. One compartments connected to the hydraulic pressure manifold, and the other is filled with compressed air or with nitrogen. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Hydraulic Accumulator © Oxford Aviation Academy An accumulator is fitted: to store hydraulic fluid under pressure. to dampen pressure fluctuations. to allow for thermal expansion. to provide an emergency supply of fluid to the system in the event of pump failure. to prolong the period between cut- out and cut-in time of the ACOV and so reduce the wear on the pump. provides the initial fluid when a selection is made and the pump is cut-out. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  To achieve the required levels of safety at least two hydraulic circuits must be incorporated to the system. © Wiley, Aircraft Systems The degree of redundancy necessary is very largely controlled by specifications and mandatory regulations issued by the national and international bodies charged with air safety. A typical dual channel hydraulic system is shown on the left. AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  © Oxford Aviation Academy System Indicators AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  Topic 04 Suggested Readings: Chapter 4 - Hydraulic Systems “Aircraft Systems: Mechanical, Electrical, and Avionics Subsystems Integration” by Ian Moir and Allan Seabridge Chapter 12 - Hydraulic and Pneumatic Power Systems “Aviation Maintenance Technician Handbook - Airframe” by the U.S. Federal Aviation Administration (2023) AE624(18) | Engr. Venice D. Portes, AE | © All Rights Reserved 2024  THANK YOU For concerns, free to contact me via the ff.: [email protected] 6th Floor Building 3, Aero/AMT Department Office