(Hydraulic System).pdf

ESA 272 Aircraft Sub-system Elements Dr. Nurul Musfirah Mazlan (Hydraulic System)  Aircraft’s Maximum Take-Off Weight (MTOW) drives aerodynamic forces that drive control Introduction surface size and loading...

ESA 272 Aircraft Sub-system Elements Dr. Nurul Musfirah Mazlan (Hydraulic System)  Aircraft’s Maximum Take-Off Weight (MTOW) drives aerodynamic forces that drive control Introduction surface size and loading ❖ A380 – 1.25 million lb MTOW – extensive use of hydraulics ❖ Cessna 172 – 2500 lb MTOW – limited hydraulic applications Hydraulic Architecture Why Hydraulic? Effective and efficient method of power amplification Precise control of load rate, position and magnitude Ability to handle multiple loads simultaneously Smooth, vibration free power output Hydraulic fluid transmission medium Typical users of Hydraulic Landing gear Extension, retraction, locking, steering, braking Primary flight controls Rudder, elevator, aileron, active (multi-function) spoiler Secondary flight controls high lift (flap / slat), horizontal stabilizer, spoiler, thrust reverser Utility systems Cargo handling, doors, ramps, emergency electrical power generation Concept Based on Pascal’s Law : Pressure exerted on a fluid in an enclosed container is transmitted equally and undiminished to all parts of the container and acts as right angles to the enclosing walls Function : To control moveable parts by transforming hydraulic energy (pressure and volume) into mechanical energy (force and stroke) Basic Hydraulic System and Operation The pump converts the available (mechanical) power 3 1 from the prime mover (electric or diesel motor) to hydraulic power at the actuator. Valves are used to control the direction of pump flow, 2 the level of power produced, and the amount of fluid and pressure supplied to the actuator. A linear actuator (cylinder) or a rotary actuator (motor) 2 3 converts the hydraulic power to usable mechanical power output at the required point. (ENGINE) The hydraulic fluid provides direct transmission and control as well as lubrication of components, sealing in valves, and cooling of the system. Connectors, which link the various system components, direct the power of the fluid under pressure and fluid- 1 flow return to the tank (reservoir). (ensures sufficient fluid is available under all demand)  Hydraulic liquid Hydraulic fluid specification between military and civil aircraft is different. Military uses DTD 585 and MIL-H-5606 Commercial uses Solutia Skydrol LD-4, Skydrol 500B-4, or Skydrol 5 Hydraulic pressure Hydraulic pressure depends on the operation (from 1500 psi to 4000 psi) Basic 3 pressures: Rated pressure; overpressure (125% over rated pressure); max full-flow pressure (no less than 95% of the rated pressure) Hydraulic Hydraulic fluid temperature Depends on operation Parameters Between 130°C to 200°C Viscosity problem as it decreases as temperature increases causing potential damages to actuator and motor Fluid flow rate Determined by aerodynamic load and load movement Pressure drop from pump to reservoir under the full-flow condition should be considered, which is usually approximately 20 - 25% of the rated pressure. Different fluid flow rate for different flight profiles  Main components of hydraulic Pump Most important part in hydraulic system To supply the pressurized hydraulic fluid for the flight control system and utility systems Engine driven pump (main) and AC moto-driven pump (auxiliary and back up) The pistons receive the low-pressure fluid from the “inlet port” and discharge it at high pressure through the “outlet port.” By changing the swash plate angle, the piston stroke can vary and the flow rate of the pump will change. EDP and ACMP have similar principle operation except for ACMP, the pump in driven by AC electrical motor. Reservoir Supply fluid for operating needs and replenishes fluid lost through leakage Serve as an overflow basin for excess fluid forced out of the system Baffles/fins are incorporated in reservoirs to keep the fluid from any random movement such as vortexing or surging Vented reservoir: Atmospheric pressure and gravity are the forces that cause fluid to flow out of the reservoir and into the pump Must be positioned at a higher location than the pump Pressurized reservoir: Used in aircraft that flies at high altitude Pressurized by bleed air from compressor or hydraulic pressure from the hydraulic system itself Incorporated with heat exchanger to avoid issue related to fluid viscous  1. Filter the hydraulic fluid before it leaves the reservoir Filter 2. Located in the suction line, pressure circuit, return line, and branch circuit. Secured to the hydraulic pipes Has pressure-operated bypass valve that route the hydraulic fluid from the inlet to outlet port in case of filter blockage 5-mm noncleanable, woven mesh, porous metal, or magnetic type element stops the particles and allows the clean fluid through Common filter made from composed of organic and inorganic fibers hold the element to the filter head  Pressure Relief Valve Used to limit the amount of pressure exerted on a confined liquid Necessary to prevent failure of components or rupture of hydraulic lines under excessive pressure Spring-loaded valves are installed in relief valve to discharge fluid from the pressure line into a reservoir return line when the pressure exceeds the predetermined maximum for which the valves is adjusted Pressure relief valves are adjusted by increasing or decreasing the spring’s tension Check Valves Have two ports – inlet port (allows fluid to enter), outlet port (fluid to leave) Check valve is designed for controlling of fluid flow in one direction Check valves are very small, simple, and/or inexpensive  Energy storage device in which one end is closed and another Accumulator is connected to the hydraulic pipes Divided into 3 parts: compressed gas (air chamber), piston, and hydraulic fluid (oil chamber). Main functions: Provide transient flow in a short time such as when controlling landing gear and flap. Maintain constant pressure for any actuation unit that does not move for a long time but needs constant pressure. Work as an emergency power supply particularly when the hydraulic power supply system suddenly stops feeding the hydraulic fluid. Absorb the impact pressure when the control valve suddenly changes the direction or the actuation system stops suddenly  3 Types accumulator Diaphragm Bladder Cylinder Cylinder Consists of cylinder and piston assembly The internal piston separates the fluid and air/nitrogen chambers Has two end caps. Attach the fluid chamber to hydraulic system. Diaphragm Bladder A filler valve to perform the same function as the filler valve installed Consists of two hollow, hemispherical Operates on the same principle as in the spherical accumulator metal sections bolted together at the diaphragm but varies in center construction Synthetic rubber diaphragm divides the Has one-piece metal sphere with a accumulator into two sections fluid pressure inlet at the top and Approximately 50% of air to initiate and an opening at the bottom for forces the diaphragm upward inserting the bladder Screw plug as a retainer for bladder and seals the unit