HYDRAULIC SYSTEMS IN AIRCRAFTSBy:Twinkle Patel

INTRODUCTION

• Hydraulics is based on the fact that liquids are incompressible

• Hydraulic system is a system where liquid under pressure is used to transmit energy

• In it a hydraulic pump converts mechanical power to hydraulic power

• An actuating cylinder converts hydraulic power to mechanical power.

Basic of hydraulics-Pascal’s theory

• In a confined stationary liquid, neglecting the effect of gravity, pressure is distributed equally and undiminished in all directions; it acts perpendicular to the surface it touches.

• Because the actuating cylinder is not vented, the force delivered through the piston to the surface of the fluid is translated into a pressure on the surface of the fluid.

• The pressure (p) acting on the incompressible oil does work

• [(pressure)*(area of piston)*(pistons stroke)=work]

• It is a system where liquid under pressure is used to transmit this energy. Hydraulic systems take engine power and convert it to hydraulic power by means of hydraulic pump.

• This power can be distributed throughout the airplane by means of tubing that runs through the aircraft.

• Hydraulic power may be reconverted to mechanical power by means of an actuating cylinder or turbine.

• If an electrical system were used instead of a hydraulic system, a generator would take the place of the pump and a motor would take the place of the actuating cylinder.

Principles of operation • Part of the hydraulic system is the actuating cylinder whose

main function is to change hydraulic(fluid) power to mechanical(shaft) power.

• Inside the actuating cylinder is a piston whose motion is regulated by oil under pressure.

• The oil is in contact with both sides of the piston head but at different pressures.

• High pressure oil may be pumped into either side of the piston head.

• The selector valve decides which side of the actuating cylinder the high pressure oil is sent.

• The piston rod of the actuating cylinder is connected to the control surface. As the piston moves out, the elevator moves down and vice versa.

• The selector valve directs the high pressure oil to the appropriate side of the piston in actuating cylinder.

• As the piston moves the oil on the low pressure side returns to the reservoir since return lines have no pressure.

• The differential in oil pressure causes movement of the piston .The force generated by this pressure difference can be sufficient to move the necessary loads.

• Each cylinder in the plane, boat etc. is designed for what it must do. It can deliver the potential it was made for no more no less.

• Air loads generally determine the force needed in a/c applications.

• Hydraulic systems made their appearance on aircraft in the early 1930s when the retractable undercarriage was introduced.

• the power demand has consequently increased greatly - an efficient means of transferring power from small low energy movements in the cockpit to high energy demands in the aircraft

• move the control surfaces with every increasing speeds and demands for manoeuvrability - hydraulics in the area of safety critical systems in which single failures could not be allowed - using multiple pumps, accumulators to store energy and methods of isolating leaks.

• more-electric systems are being considered to replace hydraulically powered systems - Even with the advent of rare earth magnetic materials, the electric motor cannot yet match the power to weight ratio of a hydraulic actuator

• microprocessors, both for monitoring system performance and to perform control functions - ‘smart’ pumps and valves

• Military aircraft frequently have two independent circuits, large civil transports and passenger aircraft invariably have three or more. In both types additional auxiliary power units and means of transferring power from one system to another are usually provided

GENERAL USES

Provides the extra force required to move large control surfaces in heavy aerodynamic loads.It is used in primary control systems,secondary controll systems and for general utilities like • landing gears• breaking systems• weapons bays • gear steering• aerial refueling probes• loading ramp(military)• passenger stairs etc.

GENERAL SPECIFICATIONS• Several different Fluids• MIL-H-5606,   MIL-H-83282,   and MIL-H-81019• General Temperature Ranges : -65°F to 295°F

• Pressures:• Airbus A380 has 5000psi hydraulic system• Typical commercial airline pressure is 3000 psi

• Systems have become standardised at 3000 psi (200 bar) or 4000 psi (275 bar). These have been chosen to keep weight to a minimum

• Pressure targets have varied from 5000 psi (340 bar) to 8000 psi (550 bar), and all resulting systems studies claim to show reduced system component mass and volume.

• Preclude the common use of well-proven components and test equipment

Problems

• High maintenance • Adds cost and creates a logistics problem

• Requires space (pumps, hydraulic lines, etc.)• Heavy• Leakage of fluid(very rare)• Alien particles in fluid may cause severe damage• Expansion and contraction of fluid with temperature

change(rare)

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