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.

General Uses

• Used for flight control, actuation of flaps, slats, weapons bays, landing gear, breaks

• Provides the extra force required to move large control surfaces in heavy aerodynamic loads.

A SIMPLE HYDRAULIC SYSTEM

DEVICES USING HYDRAULIC SYSTEMS IN AIRCRAFTS

• Gun turrets• Auto pilot• Shock absorption systems• Dive, landing, speed and flap brakes• Bomb-bay doors • Doors and hatchways• Landing gears, wing flaps, etc…

Hydraulic Mechanism in Airplane

SYSTEM A

SYSTEM B

STA

ND

BY

S

YS

TEM

PRINCIPLES OF OPERATION

• Governed by Pascal’s law• The oil is in contact with both

sides of the piston head but at different pressure. High pressure oil may be pumped into either side of the piston head.

• The selector valve determines to which side of the actuating cylinder the high pressure oil (red colored side) is sent.

RESERVOIR FUNCTIONS

• Holds reserve supply oil to account for normal leakage, emergency supply of oil, volume changes, thermal contraction of oil.

• Provides pressure head on the pump, a place to remove air or foam from liquid, air space for expansion of the oil due to temperature changes, etc…

Hydraulic Accumulator

A hydraulic accumulator is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source.

An accumulator enables a hydraulic system to cope with extremes of demand using a less powerful pump, to respond more quickly to a temporary demand, and to smooth out pulsations. It is a type of energy storage device.

ACCUMULATORS USES

• Absorbs the shocks due to rapid pressure variations in a hydraulic system

• Helps maintain a constant pressure within the hydraulic system

• Helps the hydraulic pump under peak pressure loads

• It is an emergency source of power • Spherical shape is preferred for accumulators

SYSTEM RELIEF VALVE PRINCIPLE

• The adjustment screw is set for a certain pressure value P2.

• When the pressure increases, the poppet will move up, forcing the excess liquid in.

Selector Valves

• Position (1) is the position of the selector valve, for example, upon the extension of the landing gear or the lowering of flaps.

• Position (2) is the position of the selector valve upon retraction of the landing gear or the raising of the flaps.

FLOW EQUALISER

• If wing air loads on one wing are greater than on other, during wing sweep back or sweep forward, motion occurs unevenly.

• To synchronize wing sweep back or sweep forward flow equalizer is used.

• Flow equalizer is made up of two power pumps placed side by side in which the drive gears of both pumps are connected.

ACTUATION CYLINDER

• The function is to take the pressure & hydraulic fluid flow & change them into linear or rotary motion.

• Single piston double rod is an equal displacement cylinder used in aileron system or automatic guidance system.

• Double piston-double rod actuating cylinder used in bomb bay doors.

ADVANTAGES

• It is lighter in weight than alternate existing systems.

• It is dead beat, that is, there is an absence of sloppiness in its response to demands placed on the system.

• It is reliable; either it works or doesn't.• It can be easily maintained.• It is not a shock hazard; it is not much of a

fire hazard.• It can develop practically unlimited force or

torque.

CONCLUSION

Hydraulic systems determine flight worthiness,

usability and reliabity.

Problems with Hydraulics

• Heavy

• High maintenance Adds cost and creates a

logistics problem

• Requires space (pumps, hydraulic lines, etc.)

Possible Improvements

Electric Actuators:

• Consists of a small electric motor, pump and actuator ram requiring about 1 pint of hydraulic fluid

• Flight tested by NASA’s Dryden Flight Research Center on a modified F-18.

• Provides significant weight savings by eliminating pumps and hydraulic lines

• Also could decrease required maintenance