AIRCRAFT AIRCRAFT HYDRA HYDRAULIC ULIC SYSTEM SYSTEM The word hydraulics is based on the Greek word for water, and originally meant the study of physical behavior of water at rest and in motion. Today the meaning has been expanded to include the physical behavior of all liquids, l iquids, including in cluding hydraulic fluids. Hydraulic operations are are almost 100% efficien efficient,t, with only a negligible loss due to fluid friction. Aircraft Aircraft hydraulic hydraulic systems systems belong belong to that that branch branch of physics known as a s fluid fl uid power po wer systems. syst ems. Work is is done by moving an incompressible fluid. Pneumatic Advantages: Lighter weight Ease of installation Simplification of inspection Minimun maintenance requirements Dead beat, that is, there is an absence of sloppiness in its response to demands placed placed on the system. Fire Hazard, not a Shock Hazard Develops practically unlimited force or torque.
HYDRAULIC SYSTEM COMPONENTS AND DESIGN Hydraulic FLuid This fluid must be able to flow through all of the lines with a minimum of opposition, and it must be incompressible. It must have good lubricating properties to prevent wear in the pump and valves. It must inhibit corrosion cor rosion and not chemically chemica lly attack any of the seals used in the system. Above all, it must not foam in operation, operation, becau because se air carrie carriedd into the components compon ents will give the t he system sys tem a spongy action action.. The major characteristics that must be considered when selecting a satisfactory liquid for a particular system are viscosity, chemical stability, flash point, point, and fire point.
Viscosity is internal resistance to flow. Viscosity increases as temperature decreases. The viscosity of a liquid is measured with a vis-cosimeter or viscometer Chemical Stability Stabili ty is the ability of the liquid to resist oxidation oxidation and deterioration for long periods. Flash Point is the temperatur t emperaturee at which a liquid liqui d gives off vapor in sufficient sufficient quantity quantity to ignite ignite momentarily (flash) when a flame is applied. 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.
Type Vegetable base (MIL-H-7644) Mineral base (MIL-H-5606) Synthetic Fluid (Sky-drol)
Mixture Castor oil alcohol
&
Dye color Blue
Kerosene type Red petroleum Non-petroleum Light base/phosphate- Purple ester base
Compatible seals Natural rubber
Cleanser Alcohol
Flammable Yes
Neoprene seals
Naptha, Varsol, Stoddard Solvent trichlorethylene
Yes
Natural & Synthetic Fiber
no
Typical Users of Hydraulic power: Landing gear Extension, retraction, locking, steering, braking o Primary flight controls Rudder, elevator, aileron, active (multi-function) spoiler o Secondary flight controls o high lift (flap / slat), horizontal stabilizer, spoiler, thrust reverser Utility systems Cargo handling, doors, ramps, emergency electrical power generation o
Sources of Hydraulic Power: Mechanical Engine Driven Pump (EDP) - primary hydraulic power source, mounted directly to engines o on special gearbox pads o Power Transfer Unit – mechanically transfers hydraulic power between systems Electrical Pump attached to electric motors, either AC or DC o o Generally used as backup or as auxiliary power Electric driven powerpack used for powering actuation zones o Used for ground check-out or actuating doors when o engines are not running Pneumatic Bleed Air turbine driven pump used for backup power o Ram Air Turbine driven pump deployed when all engines o are inoperative and uses ram air to drive the pump o Accumulator provides high transient power by releasing stored energy, also used for emergency and parking brake
HYDRAULIC SYSTEM COMPONENTS Reservoir
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
Pumps
provides the movement of the fluid, transmitting hydraulic power
Powered Pumps Changes mechanical horsepower to hydraulic horsepower
TYPES
Gear pumps: move fluid based upon the number of gear teeth and the volume spacing between gear teeth. Piston pumps: move fluid by pushing it through the motion of the pistons within the pump
The flow of fluid in a line is equivalent to the flow of electrons in a wire, the current. The pressure that causes the flow is the same as the voltage, and the opposition to the flow of fluid is the same as the resistance. Valves Types:
Flow control Selector valves o Check valves o Sequence valves o o Priority valves Quick-disconnect o Relief valves o Pressure control o Pressure regulator Pressure reducer o
Accumulators 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
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
Fly-by-Wire (FBW) Systems Conventional Mechanical Pilot input mechanically connected to flight control hydraulic servo-actuator by cables, linkages, bellcranks, etc. Servo-actuator follows pilot command with high force output Autopilot input mechanically summed Manual reversion in case of loss of hydraulics or autopilot malfunction
Fly-by-Wire Pilot input read by computers Computer provides input to electrohydraulic flight control actuator Control laws include Enhanced logic to automate many functions o o Artificial damping and stability Flight Envelope Protection to prevent airframe from exceeding structural limits o Multiple computers and actuators provide sufficient redundancy – no manual reversion
