Large Aircraft Hydraulics

A/C Hydraulics Brief History

Prior to WW2 few a/c required hydraulic power

Functions pilot operated

Higher speeds

Reduce drag using smaller wing

Higher take-off and landing speeds requiring flaps and brakes... loads increase

And retractable undercarriage

Hydraulic operation

System pressure up to 1500 psi

Post WW2 large jet aircraft

Hydraulic flight controls

System pressure 3000 - 5000 psi

Mechanical Advantage

Using smaller force to produce larger force

Force multiplier proportional to ratio a:b

Mechanical advantage can also be obtained in a closed circuit hydraulic system

Pascal's Law

Pressure at any one point in a static fluid is the same in every direction, and pressure exerted in an enclosed fluid is transmitted undiminished in every direction and acts with equal force on equal areas.

Pressure, p, uniform throughout

p = F1 / Area A1

F2 = p x Area A2

Pressure, p, uniform throughout the system,

Where;

p = F1 (=10lb)/ Area A1(=10in)

p = 1 psi

F2 = p(1 psi) x Area A2 (100in)

F2 = 100lb

Advantages of Hydraulics

Low weight per unit power

Low volume

Low initial cost

Low maintenance cost

High reliability

Self lubricating

Small pipe diameters

Flexibility of installation

Circuit Design

We will consider three types of system design used in light A/C:

Open centre system

Closed centre system

Power pack system

Open Centre System

Typically simpler, used on small aircraft with hydraulic landing gear and/or flaps only

Constant delivery pump, usually with electric motor, relief valve, selection valves, simple ram actuators, filter and reservoir, possibly a hand-pump.

When no functions operating, fluid flows through each selector valve in series and back to the reservoir

This system prevents overloading of the pump.

Disadvantage is that only one function can operate at a time

Closed Centre System (Pressure System)

More complex, on larger aircraft and used when simultaneous operation of more than one function is required.

Selector valves in parallel

If a constant delivery pump is used:

Unloading valve must be used to limit system pressure and pump load when no functions are operating

Constant Delivery pump is also called Constant Displacement pump

Power Pack Systems

This is a simplification of the closed-centre system

Incorporates reservoir, control valve and auxiliary valves into a single unit (power pack)

Typically uses a (reversible DC) electric motor to drive the hydraulic pump

This type of system is used on the DA 42 landing gear

Heavy Aircraft Circuit Design

Primary flight controls

Elevators, ailerons, rudders

These systems are essential to safe flight

Auxiliary flight controls

Flaps, slats, spoilers, air-brakes

Utility systems

U/C, wheel-brakes, steering, cargo doors, loading ramp, etc.

Heavy Aircraft System Redundancy

The probability of failure of a single hydraulic channel is approximately 10-3 per flight

The probability of failure for systems essential to safe flight is required to be less than 10-9

This then requires 3 hydraulic channels

The probability of failure for secondary systems is required to be less than 10-6

Thus, where only secondary functions are hydraulically powered, 2 channels are sufficient

Large Aircraft Circuit Design

Constant Delivery/Displacement pumps and Unloading Valves/Regulators often replaced with Variable Displacement pumps

Variable displacement pump

Senses system pressure and adjusts its delivery rate to maintain fixed pressure (shown shortly)

System Components

Pumps

Filters

Valves

Actuators

Accumulator

Reservoir

Fluids & Seals

Pumps

Although driven by a crank, a single acting hand pump is not dissimilar

Power driven

Operate at pressure of approx. 3000psi (200 bar) - Now even up to 5000PSI

Usually driven from Engine Gearbox

Must be capable of running over a fairly wide speed range

Electrically driven (AC and DC)

Air driven pumps

RAT (ram air turbine)

Power driven - Constant Pressure, Variable Displacement

Power driven - Electric/Auxilary Pumps

Power driven - Ram Air Turbine

Constant Delivery/Displacement

Gear type

Low flow rate and low to medium pressure

Vane type

Higher flow rates at low pressure

Radial or axial piston

For high pressure systems

Variable displacement

Axial piston

Gear type

And... Gerotor Pumps

Vane pump

Axial Piston Pump - Fixed Angle

Fixed Angle Constant Displacement Piston Pump

-Pistons fitted to angled Swashplate

-Requires a Pressure Regulator or Automatic Cutoff Valve

Piston Pump - Variable Displacement

Tilting the swashplate will vary the stroke of the pistons with each rotation. Not this exam is actually a motor

Filters

Pumps & valves have extremely close tolerances

Fluid must be clean and free from contaminants to minimise wear and clogging

Matter can enter during servicing or due to wear of moving parts

Filter can remove particles in range of 1.5 down to 15 microns

Elements can be disposable paper or re-usable metal. A by-pass indicator can warn if an element is clogged.

Valves

To control pressure

Relief valves (safety)

Pressure regulators

Pressure reducers

Valves may be operated:

Manually

Electrically

Mechanically

Hydraulically

Using a combination of two or more of the above

To control direction and rate of flow

Selector valves

Poppet type valves

Piston type valves

Check valves

Non-return valve (NRV)

Restrictor/Orifice check valve

Metering check valve (restrict the speed of flow)

Sequencing valve

Shuttle valve

Hydraulic fuses

Selector Valve

Piston Type Selector Valve (Pilot/Spool Valve),

with a single action actuator

Solenoid Driven Piston Type Selector Valve (Pilot/Spool Valve)

Shuttle Valves

Used to automatically isolate one system from the other; such as fire extinguisher or Normal/Emergency undercarriage systems.

Hydraulic Fuses

Prevent complete fluid loss should leak occur downstream of the fuse

Other fuses work on the principle of the quantity of flow needed to isolate the line

Actuators

Linear actuators (jacks)

- Single acting

- Double acting balanced/unbalanced

- Tandem

- Servo actuator

Rotary actuators

(Hydraulic motor)

Piston type

Vane type

Rack & pinion type

Linear actuators (jacks)

- Single acting

- Double acting balanced/unbalanced

- Tandem

- Servo actuator

Rotary actuators (Hydraulic motor)

Rack & pinion type

Piston type

Vane type

Accumulators

Smooths out shocks

Store fluid to aid pump in supplying peaks or during emergency operation

Accumulator consists of:

- High strength container

- Movable separator which divides the container into two compartments (see next slides)

One compartment connected to system hydraulic pressure

The other is filled with compressed air or nitrogen

N.B. An Emergency Accumulator will be locked out from the Normal System in flight and can only be used when the emergency system is selected

Reservoirs

Essentially a tank to store adequate supply of fluid for the system

Flows to pumps, is forced through system, returns to reservoir

Serves as overflow and allows fluid to purge itself of bubbles

Non-pressurised

For a/c at low altitudes

Pressurised

For aircraft at higher operating altitudes

Simplest type uses engine bleed air

Note the use of a 'standpipe' in reservoir

Hydraulic Lines

Older A/C used copper tubing

Under vibration tends to harden, crystallize and fracture

Modern systems plumbed using either aluminium alloy, stainless steel or titanium tubing.

Properties/Requirements:

Viscosity (low), bulk modulus, chemical stability, fire resistance

Operate at high pressure (200 bar) and high temp (below zero to around 200oC)

Three types currently in use

- Vegetable base

- Mineral base

- Synthetic

Vegetable base

Used in older type aircraft

Requires natural rubber seals

Castor oil and alcohol

Highly flammable

Coloured blue

Mineral base

- Widely used in general aviation

- Neoprene rubber (synthetic) seals

- Petroleum derivative

- Flammable

- Coloured red

Synthetic

- Common on civil transport a/c

- Phosphate ester base (i.e. non-petroleum)

- Very high flash point

- Will not support combustion

- Most common make is Skydrol® and Aeroshell Fluid 31

- Skydrol 500B4 and LD are currently in use

- Light purple colour

- Ethylene propylene seals (synthetic)

Skydrol - Danger Danger

Fire resistant fluid predominantly used by Boeing

Skydrol - Danger Danger (sorry, lots of words)

Extremely irritating to human tissue. If the fluid gets on the skin it creates an itchy, red rash with a persistent burning sensation. Effects subside within a few hours; egg white can be applied to the affected area to neutralize the burning. May cause urinary bladder damage. If it gets in the eyes, it creates an intense stinging sensation. Recommended treatment for this is to use eye-wash; sometimes mineral oil, castor oil or milk is used.

Incompatible with many plastics, paints and adhesives.

Softened and eventually destroyed by exposure to Skydrol.

Some materials and rubber-soled shoes may also be damaged by Skydrol.