hydraulic systems & undercarriage chapter 10. aim to review principles of operation of the...

Hydraulic Systems & Undercarriage

Chapter 10

Aim

To review principles of operation of the Hydraulic Systems & Undercarriage

Objectives1. Define Hydraulics and discuss the principles of

operation of a typical Hydraulic Systems2. State principals of operation of retractable

Undercarriage systems

Hydraulics is the transmission of force by fluid pressureUtilisation benefits:• Accessibility• Simplicity• Power versus weight

1. Hydraulic Systems

All liquids have a high resistance to compression. For the purposes of hydraulics, hydraulic fluid is considered incompressible. When under pressure, hydraulic fluid exerts equal force in all directions. Hydraulic systems apply this concept through a system of cylinders, hydraulic fluid and pipes to amplify force.

Hydraulic Principle


Desirable characteristics:• Low viscosity• Non-corrosiveness• Inflammability• Non-toxicity• Low freezing point

• Low volatility• Stability• Colouring• Incompressibility

Hydraulic Liquid

Common types:• Mineral (petroleum based) • Synthetic fluid

Do not mix hydraulic fluids as this can cause coagulation.Restrictors are small diameter sections within a hydraulic system that are used to control the amount of hydraulic fluid that enters at a specific pressure.


The hydraulic system provides the power required to activate aircraft services that require force to move.Each hydraulic system has a combination of the following common componentry:• Reservoir• Pumps and pressure regulator (cut-in, cut-out valve)• Accumulator• Check valves• Relief Valves• Selectors• Actuators

Hydraulic Components


Reservoir• Stores the hydraulic liquid.• Highest point in the system. • Features include a screened filter, a vent and

baffles. • Pressurised in high flying aircraft to prevent

cavitation. • Any fluid beneath the stand pipe can only be used

for the emergency hand pump.



Pumps• Supplies fluid pressure to the hydraulic system• Can be driven electrically, by the engine, air driven

or by handGear-type pump• Lower pressure systems up to 1500psi.• Two gears in a casing, one turned by the engine

and the other is a drive gear.• At constant RPM, constant volume.Piston-type pump• Relatively inefficient• Commonly used as a hand pump for emergencies• At constant RPM, constant volume.



Pressure Regulator• Used with constant delivery systems allowing fluid

bypass. • Maintains constant system pressure.• Relieve the pump when no load.Variable-Displacement Pump


• Varied volume of fluid dependent on angle of swash plate.

• Constant system pressure, no regulator.

• No pressure relief valve.• Outlet pressure adjusts

swash plate angle.


Accumulator – provides short-term pressure storage.• Maintains line pressure for short time following

failure of the hydraulic pump or a small leak.• Assists to maintain line pressure during high

utilisation.• Dampens pulsations.• Allows for thermal expansion. Check Valves – allows hydraulic fluid to flow in one direction only.



Relief Valves • PRVs are safety devices that limit maximum pressure to prevent

damage. • Located between pump and system components. • Thermal relief valves (TRV) relieves pressure from the effects of

thermal expansion. Selectors – selector valves or directional control valves control the distribution of hydraulic fluid to systems. Valves can be simple on/off, four-way for reserve flow or more complex arrangements.



Actuator (Hydraulic Jacks) - converts the force of the fluid pressure into movement.


Filtering & Cooling – filters are often included in the reservoir and the system return line. Cooling takes place in the reservoir and occasionally a cooler is installed in the return line.


Hydraulic Operation – Light Aircraft


Hydraulic Operation - Heavy aircraft


Requirements• Absorb the compression loads of landing.• Support the weight of the aircraft while parked.• Allow the aircraft to taxi and manoeuvre while on the ground.Arrangements• Conventional (taildragger) / Tricycle.• Fixed / Retractable.Components include wheels, shock absorbers (spring steel or gas-strut) and less commonly skis, floats and skids, dolly’s/trolley’s.

Undercarriage System Overview

1. Undercarriage Systems

Nosewheels are susceptible to a vibration termed shimmy. This is a gyroscopic reaction and occurs most commonly immediately post take-off. The shimmy damper is a shock absorber that is used to prevent this oscillation.

Undercarriage ComponentsNosewheelNosewheels are of two types: • The fully castering nosewheel• The oleo-pneumatic strut nosewheelFully castering nosewheelSteering is achieved via differential braking. Oleo-pneumatic strut nosewheelUses a telescopic stainless steel cylinder as a shock absorber. A torque link provides steerability.


MainwheelMainwheels are of two types: • The leaf spring• Oleo-pneumatic strut configuration

Undercarriage Components


BrakesBrakes are installed on the mainwheels onlyThey are hydraulically operated and typically of the self-adjusting disc type. Operated by the pilot toe pressure, fluid from a master cylinder is supplied under pressure to slave cylinders where brake callipers apply pressure to brakes pads. The brakes are independent allowing differential braking.



Common faults with the braking system include:• Air in the hydraulic lines producing springy or spongy feel and less effective

breaking.• Leaking fluid so the brake toe pedal moves without the pilot being able to

apply maximum pressure. • Water or dirt between brake pad and disc causing reduced effectiveness and

noisy brake operation. The following factors affect braking feel and effectiveness:• Tyre pressure• Tyre condition• Aircraft weight• Runway surface condition• Aircraft speed• Aircraft brake conditionLocking a wheel will result in skidding, loss of directional control and contribute towards flat spots, tyre deflation or blow out.



Parking Brake - holds whatever pressure is in the brake lines. To operate, apply the toe brakes and then set the park brake. Tyres – function to:• Support the aircraft on the ground.• Absorb part of the landing shock.• Provide traction during braking and steering.• Provide a wearing surface that is easily replaceable. Tyres typically are electrical conductors grounding the aircraft. Marked to indicate size, speed rating, ply rating, tubed or tubeless, creep and wear.Tyre creep is the tendency for the tyre to slip on the rim of the wheel.



Retractable undercarriages reduce form drag but add weight Operated most commonly hydraulically but can be electric and/or pneumaticRequired Components:• Up and down locks.• Position indicators.• Emergency extension method.Emergency operation is usually via a hand pump or an emergency extension valve (legs fall and lock via momentum/air pressure).

Retractable Undercarriage


Position IndicatorsTypically three green lights, one red, and the selector are used to control and indicate the undercarriage position. Micro switches sense the position of each strut. Some aircraft have mirrors or mechanical indicators to confirm in the case of electrical failure. Aural WarningA warning horn which activates under specific flight conditions of low airspeed, flap extension and the gear retracted to warn the pilot prior to a possible gear up landing. Undercarriage OperationThe selector is a lever with a small wheel for easy identification. It is operated by pulling out and then up/down.

Retractable Undercarriage Components


Undercarriage DisableTwo common types of systems are:• squat switch: senses weight on the wheels.• air switch: senses dynamic pressure (IAS). On the ground, the selector must be in the down position to prevent inadvertent undercarriage retraction. VLO – max. IAS when raising/lowering gear.VLE – max. IAS with gear extended.

Retractable Undercarriage Components


When the runway is wet, hydroplaning can occur, causing the tyre to plane on the water. As friction is reduced, braking effectiveness suffers and there is a threat of loss of directional control. Tyre pressure determines the speed at which hydroplaning may occur:9tyre pressure (psi) for a rotating wheel.7.7tyre pressure (psi) for a locked wheel.

Hydroplaning (Aquaplaning)


Questions?

hydraulic systems & undercarriage chapter 10. aim to review principles of operation of the...

Documents

hydraulic pump

typical hydraulic systems

hydraulic fluids

hydraulic principle

actuator hydraulic jacks

constant system pressure

line pressure

system components