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POLITECNICO DI MILANO - DIPARTIMENTO DI INGEGNERIA AEROSPAZIALE AIRCRAFT SYSTEMS LECTURE NOTES, VERSION 2004 Chapter 3 Hydraulic System

These lecture notes are available for the students of the Polytechnic of Milan for free download. No commercialisation allowed. Queste dispense possono essere gratuitamente scaricate da Internet dagli studenti del Politecnico di Milano. E vietata la commercializzazione.

3.1

Chapter 3

Hydraulic System



3.2

3.1 Introduction The hydraulic system on aircraft is aimed to control movable parts by means of transformation of hydraulic energy (pressure and volume displacement) into mechanical energy (force and stroke). Like the pneumatic and electric system, the hydraulic system is made of 4 stages: generation, control, transfer and use. Generation is obtained by pumps that pressurise a fluid; valves and other devices acting on the pumps control the energy delivered, or pressure; pipelines distribute energy to all necessary areas of the vehicle; actuators operate the final conversion of energy for the mechanical use. The main advantages of using a hydraulic system are: low weight per unit power; high efficiency in power transmission; high flexibility in installation; low damage for overloads; high reliability; low maintenance; low inertia or high frequency response; good control ability. On the opposite the drawbacks are: risk to lose the complete system for failure of one component; mostly used hydraulic fluids are not fire-resistant. The net prevalence of advantages makes the hydraulic system a very common choice for the control of most movable parts in current aircraft designs.

3.2 General layout Before describing the various hydraulic components necessary for the generation, control, distribution and uses, a general description of the system is useful. Generation is obtained by pumps. They can be operated by mechanical connection to the engines or auxiliary power unit (through gearbox, to reduce the angular velocity), by electric motors, by compressed air turbines or, in case of emergency, by external turbines or by hand from the crew. Valves or feedback to the pump mechanics stabilise the pressure. Pipelines, junctions and valves transfer the fluid to the actuators. These can be used for different movable parts, as schematically indicated in fig. 3.1. The diagram gives an idea of the importance of pipelines layout and installation: the pumps are normally located in the engine area, and the fluid must be supplied to areas that may be tens of meters off. In the previous chapter the head loss for fluid in pipes is defined as proportional, among the other parameters, to the pipe length. Then a study of the pipelines layout can bring to a limitation of weight and an increase of the hydraulic efficiency. Auxiliary components are those allowing storage of the fluid, dump of pressure peaks, emergency parts, seals, filters and heat exchangers. System components will be described more in detail in next sections.



3.3

Normally multiple independent hydraulic subsystems are present on board, operated by the different engines, sometimes by more pumps per engine, as shown the drawing of fig. 3.2. This gives considerable advantages in terms of reliability: in case of failure of one pump, the other can supply power to the subsystem; in case of failure of one engine, the power in the active subsystem can be used to pressurise the failed subsystem. This is normally operated by a Power Transfer Unit (PTU), i.e. a hydraulic motor mechanically linked to a hydraulic pump (fig. 3.2); usually it is a bi-directional device. This kind of crossfeed solution is normally adopted because the fluids of the two subsystems are not mixed in

this way, which could result in complete loss of pressure in case of leakage of one subsystem. Some movable parts are controlled by more subsystems: typically landing gear and door operation and primary flight controls (elevator, ailerons, rudder and part of the flaps), to allow a safe manoeuvre and landing in case of failure of one system. In fig. 3.2 the two subsystems are drawn in different colours, and those primary uses operated by both systems are drawn in orange. In fig. 3.2 the auxiliary power unit (APU) is also indicated. This is a turbine engine whose power is all transformed into hydraulic, pneumatic and electric power, to operate all systems on board when engines are off, or in case of emergency. The exhaust gases have then limited velocity and do not contribute significantly to the aircraft propulsion. The system shown in fig. 3.2 is more adequate for a combat aircraft, where the limited redundancy is compensated by using sophisticated components. In civil aircraft the trend is to increase the redundancy of the system.

Fig. 3.1 Hydraulic system layout

PUMPS & CONTROL

ENGINES

NLG,DOOR,STEERING

RH AILERONS

RH L.E. FLAPS

RH T.E. FLAPS

RH AIRBRAKE

RH MLG, DOOR,BRAKES

LH AILERONS

LH L.E. FLAPS

LH T.E. FLAPS

LH AIRBRAKE

LH MLG, DOOR,BRAKES

RUDDER

ELEVATOR

Fig. 3.2 System layout and crossfeed

PTU

ENGINE 1 ENGINE 2

USES 1 USES 2

PUMP 1A

PUMP 1B

PUMP 2B

PUMP 2A

APU

PUMP 1C

PUMP 2C

PRIMARY USES



3.4

Boeing 747, for instance, has 4 independent subsystems powered by 4 engine-driven pumps (EDP) located on the 4 engines. Each use can be operated at least by two subsystems and by the APU. In case of emergency, the subsystems are pressurised by pumps operated by 4 turbines, powered by the pneumatic system.

Airbus 320 has 3 subsystems; two of them are pressurised by 2 EDP, one by an electric motor driven pump (MP) and, in emergency, by a ram air turbine (RAT) that is automatically extended in case of subsystem pressure drop. Boeing 767 has similar installation, but the 2 EDPs have in parallel secondary electric motor pumps in the event that the primary generation is not able to maintain the requested pressure, and the third system has two electric pumps, one turbine pump driven by the pneumatic system (ADP) and one RAT (see fig. 3.3). As one can guess, many solutions are possible for the hydraulic system layout, mainly deriving

from the past experience of the aircraft manufacturer. The constant feature is, anyway, a redundancy able to limit dramatically the possibility of loss of the primary flight controls, which would result in a catastrophic loss of the aircraft and occupants.

3.3 Hydraulic power generation To move an actuator, or a piston in a cylinder, the fluid must have a sufficient pressure to contrast the external load on the piston; moreover a sufficient quantity of fluid must be introduced in the cylinder to obtain the requested stroke. A hydraulic pump must supply a flow of pressurised fluid. Lets start seeing how a flow can be generated, and then how the pressure is generated and controlled. Pumps able to generate a flow are called displacement pump: they displace a fluid and force it into the system. Pumps able to generate pressure are often called fluid dynamic pumps: they accelerate the fluid and then, when it is decelerated, kinetic energy is transformed into pressure. In all hydraulic systems a displacement pump is used.

Fig. 3.3 Possible system layout, twin engine, airliner

ENG.1 ENG.2

USES 1 USES 2

PUMP 1A

PUMP 1B

PUMP 2B

PUMP 2A

APU

PUMP 1C

PUMP 2C

MOT. MOT.

PRIMARY FLIGHT CONTROLS

PUMP 3A

PUMP 3B

PUMP 3C

PUMP 3D

MOT.

MOT.

PNEU.

RAT

PTU



3.5

The most common displacement pump is a piston pump, where the piston has a sinusoidal motion. The flow generated by such a pump is anyway very discontinuous, because it is a sinusoid, or better is the positive part of a sinusoid, because the flow must be in one direction (lets assume it to be the positive one). A series of pistons like that shown in fig. 3.4, in sinusoidal motion and phase displacement, gives a resultant flow rate Q that is less variable the higher the number of cylinders; theoretically it can be expressed in the form:

+=N

itQQ1

)sin( jw , (only for )sin( it jw + > 0),

where: N = number of cylinders; w = sinus frequency;

t = time; ji = phase of the i-th piston; Q = max flow per cylinder. Fig. 3.5 shows the result for a 7-cylinder pump. It can be demonstrated that a pump made by an odd number of cylinders gives a lower oscillating flow rate than that provided by a

pump made by even numbers of cylinders. Hydraulic pumps of 7 or 9-cylinders are commonly used. The most suitable way to have the cylinders assembled is to put them in a revolving drum, with a plate that controls the reciprocating piston motion. Fig. 3.6 explains the functioning principle. The drum rotates, together with the swash plate and drive shaft. The piston rods are hinged to the plate. If the plate is perfectly levelled with the cylinders, or 90 with respect to the drum axis, there is no linear displacement of the pistons. If the plate has a different orientation, the pistons follow a sinusoidal motion with phase displacement. Each piston will be in turn in suction and delivery, passing through the top and bottom dead centres. An additional plate, pressed against the drum face, brings a couple of curved slot orifices aligned with the cylinders; this plate works as port from the reservoir and to the system: one slot will always be in correspondence of cylinders in suction phase, the other in correspondence of cylinders in delivery phase. The volume of fluid displaced by each piston at each rotation is a function of the plate angle of orientation (that determines the piston stroke) and cylinder

Fig. 3.4 Multiple piston pump

TIME

FLO

W

RESULTANT FLOW

SINGLE CYLINDER FLOWS

Fig. 3.5 Flow from a 7-cylinder pump



3.6

diameter. It is easily understood that, in the ideal case that there is no loss of flow or compressibility effects, the flow rate Q generated by the pump is as follows:

VnQ =

where n is the revolving speed of the pump and V the total volume displaced per revolution. This is ideally not dependent on pressure. In real installation, loss of flow and compressibility must be taken into account. They determine a reduction of flow that is proportional to pressure through an efficiency h:

VnQ = h (Eq. 3.1) Loss of flow is determined by leakage in all the parts of the pump that are in relative motion: between piston and cylinder, between drum and slotted plate and at piston rod hinge on the swash plate. This flow is important for the lubrication of the components in relative motion. An additional loss of flow is due to the fluid compressibility. When pressure increases, h and Q are reduced, as indicated qualitatively in fig. 3.7. The leakage is anyway kept at a low level, so that the efficiency of a hydraulic pump working at 21 MPa is normally higher than 95%.

Fig 3.7 Flow vs. pressure for a displacement pump

FLOW

PRESSURE

A B C

Fig. 3.8 (A) Radial pistons, (B) Gear and (C) Vane pumps

Fig. 3.6 Displacement pump (source: Vagnarelli, Impianti Aeronautici, IBN Editore, 1991)

DELIVERY SUCTION DRUM SWASH PLATE

DRIVE SHAFT



3.7

The power W generated by the pump, if Q is the flow and p the pressure, is given by:

QpW = Other 3 kinds of displacement pumps are shown in fig. 3.8: the radial pump, with pistons oriented radially in the drum, gear pump where the fluid is captured by the counterotating gearwheels and the vane pump, where the fluid is transferred by the variable geometry chambers among the vanes.

2.4 Hydraulic power control In a hydraulic system the control philosophy is based on constant pressure. A constant pressure system has two advantages: 1. all actuators can be sized on the basis of the external load and the known

pressure; 2. more actuators in operation do not interfere. The principle is exactly the same of the electric network, where the voltage is maintained constant and a different current intensity is supplied, depending on the absorption needed by the system. To keep a constant pressure a variable flow is necessary: when no actuators are working, the pump will supply a very limited flow, necessary to maintain pressure and compensate for leakages; when one or more actuators are moving, then absorbing fluid from the system, a flow must be generated in order to maintain a constant pressure. The generated flow is given by eq. 3.1; the angular velocity n must be considered constant, because it is linearly proportional to the engine speed through the gearbox. There are two ways to tune the flow and compensate pressure changes: by using a constant pressure - variable delivery pump, or a constant delivery pump with pressure control valve on the delivery line. Both cases are schematically shown in fig. 3.9; in the first case, mostly common, pressure in the delivery is used to control the orientation of the swash plate, then controlling the pump volume (V in eq. 3.1). In the second case the pump will always deliver the max flow rate, and a fraction of it is bleed and returned to tank by a valve sensing the delivery pressure (in the drawings dotted lines represent pressure sensing, with no bleed of liquid).

Fig. 3.9 (A) Constant pressure variable delivery pump and (B) pressure control valve

In both cases any pressure change in the delivery line will affect the pump delivery; if pressure decreases (because one ore more actuators are activated, then requesting flow), the swash plate will be moved to generate more flow to compensate for the pressure reduction. If the pressure increases (because one ore more actuators are

DELIVERY

A

DELIVERY

TANK B



3.8

stopped), the swash plate is controlled in such a way to reduce flow generation to compensate for the pressure increase. The pressure commonly used in modern aircraft systems is around 21 MPa. This allows reasonably small actuators with limited problems of sealing and sizing of components to withstand pressure loads. Military and space applications may have 30 MPa hydraulic systems.

2.5 Pipelines and valves Materials and sizes of tubing are standardised by rules. When studying a hydraulic system, the objective is to use safe and efficient sizing with reasonable weight limits. As usual the two aspects are in conflict: tubing with large diameter and high thickness should be requested to limit head losses and withstand high pressures, but they are heavy. Then relatively large pipes are used just before and after the pumps, because in those sections there is the entire flow going to actuators, which could determine high head loss; diameters larger than 1 inch are commonly used, but this of course depends on the aircraft category; then junctions allow distribution of the flow to different areas (wing, tail, landing gears and so on), and then to the single actuators: these lines are of lower diameters, but normally not less that 4/16 inch As far as material is concerned, stainless steel or aluminium 6061-T4 and T6 are commonly used for the delivery (high pressure) lines, while lower quality aluminium alloys are used for the return (low pressure) lines. Flexible hose is required to bring fluid to movable parts. Tab. 3.1, shows standard tube sizing.

Tab. 3.1 Tube sizing (OD = outside diameter, ID = inside diameter)



3.9

A wide variety of valves can be found in a hydraulic system and a description of all of them is out of the purpose of these lecture notes. A quick description of the most important ones follows. Directional control valve This can be seen more or less as the switch to operate an actuator, because it controls the flow and its direction. The valve is schematically represented in fig. 3.11. HP is the high pressure line, LP the low pressure line and IN the input signal, that can be either a pressure or a mechanical or an electric signal. Depending on how the sliding spool is moved by the signal (left o right), the flow in the actuator lines will be in different direction. A spring brings the spool in neutral position when no input signal is applied. This kind of valve can also be found in rotary spool version. Check valve A check valve allows flow in one direction only, then closing whenever fluid attempts to flow back. This can be schematically described (fig. 3.11) as a spool that is kept in closing position by a soft spring, which is easily moved and opened if the fluid flows in one direction only. Relief valve A relief valve prevents over-pressurisation, by opening a line to the reservoir and

bleeding fluid from the system until normal pressure is reached. Many solutions are possible; the one shown in fig. 3.12 is just a schematic representation to explain the functioning principle. The pressure acts on a spool contrasted by a tuneable spring. When pressure produces a force on the spool higher than the spring pre-load, the spool moves and opens to the reservoir. After opening the pressure acts on a wider spool surface, so that the valve is fully opened and small reductions of pressure cannot close it: this solution prevents

oscillation of the spool. Sequence valve The sequence valve controls the sequence of operation between two branches of a

system. It can be easily found in the landing gear kinematic system, to set the sequence between door opening and landing gear strut moving. Fig. 3.13 shows schematically the functioning principle: when the pressure in branch 1 increases, this controls the spool to move and open the flow to branch 2. In other cases mechanically

operated sequence valves are used, where the spool is moved by an external contact. Pressure-reducing valve In some cases there are parts of the system that must operate at a lower pressure than normal system. This branch of the system can be connected to the main branch

Fig. 3.10 Control valve

HP LP

TO ACTUATOR

IN

Fig. 3.11 Check valve ALLOWED FLOW DIRECTION

Fig. 3.12 Relief valve

SYSTEM

TO TANK

Fig. 3.13 Priority valve

BRANCH 1

PUMP

BRANCH 2



3.10

by a pressure-reducing valve. Fig. 3.14 shows the functioning principle: the pressure in the low pressure system is used to pilot the valve spool; when pressure increases, the spool reduces the opening from the high pressure branch, and vice versa. Shuttle valve Sometimes an actuator, or a branch of hydraulic system, must be pressurised by an auxiliary or emergency unit, typically an accumulator. A very simple device allows selecting the emergency unit in case the pressure in the main system is reduced. The functioning scheme is reported in fig. 3.15 and is called shuttle valve. The spool position is determined only by the differential pressure of the main and emergency system. If the main system pressure drops to a value lower than that of the emergency system, the spool excludes the main system and allows emergency system to operate the branch. Servo valve A servo valve is usually a control valve powered with a low intensity electric signal, which allows refined positioning of actuator and high frequency response. Fig. 3.16 is one of the possible solutions.

Fig. 3.16 Servo valve

Fig. 3.14 Pressure-reducing valve

HP BRANCH

PUMP

LP BRANCH

Fig. 3.15 Shuttle valve

MAIN SYSTEM

EMERG. SYSTEM



3.11

The solenoid is energised by a low intensity signal. Lets assume that the reed between the two nozzles is moved by the solenoid to close the right nozzle, then bringing to a pressure increase in chamber A of the control valve: this increase is due to the reduction of minor loss of the flow through the fixed orifice of the right pipe. The valve spool is then pushed to the left, opening the hydraulic ways for a left movement of the actuator piston. The piston stroke is transduced to an electric signal that forms a feedback to an amplifier. The input signal to the solenoid is function of the difference between the position command and the feedback of the actual position. When the two signals are matched, the reed reaches its central position between the two nozzles and the directional control valve also reaches its neutral position: the piston is hydraulically locked in the new position.

2.6 Hydraulic actuators There are two main types of actuators: linear and rotational. Often the first ones are normally referred to as actuators, while the second ones are called motors. A linear actuator is a cylinder closed by a movable piston. It can be a single-acting

cylinder, like that in fig. 3.17A, where the hydraulic power is used only to move it in one direction: back moving is obtained by gravity, spring or other external loads. Aviation applications require double-acting cylinders, which can be controlled in both directions (fig. 3.17B). Because many actuators must be controlled by two hydraulic subsystems for redundancy, tandem cylinders can be

used like that shown in fig. 3.17C, normally operated by two subsystems but sized to work with one subsystem only. Longer strokes can be reached by telescoping actuators like that shown in fig. 3.17D, but they are not common in aircraft systems. To size the actuator the necessary parameters are the stroke and the external load, which normally is a function of the stroke. Static sizing is simply made by considering the equilibrium of the actuator under the external loads and pressure forces acting on the two piston areas. As far as motors are concerned, their functioning principle is exactly that one of the pumps: there are motors based on axial pistons, radial pistons, gears, vanes and others; the pressure is transformed into torque by the piston system and the flow controls the rotational speed of the motor, according to eq. 3.1.

A single-acting B double-acting

C tandem D telescoping

Fig. 3.17 Types of cylinders



3.12

2.7 Accumulators Accumulators are pressurised fluid storage that can be used for two main reasons: 1. short emergency operation of branches or single components of the hydraulic

system; 2. damping of pressure fluctuations (ripple accumulator). In the first case the accumulator is installed on a hydraulic line very close to the actuator to be operated, with a check valve to prevent flow to be absorbed by other components. In the second case the accumulator is sometimes located near the pump or in an area where a more regular pressure is required. Pressurisation can be easily obtained in a cylinder closed by a movable piston, if this piston can be loaded by a spring or a compressed gas. If compressed gas is used, the piston can be substituted by a bladder or a diaphragm, which are lighter solutions. As a matter of fact, a physical separation between gas and hydraulic fluid would not be necessary if the accumulator maintains a position that prevents the gas from being introduced into the hydraulic system, but this is in general not applicable to aircraft. Fig. 3.18 shows the mentioned solutions.

spring direct gas-fluid gas-piston gas-diaphragm gas-bladder

Fig. 3.18 Types of accumulators The pressure vs. volume law followed by an accumulator is given by the ideal gas law, which can be modified according to the conditions. A typical accumulator cycle

in emergency applications will be as follows: 1. adiabatic compression during hydraulic

system start, from the pre-charge pressure to system pressure: constVp = g , where p is the pressure, V the volume and g the ratio between the gas specific heat factors, that is 1.4 for ideal gases.

2. isobaric cooling to bring the gas back to the environmental temperature:

constTV = , where T is the temperature;

3. adiabatic decompression in emergency; 4. isochoric re-heating to bring the gas back

to the environmental temperature before adiabatic decompression, provided that the discharge was complete:

constTp = .

V

p

Fig. 3.19 Representative cycle for a gas accumulator

1

23

4



3.13

The p-V plot is shown in fig. 3.19. The most significant part of the plot is the third branch, that is the emergency operation. This means that, during actuator displacement, the pressure drops with a polytropic law as follows:

g

+

=ACT

SYS VVV

pp0

0 ,

where: pSYS = system pressure; V0 = initial gas volume; VACT = hydraulic fluid absorbed by actuator.

Proper sizing of the actuator must take into account this process. In general the minimum sizing can be defined by comparing the pressure vs. stroke requested by the actuator and that one supplied by the accumulator (fig. 3.20), and verifying that the accumulator is always in excess of pressure Dp with respect to actuator.

2.8 Reservoirs A hydraulic fluid reserve must be available to compensate for small leakage, thermal and compressibility expansions and operation of non-balanced actuators. Aviation hydraulic reservoirs are usually

pressurised, to reduce risk of cavitation of the fluid before the pump inlet. In many cases the pressurisation is obtained by compressed air (from the pneumatic system) acting on a piston; this solution allows a wide range of manoeuvre, including reverse flight, with no risk of air suction into the pump. A schematic representation of a hydraulic reservoir is shown in fig.3.21. A secondary effect of the reservoir is to cool the hydraulic fluid, which during the process of flowing in pipelines and valves has increased its temperature. Another common way to pressurise reservoirs is by using pressure from the delivery line after the pump.

2.9 Filters All movable parts in the hydraulic system are subjected to wearing, that is the main cause of particles contamination in the fluid. If particles reach considerable dimensions they may damage some components. Filters must provide adequate protection of the system. The filtering element, through which the fluid is forced to flow, can be made of different materials and geometries: fabric of organic and non organic fibres, woven of metal filaments, specially treated cellulose paper and, in some cases, magnetic elements to trap ferrous particles only.

Fig. 3.20 Accumulator supply and actuator request

pressure

stroke

ACCUMULATOR

ACTUATOR

Dp

Fig. 3.21 Pressurised reservoir

SUPPLY RETURN

AIR PRESSURE

Fig. 3.22 By-pass filter



3.14

Depending on the type, the sizes of the trapped particles can range from 5 to 25 mm. Since the filter determines high-pressure drops, the element uses to have a wide surface. Moreover to prevent the filter to become clogged by particles trapping, in some cases a by-pass line is integrated in the block, that operates automatically after the differential pressure before and after filtering reaches a reference value (fig. 3.22). The filter can be located everywhere in the system. A filter on the return line has the advantage of inducing pressure drops in a part of the system where they are not relevant; a filter on the delivery line has the advantage of filtering possible particles that were present in the reservoir or in the pump, before they reach valves and actuators.

2.10 Layout examples Fig. 3.23 shows an example of hydraulic system for a modern twin-engine combat/trainer aircraft. The classic symbolic representation is followed to draw the components, with exception of the colours used for the pipelines, which makes the comprehension easier here. Reflecting the philosophy of having a redundancy for the vital parts of the aircraft, most uses are operated by both systems, with exception of trailing edge flaps (system 1 only), airbrakes (system 2 only, with possibility to further exclusion via priority valve), nose wheel steering (system 2 only) and landing gears and doors movement (system 2, accumulator and emergency hand pump); main landing gear braking has the highest redundancy, being operated by both systems, hand pump and accumulator. The reservoirs are pressurised by the fluid in the delivery system itself. Next fig. 3.24 shows the hydraulic system for a short-haul three-engine airliner. The symbolic representation here reported may look clearer is but less technical than the previous one, and is often found on the system maintenance manuals. In this case, 2 systems are normally pressurised, while a third one is in stand-by. System A is pressurised by 2 parallel engine driven pumps, system B by 2 parallel electric motor driven pumps. Ailerons and elevators can be normally operated by both systems. Brakes can be operated by both systems after opening an interconnect valve, and anyway by an accumulator. The rudder surface is split into two parts, each operated by one different system, then resulting here also in a control redundancy. The reservoirs are pressurised by the pneumatic system.



3.15

Fig. 3.24 Possible solution for the hydraulic system of a short-haul three-engine airliner

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