The New Hydraulic System Engineering Essay

The major aim of the project is to investigate about the new hydraulic system of a380 and the reasons for increasing the hydraulic pressure from 3,000 psi to 5, 000 psi. The project is also intended at discussing and investigating about the new electro- hydrostatic actuators which uses this high hydraulic pressure.

This report emphasis's on how this hydraulic system with 5,000 psi works and how it is better than previous hydraulic system.

Saving Weight is the major reason for using 5000 psi hydraulic system in a380. By using higher pressure and smaller volume of fluid though the pipe of smaller radius, saves tonne of weight throughout the aircraft. Earlier used in military aircraft, this 5000 psi hydraulic system helps to reduce overall weight of the aircraft by reducing size of pipelines and other components.

Fig.1 Kevlar hydraulic hoses, used for carrying high hydraulic pressure in A380. (eaton, 2006)

Architecture of a380 hydraulic system differs extremely from other aircrafts, it uses pipeline specifically made of titanium and eight de- clutchable hydraulic pumps being used to achieve this high pressure of 350 bars. Also hydraulic power packs serve as backup for primary systems, instead of secondary hydraulic system which reduces maintenance and save weight.

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The very obvious reason that come to mind for using an enormous 5000 pounds per square inch seems to be the fact that there is need of massive power needed to fly the giant A380. But there is a lot more to it. The technical reason for that is the weight saving that the designers envisaged. In conjunction to the principles of physics, the pipelines of the hydraulic system have been designed to a smaller diameter as opposed to those used in other wide bodied aircraft, like B 777 in close enough range of A 380.

As we know

Force = A * P

Where

A is the cross section area and P is the pressure exerted by the hydraulic fluid

Smaller diameter of pipelines enables greater hydraulic pressure to be exerted. Thus apart from giving enough power to drive the controls, it also provides the adequacy for lightweight components though they needed to be stronger. So this reduces overall weight of the aircraft.

The way A380's hydraulic system is designed is completely different from those of others. The fact that it's piping system is specifically made of titanium. Eight hydraulic pumps that are clutch able add to the extraordinary feature of this beautiful machine. Also, power packs that are driven hydraulically provide back up for the main hydraulic system. His eliminates the need for a secondary system that is normally implemented for redundancy. Here we see an intelligent application of a really appreciable weight reducing technique. This not only saves weight of the aircraft, but also takes a leap across in the world of engineering, in aircraft manufacturing and provides a base for producing aircrafts with such excellent duplicity in designing power store for possibly bigger aircrafts.

Assuming the A 380 was made with the conventional 3000 psi, it is logical to say that the overall size of the now a380 would have been much bigger. As the experts say, an A380 with a working pressure of 3000 psi would have a nominal diameter of more than 3 inches as opposed to nearly 2 inches which is the actual. The reason as discussed before is the bigger size of the hydraulic components and the pipelines. This in turn would have demanded bigger housings and heavier mountings for a robust structure. Moreover the fittings that join these components would obviously be proportionally bigger and thus adding to weight of the aircraft.

All this idea of weight saving methods has been targeted to the profits that the air carriers would yield in the long run. It translates to having bigger payload capacity that generates more revenue for the business involved.

The hydraulic pump is located on the engine gearbox that runs on the mechanical power provided by the engine and turns it into hydraulic power. This hydraulic force of the fluid is exerted to the flight control surfaces like the rudders, ailerons, flaps, elevators. Transformation of the hydraulic energy into mechanical energy is felicitated by electro hydraulic actuators, which is another key feature in the making of A 380. These electro hydraulic actuators are driven through electrical power available from the aircraft generator or the TRU's (transformer rectifier units). They have got electro hydraulic static valves.

Figures reveal the advantageous design better. As we know from the mathematical relation,

Power = C * P

Where

C is the volumetric flow rate and P is the pressure exerted.

A constant power output would imply change in either of the quantities, i.e. force or pressure. Therefore an increase in pressure would lead to reduction in the rate of flow of the fluid, which is about 40% in A380's case. Consequently, the flow rate in the 5000 psi system is about 42 gallons per minute which in a hypothetical 3000 psi system would have been 70 gallons per minute. So clearly we see a remarkable difference in the amount of fluid flowing per unit time in this instance. But considering the size of A380, this would not have been feasible like it is in case of other aircraft, or even for that matter the jumbo jet B747.

Airbus successfully achieved an appreciable 20% reduction in the weight by switching to more pressure and lower flow rate that very well suited to a system that involve figures about masses and quantities, precisely 2500 pounds of weight.

But, the only drawback for using this 5000 psi is that the components are not available easily.

Invention of hydraulic power generation system

Apart from the aviation industry, there are not many industries that put emphasis on ways of weight reduction. In an industry of commercial aviation,where the sole source of profits are the flying machines. To make that happen effectively efficiently and economically, there is constant research and development going on throughout the world to develop new technologies and devise new methods of advancement in the field. Every ounce of weight of the mass of the aircraft would imply an extra amount of fuel to be consumed. Thus in an effort to reduce the weight and amplify the power to mass em and ultimately increase profits, Airbus endeavoured to save weight of A380. 

A hydraulic system with an operating pressure of nearly 5000 psi is not the latest trend though. Since 1970s, military aircraft of sophisticated kind have been using such a mean pressure, but with the advent of the giant beauty, it is the first time that such high a pressure has been used in passenger aircraft, against the conventional 3000 psi hydraulic systems, obviously with the exception of Concorde which featured a 4000 psi system. Components that are used in such military air machines need to be light weight as well as strong enough to handle such fluid conditions. Similar requirements were demanded by the design of A380. 

When the bright idea of building the world's largest commercial carrier was still on paper, Airbus did know that weight and volume of the structural parts and components involved would be the most critical factor in practical. Engineers and designers pondered upon the expertise that would go into it and discussed the latest hydraulic system technologies. These co-ordinated efforts led to a plateau phase that lasted four months, in which the criteria and the specifications of the components and materials involved were developed. The outcome was that the trade of fuel distribution system and the hydraulic power generation system was ultimately given to Eaton Corporation. 

A380 HYDRAULIC SYSTEM BLOCK DIAGRAM

As compared to other airbus hydraulic system, this A380 got only two hydraulic systems i.e. green and yellow, replacing the third hydraulic system by electro- hydraulic system which comprises of EHA and EBHA.

fig.2 block diagram (eaton, 2006)

A380 HYDRAULIC AND ELECTRIC POWER SOURCES

fig.3 hydraulic sources (tron, 2007)

electro hydrostatic actuators

fig.4 electro hydrostatic actuators (tron, 2007)

EHAs are electrically operated actuators, which replaces the third hydraulic system in an aircraft, thus it simplifies the aircraft architecture and makes it more reliable.

It has an advantage of storing the pressure internally, when there is no movement in the motor, thus it reduces the power usage.

WORKING AND CONSTRUCTION OF THE SYSTEM 

It is the first time in the civil aviation history that flight actuators are incorporated that are powered solely by electricity that give back up to the hydraulic powered flight control actuators. When required they augment the primary actuator. They contain their own hydraulic and electrical supplies. 

For aileron and elevator movement EHAs or Electro- hydrostatic actuators are used. For rudder and spoilers movement they are used as EBHAs Electrical back up hydrostatic actuators. 

As the pressure has increase from 210 bar (3000 psi) to 350 bar (5000 psi), this has led to thinner pipelines being used. The conventional metal to metal fittings have been replaced by isomeric sealing which bind the metal chemically. Tests have proved that such high pressure would not degrade any chemical or physical property of the fluid, with an added advantage of saving about a metric ton!

Hydraulic system of A3809 consists of eight Vickers PV-300-31 hydraulic pumps that are engine driven and are of variable displacement type that have a displacement that deliver at a rate of 42 gallons per minute at 3775 revolutions per minute. Their displacement is 47 ml per revolution

There are two unique features that set this kind of pump apart from traditional type of hydraulic pumps. Firstly, the disengagement clutch that is not found on any of the other commercial or military aircraft. For instance, if any of the eight pumps is not working, it can be de clutched or separated from the system. Then the pump cannot be engaged, with the exception on ground that too by manual means.

The second important characteristic of this kind of pump is pressure pulsation. Generally, there are pressure fluctuations n a pump, typically about 10%.] I.e. The value of the pressures generated can vary to about 10% above and below of the nominated value. Even in recent models, this pulsation is about 5%. But Airbus wanted the value to be as low as 1%. These results were immaculate. It led to a system which had variations to a maximum of 4900 psi and 5100 psi on both sides. Low noise levels were an added advantage. 

Another important Development with the upcoming of A380 is that they have incorporated four 150 kVa electrical generators which can be frequency controlled. Thus eliminating the use of constant speed drives (CSDs) and better reliability. Embedded in the same electrical system are the new aluminium power cables that have replaced copper. Thus saving on weight for an aircraft that requires numerous cables running in it for such complex systems. This power system of the A380 is fully digital and computerised. So the contractors and the breakers have been replaced by solid state devices for better performance. Extending discussion about the electrical power system of A380, it would be logical to mention the illumination system. The lighting is provided by the LEDs which have replaced conventional bulbs. These multi spectral LEDs are capable of producing day like effects anytime and producing shades of variable candela and contrast. 

Fig5 components operating the hydraulic system (eaton, 2006)

GROUND SERVICE PANEL: According to the fig.2, ground service panel is located at the foremost part of the aircraft in the hydraulic bay and is used for monitoring and maintaining the hydraulic system. Basically it consists of pressure and level gauges, to keep a check over the fluid levels and air pressurization.

Fig.6 ground service panel (eaton, 2006)

ENGINE DRIVEN PUMP: The main source of supplying 5000 psi is EDP, there are total eight Vickers branded engine driven pumps being installed on the airbus a380 and each being powered by the gear box, supplying the rate of flow of 160 l/min (42 gpm) at 3775 rpm.These pumps are based on clutch mechanism which enables them to disconnect from the engine gear box as required on flight or on ground.

Fig.7 engine driven pump (EDP) (eaton, 2006)

FIRE SHUT OFF VALVE: Four fire shut off valves being fitted in A380, in order to shut the hydraulic lines in case of major incidents or engine failure. The unit consist of an electrical motor, which has been designed specially for low electrical consumption, saving notable weight of the aircraft cables.

Fig.8 fire shut off valve (eaton, 2006)

SLAT DRIVE SYSTEM: This system consists of a hydraulic motor, sensors and brake release mechanism, where motor is bi- directional with variable displacement in nature and sensors are based on closed loop motor control mechanism. it is the first commercial aircraft, which uses this variable displacement motors to operate the secondary flight controls.

Fig.9 hydraulic motor (eaton, 2006)

HIGH PRESSURE MAINTENANCE FREE ACCUMULATOR: These accumulators are mostly used for maintaining the brake system of the jumbo jet during parking mode and also used as backup for emergency braking. They are charged with helium and can only operate with high pressure hydraulic fluid i.e. 5000 psi.

Fig.10 high pressure accumulator (eaton, 2006)

HYDRAULIC SLIDING UNIONS: These sliding unions are used for distribution of hydraulic power to different sections of aircraft such as rudder, elevators, landing gear etc. They are used for supporting pipe length during contraction and expansion, due to thermal changes and significant wing deflection. As shown in fig.2, they are located at rear spar area of the wings.

Fig.11 hydraulic sliding unions (eaton, 2006)

Highlights of new hydraulic system

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Fig.12 highlights of new system (bugatti)

Electro hydrostatic actuators (EHAs) 

These are for back up flight controls and are driven by mini pumps which are smaller that conventional pumps but capable of delivering fluid at 5000 psi.

Massive size of a380, brings out a new approach to its flight control system, moving these large surfaces hydraulically being proposed but later replaced by the new hydraulic plus electric architecture known as electro hydrostatic actuators.

http://www.messier-bugatti.com/IMG/jpg/hmp0033n.jpg fig.13 mini pumps (bugatti)

Hydraulic filtration and distribution 

Its components are made of titanium so it can bear high pressures, being stronger and lighter in weight at the same time.

Steering system

To impart great manoeuvrability to the aircraft, Messier Buggati developed variable architectures to enhance the same. for example,  the nose wheel was managed by a software  IMA ( Integrated Modular Avionics) and a backup circuit of the type LEHGS (local electric hydraulic generation system) to support the main circuit for the nose wheel.

The two rear wheels on each of the two central bogies are controlled by RDC (remote data concentrators) along with IMA.

http://www.messier-bugatti.com/IMG/jpg/hs0013nblocorient_ra_a380-230-2.jpg fig.14 A380 steering system (bugatti)

An application of the same is WABSIC that stands for wheel and braking system integrated components , it has two functions.one is to ensure the rotational speed of the tyre, other is to monitor the tire pressure through wireless data transmission. Moreover a brake cooling fan is part of this integrated unit.

The same manufacturer Messier Buggati is responsible for the brake temperature monitoring system and oleo pressure monitoring system on the A380 

Braking control system

This is controlled by two main circuits fed by two hydraulic circuits, both operating at 5000 psi and monitored by the IMA software. The alternate system which uses de centralised hydraulic system is also a full brake by wire system which is controlled by an analogue computer and has an emergency baking control unit (EBCU).

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landing gear - hydraulic system

As everyone knows that the world biggest airliner a380, has gained much of media headlines for building the jumbo jet with the hydraulic pressure of 5000 psi and they uses this pressure for operating their landing gear systems, which was a big challenge to work on.

Maximum take-off weight of a380 is 560 tonnes, out of which 20 tonnes is only of gear itself. Landing gear retraction and extension system is the major element, which manages the landing gear of a380 having total of 22 wheels with 5000 psi. LGERS also introduces the new electromechanical uplocks, which are used for locking and the unlocking undercarriage bay doors and landing gears. These up locks can be used in normal mode as well as in emergency mode, but because of its weight they being replaced by the electromechanical actuators which are far more Compaq and cheaper as it eliminates the use of cable riggings which saves weight and cost as well.

There are total 12 up locks being installed on a380, which are easy to maintain as it removes the need of bleed hydraulic system.