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MODERN HYDRAULIC SYSTEMS

It is all systems go in Oberpfaffenhofen. Here, as the fuselage sections of the first 728JET test aircraft materialise one by one at the hands of the most advanced robots in Europe, the system developers are still refining the last details on some of the components. Also hard at work is the hydraulics team led by Jens Nielsen and Uwe Strecker, since their system has to be flightworthy before flight testing can get under way.

The electrohydraulic equipment in the aircraft moves the rudder, flaps, landing gear and brakes. This may sound simple, but it is actually extremely complicated. First of all all the loads which need to be exercised by the individual actuators (control elements) to implement the fly-by-wire control commands rapidly and accurately have to be ascertained.

Then the layout of the pumps, valves, reservoirs, heat exchangers and piping has to be roughly determined. The latter needs to be laid in runs as short as possible with no bends or connection pieces more than are absolutely necessary, as the pipes and hoses have to endure pressures of up to 3000psi.

Finally the design has to be redundant, which means that there must be multiple versions of the system so that if one of them fails another one can take over from it without any loss of functionality. In the Fairchild Dornier 728JET the hydraulic system actually consists of three strictly separate systems. Each pump in systems 1 and 2 is mechanically controlled directly from the General Electric CF34-8D engines, while generators also power backup pumps in those systems. Finally the third system, located in the rear of the aircraft, comprises a primary pump driven by ac current and a backup pump powered by dc current. All three systems pass on the pressure via filters to their assigned controls - the rudder, internal and external flaps, spoilers, nose and main undercarriage and reverse thrust.

System 1 is responsible for retracting and lowering the landing gear. If it should unexpectedly fail, then system 2 will jump to the rescue. Both of these systems are connected to each other via a hydro-mechanical power coupling which conducts the pressure from the reserve system to the undercarriage.

Three accumulators generate the pre-load pressure for the reservoirs, and two others for the brakes. Altogether the reservoirs contain 75 litres of hydraulic fluid. A synthetic oil, phosphate ester, is used. This has the advantages of having better hydraulic properties than mineral oil and of also being hardly flammable, a characteristic which, given the proximity of the systems to the engine areas, makes it particularly attractive from the safety point of view. In addition, its operating temperature is reached at minus 7-oC, and the system can actually be operated at external temperatures as low as -40-oC.

The pumps ensure that the system is continuously maintained under pressure, even when all the control elements are in the neutral position, as this is the only way to avoid uncontrolled movements during the cruise.

All three systems are closed and only minute losses of fluid occur. If any hydraulic oil should escape, it is collected in "ecology bottles". In this way it is not possible for oil puddles to collect underneath the 728JET, an ecological advantage which should not be underestimated and will please airlines and maintenance mechanics alike. There are no fixed requirements as to when the oil must be changed, as there are for a car. Test samples are taken and investigated during normal maintenance cycles and only when contamination is found to have accumulated to predefined levels does the hydraulic fluid, which is not exactly cheap, have to be replaced.

Once the high-level design for the complete hydraulic system is complete, its dimensions and weights will also have been roughly determined. Standard components produced by Fairchild Dornier's partner Parker Aerospace Hydraulics which have already been thoroughly tested and proved their reliability in other aircraft are used for the entire system. The same applies to pipes made of stainless steel, titanium or aluminium, pumps, valves and pressure hoses. It is only now that the real brain-teasing work begins, for the amount of space available to accommodate all the components and lines is strictly finite. Heating and cooling, fuel system, electrical and other systems will also need to be installed. If every development team were to build an aircraft that satisfied its particular requirements, each one would look quite different.

In the old days a scale model made of wood and metal would be built at this stage of the development cycle. This would be time-consuming and expensive, but the mock-up was an indispensable tool enabling designers to work out the position, mounting, control and accessibility of all the systems, which they could then translate to the necessary engineering drawings.

Today development engineers use special three-dimensional software to represent the entire aircraft to achieve the same end. Every designer can accommodate his variously coloured components within a virtual model, and checks are performed on a daily basis in the multifunctional teams to eliminate any "collisions" with equipment designed by colleagues.

However, hand-built models are still used to clarify design issues in extreme cases, so that even today the designer's worktools will still include cardboard, glue and scissors.

Once room has been found for every part, the work is still a long way from completion as it is now necessary to check whether it will be possible for technicians to access the equipment freely. The fuselage, wing assembly and empennage designers prefer a smooth external skin, whereas those working on the internal systems like to have as many service flaps as possible, and as large as possible. It is inevitable that compromises will have to be made, and the hydraulics team has the major advantage of having in Louis Beaulieu a designer with many years of experience as a mechanic. His instructions are not always easy to implement, but at least the aircraft will end up with a practical design, and this is always a good sales argument.

Whether the flaps as finally agreed are also big enough and are located in the right places can once again be verified on the computer. Using a virtual manipulator, it is possible to handle a particular part that might need replacing and attempt to manoeuvre it out. If a red light comes on, that means that another part is blocking the way. Whether the part or the flap must now be moved is once again agreed in the liaison meetings.

Finally, there will inevitably be design differences between the prototype and production aircraft. This is because a considerable number of sensors and their associated cables have to be installed in the prototype which are not needed on the final aircraft, and these create additional difficulties as regards the limited space available.

While the engineers are thus puzzling over the final refinements of the hydraulic systems, a futuristic steel construction known as "Ironbird" is already standing in one of the manufacturer's hangars. With a lot of imagination it is possible to recognise in this likeness of an aircraft, with fuselage, wing assembly and empennage in precisely the positions at which the control elements for rudder and flaps will be attached later on on the "proper" aircraft.

The first tests aimed at demonstrating how the system responds under extreme loadings are already under way here. The results of these tests will flow into fine tuning of the design, which is carried out after the flight trials and the start of full production. A lot of work remains to be done by the hydraulics team which, like the other development teams, has not yet finalised the positioning of some items. Experienced development engineers are rare today. When one considers that a complete new aircraft family is to be built, whose members will include the 928 and the 528 as well as the 728, there is plenty of interesting work waiting for young engineers.

From page 80 of FLUG REVUE 5/2001