Before an aircraft can take off for realit has to prove its prowess on thehydraulic test bed. It must withstand atough set of tests representing at least2 - 3 aircraft lives with all conceivable"real situations". For this purpose noless than 90 hydraulic cylinders haveto be coordinated with a high degreeof control effort. A dry run which paysoff for all aircraft in the air. The testingmachine cylinders from Hänchen notonly ensure a "real" feeling of flight butalso permit the sustained and highlyprecise reproducibility of allmovements.The Airbus 340 already has more than 20,000 flights behindit: taxiing on the runway, operation of the landing gear onuneven ground, take-off, flight and landing in good weatheras well as in vertical turbulence with 7.6 m/s gust speed. Thisscenario takes place on a hydraulic test bed at IABG inOttobrunn near Munich. "Since 1970, since the first fatiguestrength tests on the Airbus A 300, we have been usingHänchen hydraulic cylinders. Together we defined,developed and tested the necessary units. And even onextreme special constructions such as synchronous cylinderswith a five metre stroke and six metre length the Swabiancylinder manufacturer, unlike other companies, never failed,"reports Dipl.-Ing. Siegfried Schmid, head of the hydraulicloading unit group at IABG. The hydraulic unit for the Airbustests is a fluid system in the superlative: 90 hydrauliccylinders and two hydraulic motors are supplied by six fluidpumps with up to 2,000 litres of oil per minute at a powerconsumption of 1.2 MW.The mega-characteristics of the test stand are reflected in thedetails.At a working pressure of 210 bar the cylinder transfers up to1600 kN of load - depending on the cylinder size - to thestructure. This enables conditions to be simulated in whichthe wings are subjected to a loading of up to 250 tonnes,because the A 340 is a wide-bodied long-haul aeroplane witha wingspan of 60 metres. The entire double wing with thecentral fuselage element was constructed complete in thehall of the Ottobrunn Technical Inspectorate (TÜV) - inaddition to tests on components for the European launchvehicle Ariane V and other aircraft.Despite the giant dimensions involved, the problems for thetesting facility reside more in the detail. Coordinating 90sources of power requires a highly elaborate control system,especially as the movements have to exactly reflect the realsituation: Expensive servo-valves and as far as possible lowstick-slip cylinders with a friction level distinctly below 1 % at10 to 100 % nominal loading, which even if mounted at anangle and under lateral forces of up to 10 % do not cant, arejust as important as the sophisticated control technology.The actual testing programme, a cycle of 2,000 flights withdifferent characteristics, which is run-through 20 times, drawsits values from the specifications provided by the Airbusproducers. As purely static flight phases are irrelevant for thedynamic test each simulated flight consists of five minutes totwo hours in which the dynamic sections are combined. Thetest values are stored in a matrix which defines the currentspecified force for each cylinder.High computer power essentialExtensive expert knowledge and high computer capacitiesare needed in order to create this matrix. Its values areassigned via one microprocessor per cylinder to a dedicatedelectronic control unit which controls the respective regulatoror proportional valve. The piston movement thus triggered inturn exerts an effect via a plunger on a network of pressurepoints which has the purpose of causing an as far as possibleequally distributed impact of forces. Between the plunger andthe cylinder rod end is located a load cell with a doublesensor. The first sensor closes the control circuit andsupplies the respective electronic control unit with the actualvalue, the second sensor sends its signal to the central testcomputer. This redundancy permits control through twoseparate systems which in the event of a fault can bring thetesting to a halt independently of each other.A decisive point in the testing strategy becomes clear here: Through the value of the unit beingtested, the duration of the tests, the effort involved in a repeat test and the complexity of theprocess, the unit being tested must not - unlike for instance in tests conducted in the autoindustry - be damaged by factors not envisaged in the test programme.In the two cylinders for simulating the landing gear loading this led to a special developmentwhich was a challenge even for the Hänchen designers: As the landing gear is not available inthe laboratory and after take-off positions are also simulated which are below the normalposition, the piston in the 'landed' position is not in the final position. A locking system taking upa minimal amount of space therefore had to be found to ensure this position. Given themaximum load of 90 tonnes a clamping unit could not be used. A second, end-position pistonwas therefore integrated in the cylinder which during operation is recessed in the bottom of themain cylinder. In resting position it is moved out of the bottom and thus provides an end positionfor the main cylinder.A laterally fitted auxiliary cylinder then pushes a blocking pad between the end-position pistonand the lower cylinder bottom, rendering the system statically stable. Four receivers, duplicatedvalves and a control block ensure that even if the pressure supply fails the system can bereturned to this static condition. This guarantees that if the system fails no untypical loadingsoccur in which the landing gear for a prolonged period touches down, as it were, 10 centimetresbelow the surface of the runway.IABG is a leading companies in Europe for such complex testing tasks, in particular usinghydraulic systems. In this environment the Munich-based company sees itself as a provider forcomplete testing operations. Around 1,500 staff, 50 % of them graduates, offer tailor-madestatic and dynamic testing operations according to customer specifications as a completepackage.Tailor-made testing operations as a package of servicesExperienced development teams, programmers, electronic testing experts, fluid engineers andmaintenance specialists support the customer, backed by an extensive testing Infrastructure,computer and measuring system capacities, buildings for testing extremely large assembliesand extensive hydraulic facilities.With this background of experience units of virtually any kind can be tested with the equipmentavailable. "Particularly interesting for our customers is the fact that they can outsource completetesting projects without, for example, having to make investments in expensive testing systemsas a result of new requirements on the quality control side and without having to invest inspecialists who perhaps cannot even be fully utilised. In addition to complete testingprogrammes, we can also handle any partial task, design concept or plant assignment," reportsDipl.-Ing. Wolfgang Mohr, Marketing Manager at IABG.Projects ranging from dynamic endurance tests on motorcycle forks through to the testing ofcomplex new transport technologies, for example for the Transrapid, show how many andvaried the testing tasks are which have to be performed even just as far the hydraulic testingsystems are concerned. "Hydraulic testing cylinders represent the core technology," explainshydraulics specialist Schmid. "And in this sector Hänchen offers unbeatable solutions despitethe strong competition. Because the Swabian hydraulics company has specialised to such anextent on the actual cylinder construction that unique know-how exists at Ostfildern, especiallyfor test cylinders." The many years of cooperation between the testing company and thecylinder manufacturer has produced numerous detailed solutions. "IABG with its strictrequirements prompted us as early as the mid-1970s to develop our own test cylinderprogramme," reports Hans-Dieter Fabrowsky, Sales Manager at Hänchen. "As a result we wereable to occupy major market positions at an early stage and today supply test cylinders forexample to all prominent car manufacturers in Germany. The cylinders for the Airbus testswere, however, our biggest challenge."Perfect technology through relentless testingTo handle these special tasks, tests were also carried out in cooperative partnerships. Forinstance, Schmid had tests carried out on a cylinder which was tested with 100,000 load cyclesincluding the lateral forces arising under typical testing conditions and which was thendismantled. This enabled precise specifications to be made to the Hänchen design engineers asto where further developments were necessary. "We have been doing business with thecylinder producer for all these years because the design engineers in Ostfildern-Ruit havealways managed to meet our most extreme requirements and to respond flexibly to ourexpectations," said Schmid.
