Turbulence can cause the wing tips of the A 340-600to move over four metres. Anyone observing theconstant up-and-down movement of the wingsspanning several metres in trial conditions gets aglimpse of the material stress to which commercialaeroplanes are subject. This Airbus will complete atleast 35,000 flights - and won't even leave the hangardirectly adjacent to Dresden Airport to do so. Theaeroplane is moved by 94 Hänchen hydrauliccylinders which form the core mechanicalcomponents of a time accelerator test, whichsimulates over a period of 18 months the sequencesof movements of the entire life-span of an aeroplanelasting 25 - 30 years.
30 types of aeroplane in 40 yearsThe aeroplanes which have been tested since the60's with 
Münchner IABG acting as main contractorrange from the Tornado to the Airbus - including theAirbus numbers 300, 310, 320, 330 and 340. The aimof this extended time test is to provide experimentalevidence that the airframe has sufficient serviceablelife and to eliminate any possible weak points still inevidence. Clients requiring these tests include all themajor European aircraft manufacturers. The currenttest is carried out by the IABG in conjunction withDresdener IMA Materialforschung undAnwendungstechnik GmbH. It only took less than two years to design and construct the test.The static tests were started in April 2001 and shortly before the first flight in September, theendurance tests were started with a new control technique developed by the IABG to realize thetests in a extremely short timeframe.
Münchner IABG acting as main contractorrange from the Tornado to the Airbus - including theAirbus numbers 300, 310, 320, 330 and 340. The aimof this extended time test is to provide experimentalevidence that the airframe has sufficient serviceablelife and to eliminate any possible weak points still inevidence. Clients requiring these tests include all themajor European aircraft manufacturers. The currenttest is carried out by the IABG in conjunction withDresdener IMA Materialforschung undAnwendungstechnik GmbH. It only took less than two years to design and construct the test.The static tests were started in April 2001 and shortly before the first flight in September, theendurance tests were started with a new control technique developed by the IABG to realize thetests in a extremely short timeframe.Tests with Hänchen cylindersSince 1974 Hänchen hydraulic cylinders have beenused for the majority of these dynamic tests foraircraft. The cylinders are supplied with a throughputof 4,400 litres of compressed oil per minute overmany kilometres of pipelines with diameters of up to20 cm on the main pipes. Process computers coordinatethe sequences of movements in such a waythat the loads correspond to the day-to-day flightrealities of the Airbus A 340-600. In order to makethese movements as realistic as possible, thehydraulic cylinders are driven by proportional valves.This process mainly uses testing machine cylinders with floating annular gap sealing, patentedby Hänchen. For special tasks, however, cylinders with hydrostatically mounted piston rodguides are also used.
Simulation not only on the computer"Discovering damage in this kind of test is part ofeveryday life", is how Klaus Woithe, graduateengineer and IABG branch manager in charge of theproject sums up the experience gained over 40 years."Even modern computer models with FEM, the finiteelements method, still cannot replace the dynamicendurance tests", he maintains. Indeed, computeranalysis cannot, as a general rule, reproduce certaineffects occurring in reality with the required precision.Since aeroplanes are usually designed nowadays toresist a certain amount of damage, cracks severalcentimetres long can occur in the fuselage skinwithout compromising the safety of the aircraft. Thedemonstration trials in Dresden are carried out on thewing structure with approx. 60 metres wing span anda 33 metre long fuselage segment. Undercarriageand engine pylon dummies serve to introduce theloads coming from these components.
Hydraulics as a core technology"Hydraulics is the core technology for load simulation in the dynamic test for material fatigue onaircraft. This is because control, measuring and fluid technology work hand in hand here", saysWoithe. "The computers have to make stipulations in real time which are then activated in theset-actual comparison with the help of PLCs via control circuits with load cells. Apart fromcontrol they also serve to prevent overload. High-quality testing cylinders are a basicprerequisite for guaranteeing that this test is realistic. In this respect we have, over 27 yearstogether with Hänchen, pushed the boundaries of what is possible time and time again, yet wehave nevertheless achieved a good cost-performance ratio while remaining highly committed torunning on schedule. The testing cylinders from Ostfildern were successful on account of theirstrengths, especially their low friction, optimum tightness, excellent response, low abrasion,extreme piston speed, low initial break-out torque, fatigue strength and long serviceable life.
Two and a half lives in 18 monthsFor safety reasons experimental evidence is takenover more than two and a half times the expected lifeof an aircraft. In order to test the material fatigue, 
allthe stages of flight are simulated. This includes takeoffand landing as well as all stages of flight in whichthe Airbus is subject to load alternations, namelyvertical and horizontal gusts and flight manoeuvres. Inthis way even a long-haul transatlantic flight in goodweather conditions can be condensed into asimulation program of quarter of an hour or half anhour. In the categories of short, medium and longhaulflights a series of typical flights in each casewere defined from the standard flight right through tothe difficult flight in extreme conditions. They consistof 
load data for the airframe mapped on a gradientdiagram. This is because the cabin is set at anincreased internal pressure by means of acompressor unit and two air chambers depending onthe simulated altitude, in order to simulate thedifference in pressure between the cabin and thesurroundings depending on the altitude of each flight.A flight-by-flight program sequence encompassingmore than 1,000 flights is generated from these typesof flight. It is repeated as many times as necessary to reach the stipulated total number offlights. Constant monitoring by inspectors, comprehensive, day-long inspections of the entiretest structure as well as regular measuring of 3,600 strain gauges and 80 displacementtransducers guarantee that damage is detected as soon as it occurs. Since the aircraft isconstructed in such a way as to resist a certain amount of damage, the development of cracksis observed from the point at which they arise until they reach a critical length. Then they arerepaired or the part is replaced. A cleverly devised monitoring system ensures that the aircraft isnot inadvertently exposed to unintentionally excessive loads in particular.
allthe stages of flight are simulated. This includes takeoffand landing as well as all stages of flight in whichthe Airbus is subject to load alternations, namelyvertical and horizontal gusts and flight manoeuvres. Inthis way even a long-haul transatlantic flight in goodweather conditions can be condensed into asimulation program of quarter of an hour or half anhour. In the categories of short, medium and longhaulflights a series of typical flights in each casewere defined from the standard flight right through tothe difficult flight in extreme conditions. They consistof 
load data for the airframe mapped on a gradientdiagram. This is because the cabin is set at anincreased internal pressure by means of acompressor unit and two air chambers depending onthe simulated altitude, in order to simulate thedifference in pressure between the cabin and thesurroundings depending on the altitude of each flight.A flight-by-flight program sequence encompassingmore than 1,000 flights is generated from these typesof flight. It is repeated as many times as necessary to reach the stipulated total number offlights. Constant monitoring by inspectors, comprehensive, day-long inspections of the entiretest structure as well as regular measuring of 3,600 strain gauges and 80 displacementtransducers guarantee that damage is detected as soon as it occurs. Since the aircraft isconstructed in such a way as to resist a certain amount of damage, the development of cracksis observed from the point at which they arise until they reach a critical length. Then they arerepaired or the part is replaced. A cleverly devised monitoring system ensures that the aircraft isnot inadvertently exposed to unintentionally excessive loads in particular.Floating annular GAP sealingPrecision requirements dictate that disruptive forcessuch as stick-slip effects of the cylinders should beavoided.
Very low restoring force occurs on thewingtips, for example, but they must be moved at thesame time at up to 670 mm/s. In the process thewings are moved upwards up to 2.9 m from the zeroposition and downwards up to 1.2 m. Spuriousoscillations of the flexible structures can easily occur,however, if the pistons and piston rods of thehydraulic cylinders are not as smooth running aspossible. In this case tolerances of only 3 per cent ofthe nominal load of the cylinder are accepted; inpractice they are under 2 per cent. Spuriousoscillations would lead to unwanted load variationsand would distort the test results. For this reason, thecylinders with the floating annular gap sealingpatented by Hänchen are particularly favoured for usein structure trials on aircraft, since they always havethe same friction irrespective of the pressure. A steelbush inside them deforms through a choke gap andthus produces a non-contact packed sealing gap of afew 1/100 mm. The prerequisite for this technology isa production precision in the region of a few μm, sinceotherwise the leakage would lead to high hydrauliclosses. This cylinder series (PZR) has a costadvantage of around 30 % when compared withcylinders with hydrostatically mounted piston rodguides (PLZ). This is because the PZR cylinders, onaccount of their very low friction, offer the possibilityof very high positioning and repeat precision, are stick-slip free and admirably suited to bothextremely slow and extremely fast movements alike.Another crucial factor determining the choice of the Hänchen cylinders, however, was theirstability. After all, the trial runs for 24 hours seven days a week.
Very low restoring force occurs on thewingtips, for example, but they must be moved at thesame time at up to 670 mm/s. In the process thewings are moved upwards up to 2.9 m from the zeroposition and downwards up to 1.2 m. Spuriousoscillations of the flexible structures can easily occur,however, if the pistons and piston rods of thehydraulic cylinders are not as smooth running aspossible. In this case tolerances of only 3 per cent ofthe nominal load of the cylinder are accepted; inpractice they are under 2 per cent. Spuriousoscillations would lead to unwanted load variationsand would distort the test results. For this reason, thecylinders with the floating annular gap sealingpatented by Hänchen are particularly favoured for usein structure trials on aircraft, since they always havethe same friction irrespective of the pressure. A steelbush inside them deforms through a choke gap andthus produces a non-contact packed sealing gap of afew 1/100 mm. The prerequisite for this technology isa production precision in the region of a few μm, sinceotherwise the leakage would lead to high hydrauliclosses. This cylinder series (PZR) has a costadvantage of around 30 % when compared withcylinders with hydrostatically mounted piston rodguides (PLZ). This is because the PZR cylinders, onaccount of their very low friction, offer the possibilityof very high positioning and repeat precision, are stick-slip free and admirably suited to bothextremely slow and extremely fast movements alike.Another crucial factor determining the choice of the Hänchen cylinders, however, was theirstability. After all, the trial runs for 24 hours seven days a week.
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