F-35’s performance benefits from BAE Systems integrated structural testing regime
The wide variety of roles in which the aircraft will be deployed, combined with the differing expectations of the military, means that theairframe’s structure must be thoroughly tested to ensure top performance throughout its operational life. In addition the aircraft,in common with most modern combat aircraft, is dynamically unstable so that it benefits from high manoeuvrability. This poses additional structural challenges that need to be carefully checked during the aircraft’s development and throughout its in-service life.
BAE Systems, a lead UK contractor for the aircraft’s build and development, is responsible for the structural testing of the CTOL airframe.
A lot is riding on the performance of the F-35 Joint Strike Fighter or Lightning II as it is called in the USA; in the UK the chosen name is the Joint Combat Aircraft (JCA). The combat aircraft will be manufactured in three variants – conventional take-off and landing(CTOL), short take-off and vertical landing (STOVL) and a carrier variant (CV) –with expectations that some 3,000 aircraft in totalwill be built.
BAE Systems, a lead UK contractor for the aircraft’s build and development, is responsible for the structural testing of the CTOL airframe. It continues to undertake and monitor structural developments on two F-35 airframes at its Structural and Dynamic Test Facility (S&DT) based at Brough, West Yorkshire.
S&DT aims to provide quantitative data that airframes meet their structural strength design specifications over the anticipated operational lifetime. It specialises in the design, manufacture and operation of large-scale structural tests throughout the entire test life cycle.
A key aspect of BAE Systems’ approach to the F-35’s structural testing regime was the need to develop improved methods of managing test information. The company wanted to streamline its approach by adopting a new mind-set that challenged previous methods.
Static and dynamic tests
Both fatigue tests and static tests are undertaken by S&DT. Fatigue tests take much longer to complete than static tests and are based on data trending which looks for changes in the structural response over time. These can easily signify the possibility of cracks or other in-service related conditions.
The tests are undertaken by applying various loads to different structural components in an accelerated timeframe to reproduce potential operational damage. Complex PC based test control and data acquisition systems are used to control how the forces are applied to the different test structures and the responses carefully monitored. Test specimens are usually instrumented with strain gauges enabling real-time recording of strains generated.
BAE Systems’ structural tests on the F-35 utilise HBM’s equipment to gather data from some 3,800 channels in one of the world’s largest structural DAQ systems.
S&DT’s structural tests utilise HBM’s equipment to capture and analyse data. The equipment includes a data acquisition system from HBM comprising catman ® Enterprise software connected to 32 MGCplus devices distributed in eight cabinets across the test rig. The HBM equipment is fully synchronised giving BAE Systems one of the world’s largest structural DAQ systems.
The MGCplus supports a wide spectrum of sensors, transducers, fieldbus connections and standard PC interfaces making it a suitable choice for this application. The modular design ensures that it can be easily expanded or modified should BAE Systems’ requirements change.
The catman® Enterprise software suite is specifically developed for high channel-count measurements using the MGCplus DAQ system while its dedicated Measurement Data Server (MDS) performs the actual measurement enabling monitoring and evaluation in real-time. This set-up gives test engineers access to some 3,800 channels populated with HBM’s ML801 cards and AP815 connection boards terminating in RJ45 connectors for data acquisition.
Another challenge was to successfully share test data in a timely manner with multiple sites. These included the BAE Systems’ sites at Brough and Samlesbury along with three Lockheed Martin plants and Northrop Grumman’s El Segundo factory in the USA. This was important because it meant the test teams could take immediate decisions in a collaborative manner regardless of their actual location. HBM supplied its MD Client software which is used to share test data in real-time and compare these against predictions and upper/lower alarm limits with the various structural integrity teams.
BAE Systems has a Local Area Network (LAN) to facilitate communications between each test system computer. HBM’s Measurement Data Server (MDS) is configured to communicate with BAE Systems’ control network over Ethernet using industry standard TCP-IP protocol.
BAE Systems also uses nCode Automation for data storage and to produce statistical data automatically during the upload process while nCode GlyphWorks is used to generate graphical plots. nCode Automation provides automated data storage, analysis and reporting as well as a web-based collaborative interface for sharing test data and associated information. Using this, BAE Systems was able to release data to the different teams within two hours of completing a test. nCode product range is designed to complement HBM’s DAQ and analysis tools.
The fully traceable system is kept constantly calibrated and fully operational by HBM UK staff working in conjunction with BAE Systems. This gives engineers additional confidence and security in any results acquired while testing the airframes. HBM UK has ISO 9001 accreditation ensuring optimal support for critical engineering challenges.
Fig. 3 : Fatigue analysis of an engine cradle with nCode DesignLife and the Dang Van criterion, results on plain metal sheet
Fig. 4 : Fatigue analysis of an engine cradle with nCode DesignLife and the Dang Van criterion, results on electric spot welds (ESW)
Another area in the development of the F-35 where BAE Systems’ expertise is important is in de-risking the Flight Test Instrumentation (FTI) system on the aircraft. This proved especially challenging and required a unique approach. Normally the crucial task of testing the data acquisition system on modern military aircraft is completed using the aircraft’s own FTI system.
In the case of the F-35, however, testing the on-board systems needed to be carefully co-ordinated as the system is fitted at Fort Worth, Texas, USA but the tail sections are built in the UK. This means that BAE Systems at Samlesbury has to power up and function check every fitted transducer and the associated aircraft wiring during building. The approach gives a high level of confidence that the whole system is functioning.
Yet again HBM’s equipment provides BAE Systems’ engineers with the tools they need. By using HBM’s MGCplus and catman® Professional software to capture and analyse the data, BAE Systems is able to conduct the tests easily and accurately.
The equipment is used to gather data from a wide range of transducers including strain gauges, pressure transducers and accelerometers. After consulting with HBM, BAE Systems chose a system centred on 48 channels for strain gauge input, 16 channels for accelerometer input and eight thermocouples input. In addition HBM supplied on-site training and support.
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