BAE Systems has just completed the durability testing of the conventional take-off and landing (CTOL) variant of the Lockheed Martin F-35A Joint Strike Fighter.
The F-35A test article completed its third lifetime in the test rig at BAE Systems’ Military Air and Information (MAI) facility at Brough in East Yorkshire, UK, in November (2017). The Structural and Dynamic Test Department is the center of excellence for the testing of military aircraft in the UK.
The CTOL variant of the Joint Strike Fighter has an anticipated life of 8,000 flying hours and therefore when the durability tests were completed, the test article had carried out 24,000 hours of ‘flying’ representative flight profiles.
After the completion of testing over two normal lifetimes, or 16,000 flight hours, as required by the System Design and Development (SDD) phase of the international F-35 program, testing for the third and final life began at Brough in February 2016.
BAE Systems is the structural test lead for the F-35A and Lockheed Martin is responsible for testing the short take-off and vertical landing (STOVL) F35B and Carrier Variant (F-35C) aircraft at its own test facility in Fort Worth, Texas. The Brough test facility is a purpose-designed complex with four large test halls, together with associated development laboratories and supporting infrastructure. The facility has been used for structural testing since the 1970s, and prior to the F-35A, it has been used to support Buccaneer, Harrier, Hawk and Typhoon aircraft. Today, work in support of the Hawk and Typhoon, including a Typhoon flight test article, continues alongside the F-35 test campaign.
Durability testing
The structural test facility at Brough
Durability – or fatigue – testing is designed to show that the aircraft structure is able to safely handle the spectrum of flight conditions it is expected to experience in service. The purpose of this testing is to highlight any issues resulting from the stresses placed on the airframe before they become an issue on the real flying fleet.
“It is linked to how we maintain structural integrity,” explains Andy Acklam, structures lead for F-35 aft fuselage and empennage at BAE Systems. In his role as structures lead, Acklam liaises with the structural test engineering team to interpret the results.
“You test it to a spectrum, you monitor how they fly with respect to that spectrum, and then you can make sure that you’ve stayed within those defined life parameters to ensure we have a safe product.”
The fatigue test article, known as AJ-1, is being tested in a 350 metric ton structural test rig at Brough and the representative flight loads are being imparted to the airframe via 160 loading actuators, monitored by 2,500 strain gauges and altogether there is more than 20 miles (32km) of wiring.
Each phase of testing lasts a total of 8,000 hours and is equivalent to an entire lifetime of the airframe
Testing is carried out in blocks of 1,000 hours and the third 8,000-hour lifetime will provide additional information on the robustness of the airframe. “You have to have a safety margin. So, we over-test because there’s always scatter and we have to factor that in,” Acklam says.
“There are undercarriage loads, which also cover taxiing loads, and there are various phases where we model a series of flight profiles. So, for example there might be a 3g maneuver, followed by a roll, and we build up a series of scenarios. It’s all about ensuring structural integrity and making sure the airframe is safe by monitoring it.
“There’s a data acquisition system from the rig, so every time each load case is loaded up, you sample the strain gauges you want; you then offload it and all that data is recorded and we can track the response of every gauge for these load cases,” he continues. “So then, if there was a problem you would see a trend in that gauge. If a frame started cracking, for example, then a gauge response might go down and the neighboring one might go up. We track that to understand if there is a problem with the test.”
Pressurization loads are also an aspect of the fatigue testing and pressurized bays can be pumped up with air, which is then released, to represent a flight cycle. To ensure the safety of the test, the cockpit area is filled with polystyrene, to limit the volume of air inside the pressurized area and a safety net is fixed over the outside of the canopy.
Acklam says there are no plans to conduct further load testing once the third lifetime is completed and AJ-1 will be removed from the rig and returned to the USA. Once back in Fort Worth, it will be subjected to a tear-down inspection to gain an understanding of any damaged areas and these individual components may then be subject to redesign.
“Once the durability testing is completed, the structure will be torn down and any anomalies will be understood and changes embodied into production aircraft, so there will no longer be a requirement for any further structural testing,” he says.
BAE Systems has also carried out fatigue testing of the F-35s empennage – vertical and horizontal tails – at Brough for a total of 24,000 hours each. For the vertical tails, a vertical tail maneuver rig and a buffet rig was designed to simulate the dynamic loads that the units will undergo during actual flight.
Static testing
Prior to the commencement of fatigue testing using AJ-1, a static load test campaign was carried out using a different F-35A airframe, known as AG-1.
AG-1 was transported from the USA by sea in early April 2009, arriving at the port of Hull on the UK’s northeast coast. From there, it was transported by barge up the Humber Estuary, before being unloaded by crane at the Brough facility on April 25. The aircraft was one of three static test and three durability test airframes manufactured by Lockheed Martin to support the SDD phase of the F-35 program. A further 13 aircraft have been dedicated to flight testing in the USA.
For the static testing, the airframe was installed in the test rig, 165 actuators replicated the loads that the airframe would see in flight, and the data was captured by 4,000 individual sensors bonded to the airframe, connected by 53 miles (85km) of wiring. At the time testing began in August 2009, BAE Systems said the computing power available to control the load applications was approximately the equivalent of 25 high-end personal computers.
“The purpose of the static test article is to provide flight clearance for the flying aircraft, so we were applying limit and ultimate loads to the airframe, to verify that it behaved as we expected it to behave,” Acklam recounts.
“We gathered data from the thousands of strain gauges attached to the airframe and correlated our analysis with those strain gauge results. We picked gauges to verify against our models, to make sure that the model behaved in the same way that the true structure did.”
Acklam says that although the F-35 testing – both static and durability programs – followed the traditional qualification certification path, advances in software have meant that testing is more automated than it has been for other aircraft.
Static testing of AG-1 was concluded in July 2010 after 295 days, which was five months earlier than originally planned, and validated that the F-35A airframe can withstand aerodynamic forces 50% beyond its structural design limits.
The strength and stability of the airframe structure was verified to 150% of design limits, or 13.5g. BAE Systems said that 174 critical load conditions, or pressures, were applied to the airframe.
Following completion of this phase of testing, AG-1 was removed from the test rig and shipped back to the USA by sea, clearing the way for the aforementioned fatigue trials on AJ-1 to begin.
A range of F-35 testing has been carried out at Brough, in addition to the more recent structural and fatigue campaigns described, including the now-complete thermo-acoustic analysis of the F-35B STOVL and F-35C CV variants. The Thermo-Acoustic Facility (TAF) at Brough was reportedly the only facility in the world designed to measure the footprint of a vertical take-off aircraft, albeit at model scale. In the case of the F-35, a 1/15 scale model was used for the test campaign.
The F-35B variant had been the subject of some criticism during flight testing with the US Marine Corps, aboard the US Navy’s amphibious warfare ships and the thermo-acoustic modeling undertaken by BAE Systems in the UK was critical to the understanding of the jet outwash generated when the aircraft is in the hover.
During the testing, the model was mounted in an anechoic chamber and its underside instrumented with 48 miniature transducers. Hot gases were then piped in to recreate the temperatures and pressures generated by the F-35Bs vectoring engine nozzle and forward lift fan. Following the F-35B trials, the TAF was used to study the effects of jet efflux on the deck of the larger US Navy aircraft carriers from the conventional exhaust nozzle of the F-35C Carrier Variant.
BAE Systems Brough
Brough is situated on the north bank of the Humber Estuary in East Yorkshire, UK, and can trace its history back to World War I, when Blackburn Aeroplane and Motor Company established an aircraft manufacturing factory and seaplane testing facility on the site.
During World War II the facility produced a number of different aircraft types for the Royal Navy, including license production of the Fairey Swordfish and Barracuda torpedo bombers. It also developed its own designs during the period, including the Firebrand single-seat strike fighter.
In 1949, Blackburn merged with General Aircraft Limited and for a decade the company was known as Blackburn and General Aircraft Limited, before again being renamed in 1959, as Blackburn Aircraft Limited.
During the post-war period, the company produced aircraft including the Beverley transport and Buccaneer maritime strike aircraft. Blackburn was absorbed into the Hawker Siddeley Group in 1960 and the name had vanished by 1963. In 1977, Hawker Siddeley and the British Aircraft Corporation were merged to form British Aerospace – today BAE Systems.
The Buccaneer was the final aircraft to be manufactured entirely at Brough, but the facility manufactured parts for other aircraft, including the Harrier and Hawk and as a result it can lay claim to being one of the longest continuous aircraft manufacturing sites in history.
Brough has also been used for structural testing since the 1970s and work has been performed in support of Buccaneer, Harrier, Hawk and Typhoon aircraft. Today the Structural and Dynamic Test Department is the structural testing center of excellence within BAE Systems and is currently being used to support the Joint Strike Fighter, Hawk and Typhoon programs.
