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Davidson Measurement - Defence Product Article

AMRL & Davidson Measurement give the F/A-18 Hornet an extra buzz

Australian engineers have assembled an extremely effective fatigue test rig which is able to subject the aft fuselage, empennage (horizontal and vertical tail surfaces) and engines of an F/A-18 Hornet jet fighter to realistic inflight loading conditions without the aircraft having to leave the ground.

The full scale fatigue tests are carried out at the Aeronautical & Maritime Research Laboratory (AMRL) Melbourne by the Defence Service and Technology Organisation (DSTO) and are part of what is referred to as International Follow-on Structural Test Project (IFOSTP). This is part of a collaborative program with Canada where more conventional fatigue tests are being conducted on the centre fuselage and wings.

Australia's F/A-18's valued at approximately $35 million each, are the nation's front line air defence asset and the government plans to keep them in service until 2015. This requires each aircraft to have a flight life availability of approximately 6000 hours and many of the aircraft are now approaching 3000 hours. The IFOSTP will confirm the F/A-18's flight life availability to be 6,000 hours or greater. In fact the test program is scheduled to run to 20,000 hours to cover material fatigue variability. DSTO's Vibration Laboratory Manager, Chris Rider, said "at a program cost ranging from $30 to 40 million this is the biggest single funded test program in the DSTO but it represents excellent value for money for taxpayers as it establishes the life of the aircraft and reduces the possibility of aircraft loss due to airframe cracking and ultimate failure.

The significant feature of the test procedures is the complex loading environment on the empennage which combines broad based buffet loading with manoeuvre loading. The loading method for the tests is a combination of pneumatic actuators (air springs) for the manoeuvring loading and eight Ling Electronics electromagnetic shakers supplied by Davidson Measurement for the buffet loadings.

The shakers are identical units which were custom made for the test rig by the Ling Electronics Corporation in the USA. They have a transient force capacity of 5,000 lbf, a continuous force capacity of 1,700 lbf RMS, a working stroke of 4 ins. and a full armature travel of 4.3ins. The fin and stabiliser shakers are attached to seismic masses (lead cylinders) and all shakers are isolated from the rig.

The initial configuration of the electromagnetic shakers was a single shaker on each stabiliser and a push-pull pair of shakers on each fin. However, development testing revealed the engine vibration was not being excited to a representative level. As a back-up measure two more shakers were added to apply complementary vertical dynamic loading to the engines.

The test program is looking for cracks and defects in the airframe before the RAAF fleet reaches the number of hours achieved by the test rig. If the test program reveals a structural failure AMRL aeronautical engineers design a repair, rectify the test aircraft section and keep running the test to ensure the integrity of the air-frame.

The test rig also incorporates PCB accelerometers and a PCB modal data acquisition system supplied by Davidson Measurement. Davidson Measurement is a leading supplier of vibration excitation equipment covering everything from 10 Newtons full scale to 178,000 Newtons.

Test load capabilities extend from very small components to larger, complex assemblies like compressors, generators and even defence force missiles. Davidson Measurement also trains technicians to use the test equipment it supplies and provides full service and support for the shakers and amplifiers.