Using Motion Control Technology to Minimize Fatigue Related Accidents and Extend Operational Life
Fatigue testing is a critical requirement for military aircraft to determine the life span of safe, economical service and extend the fleet beyond the specified flying hours. This testing can save governments many millions of dollars by delaying the purchase of replacement aircraft
Let me provide some background on the challenges associated with this testing. You may think that one set of tests can be used for a particular aircraft around the world. Unfortunately, the mission profiles and usage varies significantly between countries such that individual testing is normally required to determine a safe life span and the extended operating hours.
Another challenge is simulating fatigue for a military aircraft. Load spectrums are gathered from instrumentation of actual flights over a period of about 12 months. From this flight data a compressed load profile is created using only the significant manoeuvre loads that cause fatigue. This loading is applied for the long term testing of an actual aircraft structure. For each hour of actual flying time the tests equate to approximately 10 minutes of testing.
This saving in test time is increased by the factors needed to validate appropriate life span providing for a margin of error on fatigue failures. Most new aircraft are provided with strain gauges for monitoring flight loading. Hence for a monitored aircraft the test must achieve 3 times and for a non-monitored aircraft 5 times the actual lifetime.
By operating the test continuously for 3-5 years or more, it is possible to complete the necessary full-scale testing hours ahead of the real flight time. It is interesting to note that more than half the test period is taken up with inspections to detect any cracking.
For more than 20 years Moog's facility in Australia has been designing and supplying custom low friction servoactuators for aircraft testing applications. We have partnered with many customers on important programs such as the FA18, Pilatus, F111 and more recently the P3 Orion test programme. The challenge Moog met was to create a specialized actuator design capable of achieving critical performance parameters required that are not possible to achieve with standard industrial cylinders. In addition, Moog developed a unique abort manifold for static and dynamic testing that is superior to that previously available.
BAE Hawk Mk127 Lead-in Fighter
Over the past 50 years, the Defence Science and Technology Organization [DSTO] in Melbourne has been widely recognized for its expertise as a world leader in the fatigue testing of defence platforms. When the Australian Government made the decision to purchase the BAE Hawk Mk 127 Lead-in fighter for the Royal Australian Air Force, DSTO was tasked with completing the programme in conjunction with BAE Systems. Hydraulic servoactuators and controlled abort manifolds for the project were designed and manufactured by Moog in Australia.
Test System Hardware
The main control system applies and monitors loads to the test structure from 83 hydraulic and 7 pneumatic channels simultaneously. Hydraulics is used to apply simulated flight loads across the complete airframe and pneumatics to pressurise the cockpit and fuel tanks. It also includes a 1,200 channel data acquisition system.
Servoactuators
With testing applications it is important to achieve critical performance parameters that are not possible with standard industrial cylinders. These include low friction, high duty cycles and structural rigidity.
For the multi-axis load test of an aircraft structure it is critical to predict and guarantee breakout and running friction for each servoactuator design. Essential factors for achieving these goals are:
- Realistic and repeatable test processes.
- An extensive database of measured values for a variety of solutions.
- Ability to design and produce small batches of custom servoactuators.
- Design and manufacture of customized sealing and bearing solutions including elastomer seals, laminar and hydrostatic.
Analysis of the Hawk test requirements indicated that most would require special design low profile elastomer seals and 15 would need "seal-less" hydrostatic bearings to achieve the extremely low friction levels specified.
Abort Manifold
About 3 years ago, Moog embarked on a development project for a new generation abort manifold to manage the controlled abort of the test should a fault condition occur.
Traditional abort manifolds use a conventional flow control valve to regulate the relaxation of the servoactuators. The new design has closed-loop pressure control using a Direct Drive Servo-Proportional Valve [DDV] so that the load is ramped at a controlled pressure using pressure transducers for feedback to the abort control system.
As part of the contract qualification, abort test rigs at DSTO were used for static and dynamic simulation of this critical aspect of the full-scale testing. Incorrect abort has the potential to corrupt test results, damage the full-scale test specimen and ground the fleet of aircraft.
Key features of the Abort Manifold design are:
- Modular construction for "active" [closed-loop] abort or conventional "passive" fixed orifice abort.
- Optimized transition from normal control to abort to minimize structural disturbances.
- Developed under aircraft standard Failure Mode, Effects and Criticality Analysis for optimal reliability.
- Extensive static and dynamic performance testing.
The contract was recently completed and integration is well advanced at DSTO for full system start up during 2005.
Aircraft test systems demonstrate Moog capabilities for improving motion control and safety for material test applications. The requirements for high performance servoactuators, controlled abort and digital system control are also common for a variety of applications ranging from flight simulators to turbine controls to high speed injection moulding machines. Moog provides high performance hydraulic and electric motion control solutions for some of the world's most challenging machine designs.
Author
Roy Park has 32 years experience in engineering, marketing and management in the hydraulics industry including the past 21 years as Managing Director and Site Manager for Moog Australia. He has a B.E. honors degree in mechanical engineering from Monash University.