Jonathan Newell talks to Siemens PLM about how digitally linked testing and verification methods are helping the aerospace industry meet new design challenges without getting in a flap over aeroelastic flutter.
Test pilots in the heyday of aviation routinely ventured into unknown territory, trusting the calculations of the engineers and designers and exploring the limits of performance and structural integrity.
With decades of improvement in design software, ground testing, simulation and experience, it still isn’t possible to eliminate the need to perform test flights but now the pilots have high levels of confidence that the aircraft will perform as intended.
One highly complex phenomenon that needs to be tested is aeroelastic flutter, something the test pilot wants to avoid and which is most likely to occur at the boundaries of the flight envelope.
Flutter is a dynamic instability of an elastic structure in a fluid flow, It occurs when there is a convergence of multiple modes of resonance and damping. This causes the wings or control surfaces or other airframe components to vibrate uncontrollably and can result in catastrophic airframe failure.
Verification and Testing
According to Raphael Van der Vorst, Aerospace & Defence Business Development Manager at Siemens PLM Software, the industry has the constant challenge of meeting tighter regulations, facing increasing competition and catering for emerging technology. The need to rapidly innovate doesn’t fit well with extended testing and verification processes, which is why integration of data throughout the design, test and verification is so important.
The Siemens PLM approach is to support verification through a number of methods, which include analysis (CAE, Simulation, FEA) and physical testing (wind tunnel testing, ground vibration testing, flight testing). Data management is a critical element of this, as explained by Van der Vorst.
“The industry has seen a lot of data disconnect in the past with design, test and engineering working in silos. Within the PLM software, design, CAE- and test data are synchronised to the initial requirements so that the purpose and importance of the data isn’t lost,” he explains.
This data traceability enables closed-loop requirement traceability into all activities of the verification process to confirm requirement compliance.
An example illustrated by Arne Vollan, a partner and Senior Engineer at AeroFEM GmbH, is the flutter analysis performed with Finite Element Analysis using NX NASTRAN. AeroFEM uses the software to create complex structural and aerodynamic models. The models are used to determine responses at certain frequencies that are compared to Ground Vibration Test (GVT) and flight flutter test results obtained with Simcenter Testlab.
Frequency / velocity curves are plotted to produce flutter diagrammes and predict instabilities. The data from these models can be used to determine where accelerometers should be positioned for verification testing.
The aviation industry is going through significant change with electrification, new materials and modern manufacturing technology aligned to alternative structural compositions as a result of 3D Printing.
All of these technology innovations affect susceptibility to flutter. “Electric motors in general aviation are more susceptible to the phenomenon of whirl flutter due to their lower inertia,” Vorst tells me.
Whirl flutter is the same phenomenon applied to rotating systems, such as aviation engines, and introduces additional complexity to the overall consideration of flutter. As this complexity increases, the imperative grows to apply good data management practices to the whole test and verification process.