Static Structural Testing
Perhaps the most important reason for carrying out structural testing is to validate the previous sets of calculations used when designing the structure. Ideally, one would replicate the load
Figure 16.12 Clamping system for main spar.
Figure 16.13 Wing assembly under sandbag load test.
case and support conditions used during analysis in the structural experiment. This is rarely possible for full flight conditions without access to a wind tunnel, as generating detailed pressure variations across wing surfaces is very difficult using weights or force actuators. If a wind tunnel is available, attempts can be made to set up displacement measurement indicators at wing tips and then record deflections at various air speeds and AoAs. If this approach is to be followed, care needs to be taken to ensure that the measurement system does not significantly impact the flow field. It is also difficult to generate significant airframe loads in a tunnel test without exciting noticeable vibrations in the structure. Consequently, we normally restrict our wing structural tests to sandbag loadings and match these against calculations set up to match what we can achieve in the laboratory. Then, if good agreement is gained, one can be confident that results for other load cases will also be acceptably accurate.
Another key purpose of structural tests is to establish critically loaded parts and failure mechanisms by increasing loads until the point of failure is reached. In general, airframe structures do not fail completely catastrophically without there first being significant signs of distress. While these may not be readily visible during flight, they should be observable during a controlled lab test. Such tests can reveal those areas of the structure that need further design effort. They will also show which parts of the structure are relatively unaffected by extreme loading and are thus good candidates for weight-saving exercises. Figure 16.14 shows the onset of failure in an selective laser sintered (SLS) nylon wing part in the area between the main wing spar penetration and the torque reaction peg. Figure 16.15 shows an undercarriage leg and SLS mounting structure under load test.
Figure 16.14 Partial failure of SLS nylon structural component during sandbag load test. Note the significant cracks and large deformations.
Figure 16.15 Load testing of an undercarriage leg and associated SLS nylon mounting structure. Note the dummy carbon-fiber tubes present to allow the SLS structure to be correctly set up.