Structural Mounting and Loading
To carry out a load test on an airframe or airframe part, it is important that a safe and secure mounting system is devised that not only holds the parts under test but also prevents any damage being caused to the airframe by the actions of the mount itself. It is very easy to crush fragile aircraft fittings by the clumsy application of clamps or by resting parts over the sharp edges of hard work surfaces where stress concentrations can occur. Equally, however, it is important that any flexibility in the mounting system is accounted for when taking measurements of part deflections. So, for example, when testing a wing or wing spar, a mounting has to be provided that stands in for the fitting to the main aircraft fuselage. We typically use wooden supports that surround the Carbon-fiber-reinforced plastic (CFRP) spar tube, which can then be clamped to the spar and a hard point in the laboratory, sometimes with the extra precaution of a binding of tape between the wood and the spar, see Figure 16.11, or using a specially made
Figure 16.11 Mounting system for wing and main spar assembly under sandbag load test.
clamping block with foam liner, see Figure 16.12. We would never apply clamps directly to the spar itself for fear of causing local damage. An alternative, better but more expensive, solution is to make a subsection of the fuselage mounting in the same material as the final design will use and treat this part as being sacrificial: this more accurately simulates the actual support the spar would see, of course. We also ensure when carrying out tests with dead weights that should anything fail during test, the weighted items cannot fall very far, that they will land on suitable soft surfaces, and that staff will not be injured should this happen. Although we do not always deliberately cause parts to fail under structural test, one must always allow for this to happen and take appropriate safety precautions. Figure 16.13 shows a wing assembly being tested with sandbag loading.