I have a guest blogger today, Presson Sr. who shared some impressive information about the wing tests conducted on Boeing’s new 787 Deamlinder. I added some notes in <>.
And now to his first post:
All our planes, commercial and military, undergo both static (stationary) and fatigue (lifetime) testing before the design is considered satisfactory. The static test gradually increases loading on surfaces, and testing to 1.5 times the "design maximum" is standard, to provide a safety margin. Here in St Louis, we usually refer to that as the design limit loads test. We reserve the term "ultimate load" test for the next level, which is to continue adding loads until the item fails (breaks) – hence, the term "ultimate". It is amazing to see how much the wings bend as it gets to that ends of the testing – without breaking.
The video illustrates how the loading is applied, with hydraulic forces being spread out across the wing length to smoothly load the whole wing, not just the tip or some specific point. <The voiceover gets a bit too rah-rah for the team, but it’s still interesting to watch.>
The other major kind of test, fatigue, generates alternating up and down loads (and other kinds* of loading) similar to what the plane will experience in service, and continues the test for days, weeks, and longer, so that the structure gets – literally – a ‘lifetime’ of wear and tear. A month of testing may represent a year of actual service, depending on how many hours per day it is expected to fly. And it’s more complex, since the spectrum of loads used in the test varies to represent takeoffs and landings, climb and descent, cruise, maneuvering, dogfights, and whatever – the so-called "operational mission environment."
Metals do wear out from repeated bending – microscopic discontinuities in material become small cracks after continued service, and small cracks continue grow to become bigger ones. It always surprised me that a crack inspection (no giggles, now) of an airplane in service (done periodically) might result in a note to take another look at some area in between normal inspections. <In other words, small cracks can be ok – don’t think about it when you’re on an airplane.> Crack propagation (how they grow) is a whole field of interest. During the fatigue testing, inspections are done, and this testing also goes "beyond nominal" – I think they do at least 2 lifetimes to be sure. And maybe more.
It can be a great selling point that a plane lasts and lasts (kind of like the bunny). However, we also have to watch out for overdesign. If it lasts too long, we obviously made it too strong, which means too heavy (nasty, nasty), and too expensive (also nasty). The key is continual feedback so we know that we can trust our estimates and our tools – so that when we design for XX performance, the item performs at XX.
* Some are "symmetric", as when both wings bend up, others are not, as during a roll, when one wing bends up and the other down. Obviously, military aircraft tests are a lot wilder. There’s also a drop test, to simulate the vertical impact of an extreme landing (as on a carrier) – which is referred to as a "controlled crash".”