Aircraft Structural Strength
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Aircraft Structural Strength
I don't know whether you clever folk have seen this thread over on Mil Av but I suspect some of you could give us some really sensible answers (preferably in words of 1 syllable, and without an MSc in maths).
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A copy of the post I just made on the referenced thread:
Sorry, multiple syllables, but I'm far from a math expert!
"Happily, the "strength" of an aircraft structure is complex and multi faceted. There are many design opportunities to optimize the design. Strong is nice, but it's usually either very heavy, or very expensive. But too strong may eventually break, either by overload, or fatigue. As India Two Four shows, "strong" is better when it is carefully balanced with flexible, as the flexing will relieve the need to carry the load somewhat.
Changing an airframe by "strengthening" requires a lot of applied knowledge. Just adding a gusset or doubler here and there can have the effect of shifting the load to be carried to another part of the airframe, which is perhaps even less able to sustain those loads, and possibly eve concentrating the load at that more vulnerable place.
The change to the structure can also cause the intended flexing to happen differently, and disastrously. I recall reading of an early German monoplane, which suffered wing failure during test flight. The designer strengthened the wing spars, and failed at even a lesser load. By strengthening the spar, the designer unwittingly changed changed how the wing flexed under G load, and now, as G was pulled, the wings flexed so as to increase their angle of incidence near the tips, and ripped them off. The spar had been strengthened behind the flexing axis of the wing. So if you're going to stiffen a wing structure, the leading edge is probably the place to do it.
The "get you home" properties of an airframe are often as much about multiple load paths, as a "strong" or "tough" structure. It's nice to have a wing with many spars (Lear Jet, for example), so if a spar fails, the rest will get you home. COnsidering the durability of the dH Mosquito. it was wood, with so many load paths that if you shot away some, the rest would carry the load, hoping of course that the pilot then recognized the need to handle compromised airframe with a gentle touch.
This nice characteristic of a well designed aircraft is found in many good designs as layered structure, so one layer can fail, and the others carry the load. "Built up" wing spars are generally an example of this, where multiple pieces make up the spar caps, so if one is broken the others carry the load until you get home.
The back side of this is the possibility of a "latent failure" such that an element of the structure has been damaged (fatigue/corrosion/overload) but the associated structure is still carrying the load, so the failure is undetected. Good inspectability is vital.
Aircraft structure is a fascinating subject to learn, and there is seemingly endless material."
Changing an airframe by "strengthening" requires a lot of applied knowledge. Just adding a gusset or doubler here and there can have the effect of shifting the load to be carried to another part of the airframe, which is perhaps even less able to sustain those loads, and possibly eve concentrating the load at that more vulnerable place.
The change to the structure can also cause the intended flexing to happen differently, and disastrously. I recall reading of an early German monoplane, which suffered wing failure during test flight. The designer strengthened the wing spars, and failed at even a lesser load. By strengthening the spar, the designer unwittingly changed changed how the wing flexed under G load, and now, as G was pulled, the wings flexed so as to increase their angle of incidence near the tips, and ripped them off. The spar had been strengthened behind the flexing axis of the wing. So if you're going to stiffen a wing structure, the leading edge is probably the place to do it.
The "get you home" properties of an airframe are often as much about multiple load paths, as a "strong" or "tough" structure. It's nice to have a wing with many spars (Lear Jet, for example), so if a spar fails, the rest will get you home. COnsidering the durability of the dH Mosquito. it was wood, with so many load paths that if you shot away some, the rest would carry the load, hoping of course that the pilot then recognized the need to handle compromised airframe with a gentle touch.
This nice characteristic of a well designed aircraft is found in many good designs as layered structure, so one layer can fail, and the others carry the load. "Built up" wing spars are generally an example of this, where multiple pieces make up the spar caps, so if one is broken the others carry the load until you get home.
The back side of this is the possibility of a "latent failure" such that an element of the structure has been damaged (fatigue/corrosion/overload) but the associated structure is still carrying the load, so the failure is undetected. Good inspectability is vital.
Aircraft structure is a fascinating subject to learn, and there is seemingly endless material."
Educated Hillbilly
Stress analysis is one of those subjects where you never stop learning (bit like flying in one sense), as you learn more you begin to realise how little you actually know.
