Quote:
Originally Posted by TexasGR
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So, for aircraft using original construction methods that are all wood (e.g. Nick's Snipe) or aircraft with wooden spars (e.g. Jeff Brooks' DVII projects and the various DR I projects), would the repeated stresses eventually lead to weakening of the spars or other critical structural parts?
How would an owner/builder determine an acceptable lifetime for use in aerobatics?
Or is it just a matter over never over-stressing and routine inspections?
Can inspections reveal the most likely issues and/or incremental damage?
...and then what about metal fatigue, particularly on welds?
I read about replicas involved in aerobatic displays, so I know it can be done.
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Thanks,
Brian
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Brian, you have written quite a bit, so I think I'll try to answer some of your easier questions.
As far as wood structure goes, my understanding is that wood does not fatigue but it can deteriorate and it can be overstressed. Inspections of wood structures would then be oriented to detecting these latter two conditions.
Metal fatigue of the welds themselves should not be a problem since properly executed welds are stronger than the adjacent tube. You do have to worry about the heat affected zone next to the welds because this can become embrittled if the weld is not done properly. Once the area around a weld is cleaned and painted, about the best you can hope for is to detect cracks in the structure using non destructive test methods.
The whole subject of strength of experimental aircraft and particularly replica aircraft is something to approach cautiously. Historically, aircraft load limits have been set at 2/3 of ultimate strength of the structure.
If you can test enough of your structure to determine it's ultimate load capability then you have a starting point. If you can't test enough of your structure to make an ultimate load determination. Pick an operating g load limit that seems reasonable but cautious and then monitor for damage while operating at that limit. Look at Achim Engels thread. He has done a lot of structural proof testing of sub-components.
I've seen an F-4 come back after pulling 12.5 g or so, but there was some damage. Most planes aren't quite that tough.
Sid