wandersen
Well-Known Member
- Joined
- Jul 31, 2007
- Messages
- 519
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Hello Everyone,
I've started my build on my Skybolt. I've purchased the fine Builder's Manual - thanks for all the great information.
I've got a question from the Builder's Manual, Section X-C2 - Problem Areas.
Problem 11 talks about the increasing the pull-out strength of the lower spar butt attach fitting bolts by installing 5/8" phenolic bushings into the lower wing spar butts. I've done some searches in the archives, but didn't come up with any discussions on this mod. I think this concern is based on the CEAA evaluation and analysis of the Skybolt from the late 70s. The CEAA evaluation indicated that this is one of the potential weak points in the Skybolt structure.
The intro paragraph of Section 11 indicates that this modification increases the spar butt bolt pull-out strength by a factor of 8.
I'm interested in thoughts and best practices around this modification. An engineering analysis would be ideal
My back of the envelop calculation indicate that this potentially could increase the pull-out strength by 2.5 times (not 8x), given the increase in area of the 5/8" bushing vs the 1/4" bolts.
However, I've read some rules of thumb that you don't want structural bolt/bushing holes closer than 3x the diameter from the edges. Also this rule applies between the bolts/bushings as well (subject to some relief given grain direction). One old reference for this is ANC-18. Unfortunately, this reference doesn't address allowances for decreased edge/bolt spacing. The question becomes what is the actual increase in strength from this mod. The concern is could this modification actually decrease the spar butt attach strength given the decreased edge and bolt/bushing spacing distances.
I'd be interested in thoughts on this mod. Any information on whether this is an acceptable practice?
Thanks,
Ward
I've started my build on my Skybolt. I've purchased the fine Builder's Manual - thanks for all the great information.
I've got a question from the Builder's Manual, Section X-C2 - Problem Areas.
Problem 11 talks about the increasing the pull-out strength of the lower spar butt attach fitting bolts by installing 5/8" phenolic bushings into the lower wing spar butts. I've done some searches in the archives, but didn't come up with any discussions on this mod. I think this concern is based on the CEAA evaluation and analysis of the Skybolt from the late 70s. The CEAA evaluation indicated that this is one of the potential weak points in the Skybolt structure.
The intro paragraph of Section 11 indicates that this modification increases the spar butt bolt pull-out strength by a factor of 8.
I'm interested in thoughts and best practices around this modification. An engineering analysis would be ideal
My back of the envelop calculation indicate that this potentially could increase the pull-out strength by 2.5 times (not 8x), given the increase in area of the 5/8" bushing vs the 1/4" bolts.
However, I've read some rules of thumb that you don't want structural bolt/bushing holes closer than 3x the diameter from the edges. Also this rule applies between the bolts/bushings as well (subject to some relief given grain direction). One old reference for this is ANC-18. Unfortunately, this reference doesn't address allowances for decreased edge/bolt spacing. The question becomes what is the actual increase in strength from this mod. The concern is could this modification actually decrease the spar butt attach strength given the decreased edge and bolt/bushing spacing distances.
I'd be interested in thoughts on this mod. Any information on whether this is an acceptable practice?
Thanks,
Ward
