• Become a Subscribing Member today!

    The Biplane Forum is a large global active community of biplane builders, owners and pilots. From Pitts to Skybolts, to older barnstormers, all types are welcome.

    The Biplane Forum is a private community. Subscriptions are only $49.99/year or $6.99/month to gain access to this great community and unmatched source of information not found anywhere else on the web.

    Why become a Subscribing Member?

    • In addition to our active community, our content boasts exhaustive technical information which is often sought after for projects and maintenance. This information has accumulated over the 12+ years the forum has been in existence.
    • We are also a great resource for non biplane users, since many GA aircraft are built the same way (fabric and tube construction).
    • Annual membership also comes with two BiplaneForum.com decals.

    Become a Subscribing Member and access the Biplane Forum in full!

    Subscribe Now

Builders Manual - Problem Areas -Spar Butt Attach

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

wandersen

Well-Known Member
Joined
Jul 31, 2007
Messages
519
Reaction score
140
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





 

Latest posts

Back
Top