main spar bolts - Sling uses AN - sacrilege in the RV world

[johnbright]

Helped a fellow put wings on a Sling today. I was surprised to learn Sling main spar bolts are AN, 125 ksi tensile, 76 Ksi shear, +/- .0015” on the diameter whereas RVs use NAS66, 170 ksi tensile, 95 ksi shear, +/- .0005” on the diameter.

I’m curious what those on VAF who know about bolted joints think. Maybe the aluminum spar plastically deforms until the bolts share the load?

 

[PaulvS]

Helped a fellow put wings on a Sling today. I was surprised to learn Sling main spar bolts are AN, 125 ksi tensile, 76 Ksi shear, +/- .0015” on the diameter whereas RVs use NAS66, 170 ksi tensile, 95 ksi shear, +/- .0005” on the diameter.

I’m curious what those on VAF who know about bolted joints think. Maybe the aluminum spar plastically deforms until the bolts share the load?

It is all in the engineering and the bolts may be designed around their clamping force, just like many of the bolts in some RV wings.

The Sling wing bolts are the same as those that hold on my RV engine. The wing diagram shows twice as many in each wing.

 

[Freemasm]

I'll see if I can make the structural engineers choke a little.

This is a pure shear application. The joint service life integrity is mainly a product of the fastener/hole geometry -> the bolt torque is basically along for the ride. Can't speak for the designers but this is most likely why Vans chose close tolerance bolts here. Loading cycles, especially reversing loads that would be more associated with aerobatics, could cause fretting over time and would invite associated fretting corrosion. You didn't mention the Sling model and I'm not familiar with the product line but I'm going to assume the subject airframe is not intended for aerobatics. That would be the most obvious reason to allow a "standard" fastener selection, to me at least. Even if this assumption is incorrect, the designer could have applied different techniques to account for such but it's not the way I'd bet. The last thing to get worried over would be teh relative fastener strengths. These are very easy and honest parameters to design with/around.

Hopefully someone specifically skilled in this art such as Mr. David Paule from CO or possibly SCSmith will weigh in.

 

[Snowflake]

You can design and manufacture holes to fit any bolt, so there's no reason to believe the bolts are rattling around in their spots. And you can use proportionally more of them to account for the lower shear strength per bolt. The wings on my -6 are held on with something like 30 bolts a side, vs. an RV-7 with four (I think?). As someone said, it's all in the engineering.