Torque value - bolts through wood spar

[Ed Haywood]

I am trying to find torque specs for the AN5 bolts that attach the strut-to-spar brackets to the wood spar on a Decathlon.

My efforts are being hampered by the fact that American Champion does not service wood spar aircraft and will not provide tech support on wood spars.

I vaguely recall reading service guidance at one time to be careful not to over torque and crush the wood. I also ran across a statement from SGOTI that there are bushings to prevent crushing. However, I cannot find anything in the service manual or parts manual. At one time there was a service letter for the bracket, but it has been rescinded. I have a factory drawing for the bracket but it has been rescinded.

I'm probably just going to use general torque values. Minimum torque for AN-5 with AN-310 primarily in shear is 120-140 in-lbs. Before I go with that, does anyone have specific experience with wood spars to address the crushing concern?

 

[Tools]

The bushings are also there to provide pull out strength…

I’ve had the wood spar wings off my biplane and my Aeronca Chief. Maybe check the manual for a Chief, as they’re still around and haven’t been deliberately pulled.

Also maybe find an older 43.13. I have an older one and can check after work today.

When I’ve removed these bolts, the “tightness” felt the same as through bolts on metal spar wings.

 

[geezer]

SGOTTI here

If there is a bushing, the torque is much higher than if none.

Crushing wood with the higher torque will ruin the spar, a friend did that.

Checking for a bushing is the first step in doing this correctly.

Maybe Aircraft Spruce has tables of correct torque for various woods and dimensions?

I crushed some wood on my boat, fine thread bolt, spec. was for coarse threads, crushing pressure was double what it should have been. Over drilled the hole, epoxy filled, redrilled with proper size, and good to go. That was not an aircraft spar.

 

[Ed Haywood]

I think I have enough confirmation that bushings are present to proceed with standard torque.

 

[kyleb]

There are bushings. Aeronca used phenolic, American Champion uses aluminum.The question remains about torque - this is a pinned triangular truss. Do the bolts get firmly tightened or are the joints designed to wiggle a bit with the bolts finger tight or something like that?

 

[Bell206]

At one time there was a service letter for the bracket, but it has been rescinded. I have a factory drawing for the bracket but it has been rescinded.

Just because it has been "rescinded" does not mean it cannot be used as a reference. The "current" term used in Part 43.13 applies to what was current at the date of aircraft mfg and not what is current today. If that makes sense. Post what you have.

I'm probably just going to use general torque values. Minimum torque for AN-5 with AN-310 primarily in shear is 120-140 in-lbs. Before I go with that, does anyone have specific experience with wood spars to address the crushing concern?

Guidance states if the OEM does not present a specific torque then you are to use standard torque. Crushing the spar is a concern just like with wooden props. Perhaps read up on torquing wood props to give you some more background on this topic? I also find it interesting that American Champ will not give you torque values for installing their wings. I would push on that point a bit more if it were me.

 

[Ed Haywood]

The question remains about torque - this is a pinned triangular truss. Do the bolts get firmly tightened or are the joints designed to wiggle a bit with the bolts finger tight or something like that?

That's what I'm trying to find out.

 

[Ed Haywood]

Just because it has been "rescinded" does not mean it cannot be used as a reference.

The issue is finding it. The SL was rescinded 25 years ago. I have found enough references to confirm it existed, but no actual copies. I have the part drawing, but no torque values are on it.

Crushing the spar is a concern just like with wooden props. Perhaps read up on torquing wood props to give you some more background on this topic?

I did that. However, there are also some differences. Loosening caused by shrinkage can be a big deal with wood props, but is less of a concern for spars. The torque values I found for wood props, 22-27 ft lb, are probably excessive for a spar with an AN5 bolt.

I also find it interesting that American Champ will not give you torque values for installing their wings. I would push on that point a bit more if it were me.

These are Bellanca wings. American Champion has never made or sold a wood wing. After purchasing the defunct Bellanca assets, they redesigned the spar to aluminum before resuming production.

I think they supported older wood wing aircraft in the early years, but that stopped when they were dragged into court for a couple of crashes, followed by the FAA AD on wood spars. Now their official position is not to touch them. When they first started selling metal wing upgrades, they required wood cores in exchange, which they destroyed. All about liability.

 

[Bell206]

These are Bellanca wings. American Champion has never made or sold a wood wing. After purchasing the defunct Bellanca assets, they redesigned the spar to aluminum before resuming production.

Technically, hardware on an aircraft requires standard torque unless specified by the OEM. So at a minimum, if you can't find any OEM torque values go with standard torque on the spar bolts from an acceptable reference like AC43.13-1B, Standard Aircraft Handbook, etc. But since you reference Bellanca, perhaps PM @Rob58 since he owns the Bellanca company and he may have some old guidance in the archives for you? The Vikings had wood spars so he might have something applicable.

 

[MauleSkinner]

I’m recalling from my attempt at Staggerwing restoration that the bushings were slightly shorter than the thickness of the spar so that the bolt held the spar, not just the bushing. Would that present the possibility of over torquing the fastener and crushing the wood?

 

[Ed Haywood]

I’m recalling from my attempt at Staggerwing restoration that the bushings were slightly shorter than the thickness of the spar so that the bolt held the spar, not just the bushing. Would that present the possibility of over torquing the fastener and crushing the wood?

Damned if I know. Wood spars are a dark art, apparently.

 

[MauleSkinner]

Damned if I know. Wood spars are a dark art, apparently.

If you end up using “standard” torque values, I’d probably try a test piece first.

 

[Bell206]

Would that present the possibility of over torquing the fastener and crushing the wood?

No. The use of a bushing in most cases is to provide a larger load bearing surface for the attachment. And the requirement for the bushings to be "slightly shorter" is to ensure the bolt clamp up is on the spar and not only the bushing. And just to note, I've installed more spar bolts at standard torques than any other value.

 

[Ed Haywood]

No. The use of a bushing in most cases is to provide a larger load bearing surface for the attachment. And the requirement for the bushings to be "slightly shorter" is to ensure the bolt clamp up is on the spar and not only the bushing. And just to note, I've installed more spar bolts at standard torques than any other value.

At this point I'm comfortable going with standard torque and seeing if that resolves my issue.

 

[Dan Thomas]

The question remains about torque - this is a pinned triangular truss. Do the bolts get firmly tightened or are the joints designed to wiggle a bit with the bolts finger tight or something like that?

No wiggle allowed between the brackets and wood. If it starts to move it will destroy itself.

The torque values I found for wood props, 22-27 ft lb, are probably excessive for a spar with an AN5 bolt.

The hard maple Colin Walker prop I had on my Jodel used 3/8" bolts, torqued to 175 inch-pounds, a hair under 15 foot-pounds. But there were no bushings to take the compression by the bolt.

I’m recalling from my attempt at Staggerwing restoration that the bushings were slightly shorter than the thickness of the spar so that the bolt held the spar, not just the bushing. Would that present the possibility of over torquing the fastener and crushing the wood?

The wood needs to be held firmly, but not so much that it gets crushed more than a tiny bit.

 

[Lindberg]

Have you asked in the Citabria/Decathlon FB group?

 

[kyleb]

No wiggle allowed between the brackets and wood. If it starts to move it will destroy itself.

In Aeroncas, the he strut (and fuselage attach) bolts don't pass through the wood at all. They attach to steel weldments that are bolted to the spar. Certainly you can't have movement there. Those weldments have cantilevered ears or lugs where the struts and fuselage attach. I assume (?) Citabrias and Decathlons are the same.

 

[Dan Thomas]

In Aeroncas, the he strut (and fuselage attach) bolts don't pass through the wood at all. They attach to steel weldments that are bolted to the spar. Certainly you can't have movement there. Those weldments have cantilevered ears or lugs where the struts and fuselage attach. I assume (?) Citabrias and Decathlons are the same.

That's what I meant. The spar has to be free to pivot on the bolt holding the bracket to the fuselage carrythrough. The spar bends in flight somewhat. We don't want the wood moving inside the bracket.

Pretty much all strut-braced airplanes are the same.

 

[Silvaire]

The general rule, learned through decades of trial and error is: tight is good, tighter is better, until it isn't.