Nut tensile required for forward horrozontal stab mount point

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Flow

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Hi folks, we are looking to add an other 1/4" of 5ply on top of this doubler and struggling to get the right length AN bolts. If we use a thin series nut MS21083-428A with a min tensile strength of 2290lbs and one washer rather than two in the photo below then an AN4-66A will work we cant get a -67 unfortunately.

The existing MS21044-428A nut has a tensile strength of 4580lbs. The question is will the wood break before the thin series nut or do I need to stick with the 4580lbs nut.

Here is a photo of the doubler we are planning to add an additional 1/4" of 5ply to.

200226_023B.jpg
 

BoKu

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Perhaps not very helpful, but:

* I would use a thin AN364 nut only in an application loaded primarily in shear.

* I would be leery about using a thin washer directly against wood. Or rather, I would be leery about using an AN960 washer of either thickness in wood where the attachment is torqued to any substantial fraction of the spec value for the bolt and nut. I'd want to see an AN970 large-diameter washer, and those only come in one thickness.

* That is one long-ass bolt! I see that Aircraft Spruce has AN4-72As, I'd probably use one of those and stack a disk of plywood under the far end to make it look like it was supposed to be that way.
 

Dana

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The wood will probably break before the nut gives way, but it's still bad practice to use a thin nut for tension applications. It's also bad practice to have threads inside of the parts you're fastening together. Of course, you're also not supposed to use more than two washers, but sometimes you have to.

If it were me, I'd get the shortest available bolt longer that keeps all the threads outside the wood, and machine an aluminum spacer to make up the difference either under the nut or under the head (or both).
 

Dan Thomas

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If it were me, I'd get the shortest available bolt longer that keeps all the threads outside the wood, and machine an aluminum spacer to make up the difference either under the nut or under the head (or both).
That's what I would do. Bandsaw a bit of 1/8" aluminum sheet or a piece of angle, drill it, and use it as a shim. Use a 7" bolt (AN4-70A).
 

Flow

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Great advice guys thanks,

So the preference is for 1/8th Aluminum over 1/16 x 1.125" AN970 washers?
I currently have the AN970 washers on the top.
I will look into MS or NAS bolts in the 6.906" range.
I could also use the flanged MS21042-4 self locking nuts with ~6000lb tensile. These are short and stronger than the full length nylocks but don't do well when adjusting for the seasons.
 

Flow

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That sound like a great idea, any recommendation on items compatible with AN4 and 5 ply, are you supposed to wind them up to flat in winter? Any issues with point loading on the edges?
 

Dana

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I've never heard of adjusting airframe bolts for the seasons. Propellers, sometimes, but the clamping force is a lot more critical for a prop.
 

Hot Wings

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I've never heard of adjusting airframe bolts for the seasons.
Nether had I until about 6 months ago. Found a note in my AV-36 documentation recommend checking the aileron hinge bolts periodically for this reason. They only go through 12mm of wood so I'd not have expected them to be a problem!?

With a 7 inch-ish long bolt if it needs the tension well controlled then I can see understand considering spring washers.
 

Mad MAC

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That sound like a great idea, any recommendation on items compatible with AN4 and 5 ply, are you supposed to wind them up to flat in winter? Any issues with point loading on the edges?
I have to admit that I have never seen it in the wild, only heard about it. Given the crush strength of plywood / spruce, the preload would be comparatively low. I had a dig through the Brown catalogue (1950's English aerospace hardware catalogue) and couldn't see anything likely so possible they were all airframe specific). I guess could you make nice ones from flat discs punching the centre down to give the spring (maybe 6061-T6 or 304 stainless) once you know the required preload and the relative movement required.
 

TFF

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I would be putting a steel or aluminum crush plate backing the wood. I would use the stronger hardware just because. The other is probably strong enough, but because you are pulling through wood, it will cut itself through in time without a large spread. At least a square that matches the doubler width.
 

wsimpso1

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Just looking at nut strength is not adequate for determining if you could or could not use a particular nut in an application. A little fastener theory is required in understanding joints where the bolt and nut are put in torqued and clamp the joint:
  • Bolts and threaded rods are under tension - they get longer when torqued, and the increased length means thread spacing is increased slightly. This includes the threads inside the nut;
  • Nuts are under compression - they get shorter when torqued, and the decreased length means thread spacing is decreased slightly. This includes the threads around the bolt;
  • The change in thread spacing is highest at the face of the nut in contact with the bolted members, and drops as you go away from that surface. Usual theory says the load carrying is all done in the first three or four threads;
  • Max strength in a nut is achieved when the first couple threads yield, distributing the load much better in both the nut and bolt. Unfortunately, you can not use this level of strength in a fastener that will be adjusted over time:
    • The nut will now be shorter and thus stiff in rotation and will not adjust well, nor will torque be usable for determining load in the joint;
    • The first three to four threads of the nut are very close to stripping out;
    • Shear nuts have few threads - if the first three to four threads are close to stripping, the whole shear nut is close to stripping. Think about it, if the first three are at yield and you load the joint a little bit more, the first three threads move and now the fourth thread is at yield. A tiny bit more load and a shear nut's threads unzip completely, stripping the nut and failing the joint;
  • Now if you make the female thread harder than the bolt, you can torque the bolt to yield, but you are usually using a longer female thread for this and accept bolt thread damage on each tightening cycle, which usually makes the bolt a one use item. Loosen it and you have to throw it away and use another one.
The upshot of all this is that for bolted joints that are clamping a joint, we use longer nuts. Shear nuts are used where we have the bolt used as a pin in shear.

Could you get away with a shear nut on a clamped joint that has low loads? Maybe it would work for a while. If the loads are that small, maybe you should ask yourself why the designer did not go one size smaller on the bolt diameters. I seriously doubt that the loads are that low or the designer overspecified the bolts that much... Let's also remember that our best practices were usually written in the blood of wreck victims. Find the right length bolt. Use the right type nut. Safety the joints per design. Then smile because you know you will be able to disassemble and reassemble it later if need be, and because you did it the right way...

Billski
 

Flow

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Epic as always Bill, after installation it is looking like the 66s have been a better call than 67s as the previous 65s were pretty much thread bound.

So the MS21042-4 and MS21044-428A have the same number of threads while the 42s have a more tapered lead thread and the 44s are cut more abruptly. The 42s use a metal self locker the 44s use nylon.

Would the 42s be suitable for this application in your mind?
Also what are your thoughts on using a conical washer to manage seasonal changes?

MS21042-4 - 6200lbs yield, 5 threads


MS21044-428A - 4580lbs yield, 5 threads
 

Hot Wings

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Also what are your thoughts on using a conical washer to manage seasonal changes?
Not going to answer for him, but it matters what this bolt is doing. How critical is it to maintain clamping pressure? We don't normally worry about this with bolted wood structures, other than props which are very sensitive to clamping load to maintain friction. Given the extreme bolt length in this case it may be worth considering the spring washer method?

If seasonal adjustment is needed then I'd stay away from the all metal lock nuts unless you plan to use new every time. In my experience they have a greater change in 'residual' torque than the nylocks on the second use.
 

Flow

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2020-03-03 13.22.22.jpg This is the application in more detail.

It is one of two the front attach point bolts for the tailplane.
 
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