Wing attach bolts in tension

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BJC

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(New thread so as not to hijack the one about U bolts.)

The Cassutt spar sits on wood shims on the top longerons with a wood shim and an aluminum strap across the top. Four 5/16 (IIRC) bolts, two per side, tie the strap to bushings welded vertically into the top longeron. There is additional bracing below the longerons, around the bushings.

Really simple to build, really easy to shim the spar for wing alignment, really easy to remove the wing for trailering, really strong.

What other aircraft use bolts in tension to attach the wing?


BJC
 

wsimpso1

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I won't speak for the designers of the above mentioned airplanes... Torqueing a bolt puts it in tension. The question becomes how much tension? If the joints are solidly torqued, like the F-16 and Bonanza/Baron, the joint appears to be loaded up by torqueing the bolts.

In this sort of design, bolt torque takes the bolt to around 90% of yield strength. The combined joint forces are usually greater than the live loads in the joint, so the joint does not open and close and the bolts see only a small alternating load from g cycles. This prevents fatiguing of the bolts. If designed properly, the compression on the mating faces is large enough that friction carries the shear load without movement within the joint. This is common in many other assemblies that we count on every day - the wheels are held on the hub of our cars and trucks by the studs being under preload from nut torque. The friction of the wheel being pressed against the hub from the stud tension keeps the wheel from shifting on the hub.

I suspect that both the F-16 and the Baron/Bonanza wing attach points carry shear with the massive friction, and the bolts see only tension... We can check this assumption. Anybody have any pictures of the attach bolts after they have been in use for a while? I anticipate that the bolt bodies only show marks from install/remove, not marks on the shanks of the bolts from bearing on the holes of the fittings.

Another problem with using a set of bolts to carry shear, as some of us think the F-16 does, is that statistically only a modest fraction of those bolts will bear up on their fittings and carry shear. It is even possible for only one or two bolts to carry all of the shear, while the rest carry none. The bolted joints would have to be sized to carry all of the shear on one bolt.

Now if any of these applications does carry the shear load through the bolts, and the bolts carry live bending loads too, that drives up bolt size and fitting weights a bunch. The bolts have to be sized to both carry tension from wing bending moments and the shear from lift. Might be done, but not usually. I do not have the pertinent design dimensions and bolt sizes/torques, but if some one does, I would be happy to run the various calculations.

On the DC-3/C-47 and even the NA AT-6 wing joints, the ring of bolts attaching the outer wing panels to the center sections are all torqued up and under big tension. I am willing to bet that these bolts too are under a significant tension and the shear is actually carried by friction from the joint compression. Somebody have the bolt/nut torques and count for these joints? I am willing to run the calcs here too.

Billski
 

TFF

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Most of my model airplanes. Same reason as the Cassutt, to be able to transport it easily. While every bolt tightened down is in tension, I see the Bonanza reference really single shear. Engineering has to take into account all the forces to work, but that doesn’t make it the primary reason for choice. Ease of bolting on. I see the heart of the original question as how many planes flying are hung from the bolt head or nut?
 
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Bellaire MK

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Lift strut bolts, Kitfox, Avid and Challengers. There was an SB on the latter. I was never keen on this concept and then to see AN-4 studs / bolts welded to the ends of the lift struts, not easy to inspect for cracks, also bending issues are a problem.
 
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cvairwerks

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C-47 uses approximately 650 AN4 bolts at the wing splice, with a nominal install torque of 140 pound/inches.

I don't remember the exact sizes we use on the F-16, but these are close... (8) 1" or so diameter bolts on the upper fittings and (8) 1 1/4" or so bolts on the lower. Torque is in excess of 1000 pound/inches, wet install.
 
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Tiger Tim

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What other aircraft use bolts in tension to attach the wing?
I know of one Longster that was built that way but IIRC it was a deviation from the plans. Beyond that I wonder if other mid wing cantilever Goodyear racers had about the same wing attach as the Cassutt, maybe the Shoestring is the same? For that matter how about Folkerts’ racers? Maybe even Monocoupes?

If I understand right, the Bleriot’s front spar is held in a similar fashion to what’s on the Cassutt while the rear spar has a sort of flapping hinge to allow for wing warping.

My racer project had vertical bolts holding the root ends of the wing spars but they were in shear, and I think the Gere Sport was similar though I’m not sure the logic of doing it that way. Maybe saves having to have a separate fairing to go over the longitudinal attachments you see in so many other planes.
 
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Hawk81A

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Going to have to disagree with you on this point. The wheel is held from shifting by the taper that is machined into the rim and on the surface of the lug nuts. Think of it like a taper pin. On an OEM application, the load is carried by the center of the rim against the hub. This was the problem with "Unilug" custom wheels a few years back. Designed to "fit many", these transferred the twisting (friction) and load carrying to the actual lug studs instead of the hubs. Dennis
 

pylon500

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I won't speak for the designers of the above mentioned airplanes... Torqueing a bolt puts it in tension. The question becomes how much tension? If the joints are solidly torqued, like the F-16 and Bonanza/Baron, the joint appears to be loaded up by torqueing the bolts.

In this sort of design, bolt torque takes the bolt to around 90% of yield strength. The combined joint forces are usually greater than the live loads in the joint, so the joint does not open and close and the bolts see only a small alternating load from g cycles. This prevents fatiguing of the bolts. If designed properly, the compression on the mating faces is large enough that friction carries the shear load without movement within the joint. This is common in many other assemblies that we count on every day - the wheels are held on the hub of our cars and trucks by the studs being under preload from nut torque. The friction of the wheel being pressed against the hub from the stud tension keeps the wheel from shifting on the hub.

I suspect that both the F-16 and the Baron/Bonanza wing attach points carry shear with the massive friction, and the bolts see only tension... We can check this assumption. Anybody have any pictures of the attach bolts after they have been in use for a while? I anticipate that the bolt bodies only show marks from install/remove, not marks on the shanks of the bolts from bearing on the holes of the fittings.

Another problem with using a set of bolts to carry shear, as some of us think the F-16 does, is that statistically only a modest fraction of those bolts will bear up on their fittings and carry shear. It is even possible for only one or two bolts to carry all of the shear, while the rest carry none. The bolted joints would have to be sized to carry all of the shear on one bolt.

Now if any of these applications does carry the shear load through the bolts, and the bolts carry live bending loads too, that drives up bolt size and fitting weights a bunch. The bolts have to be sized to both carry tension from wing bending moments and the shear from lift. Might be done, but not usually. I do not have the pertinent design dimensions and bolt sizes/torques, but if some one does, I would be happy to run the various calculations.

On the DC-3/C-47 and even the NA AT-6 wing joints, the ring of bolts attaching the outer wing panels to the center sections are all torqued up and under big tension. I am willing to bet that these bolts too are under a significant tension and the shear is actually carried by friction from the joint compression. Somebody have the bolt/nut torques and count for these joints? I am willing to run the calcs here too.

Billski
This is basically my understanding of tension fitted bolts holding wings on.
DC-3, AT-6 are basically tensile use with people assuming some shear because the wing join is a part way out along the wing. However, P-51 and P-40 (that I know of also have major angle strip joining the two wing halves, and in that situation there is no 'lift' generated shear load at that joint.
C-130 also uses bolts in tension to hold the wings on (and they're huge), and again the shear load is more or less taken by the clamping surface, a massive machined part (rainbow fitting) with neat fitted bolts, 3/4'ø on top and 7/8"ø on the bottom, proving the tensile requirement over shear.
Check out this study on the Rainbow Fitting.
Meanwhile (and I've spoken of this earlier), the fittings on a Bonanza (Baron and all derivatives), use a loose fit bolt that clamps a soft alloy washer between two textured (scrolled) surfaces. This adds to the friction in the joint to carry whatever shear-load may occur.
Snapshot_20170303_105-20170303-115904.jpg
Note the spiralling pattern on the two large washers on the left.
Interestingly, I got this image from an aircraft salvage website where they are trying to sell second hand wing bolts, whereas I was always led to believe that these bolts have to be replaced at a specified schedule, the old bolts AND washers being discarded...?

As for the original purpose of this thread, using bolts in DIRECT tension (eight I assume) to hold a wing down, they only have to hold the G factored loads of the gross aircraft MINUS the weight of the wing. The Cassutt spec sheet claims around 850lb gross (which includes the wing, but I'll go with it), so if pulling 6 G's around a pylon, the aircraft weighs 5100lb. After much hunting I found a chart that gave the yield strength of a 5/16 AN% bolt as being around 4890lb, so technically, two bolts could hold the wing on up to 6G!
Could not determine how strong the NUT is...
 

Marc Zeitlin

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...The Cassutt spec sheet claims around 850lb gross (which includes the wing, but I'll go with it), so if pulling 6 G's around a pylon, the aircraft weighs 5100lb. After much hunting I found a chart that gave the yield strength of a 5/16 AN% bolt as being around 4890lb, so technically, two bolts could hold the wing on up to 6G!
That's not how moments work. After determining the moment carried by the wing at the point of bolt attachment, you can then determine what the loads in the bolts are.
 

Richard Roller

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I won't speak for the designers of the above mentioned airplanes... Torqueing a bolt puts it in tension. The question becomes how much tension? If the joints are solidly torqued, like the F-16 and Bonanza/Baron, the joint appears to be loaded up by torqueing the bolts.

In this sort of design, bolt torque takes the bolt to around 90% of yield strength. The combined joint forces are usually greater than the live loads in the joint, so the joint does not open and close and the bolts see only a small alternating load from g cycles. This prevents fatiguing of the bolts. If designed properly, the compression on the mating faces is large enough that friction carries the shear load without movement within the joint. This is common in many other assemblies that we count on every day - the wheels are held on the hub of our cars and trucks by the studs being under preload from nut torque. The friction of the wheel being pressed against the hub from the stud tension keeps the wheel from shifting on the hub.

I suspect that both the F-16 and the Baron/Bonanza wing attach points carry shear with the massive friction, and the bolts see only tension... We can check this assumption. Anybody have any pictures of the attach bolts after they have been in use for a while? I anticipate that the bolt bodies only show marks from install/remove, not marks on the shanks of the bolts from bearing on the holes of the fittings.

Another problem with using a set of bolts to carry shear, as some of us think the F-16 does, is that statistically only a modest fraction of those bolts will bear up on their fittings and carry shear. It is even possible for only one or two bolts to carry all of the shear, while the rest carry none. The bolted joints would have to be sized to carry all of the shear on one bolt.

Now if any of these applications does carry the shear load through the bolts, and the bolts carry live bending loads too, that drives up bolt size and fitting weights a bunch. The bolts have to be sized to both carry tension from wing bending moments and the shear from lift. Might be done, but not usually. I do not have the pertinent design dimensions and bolt sizes/torques, but if some one does, I would be happy to run the various calculations.

On the DC-3/C-47 and even the NA AT-6 wing joints, the ring of bolts attaching the outer wing panels to the center sections are all torqued up and under big tension. I am willing to bet that these bolts too are under a significant tension and the shear is actually carried by friction from the joint compression. Somebody have the bolt/nut torques and count for these joints? I am willing to run the calcs here too.

Billski
I don't have pictures, but I've installed many bolts in the Bonanza/Baron a/c. The wing attach "bathtubs" have machined rings in the faces of the two bathtubs at each attach point (2 each, main and rear spars). A soft aluminum washer goes between the fittings and the bolt through all pieces. The bolt has a double washer set, a large washer around a smaller diameter washer, under it's head. The outer washer has holes drilled in it. As you apply torque to the bolt head you apply rotation to the outer washer with a pin inserted into one of the holes. When the inner washer swells to the point it takes a certain force to move the outer washer, the bolt is torqued. The machined rings in the bathtub fittings bite into the soft washer between them for vertical position. The soft washers are only used once. The bolts are a loose fit in the bathtub fittings.
 

mcrae0104

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That's not how moments work. After determining the moment carried by the wing at the point of bolt attachment, you can then determine what the loads in the bolts are.
The Cassutt is a little different from what you may have in mind. The one-piece spar sits on top of the longerons and is clamped down by two bolts on each side.
943B6AFA-F1F0-4E1B-AC8D-0D10B780D2C6.jpeg
 
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