Wing Alignment holes
Posted: Mon Jun 08, 2020 9:50 am
by racaldwell
I see in the plans that the three 1/4" front spar alignment holes are not used after the spars are drilled. Anybody know why 1/4" bolts or pins are not used permanently in those alignment holes in addition to the two 1/2" bolts?
Rick Caldwell
Xenos 0057
Re: Wing Alignment holes
Posted: Mon Jun 08, 2020 1:48 pm
by Rynoth
Since they’re not necessary I can think of a few reasons why not. The main reason being: how will you install/remove the wings once the spar tunnel is riveted up? Bolts/pins in those holes would be pretty inaccessible without further modification to the spar tunnel. And since those holes are not reinforced with blocks like the main holes you might be putting unnecessary stress on a weaker part of the spar.
Re: Wing Alignment holes
Posted: Mon Jun 08, 2020 4:24 pm
by racaldwell
On the Xenos, the spar tunnel has clearance holes aligned with the spar alignment holes. It would be easy to install pins all the way through the spar tunnel. It was designed to be trailerable so the wings come off relatively easily.
I agree about the spar not being beefed up around these holes. It is just hard to think that spar relative movement is a good thing. My spars are in contact with the fuselage side skin at the top and the lower longeron at the bottom. Any movement of the spars in relation to the fuselage will be bending something. Maybe the spar overlap is designed to restrain movement when the 1/2" bolts are in place. If that is true, then three pins in the alignment holes should not carry much load. On the Xenos, the spar web is quite thick at that point. On the surface, seems to me that five pins in the spar overlap is safer than two.
But, maybe not. Just askin’
Rick Caldwell
Xenos 0057
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 3:53 am
by NWade
Rick -
The answer to your question is dependent upon understanding how wing spars like ours work.
An “I”-beam or “C”-beam spar has 3 main elements:
- The top spar cap (this is the horizontal piece at the top of the “I”)
- The middle shear webs (the vertical part of the “I”)
- The bottom spar cap (the horizontal piece at the base of the “I”)
The spar caps are what transfer the load spanwise. The shear web is just there to stabilize the upper and lower spar cap relative to each other. The shear web helps fight torsion and can help spread a bit of load between the top and bottom spar-cap (especially when the web is in tension - with the caps trying to pull vertically away from each other). But the shear web does not carry much of the load at all and does not do much of the spanwise load spreading. Essentially, the spar caps do the vast majority of the work!
In flight, the wings are essentially supporting their own weight via lift, and the weight of the fuselage is essentially “hanging” on the Two main spar pins, which are tied to the upper and lower spar caps with a beefy extrusion (or fabricated block). The bending forces of the fuselage “pulling down” on the wing spar at the main pins is the load that must be transferred spanwise out along the wing via the spar caps.
Since the main spar pins are near the farthest outboard part of the fuselage, the weight of the fuselage is already being applied as far out along the wing spar as possible (without adding external wing struts). So
- Adding additional pins near the center of the fuselage doesn’t really help to spread the load at all. You might be further immobilizing the spars relative to each other, but that doesn’t actually spread the weight/load across the wings.
- The shear webs are not very thick. If they are exposed to strong bending loads the bolt or pin through the hole will simply squash or tear the thinner material and “wallow out” or elongate the hole.
- This is hard to explain without a good animation or diagram, but each wing’s spar is going to try to bend into a shallow curve. If you think about how each spar is pinned and where the bending loads are applied, you’ll see that any holes in the shear webs near the centerline of the fuselage may experience movement in slightly different directions (because of the relative position between each shear web’s hole and the distance to that spar’s wingtip versus the hole in the other spar, as well as the fact that each spar tapers to allow for different loading and bending moments as you move spanwise along the spar). The bottom line is that these holes in the central part of the shear webs are going to want to move relative to each other, as loads change. Putting something through these holes just adds stress to the shear webs as you now force them to work against each other; but not in a place or a way that necessarily transfers the stress/load to the spar caps.
And again, even if it “beefed up” this part of the spar - which is doesn’t really - it still doesn’t help the fuselage be attached to the spar any stronger than what the two main spar bolts/pins provide. And it doesn’t help the spar transfer loads spanwise outboard along the wings any better than what the main bolts and spar caps are already doing.
Hope this helps clarify things!
—Noel
P.S. I’m writing this on my iPhone in bed, after midnight… so if I wake and find typos or problems I’ll correct them. 
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 8:58 am
by racaldwell
Noel,
Glad I could give you something to occupy your sleepless night. Thanks for the reply. I couldn’t imagine writing that on an iphone.
I hear what you say.
You would have to see a Xenos spar to see how thick the shear web is at the root. The two 1/2" bolts are attached to that massive shear web pretty much in the center and not the cap strips. Yes, the attach point is beefed up with the mount blocks. The mount blocks are only connected to the shear web.
I guess the spar is designed to flex between the two mount bolts and is stationary at the fuselage sidewalls so it doesn’t flex the skin skins any noticeable amount. At least I hope so.
Rick Caldwell
Xenos 0057
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 9:09 am
by Rynoth
racaldwell wrote:You would have to see a Xenos spar to see how thick the shear web is at the root. The two 1/2" bolts are attached to that massive shear web pretty much in the center and not the cap strips. Yes, the attach point is beefed up with the mount blocks. The mount blocks are only connected to the shear web.
Xenos 0057
If the Xenos attach points are like the Sonex/Waiex (and I presume they are similar if not nearly identical), the “blocks” are bolted to a beefy angle that is bolted directly to the “T” of the spar caps (with some additional bolts into the spar web as well.) There is no such reinforcement at the 3 alignment holes.
Bolts in the alignment holes would be through the center of the shear web only (arguably the weakest point of the web in terms of rigidity.)
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 9:37 am
by builderflyer
When I built my Sonex almost 20 years ago, I asked Pete Buck that very question as to whether or not he had any concerns regarding leaving the 3 alignment bolts in place in a completed wing. The question was asked recognizing that leaving the 3 bolts in place would, in a small way, alter the manner in which the loads are transferred from the left wing spar to the right wing spar and vice versa. Pete had no objection to leaving the 3 bolts in place.
So for 14 years my Sonex flew with the 3 alignment bolts in place until last year when I removed the wings for the very first time. My Sonex now flies without those 3 bolts installed only because of the hassle of putting those bolts back in place.
For whatever it’s worth,
Art,Sonex taildragger #95,Jabiru 3300 #261
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 1:37 pm
by racaldwell
Thanks, Art.
Glad you were wondering the same as I am now all those years ago. Pete would be the guy to answer.
Ryan, The Xenos attach blocks are not like that. There is no angle on the side that attaches to the cap strips. The shear web is very beefy at the root on the Xenos and that shear web carries the load from the mount block to the cap strips. By going from memory, I’d say 3/8" thick shear web x 2 so the alignment pins would be in 3/4" thick plate if I were to fly with them installed.
Which I am thinking about doing now that I know what P. Buck said.
Rick Caldwell
Xenos 0057
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 5:49 pm
by drPete
Some photos of a Xenos-B spar. The caps nest together, locking the spars.
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 5:57 pm
by Rynoth
Thanks for the pictures Pete, I was scouring the internet trying to find pics of the Xenos spar attach points. Definitely very different than Sonex/Waiex, with a lot more BEEF added to the middle spar web section.
I’d still caution against putting extra bolts/pins in the alignment holes. I just don’t see a benefit, only downsides. Probably a question for Sonex.
Re: Wing Alignment holes
Posted: Tue Jun 09, 2020 7:55 pm
by racaldwell
I forgot one of the mount block bolts went into the web of the Tee cap strip. It’s been an eternity it seems since making the spars.
Rick Caldwell
Xenos 0057
Re: Wing Alignment holes
Posted: Wed Jun 10, 2020 10:08 am
by sonex1374
builderflyer wrote:I asked Pete Buck that very question as to whether or not he had any concerns regarding leaving the 3 alignment bolts in place in a completed wing. The question was asked recognizing that leaving the 3 bolts in place would, in a small way, alter the manner in which the loads are transferred from the left wing spar to the right wing spar and vice versa. Pete had no objection to leaving the 3 bolts in place.
Those 3 holes are located in the mid-point between the top and bottom spar caps, and lie very nearly on the “neutral axis” of the spar. The web material on the neutral axis is neither in tension or compression, but rather just sort of idle from a bending load perspective. Locations holes on the neutral axis is preferred because the holes don’t detract from the strength of the web when placed there. Correspondingly, filling in those holes doesn’t really make the spar any stronger either. So if you analyze the spar strength three ways, 1) no holes, 2) open holes, and 3) filled holes with bolts the results don’t really change.
There are some other dynamics going on in the web that can be affected by holes, even ones located on the (bending) neutral axis, but those other conditions are not the limiting design condition on a Sonex spar. Essentially, the spar is so overbuilt that it just doesn’t matter.
All this is to say, those holes aren’t a problem, but filling them with bolts isn’t either. Take your pick… it’s ok.
Jeff