Citabria Coordinating Milman Spars and Leading Edges Installation

[Bartman]

This is a side entry to my larger wing spar conversion thread. If you're converting your Citabria's wood spar wings to aluminum using Milman's STC then you'll follow their directions and move things right along. Milman also makes a leading edge STC that can only be installed if you also have their spars. The leading edges wrap around the back of the top of the front spar, around the nose ribs and then down and around the back of the bottom of the front spar.

The problem is, it's a little bit of a dance to get both STC's installed and the information provided by Milman leaves room for some creativity and interpretation.

In my case, I took the lack of reference to the Milman leading edges STC in the spars STC instructions to mean that the two could be installed one after the other. Once I was close to finishing the first wing I went looking for the instructions for the leading edges STC and it became clear I'd have to back track on the first wing to get the leading edges sections into place. The directions for the leading edges refer to some of the coordination required but I thought I'd try to map it all out for any of you, or for any of our future members, that will be going down the same road at some point.

I'll post here as I get the details worked out but it seems that you have to build your wing through installing the fuel tank, trammeling the three outboard bays, and having all of the minor trimmings in place(strut attach brackets, trailing edge, aileron cove, wingtip, etc.). The inboard bay doesn't have to be trammeled yet because you'll be taking out the fuel tank and inboard drag wires to install the leading edges.

So, get the wings mostly done and check the installation of the fuel tank and other bits near the wing root.

Then remove the inboard drag wires, the fuel tank, the two wing attach fittings, and the root rib so you slide the fuel tank support out also. With the fuel tank and tank support out of the way, reinstall the wing attach fittings, the root rib, and the inboard drag wires.

I made a brace to act as a compression strut between the two wing attach fittings so I can trammel the inboard bay before reinstalling the wing.

With the brace installed, I'll trammel the whole wing one more time and then get it set up on my work table so it's perfectly flat. At that point it will be ready for installation of the leading edges. The inboard leading edge section will be installed first so, with it in place I'll be able to reassemble the inboard area of the wing before installing the outboard leading edge sections.

Does that make sense? I'm going to keep going and post as this works itself out. I'll also revise this post as I get the details worked out. That's where I'm at though.

 

[aftCG]

Hey at least you didn't scrap any parts. Thanks for sharing

 

[Bartman]

hey thanks! It's time to make some straps from ratchet strap straps to hold the new leading edge material in place while I (stare at it and glow in its shinyness) try to check and double check it's going on correctly before drilling holes.

I suppose I could just do it this way and save the straps to live another day (Photo credit, https://jasonbeaver.com/rv7/2009/05/ )


I've seen this method work pretty well but it'll take at least one strap's worth of strapping to make three tools (Photo credit, https://yellowairplane.com/Pitts/build/Pitts_wings_13.html )

 

[Bartman]

Getting ready to do this with the other wing and feeling grateful that I posted because it's all just as confusing the second time around!

I found out today that the little aluminum angle that is under the factory leading edge material doesn't have to be reinstalled under the Milman leading edges since they already add stiffness by wrapping around the top and bottom of the leading edge.

 

[morgan]

Bart, I am starting on my leading edges and finding the instructions a bit lacking. I acquired 100 clecos to assist in the task and they work fine with the #39 or #40 drill bit on the spar. They do not fit through the #50 holes that are to be drilled through the ribs and the skin.

I’m wondering what you did during the install? I don’t think they make a size smaller cleco than the 3/32. I also don’t want to put screws in yet as I need to pull it back off to finish the tank install and some plumbing.

Definitely a bit frustrating to constantly be struggling to figure out the secrets of these kits.

Morgan

 

[morgan]

I think I answered my own question with a bit more google work. 1/16 clecos... Damn they are expensive compared to others. $1.80ea.

 

[Bob Turner]

That is an impressive number of Clecos. On a Cub, the leading edge has maybe 3 PK screws per rib. Metal spar cubs have maybe four more holding the skin to the back of the spar cap. Still a lot of screwing.

 

[Bartman]

Morgan, I owe you a huge apology for not following through on my offer to explain what I did to get my leading edge sections on. I uploaded the photos to the first post and was going to make a video on my desktop explaining the photos one by one instead of typing explanations but forgot all about it.

I didn't use clecos even though i started out by buying a bunch of them. I put each section in place and held it there with ratchet straps. With the alignment as close to perfect as I could get it I drilled and screwed the top or bottom flange to the back of the spar first making sure I got it perfectly straight. With one flange attached I then removed the straps and used a block of wood in the palm of my hand to hold the leading edge material against the rib while match drilling the holes from the inside out. I'd drill the first row of holes working from the center of the sheet out to the edges and put the screws in after drilling and then deburring each row of holes. Once the one side was done I'd put the straps back on and then attach the other flange before removing everything except that last flange. Then I'd match drill and attach the other face of the leading edge.

To get down into the interior of the leading edge with the drill bit I used an X-acto knife as a drill extension by drilling the center of where the blade would go slightly undersize and then loosening and inserting the drill bit. When you tighten up the collar of the knife's chuck it held the drill bit perfectly and was just long enough. I was rather proud of that little solution after shopping around for a drill extender!

I suppose you could use clecos to hold each side of the leading edge in place but I just used screws after coming to the conclusion that the really small clecos weren't worth it. I used the larger ones for the flange screws I think, I'd have to look at the photos again but right now I have to get ready for our van pick up time and get to the airport. Will check in again and add to this when we get to the hotel later this evening.

 

[morgan]

Thanks Bart. I am going to go ahead and order some clecos. They are expensive, but I think it will save me time to have them available since I still have some work to do on the fuel tank, lines and vents. As you discovered, the leading edges go on before the tanks go in, so I am maybe doing things a little out of order, but I didn't want to get everything in the wing, then yank the tank and try to fit the leading edge only to find some problem.

I think I've found that problem as it stands. The bottom skin isn't fitting flush and I think my short nose ribs are too short. Talked with Mike at Milman this morning and he suggested shimming the ribs or possibly I need to use the thick wood block spacers. So it is my typical 2 forward 3 back, 1 forward approach to the project. So now I need to pull the LE off and try shimming the nose ribs by the tank.

 

[Bartman]

There's only one nose rib on the 7ECA with the smaller tanks and it can either be a full or undersized nose rib so Milman provides two spacers, mine was one of the undersized ones so my wings needed the thicker spacer. If your nose ribs can be made to match the full size ribs just by sticking a thicker spacer in there then that would be a pretty straightforward solution and not that surprising. Are both wings' nose ribs like that?

As for sequence of how the spars and leading edges got done, it went like this though I suspect you're already all caught up at this point

• Follow the spar instructions and get everything trammeled with the fuel tank installed
• Remove drag wires through the tank and remove the tank (I did the complete assembly with the tank for each wing as confirmation I wasn't going to have issues with tank fitment later even though some people might be confident enough to think it will go in when the time came)
• Fit and attach inboard leading edge section
• Reinstall fuel tank and drag wires (you may have to remove a lot of screws to be able to raise the leading edge section up high enough to get the wires back in but you can't remove any screws that would be impossible to get back in with the tank in place and be careful not to kink the leading edge material when lifting it to get access to the drag wires)
• Trammel inboard section, check other sections before finishing leading edge installation

 

[morgan]

I think I have set a new record for lack of accomplishment on this root leading edge segment. I cannot get it to fit correctly. I've spoken with Mike at Milman a few times now and yet I'm unable to get the skins to lay flat on the lower surface of the ribs by the tank.

After talking with Milman, I thought it was a rib alignment problem. So I brought my 6ft beam level to the hangar and checked the ribs. All are within 1/16" of each other. I shimmed the #2 rib to bring it in alignment with the rest of the ribs a little better. That helped it a little, but chased the bubble farther out the wing.

I think I've spent about 8 hours or more on just trying to figure out this one segment. Super frustrating. And also disconcerting when these tank ribs are the only ones that you can shim.

I built a template out of cardstock to check the shape of the ribs and they seem fine. I've moved them up and down the slight amount that the bolt holes will allow. Nothing quite does the trick. Some of the ribs, namely the first and last are looking ok as of my last test fitting. But not the middle ones.

I'll probably have another go at it tomorrow. I can only do so much until my 1/16th clecos arrive, but I was hoping to at least get it matching the curve of the ribs. One thought is that somehow the curvature isn't quite right and it's also flexing the nose ribs instead of sliding along the surface. They are pretty flimsy and the new material is thicker than the stock.

The other thing I have to look forward to is cutting the inspection holes into the leading edge material. I had wondered about that element since the new leading edges cover the bottom all the way back behind the spar and eliminate access to the drag/antidrag wire nuts.

Hopefully will get past this frustration soon. I put the original leading edge on and realized that it doesn't even fit as nicely as I'm trying to get the new leading edges. So I may just be holding myself to a standard that the factory didn't bother in 74. Sure hope that the rest of the LE pieces go smoother than the first one.

Morgan

 

[Bartman]

Morgan,
My approach was to get the fit straight and tight to the back of the spar flange and tight along the ribs to the most forward line of screws. If there was any slack I left it to bubble off the fronts of the ribs but there was actually very little. I figured at 105 mph the nose of the leading edge would be ok with it.

 

[Bartman]

Also, I posted a photo above of the reflection in the aluminum which I used to gauge how straight I had the piece in place. If the reflection is all warped then so is the aluminum.

Take your time! I have built a lot of different things and I still sweat that first hole I had to drill in the new spars! Hardest drill hole of my life!

 

[morgan]

Bart,

Question on the access holes for inspection. How did you set up your access holes for the drag wire access? Milman recommended cutting a hole with the center of the hole at the back of the spar. This creates a semi circle for the inspection port. A 3.5” access hole gives you room for inspection with a mirror, but I can’t get my hand in front of the spar with that size of opening.

I’m at the point where I just need to cut the access ports. I think I’m going to go with full circles in front and behind the spar. This is how the original fabric was. The semicircle just doesn’t seem viable to me.

Morgan