'68 Citabria 7ECA Milman Spar Conversion

[Bartman]

A little side piece, there are questions around the internet about the suitability of Stewart System's ability to resist fuel so I took a piece of spruce and coated it with the same coating that I used to seal my new fuel tank support panels, Stewart System clear EkoCrylic. After a couple of weeks of soaking in 100LL there was no degradation of the clear coating. I haven't tried soaking it in auto gas with ethanol but I don't see myself ever using auto gas.

 

[Bartman]

Got the new left rear spar into place this evening. Maybe other people have had a different experience but my spars aren't sliding into place very easily. In fact, and maybe it's the primer making things less slick, I've had to tap the spars with a wood block to get them to slide into place.

Because I'm working by myself, I've had to brace everything to the table so I could tap the spar without knocking the rest of the wing across the table. I clamped a piece of wood across the top of the ribs and screwed blocks to the table to keep the rib bottoms from sliding. The whole assembly is strapped to the table to keep it from sliding.



As the spar is being slid into place you have to make sure you are maintaining clearance in the rib openings so you aren't scraping the spars on the sharp edges of the ribs. There is only a tiny little bit of room to play with


You can also see the blocks I have in place to keep the spar aligned with the table so it isn't pointing up or down as it's moving through the ribs.


And finally, after hours of preparation and careful execution, the new spar is in place!

 

[Clifford Daly]

Got the new left rear spar into place this evening. Maybe other people have had a different experience but my spars aren't sliding into place very easily. In fact, and maybe it's the primer making things less slick, I've had to tap the spars with a wood block to get them to slide into place.

Because I'm working by myself, I've had to brace everything to the table so I could tap the spar without knocking the rest of the wing across the table. I clamped a piece of wood across the top of the ribs and screwed blocks to the table to keep the rib bottoms from sliding. The whole assembly is strapped to the table to keep it from sliding.

View attachment 947View attachment 946

As the spar is being slid into place you have to make sure you are maintaining clearance in the rib openings so you aren't scraping the spars on the sharp edges of the ribs. There is only a tiny little bit of room to play with
View attachment 948

You can also see the blocks I have in place to keep the spar aligned with the table so it isn't pointing up or down as it's moving through the ribs.
View attachment 945

And finally, after hours of preparation and careful execution, the new spar is in place!
View attachment 944

After doing the job even with two people, the wood blocks and wood across the top looks like it would help a lot. Mine didn’t slide in easy either and just took lots of patience, only issue was it was cold outside and the garage door had to be open haha

 

[Bartman]

I adjusted everything after that photo was taken so it would be even more rigid and I'm going to spray the front spar with soapy water as I'm tapping it in so maybe it will slide better. I didn't have to hit it very hard, just tap tap tap with the hammer against a block of soft wood to move it along.

I can't wait to transition to doing fabric!

 

[Bartman]

I have the nail removal down to about five minutes per rib flange. I'm using an old soldering iron that I pulled from my grandfather's garage in Kenilworth, NJ after he passed away, a pry bar that has been sharpened to fit between the rib flange and the spruce spar, and a little hammer.


Whoever assembled the wings, or maybe the last time it was covered, they put epoxy over the nail heads so the heating iron melts the epoxy and is probably also loosening up the natural resins in the wood that are holding the nails in like glue.


It takes about 25 seconds to get the nail hot enough so it will come out easily. After that the pry bar goes under the flange to get the nail loose, the flange gets pushed back into place, the pry bar goes under the nail, and out it comes. The best part of this is that no nails holes are harmed in the process of removing the nails.


Before I started doing this I was wrestling with each and every nail, minutes each and the flanges were getting roughed up a little. All of my nails are out now so we're putting the soldering iron away until the next project.

 

[Bartman]

Milman claims only some of the ribs in Citabria wings will have front spar openings that are too short but all of my ribs were like that, with front spar slots that were not tall enough. The problem is that the wood spars fit through but the new aluminum ones won't and any contact between the ribs and the spars is bad for the spars and, therefore, bad for you. As Milman explains it, there were multiple dies for the ribs and some have the larger opening and some don't so you may or may not encounter this problem on one, some, or all of your ribs.

The solution is to open up the top of the rib opening with a round file. I made a block of wood to use as a guage and then roughed the opening quickly with a kinda coarse tapered round file and then cleaned it up with the finer cut tapered round file in the photo. You can see from the outline of the block on the aluminum that the opening only needs to be enlarged a little bit to be ready. Once you're good at it it only takes a few minutes per rib.

It's very important to ONLY USE A ROUND FILE so as to not leave any sharp notches filed into the opening which would lead to a crack and a failed rib later on.

 

[Bartman]

In earlier posts I showed how I used wood blocks to anchor the ribs to the table so I could work alone to get the old spars out and the new spars in. With this last spar I thought maybe the single wooden block was allowing the ribs to flex too much thus making it harder to get the new spars in. I tried adding a second wood block to keep the ribs from flexing and it seemed to help the new spar to slide in more easily. I still had to tap it in with a hammer and wood block but it went in more easily. Everything was sprayed with a mild soapy water mix so that probably helped as well.

 

[Bartman]

Last of the new Milman aluminum spars has gone in and not long after the ribs were put back into place. The second wing is going so much faster!

 

[Bob Turner]

It does look nice. I wonder if those are Super Cub extrusions? If you measure them I can tell.

 

[Bartman]

I'll try to remember to measure them in the morning before heading out.

 

[Bartman]

@Bob Turner

Both spars are about 13/16 wide, the rears are about 4" tall and the fronts are about 5 11/16" tall.

 

[Guy]

Got my spars in the first wing. No major problems with that. But I am having a problem re-installing the front spar butt fitting. Did anyone else have this problem? The -111 spacers when on the spar are thicker than the fitting.

 

[Bartman]

Got my spars in the first wing. No major problems with that. But I am having a problem re-installing the front spar butt fitting. Did anyone else have this problem? The -111 spacers when on the spar are thicker than the fitting.

is the fitting pinched in at the open end? it's a pretty snug fit as I recall but it should slide on. or just pull the open ends of the fitting a bit to unpinch them? i'll go take a look, give me a minute. just spent all day working on it, ready to trammel the left wing then finish up with putting stuff back on before the new leading edge sections get installed

 

[Bartman]

Guy,

I went and looked at my project, there's no trick to it, two spacers, the spar and the fitting, it should all just slide together. Did you powder coat the spars or go heavy on the primer at the butt end? Maybe that would explain the tightness? I don't think the gap in the rear fitting is a different value but maybe you're trying to put the rear one on the front? Don't know, just throwing stuff out there.

 

[Guy]

Guy,

I went and looked at my project, there's no trick to it, two spacers, the spar and the fitting, it should all just slide together. Did you powder coat the spars or go heavy on the primer at the butt end? Maybe that would explain the tightness? I don't think the gap in the rear fitting is a different value but maybe you're trying to put the rear one on the front? Don't know, just throwing stuff out there.

Bart,
Thanks for that. I didn't put anything on the spars but repainted the fittings. I'll take a real close look at that fitting again. Your input is very helpful.

Thanks again.

 

[Bartman]

Bart,
Thanks for that. I didn't put anything on the spars but repainted the fittings. I'll take a real close look at that fitting again. Your input is very helpful.

Thanks again.

Believe it or not, I was out there this morning putting things back into place and my forward spar butt fitting wouldn't slide on! There was some leftover fabric adhesive that had wicked where it wasn't supposed to be so I had to clean it out but even still the fit was tight and required some persuasion. Putting a 3/16 bolt through the spar and spacer plates (through the root rib mounting hole below the fitting holes) helped hold the plates while I worked the fitting into place.

@Guy If you're also doing the Milman leading edges STC you'll want to plan ahead as the wing needs to be trammeled without the fuel tank in place before the leading edges can be installed. I have another thread going just about coordinating the two STC's.

 

[Bob Turner]

Cub spars are so close it makes me think they are the same: front 5 5/8" x 3/4" with .110 web
Rear 4" x 3/4" with .085 web. The cap is a "U" section of about 1/2" height.

 

[Bartman]

@Bob, I measured again, it's definitely 13/16 across the top/bottom, the web of the rear is thinner than the front....maybe it's the same manufacturer but slightly different extrusion?

@Guy, my rear spar butt fitting was a struggle to get on tonight, it had no wiggle room so it took multiple tries putting through different combinations of bolts in different orders to get them to all push through. Funny that my first wing was a breeze and then you posted with an issue and I happened to then have the same issue.

 

[Bartman]

Some photos of the aileron bellcrank assembly as I was taking it apart to clean and prime the hinge area. Figured someone might need to see something in there at some point so here they are. I'm trying to not get carried away with the whole remove/restore thing so I only cleaned this up and primed it because it gets painted where it sticks out of the wing and the last one's paint was all peeling off and kind of shitty looking. Maybe the primer will hold the paint and keep it looking saweeeet!

 

[Bartman]

You know the end is getting closer when your giant pile of spacers is mostly gone.


Here are a couple of pics of the fwd and aft compression strut spacers, the metal ones bear against the strut, the wood ones go on the outsides of the spar for the hardware to bear against.


Making progress and trying to keep the bench clean as I go! It's a new habit I've been trying to maintain.