1/4 Scale Bud Nosen Designed Citabria

[Joker 53150]

More uncovering and disassembly. The vertical stab is removed completely and will be a full re-build. The horizontal stabs appear to be usable as-is, although new hinges will be needed. The plane's builder did use the hinges I like, with removable pins, however, it looks like he built and covered the assemblies and then glued the hinges in with so little clearance between the stab and elevator that the pin couldn't be removed without damaging the surrounding surfaces. Hopefully I can get the old hinges out without too much damage. Also, the spot in the TE of the stab where the guide wires are attached looks very solid. The balsa used is very dense & strong, so I may just keep that as-is.

The elevator is in OK condition, except for the leading edge where the hinges are installed. That piece got chewed up a bit during removal so I will either replace that one piece or trim it back and simply re-face it with a new piece of balsa. Also note the three holes for the control horn, once again not in the hardwood block included with the kit! I'm surprised there isn't more wear from use, although that balsa is pretty dense & heavy. I'm tempted to just build new and lighter pieces which would help the overall balance of the plane.

 

[Joker 53150]

...And shortly later all of the covering has been removed! Doing this achieves a few goals:
- It allows me to re-cover the plane with a better material.
- I can now do some surgery to shed weight from the tail. Note the solid sheet of balsa along the bottom of the fuselage - it's thick and heavy!
- Any other damage should be visible - so far nothing jumps out at me.
- I can plan the re-routing of the pull-pull cables and elevator pushrods.
- I have better access inside to mount the fuel tank more securely.
- Fixing the damage to the stabilizers will be much easier.

The covering I'm going to use is heavier than the stuff I removed, so tail weight will be a big concern.

 

[rockyboy]

On the subject of covering, I was surprised to see a lot of great building content in the AMA magazine this month - and not just basics either. There is a couple page article on more advanced heat-shrink covering tips that I learned a good bit from - it's a follow on to their March covering basics piece. I think my next set of wing-tips are going to come out much better! However I haven't seen them tackle the "choosing a covering" yet - would like to see some tables comparing covering weights and other properties objectively.

 

[Joker 53150]

I did some comparison between the plans and the plane as it was built, and found a couple things which were either ad-libbed, skipped, or missed during the construction. First is a 1/4" hardwood dowel which is supposed to travel from the cockpit area to the tail. The upper one at mid-height on the fuselage is there, but no lower one.

Although not really needed for strength, it'll make the covering look better so I'll add it once I'm done stripping the fuselage down.

Here you can see how the tail of the fuselage was designed, with open structure except for F7.

The builder added sheet material on both sides above & below the dowel stringer. My guess is that he did it to have surface around the elevator pushrods to stick covering to.

Whatever the reason, I removed it as I'm planning to re-do the pushrods. I may need to add a little material back in, but certainly won't need as much as he had included. As I study the horizontal stabilizer and elevators more, I think I've decided to just go ahead and build new pieces. The original pieces can be saved, but it'll take quite a bit of work and I think I can make them a little lighter. Speaking of lighter, there is a fair amount of Bondo filler around the tail. It'll probably be a pain to do, but removing the entire horizontal stab is the best option, and with all the epoxy I'm finding it won't be minor surgery!

Last is something that was done per plans, sheeting the entire bottom of the plane. In my opinion that's overkill, so I'm planning to cut a series of holes to lighten the tail a bit. If all goes right, this won't be noticeable once the covering is on.

 

[Joker 53150]

I'm taking a short break from my Rascal 110 work, thanks to my BADD (Balsa ADD). In truth, I really hate working on battery and fuel tank placement for some reason, so attention is being turned back to the Citabria.

Mentioned previously, the tail on this plane was in pretty rough condition. Lots of Bondo, heavy repairs, and the balsa was a bit rough. If I'm going to do the work, I may as well make it right. With that in mind, I've come to the conclusion that the tail is going to need some fairly major surgery. First step is removing the horizontal stabilizer and the stringers under it. Vertical and diagonal braces will follow soon enough, followed by re-building the structure.

The rudder and vertical stabilizer are being completely re-built. You can see a couple breaks towards the top of the rudder which are the result of a nose-over upon landing after the last flight the plane had. I'm taking what I learned from the breaks into account on the new parts. It doesn't show well in the picture, but the vertical stab has a small hole in it which is where the bracing wires are attached. The hardware for the braces have deformed/crushed the balsa a bit and wallowed out the hole a little.

And here are the new pieces waiting for shaping and sanding. Note I added some thin light ply to both sides where the brace wires go. These will spread the load a bit and also help me from crushing the balsa when the hardware is installed. I'm also thinking about drilling it out more and sliding a short section of brass tube inside to act as a crush sleeve. To be determined... I've also changed the fillets a bit, adding smaller pieces of basswood in three spots on the rudder where I think it could break on a nose-over.

The horizontal stab and elevators are all being built together as a single assembly. Once complete, two cuts along the leading edge will separate the stab from the combined elevators. Then, two more cuts will separate the elevators from each other. In theory this should help make sure everything lines up properly later on.

 

[Joker 53150]

Here's a "teachable moment" for balsa building - grain direction really matters, a LOT! Most kits take the grain direction into account when the pieces are laid out, but if you're scratch-building or doing some repairs it's up to you to decide how you want to cut the parts. In the picture below (elevator for the Citabria), I've outlined where pieces were glued together for clarity. You can see the direction of the grain as the part was built, along with arrows showing how it should have been orientated. While the parts did function as-built, the grain direction made them much weaker than if the grain was running as the arrows show. To test this yourself take a piece of balsa and cut pieces like this out with grains running both directions. Run the grain the long direction and the piece is much stronger and doesn't flex nearly as much (or as easily). I've made this mistake more times than I'll admit, and this also pertains to sheeting a wing, fuselage, etc.

 

[Joker 53150]

It took some time, but the new pieces are assembled. There is plenty of sanding coming up to get the final shapes and taper right, but it'll be worth the effort. I also cut out the plywood control horn blocks from the original pieces and re-used them. The previous builder somehow cut them and installed a hinge into each, which must have been a massive PITA. My plan is to add a little extra balsa to beef up the areas where the hinges will go.

 

[Tench745]

Here's a "teachable moment" for balsa building - grain direction really matters, a LOT! Most kits take the grain direction into account when the pieces are laid out, but if you're scratch-building or doing some repairs it's up to you to decide how you want to cut the parts. In the picture below (elevator for the Citabria), I've outlined where pieces were glued together for clarity. You can see the direction of the grain as the part was built, along with arrows showing how it should have been orientated. While the parts did function as-built, the grain direction made them much weaker than if the grain was running as the arrows show. To test this yourself take a piece of balsa and cut pieces like this out with grains running both directions. Run the grain the long direction and the piece is much stronger and doesn't flex nearly as much (or as easily). I've made this mistake more times than I'll admit, and this also pertains to sheeting a wing, fuselage, etc.

View attachment 146419

It is worth noting that, while the part itself is weaker because of this grain orientation, the glue joints are stronger. Glue joints like parallel grain. I believe this is part of why gussets are so effective.

 

[Joker 53150]

No argument from me on the strength of the joint when flying along the grain. The glue joint is often far stronger than surrounding balsa. I’ve found the Titebond II glue does well even with the end grain, maybe due to the slower cure time compared to CA glue.

In the case of this plane, I see no advantage with the grain as it was built. With my rebuild a couple small gussets may be added, but I think it’ll be plenty strong and rigid without.

 

[Tench745]

No argument from me on the strength of the joint when flying along the grain. The glue joint is often far stronger than surrounding balsa. I’ve found the Titebond II glue does well even with the end grain, maybe due to the slower cure time compared to CA glue.
In the case of this plane, I see no advantage with the grain as it was built. With my rebuild a couple small gussets may be added, but I think it’ll be plenty strong and rigid without.

I didn't intend to imply there would be any problems with the new surfaces. I would have made the same changes you did, the old grain direction seems like they had to work harder only to make a weaker piece.

I just wanted to impart some information that others might not know.

 

[Joker 53150]

It's all good info to share, no argument from me at all. Some cases may be better off as this plane was built, just not this one.

Before I wrapped up work on the plane last night I started taking the fuselage tail apart. Well, I used a saw and cut a lot of it off... I figure I'll end up replacing about the last 1/6 of the fuselage, and need to splice it in to make it structurally sound again. Shouldn't be too hard, although removing a lot of the old glue is a bit of a pain. I don't know what was used, but it's similar to epoxy - maybe the old Ambroid or similar? Thick, clearish, and a real PITA to remove.

 

[Joker 53150]

Disassembly of the Citabria continues. I've removed the landing gear, which required cutting out some sheeting on the bottom of the fuselage.

The tail has also been trimmed back quite a bit, and may be trimmed even further. My thought with this is that I want to spread out the upcoming glue joints and new structure as much as possible to avoid accidentally creating a weak spot. I plan to introduce some small pieces of carbon fiber to help strengthen the joints. Note I also removed the sheeting from the top of the fuselage. It had a lot of CA soaked spots and was very uneven. Replacing it is far easier than fixing the old piece.

Here's an example of the lightening holes that will be added to the fuselage bottom. The bottom is flat, so removing material won't be noticeable once it's covered. The amount of weight removed will be pretty small, but it's all aft of the CoG on a plane that is already tail heavy.

 

[Joker 53150]

I won't say that all of the disassembly and removal of material is done, but it's done enough that I can start building new parts and install them! First up is construction of a new fuselage tail section. I need to build this assembly twice (left and right sides), and construction is pretty quick. Note I'm adding the same style gusset/reinforcement pieces the plane was originally built with. They add a tiny amount of weight, but really help strengthen the build. When both sides are done I'll offer them up to the fuselage to see how they fit.

Also under construction is a new leading edge of the wing saddle portion of the wing. The original was pretty ugly and the fit wasn't that great. The new one will get some finish sanding to smooth the curves a bit, but it's already much better looking than the original. Note I added a couple hardwood "pins" to help secure it to the bottom of the wing saddle. A second set of pins will be added after this epoxy dries.

 

[Joker 53150]

With the front of the wing saddle cured in place I could test-fit the wings and prepare to fix some other stuff that's been bothering me since I got this plane. The center section for the wing is built to plans (well, re-built since I repaired and re-covered it a while back) as is the leading edge where the windshield attaches. However, the leading edges of both wings have always been much lower. Probably due to the way the wings were sheeted. To fix this I could tear apart the sheeting on both wings and try to make it fit the center section, or, I could just adjust the center section.

Here you can see where I marked the center section to match the wing. I also marked an "X" on some damaged sheet to be repaired.

The other wing was about the same. This pic is after quite a bit of sanding and shaping - much closer than where I started. I also need to sand & shape the center section of the wings.

The wings are pretty solid, and need only minor sanding and a single (so far) patch to the sheeting. They should be solid, as the builder used a TON of glue, added more shear webbing than is necessary, and ran a heavy PVC pipe down the length of the wing for the servo wire. No wonder the plane feels heavy, it was built to survive a crash, not to fly!

Back to sanding, I used a random orbital sander and matched the center section to the section above the windshield. This should give me a good, even fit once it's covered.

I then installed the wings and did the same to even the height between the wings and center section. Some more tweaking and it should be good.

 

[Joker 53150]

This will be the last bit of work for the day - hopefully tomorrow gives me a little time to play. The new construction has been added to the tail, and fit extremely well with minor tweaking & sanding - I'm very happy with it. My plan is to get a few more things done to the fuselage before installing the horizontal and vertical stabs.

 

[TooJung2Die]

ran a heavy PVC pipe down the length of the wing for the servo wire

No joke, that looks like schedule 40 PVC pipe. If it's just for tidying up the servo wire that's heavy. There's thin wall irrigation PVC pipe. You've probably seen card stock paper rolled into a tube to serve the same purpose.

 

[rockyboy]

The repairs are looking good! Especially like that hardwood pin in the saddle - good idea there.

And I agree with @TooJung2Die that PVC pipe should go. It's not glued into the ribs securely enough to be adding any strength, and it's close enough to the CG it's not helping the balance - it's just sitting in there increasing the stall speed.

 

[Joker 53150]

As I start adding new construction on to the old there are a few spots which jump out as needing attention. I didn't take a "before" picture of the trailing edge of the wing saddle, but here's the "after". Previously this piece was about half the new height and flat, meaning there wasn't a smooth transition from the wing to the fuselage. My solution was to add a piece of angle-stock and a few small fillers, and then sand to shape. I'm going to add a little balsa filler before doing the final shaping, but it already looks much better.

Test-fitting of the tail feathers was also done, and all parts appear to fit as they should. Top and bottom sheeting will be next, along with adding the two missing 1/4" longeron dowels. There is also a lot of sanding in my near future...

 

[rockyboy]

All those cross pieces in the tail look like bridge girders - really hefty! It's almost getting to the size where the tail could be built more simply using wing ribs

 

[Joker 53150]

Lucky for me the PVC pipe in the wings were the only pieces NOT overbuilt and swimming in glue! A little twisting with a pliers and they both came out without damaging any ribs. Total weight savings is 7 ounces. A little of that will return if I make new (light weight) tubes for the wires.

In the meantime, sheeting is finished on the bottom so it bridges the old and new construction. Holes were added to lighten the tail a bit as well. Hopefully I didn't overdo it!

More test-fitting of the tail, now that the top sheet is done. I haven't decided if I'm going to add holes to lighten this sheet or not yet, but am leaning towards cutting them. With the covering I'm going to use I don't think they'll be visible at all.

Here's the newly lightened wing! About 3.5 ounces per wing was removed by pulling the PVC pipe out of each. A considerable amount of weight could have been saved on the wings with better original construction. The two metal rods which support each wing are held in by a lot of hardwood and even more epoxy. Soooooo over-built!

Oh, and I put her back on wheels to make work easier. The heavy aluminum gear had a slight bend from a "heavy landing" a few years ago, and I was able to get it back into shape while it was out. Seeing it like this really gives me the desire to keep pushing forward!