As regular readers will recall, I'm working on repairing a damaged Zenair 701 that shop owner Ron bought as a rebuildable scrap project. Eventually, it will be reinspected, painted and sold to fund other projects. Ron is giving me the opportunity to learn the skills I need for my build by doing this rebuild and it's been really enlightening.
We are both starting to guess that whomever inspected the original build must have either been in a huge hurry or not very aware of acceptable building practices. Through this post, I'll point out some of the more obvious clues. It's lucky no one was killed flying this airplane (I use the term loosely).
These photos were taken over the course of the last couple of weeks, but are generally in order that I accomplished them.
Ron decided that the rear tank channel needed to be moved back to accommodate a bigger fuel tank, so I worked on removing it. This first picture was from several weeks ago, when I was still stripping paint and the rear fuel channel was still in place:
Here is a more recent one. The paint strip is mostly complete and the rear fuel channel is removed. I've also removed the silicone caulking the builder used for "anti-oil-canning":
The channel came out without too much effort. It's in rough shape and covered in the remnants of adhesive from the tank mounting cork, silicone (surprise) and, get this.... crack filling foam! Ron seems to think that the plane at some point probably developed a fuel leak and the silicone and crack foam were a red-neck repair (clue # 1). Ron also let me know that the suspected cause of the crash that damaged this 701 was the plane running out of fuel. More on this later. Here is the removed channel:
As the wing gets thinner top to bottom the further to the rear you move, so does the space where the channel will go. After measuring very carefully considering what size it will be in it's new location, I cut the top edge off. Here is the modified channel, all cleaned up, cut down and straightened:
In order to bolster the strength of the channel top edge, I created an "L" flange out of flat stock using the same thickness as the channel:
Next was a series of trial fits, removal, trimming, fit, removal, and fit again to get the new channel in the right place to ensure the new flange was at the correct height to meet the wing skin. I predrilled the new mounting holes in the ribs at this time. This is where clecos earn their value!
It took some more working of the edge of the channel to remove any interference with the rib lightening flange hole and working around the rib repair piece:
Once I was happy with the fit of the modified channel, it was time to attach the reinforcement L flange. Measure two rivets per section, cleco, drill and deburr:
The new L flange really tightened everything up!
I've put it aside for now to work on the next repair item, the wing attach point.
When this plane crashed into the trees, this wing was ripped backwards, bending the wing attach bracket. No small feat, as these are obviously designed to be stout:
Even if it wasn't bent, it would need replacing anyhow for a couple of very obvious reasons. Look at the shape of that hole! There is no way this was an acceptable mount. It looks to me like someone got lazy and just made the hole bigger instead of rigging the wing properly in the first place (clue #3). Another issue is the lack of corrosion protection at this joint. The aluminum of this wing attach point was obviously in direct contact with the steel of the fuselage frame. Anytime two dissimilar metals are in contact, there needs to be corrosion protection. It's called Galvanic Corrosion, and you can read about it here.
To remove the wing attach bracket is going to take some work. These are installed using bucked rivets, not the pulled rivets I'm used to dealing with. That root nose rib is also really trashed from the accident, so it will also need to be replaced:
To make access easier, I tied back the top of the nose skin with a piece of string, being careful not to crease it. Sometimes it's the simple way that's best!
Using a centering punch and a drill bit, the heads of the rivets are drilled off. After some gentle persuasion, the wing attach bracket came off without much trouble.... hmmm. It shouldn't be that easy, should it?:
Pulled rivets have a hollow core, which means they can be drilled out and removed with ease. Bucked rivets are solid all the way through and what a pain they are to remove even after the heads are removed. Reminder to self - don't screw these up when doing mine!
Next, remove the root nose rib and it's support bracket. This is when I noticed this wing spar is missing the root doubler (clue #4)! A root doubler is another layer of aluminum plate sandwiched to the main spar web to increase the strength and integrity of the wing attach points. This is missing on this build. How it could have been omitted, I don't know. What drew my attention to the issue was the L bracket riveted on the spar web. This doesn't appear anywhere in the plans. The root doubler has this built right in!
As far as I am concerned, this was an accident waiting to happen, all the clues add up. It's a wonder this wing (or the other one as I suspect it's the same) never folded or failed in some way, even on the ground.
Running out of fuel (the actual cause of the accident), probably saved this guy's life and perhaps that of his passenger. I'm not going to even try and guess the reason they skipped such an important assembly, but I guess I'm fixing that too!
I decided to drill out the next row of nose skin rivets, as I'll have to remove everything up to and including the next outboard rib to install the missing doubler:
Next I removed the bent L that supports the root skin at the rib and I removed the slat support bracket. Next was the nose rib and the mystery L on the spar web:
Definately a ton of work (and learning) done and still more to come. I'm really enjoying this process!
During my free time away from the shop I'll continue to get the plans digitized into CAD. I've repurposed an old laptop we had lying around. I installed the Ubuntu (Linux) operating system and the CAD program LibreCad. It's really fun, easy to use/learn and best of all it's free. It's also available for Windows and OSX if Linux isn't your thing:
Back to the shop tomorrow, thanks for reading!
Haven't done much in the shop on my own stuff lately, as I've been helping Ron with his Aeronca Scout rebuild and puttering away on the 701 wing rebuild. I'm gaining confidence in my ability to fabricate simple aluminum channels and web caps and learning the value of measure twice, cut once, fit and debur before riveting. Pictures to come!
I've also been talking at length with Ron and he's all but convinced me that building my 750 from "scratch" (hand make all the parts from raw aluminum stock) rather than via prefabricated kit components from Zenair is not only doable, but really economical. It's what he's done with his 701 and has shown me how easy the process actually is. The monetary savings to be had by forming the parts myself is nothing to scoff at either, perhaps saving upwards of 75% on what Zenair produced parts cost! Sure, there are some things it makes more sense to buy direct (windshield comes to mind), but the more I think about it, the more I like the idea of scratch building (and it means more learning!)
So what does scratch building entail? Basically it means making forms out of wood from the plans which in turn will be used to form the raw sheet aluminum into ribs, channels and other associated parts. Other flat parts (wing skins, fuselage panels) are measured and cut directly from aluminum sheet stock.
The obvious trade off is time, but Ron and I both believe the goal of 3 years building is easily achieved.
In order to determine how much aluminum sheet to order (the Zenair plans unfortunately don't include a comprehensive material list), I'm working on converting the appropriate plan drawings to scale CAD files. In the spirit of money savings, I've found an excellent free online CAD program called LibreCAD that makes converting the dimensional drawings in the plans to CAD files easy.
For example, I can take the drawing of the Horizontal Stabilizer Nose Rib from the plans:
.....and use LibreCAD to turn the information from the drawing to a CAD file, suitable for printing, exporting to a CNC machine, etc:
As you can see from the screenshot above, I'm not including the dimensions on the CAD drawing, as they are just as easily referenced on the paper copy. I've also created the CAD file with 3 different layers; form, aluminum and a label layer. Each of these can be toggled on or off for easy viewing should the need arise.
I am however keeping an Excel spreadsheet to document each drawing that has been converted. Once I have the drawings all converted, I'll be able to place parts of the same material thickness onto a page representing a complete sheet of raw aluminum. This should save material and money as I can nest smaller parts (ribs, channels, etc) among the larger ones, saving material waste.
The spreadsheet also tracks what forms I've got made and what parts are completed and ready for assembly.
I never had CAD available to me as a highschool drafting student, but that part of my brain that knows how to interpret drawings still works it seems! Is this worth the effort? Who knows. I'm just enjoying the journey!
Time until takeoff
Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.