Restoring the A-10D Mitchell Wing
SN# 275
By
Episode II
“The Wing and I”
As Episode 2 opens we find our hero no longer rhythmically banging his head on hard objects. Determination has replaced disappointment and the restoration has begun with the need for information, lots of it. Once again I enlisted the assistance of my kitplane builder friend who provided me a list of A&P Mechanic’s reference documents that he found useful along with the 800 numbers and e-mail addresses for every aircraft parts supplier known to the kit building and ultralight fraternities. A few weeks later I was armed with an impressive stack of reference books, catalogs, magazines and flyers that nearly equaled a Microsoft user’s manual. Fortunately, they were far more understandable.
Back at the “hanger” behind my garage, I’d spent several nights with a six inch caliper and an AN bolt pocket reference cataloging every nut, bolt and washer on the A-10D. During the day, when time allowed, I hovered over catalogs locating replacement hardware. Like most ultralights the Mitchell Wing’s pilot’s cage was mostly straight sections of aluminum tubing bolted together with AN aircraft bolts. There were, however, some unique parts that would have to be obtained from Ameriplanes in Iowa. A few phone calls later and with some hefty new charges to my credit card, the first of many orders had been made.
While I was waiting for the parts and hardware to arrive I decided to begin the restoration by repairing the small hole in the underside of the right wing caused by road debris during the trailer transport from Florida. Imposing on the good graces of my kitplane friend I got a scrap of aluminum, some clecos, pop-rivets and a crash course in simple metal repair techniques. It looked good. Buoyed by this tiny success I began planning the next move, which would be the disassembly of the wing into its three sections. Recognizing the need for space, I hastily reorganized “my side” of the garage and moved my trusty Subaru station wagon out of its hallowed spot and into the driveway. This was a small drama, but one common to all kit builders.
I decided to start small, so restoring the tip rudders became my first project. Removing them turned out to be fairly straightforward and allowed a better look at the rudder hinge assemblies. They were in even worse shape than the preliminary inspection had revealed. New ones were on order, however, so replacing them would have to wait. It was here that I discovered the “Prime Directive” of restoration: Work with what you have. Don’t obsess on one area. There’s always something else that needs doing. On the other hand, the corollary to this is that what you want to do can’t be done without several something elses that need doing first. I call it the “Domino Theory of Frustration”. Before I could re-tape the rudders it was logical that I should remove the corrosion and before I could do that I had to remove the large, faded “Mitchell Wing” logo decals and serial number transfers. I had not expected the seven labors of Hercules doing something so mundane, but I was wrong, as I am so often.
A quick trip to the local hardware store yielded quart cans of every solvent I could think of. Soon the transfers were history but the large logo decals resisted every chemical attack. Even though the graphics were wiped slick the tough plastic backing remained. The situation looked hopeless until I stumbled upon a spray can of Decal Remover at a local auto parts store. It didn’t work worth a damn either, from above. But injecting it beneath the decal through a bubbled area with a plastic syringe slowly began to have an effect. Finally, using a razorblade scraper wielded with the patience of a saint, the decals were reduced to small pieces in the garbage bag.
Removing the corrosion and polishing the aluminum was the next effort. After considerable research and reading several articles, including an old one by another Mitchell Wing owner, I decide to experiment on the corroded aluminum stabilators that were to be replaced. Searching the catalogs I purchased a wide variety of metal polishes all promising to “melt away corrosion” and leave a mirror finish. Almost all of them left the corrosion intact while doing a great job polishing the tiny areas that were already smooth. After that I tried a collection of abrasive pads but found them too harsh. Then, one day while thumbing through the A&P books, I came across the mention of phosphoric acid to remove corrosion as a preparation for painting aluminum. A quick check of the catalogs, in the paint section, turned up such a product. Cheap, it was not at $40.00 a gallon plus “special” shipping. Nevertheless, I bit the monetary bullet and ordered a bottle. It showed up on a UPS truck with a very nervous driver who donned a yellow rubber suit, cordoned off my yard with yellow plastic tape and then carefully tip-toed to my door with a box. The box was festooned with every possible hazard label, each promising horrible consequences if it was opened. The driver thrust a gloved hand at me holding a crumpled wad of delivery forms and mumbled something that sounded like “sign this” from behind the breathing apparatus of his yellow hood. I managed to wedge my name between all the OSHA warnings and he bounded away like a jackrabbit, yellow tape still clinging to the truck as he accelerated away from what was sure to be “ground zero”. This little episode gave me pause, and after a moment’s reflection I decided this stuff might not be what I was looking for. I did eventually open the box, just to ensure there wasn’t an EPA guy taped inside, but the bottle is still on the shelf, sealed tight.
It is said that civilization’s most important discoveries were accidental. Such was the case with me and the art of polishing aluminum. One evening while trying to remove a particularly stubborn swatch of rudder decal residue I spritzed it with a citrus and aloe based product called “De-Solv-It” that I’d picked up at the local Wal-Mart. Tired, frustrated and expecting the usual battle with the glue, I mistakenly picked up one of the brown abrasive “metal polishing” pads that I’d abandoned on the stabilators and began scrubbing. In seconds, the glue had become mush and I was down to metal. Aghast at the prospect of seriously scratching the surface of the rudder I stopped and wiped away the goo with a paper towel. Amazingly, an area of corrosion on the perimeter of the residue was gone! In its place was a smooth, satin finish. It was not at all unattractive. I so impressed that I decided to risk doing the entire rudder. The results were very satisfying. Apparently, the aloe in the solution was acting as a lubricant that slowed the abrasive action of the polishing pad. As I rubbed the surface with a circular motion the solution gradually became a dull gray as the aluminum oxide was trapped in the liquid. It rinsed off quickly with water and even smelled good. After two applications the surface was corrosion free. Some of the larger pitted areas remained as unusually shaped micro depressions but I decided against removing any additional surface material. After all, the wing surface is only .016” to begin with.
It was here that I decided to go one additional step, purely on the basis of cosmetic appearance. While wandering through a local boating supply I happened upon a product called 3M Marine Aluminum Restorer and Polish (3M part #09020). It was a favorite of the aluminum boat crowd. So, being still financially numb from the expensive phosphoric acid adventure, I bought a 20 oz jar for $25.00 without flinching and took it to the garage. That night, after reading the instructions, I carefully smeared a tiny amount on a test area and began rubbing with a 0000 equivalent plastic abrasive pad. The results were amazing. Not exactly a mirror finish, but not bad at all. As a result, both rudders got the same polish. Then, as final experiment, I did a small section with Blue Magic metal polish. After some work it became obvious that with enough elbow grease I could probably attain the coveted mirror look. But, considering the work involved it wasn’t worth the investment of time. At this point I made the command decision to stop the process after the application of the 3M polish. Even that was more work than required, but the cosmetic results seemed worth the effort. So, the plan was to do the entire wing with the De-Solve-It and metal polishing pads to remove the bulk of the corrosion and then polish with the 3M. If I decided to keep the A-10 I could always do a yearly application of the Blue Magic to bring it a bit closer to the ideal. The final step would be a coating of auto wax to slow or stop the corrosion process and perhaps buy a tiny bit more efficiency from the wing. Unfortunately, this was still a daunting project given the large area of the A-10’s wing. Nevertheless, having not much else to work on, I opted to start the process. About this same time a sailing friend dropped by to observe and confirm the rumors that I had indeed gone over the edge and was hiding an airplane in my garage. A veteran of several sailboat restorations he quickly convinced me of the need for an orbital buffer to assist in the polishing process. He also demonstrated the efficacy of using my electric sander with the abrasive pad clinched to the bottom instead of sandpaper. After spraying an area with De-Solv-It the sander glided over the surface, slowly turning the lemon scented liquid to gray goo using only the tool’s own weight and a guiding hand from the operator. More stubborn areas called for a bit of additional pressure.
And so, I was off. The polishing effort would occupy most of the next three months until cold weather forced it to be an indoor activity.
But, back to the drag rudders. Having removed the corrosion, the next step was to replace the torn and tattered silver Mylar tape on the top and bottom of each rudder. Now an expert at removing adhesives I carefully stripped off the old tape and then removed the various fauna that had made their homes inside the rudder cavities. But where to find replacement tape? The aircraft supply catalogs were not much help. The local auto parts store came to the rescue with a product called “Chromebrite” exterior trim tape. Unfortunately they were down to their last roll. Some elementary math revealed it would cover only one rudder. More bad news, none of the other stores in the area carried it. I needed another source. And again, Providence stepped in. While cruising the aisles at Home Depot what should I find but mega rolls of aluminum tape meant for sealing heating ducts. It worked. I was back in the rudder repair business.
Back at the garage I carefully dug out damaged pieces of Styrofoam rudder core and replaced them with fitted pieces of Styrofoam insulation board. They were glued in place with the special blue adhesive used by builders to attach the foam boards to concrete walls. It worked very well with no risk of adhesive solvent melting the foam. This done I replaced each rudder’s AN hardware (the components that were reachable) and then applied the tape. The finished product was quite acceptable.
By now the new rudder hinge support brackets had arrived from Ameriplanes along with a variety of other items. So, I set about removing the old, broken assemblies. A new skill was developed in the process, that of drilling out the old rivets while leaving the holes intact. After two hours of surgery on the right wing tip with drill and punch, the old bracket dropped into my hand. I pulled the new one from it’s wrapping and did a quick comparison. Bad news. The aluminum web on the new unit was cracked. Not only that, the new one had a slightly different geometry that raked the rudder aft more than the original. This raised the question of whether the base of the rudder would mate with the fiberglass fairing without leaving a large gap. It also did not come with the hinge sections attached, or for that matter with any hinges at all. An e-mail to Larry Smith at Ameriplanes confirmed that the assembly was correct and used on the new A-10. The factory no longer supplied the assemblies with the hinges attached since each installation was slightly different. He apologized for the cracked web and a swap was made. While that was in progress I attacked the other wing tip. The new bracket assembly was intact. Using the old unit as a guide and some short sections of piano hinge found in the “spares box”, inherited from the previous owner, I carefully fit the new assembly to the end plate. In the process I discovered the reason that the old hinges had cracked. They were attached by five or six closely spaced pop rivets placed in a straight line down the center of the hinge face. This left it largely unsupported when the rudder was locked in place. I corrected this by using the same number of rivets but with wider spacing and in a zigzag pattern to distribute the forces. In one case, due to the rake of the new assembly I used rivets and aluminum brazing to strengthen the hinge attachment.
The next move was to attempt a repair of the top surface of the right outer wing section where the aluminum skin seemed to have pulled away from the foam wing core. Engaging my rusty memory I searched for and found the old article on maintenance of the A-10. The author had reattached some sections of his wing skin by drilling small holes in the aluminum and injecting adhesive with a syringe. Assuming I would do the same, I ordered a quart of Hysol EA-9430 for a painful $45.00 and then made a trip to the local marine supply house where I picked up some plastic syringes and mixing cups. While waiting for the adhesive I carefully marked out the roughly oval shaped section that appeared to be “oil-canning” and then set about removing the small section of aluminum hinge that was used to lock the rudder to the wing when folded. It was nearly in the center of the affected area. After removal, however, I noted a curious thing. Looking through the rivet holes in the wing skin I could see the white foam core. Pressing the surface moved both the aluminum skin and the core. They were still bonded! Using an inspection mirror from the wing tip while depressing the troubled area in strategic places I discovered that there was no delamination. Instead, the foam wing core had fractured in multiple places allowing the wing surface to move when pressed. Several vicious oaths later, a moment’s calm reflection revealed a possible source of the damage. During transport with the rudder folded and locked in place on the upper surface of the wing, it can still flutter. Over time it could cause such a fracture. The question now was what to do about it. Replacing the entire outer wing section was prohibitively expensive and a call to the local recycling center indicated that scrap aluminum prices were still not high enough to dump the whole thing and take up bowling.
At this point I removed the aluminized Mylar seal from the root end of the wing section and studied the construction. I needed a way to fill all the fractures and bond them together, making that section of the wing surface rigid again. After several hours of staring into the hollow void, I had an idea. The core was Styrofoam extruded in the shape of the airfoil with a single foam spar down the length of the wing section. Interestingly, the spar was a separate piece held in place by yellow polyurethane construction foam that acted as an adhesive. The fractures stopped at the spar telling me that the polyurethane was quite effective in holding things together. So why not use it on the entire fractured area? Using an old piece of Styrofoam board, I carefully fabricated a rib-like dam that I forced into the inside of the wing section using a broomstick as a ramrod. I positioned it toward the wingtip until it isolated the damaged core. I then purchased a few cans of expanding foam at the local hardware along with a three-foot length of flexible plastic tubing that would seal over the cans’ nozzles. This I duct-taped to an old yardstick and slid it into the void from the open wingtip. It took two cans to fill the void with urethane. The yardstick allowed me to position the outlet of the tubing to ensure the entire volume was filled from trailing edge to spar, all the way to the wingtip. Leaving the wingtip open allowed excess foam to flow out as it slowly continued to expand over the next few hours. Naturally the process was not without some learning curve problems. The fact that the first attempt left me covered in sticky, yellow, expanding goo will not be dealt with. Nor will I elaborate on the humor with which my family greeted the fact that I entered the house looking like a shaving cream commercial gone bad. Let’s just say that, in essence, the idea worked, and over the next several weeks I was able to get most of the stuff off exposed skin. I was in need of a haircut, anyway. And, yes, I did decline to allow the incident to be used as a segment in the New Red Green Show.
Now that the core fracture had been repaired the next move was to reduce the odds that it wouldn’t happen again. To do this I decided to pop rivet a large “patch” of .016” 2024T3 aluminum as doubler over the fractured area before reinstalling the small section of hinge that locked down the folded rudder. The patch also nicely covered the large scar in the wing surface from the loose rudder working against the skin during transport. A mirror image of the patch was then made and installed on the opposite wingtip.
The final step was to remount the rudders to the new wingtip hinge bracket assemblies. During the disassembly I’d noted some nasty scoring in the brass rods around which the rudders rotated. A brief search of local marine suppliers turned up a length of rod that I cut and drilled to make the replacements. The rudder locking pins, however, were another matter. Short lengths of rudder hinge pin of the right size were not easy to find, and what I did find was remarkably expensive. This prompted another e-mail to Larry Smith who suggested using short pieces of 3/32”welding rod. Fortunately the local hardware had a well-stocked welding supply department and the rod was acquired in less than a day. The finger loop in the pin was made by bending the rod around an old squeegee handle. The process was aided by first putting the end of the rod into a small hole drilled into the handle and then starting the bending process. After bending, a few minutes with a hacksaw and file produced finished products that fit like a champ. The rudders were now back in place and once again functional.
The final metal repair involved replacing a short section of trailing edge on the right half of the wing center section. According to the previous owner, he’d hit a deeper than expected puddle of water while taxiing. The tips of the old composite prop struck the water and were deflected forward on their way up, chewing out a section of the trailing edge. It was an ugly situation, exceeded only by the repair itself. My first move, therefore, was to remove the piece of aluminum that had been bent around the damaged trailing edge. A variety of pop rivets were removed along with some type of amber colored glue that had been used to seal the old repair. Underneath this was yet another repair that had signs of prop contact, and underneath that, finally, was the mangled, original trailing edge. The damage extended an inch or so forward into the wing skin above and below, and there was a nasty crease in the lower skin that projected about a foot forward. Obviously there had been more than one puddle incident. Amazing.
I first marked out the area to be repaired and studied how the wing had been assembled. Next, I drilled out the pop-rivets within the damaged section and carefully separated the wing skins from the trailing edge with a paint scrapper (they appeared to be glued with the same adhesive use to bond the skin to the core). Using a small rotary tool with a diamond-surfaced cutting blade, I cut out the ragged sections of wing skin on the upper and lower surfaces. Small chunks of Styrofoam wing core that had been damaged in the accidents now fluttered to the floor leaving the torn and distorted trailing edge exposed. Using the rotary tool again, I cut away the old section of trailing edge and replaced it with a new section that Ameriplanes had provided. I considered cutting and shaping tiny new sections of Styrofoam and then gluing them into the void, but decided the effort was too time consuming for so small an area. I then drilled matching holes in the new trailing edge using a strap type hole replicator and secured it in place with aluminum pop rivets. Next came a small section of .016” 2024-T6 aluminum cut to cover the damaged upper wing skin. This was also pop-riveted in place using Clecos for the preliminary fitting as my builder friend had recommended. Moving to the lower wing surface I decided to cover, and at the same time double, the area with the crease in the original skin using a triangular patch with rounded tips. The result was quite effective. As a cosmetic measure I then did the same operations to the left wing. The result was not perfect, but much better than the original repairs.
The only remaining areas of the wing that might receive additional attention were the fabric covering on the top center section and some of the small Mylar sections on the lower surface near the trailing edge. The Mylar could be replaced if adequate tape could be found and the fabric re-painted. But, this would be done later, since these were mostly cosmetic efforts.
I had more important problems to deal with as Episode III describes.
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