Restoring the A-10D Mitchell Wing

SN# 275

By

Scott McCarthy

Episode I

In the Beginning

 

            When I purchased a used Mitchell Wing A-10D in July of 2001 I’d been assured that it had been flown on a fairly regular basis and was nominally functional. Prior to handing over cash, however, I’d gone over the plane with a list of “bad things to look for” that I’d gotten from Larry Smith at Ameriplanes. This first, cursory inspection found a few things on the checklist and some others that gave me pause to the point where I was uncertain whether or not to go through with the deal. My original plan had been to buy a used but flyable Mitchell Wing that could be brought back to nearly new over a period of time depending upon motivation and funds available. #275 was definitely in need of something more than TLC before I’d risk even a taxi test. Still, the price was good and what I’d found seemed reasonably easy to repair, even for a novice like me. And so, I returned to the land of the big mouse, hooked up the trailer and hauled the A-10 some 1,500 miles north to Rhode Island.

            The first step was to push it into the new tube-framed temporary tent structure behind my garage that would serve as hanger and repair shop. That done, I sat down and watched the Mitchell Wing assembly video four times before calling upon the assistance of a fellow engineer who was in the process of completing a Pelican homebuilt aircraft. The plan was to have him assist with a more thorough inspection. The results were not encouraging.

 

The Wing Surface - The wing itself appeared to be in fairly decent condition except for a rough, gray patina of aluminum oxide and a badly patched section of trailing edge. The oxide coating would somehow have to be removed and the pitting stopped. There were also some nasty scars in the aluminum out near the wing tips. It appeared that the leading edges of the drag rudders had, in the past, been flopping about against the wing surface when folded for transport. While inspecting the rudder lockdown point on the outer right wing section I noted that a two square foot section of the upper aluminum surface appeared to have delaminated from the foam core of the wing. It produced a curious “oil can” effect when it was pressed. The Stits fabric on the top of the center section appeared to be intact except for the paint that had been touched up with a paintbrush in a variety of places to correct cracks and lost chips. The Mylar on the bottom surface, however, was in very good shape.  Moving out from the center, we found a quarter-sized hole that had been punched into the bottom surface of the right wing. Then came the trailing edge. The previous owner had explained the patch on the trailing edge of the right wing. According to the story, he’d hit a deeper than expected puddle while taxiing. The composite prop blades distorted, bending forward and chewing up the right trailing edge. What lay beneath the patch was unknown. Any repairs by me would have to be carefully considered at a later date. For now, it looked bad but airworthy.

 

The Stabilators – The A-10D is, of course, a flying wing. Since it has no tail it also lacks the traditional rudder and elevators.  “Stabilators” is the company term for the control surfaces slung below the trailing edges of the wing that provide pitch and roll, similar to “elevons” in some jet aircraft.  Both the left and right stabilators had multiple repairs to their undersurfaces to reattach the counterweight arms. Judging by the repair patterns, there was a ground loop or two in #275’s past since the low slung weights are often the first items to get wiped off. When the wings are folded for transport, they’re also the highest points on the trailered load. Inattention to “Max Height” signs can leave them embedded in a variety of roof structures.  Complete replacement with new units looked like the most straightforward answer.

 

The Rudders – The Mitchell Wing, like many other flying wing designs uses wing tip mounted drag rudders to control yaw and, to a great extent, roll. They were unusually loose. Removal of the fiberglass tip fairings revealed that the left and right rudder support structures had cracked through the aluminum webs to which the rudders were attached. One had been “repaired” with an odd shaped piece of scrap aluminum that spanned the crack and was held in place by two different sized pop-rivets, one of which was loose. The wing tip hinges, which allow the rudders to fold for transport, were both cracked and distorted (Holy ground loop!). And, the small hinge sections that mate to the main wing to lock the rudders in the vertical position were both cracked nearly through. It was obvious that both left and right rudder hinge assemblies would have to be replaced. The rudders themselves were in pretty good shape, aside from the ubiquitous oxide coating, a dent or two and the fact that the silver Mylar tape on top and bottom was in tatters.

 

AN Hardware - Built at the Mitchell Wing factory in 1984 the aircraft had spent the bulk of its nearly twenty years in the deep south near the ocean, (The original owner lived in Louisiana, the second in Florida. Curiously, I too live within a few miles of the Atlantic.) Unfortunately this had taken a major toll in corrosion of the steel hardware, all of which would have to be replaced.

 

The Main Landing Gear - The next item was replacement of the steel sleeves that held the main gear legs to the pilot cage frame. The originals were badly distorted due to some landings that must have pushed local seismometers near the middle of the Richter Scale. Thus the reason the aircraft had a severe list to port when on level ground and the propeller arc came perilously close to mother earth. Interestingly the legs themselves appeared to be intact. Close examination of the “spare legs”, provided by the second owner, showed a variety of bends and kinks that rendered them useless. The fiberglass wheel pants were also loose, one being held in place by a single, rusting pop rivet.

 

The Nose Wheel – The nose wheel assembly, aside from a splotched coating of rust, appeared to be in reasonably good shape. The drum brake was also functional. Following the nose gear pivot shaft up into its bearing assembly revealed a set of bicycle headset bearings and a compression nut familiar to any cycle mechanic.  The hard landing theory got another boost when inspection of the lower bearing showed that it had been pushed upward with enough force to tear the vertical steel sleeve that housed it. This and the rust dictated replacement. There was also a small hole punched through the fiberglass wheel pant. This appeared to be road damage from some flying debris during trailering. Like the main wheel pants it too was very loose.

 

The Pod and Podsterior – The A-10D uses a fiberglass pod on the forward part of the pilot’s cage to protect the pilot from windblast. Attached behind the pilot’s seat is the fiberglass “Podsterior” which streamlines the after part of the pilot’s cage. The red gel-coat on both was faded and riddled with tiny cracks at the stress points where the fiberglass was attached to the frame. Large sections had been spray painted with hardware store red enamel that had faded several shades. The same was true for the wheel pants and the wing tip rudder fairings. There was also an unsightly “hump” on the top of the nose where the previous owner had cut out a section of the fiberglass in an unsuccessful effort to install a ballistic parachute.  A bit of probing with a knife blade showed that the section had been put back and held in place with a copious layer of Bondo, which was now cracking and chipping away.  The front of the Plexiglas windshield was attached to the Pod where the Bondo ended. It clung valiantly to the fiberglass despite a variety of stress cracks around the pop-rivets. While the means of dealing with the fiberglass was still in question, there was no doubt about the windshield. It would definitely have to be replaced

 

The Engine and Fuel System – The free air model Rotax 277 and its belt reduction drive appeared to be in pretty good shape as was the electric starter. The previous owner indicated that it had only a bit over thirty hours since a rebuild. The aluminum fins had some corrosion and they had accumulated a tiny chip or two, but nothing serious. The engine mounting plate was in good shape but close examination of the Lord anti-vibration mounts revealed cracks in the elastic absorption material. Not surprising after nearly twenty years on the job. The Bing carburetor was next. It looked pretty good too, except for a tiny blob of white stuff that seemed to have been forced out where the choke cable screwed into the carburetor body. As I loosened the choke fitting for inspection a small piece of metal detached itself from the body and fell to the ground. It and the cable end had been held in place with white epoxy. Not a good sign. Note to self: buy new carburetor, new fuel pump and have entire engine torn down and inspected. !@#%$@!!!

 

The Fuel Tank – I was told that there was a plastic fuel tank attached to the back of the pilot’s seat, housed within the Podsterior. This was a feature unusual to most Mitchell Wings. All the other units I’d seen had torque tubes that ran vertically behind the seat for control of the stabilators. #275 was different. It had been factory built using Teleflex cables that left the back of the seat open, an inviting arrangement for a larger fuel tank. I’d been told that the tank had a five-gallon capacity in keeping with Part 103, (When I asked where the fuel gauge was, the former owner handed me a calibrated wooden stick, pointing out that wearing a reliable watch was a good idea while flying.). There were, however, some fuel stains on the bottom of the Podsterior that led me to believe that fuel grommet may be leaking. Again, this would not be an unusual thing considering the age of the plane, so I decided to take a closer look. Removing the fiberglass rear end proved a simpler operation than I had expected. Inside was quite interesting. The tank turned out to be a Rubbermaid oil collection container that was marked 3.5 gallons. And it had indeed leaked, both at the fuel outlet grommet and around the large white plastic plug that had been glued into the hole on the side where motor oil is generally poured in. The bottom of the container sat on a small shelf formed by a piece of steel angle stock. It was attached to the back of the seat with a broken rubber bungee and a length of thin, oil-stained nylon line.  Overall, it was a pretty clever installation, if a bit crude. Nevertheless, it would have to be replaced.

 

Into the Pod! The Rudder Pedals – Buoyed by the success in removing the Podsterior and at the same time suffering from a growing pessimism, I decided it would be wise to take a careful look at the entire pilot’s cage with the Pod removed. After a bit of a struggle removing the nose wheel, the windshield and instrument panel, the fiberglass pod finally yielded and I had access to the entire frame. Starting at the nose, the first thing I noticed was the rudder pedals. In their neutral position the right was several inches forward of the left. Obviously the previous owners either had severe shortening of one leg, or the airplane was designed to only turn left. The fore and aft pedal adjustment feature was only partially successful in bringing the pedals into the same plane. Part of the reason for this was that the pedals were each attached to, and rotated around, individual aluminum tube axles that turned in a set of aluminum bushing tubes below the frame. Pedal action was stiff with rust and aluminum corrosion. “Replace pedals” was scribbled into the small notebook that I began to keep on the inspection process.

 

The Electrical System – Moving aft, the next thing I ran into was the rat’s nest of wire, two switches, a charging module and battery that served as the A-10’s electrical system. As an electronics engineer by trade I was finally in my element. Reaching for my wire cutters I gleefully began snipping until the last strand fell neatly into a nearby garbage bag. If nothing else, at least the new wiring would be a work of art.  The next thing to go was the small, lead-acid motorcycle battery. In case of an overly energetic landing, the last thing I wanted was battery acid splashing on my legs. “Replace with gel cell” went into the notebook.

 

The Control Stick – The control stick on the A-10D is a two-piece affair with the lower portion moving fore and aft for pitch control and the upper section pivoting left and right for roll. It’s a clever solution to a very narrow cockpit where a conventional design cannot be used. Aluminum push-tubes attached to the sides of the upper section of the stick lead down to a mechanical mixer for control of the stabilators. The design is remarkably elegant in its simplicity but, as I later learned, can be a problem for experienced 3-axis pilots. Preliminary inspection of the stick showed it to be in good shape except for the badly rusted ball joints that connected the push rods to the upper stick. One of them disjointed when I pulled on it allowing the push rod to fall down against the frame. It was probably not a good thing to have happen in flight. The notebook entry reads “Replace ball-joints. There must be a better way”.

 

Throttle, Enrichener (Choke) and Brake Controls – The normal throttle location for an A-10D is on the left down tube, co-located with the brake lever which acts as a throttle stop. The enrichener could be in a variety of places depending upon the year of manufacture and the engine installed. My A-10 had only a hole through the tube and a Rivnut installed a few inches below that. #275 had obviously been customized by a previous owner. It had a pair of bicycle shift levers on the left diagonal tube next to the seat for throttle and choke controls. The brake lever was attached to the diagonal tube on the other side of the seat. I found this arrangement to be inconvenient and noted that the original factory locations for brake and throttle were more to my liking. The bicycle parts would have to go.

 

The Frame and Seat – Moving further back I found that the aluminum seat pan had been torn in two places on the left side. The tears appeared to be remnants of an attempt to replace the steel main gear sleeves and were slowly advancing across the thin metal. Another replacement job. Carefully examining the extruded aluminum brackets that anchored the down tubes and the front of the seat, I noted that the left bracket had been bent and the seat anchor distorted. It was difficult to imagine just how much force this would require, since both these mating brackets were sturdy 1/8th inch aluminum. I decided to forego imagining what might have happened and scribbled another “replace” line in the advancing pages of the tiny notebook. Dreading what I might find, I pulled the bolts that anchored the down tubes and discovered that both bases had been squeezed to distorted ovals by an over enthusiastic individual with a wrench. The tubing was kinked and the bolt holes very long ovals. “Replace both.” The rear cross tube not only anchored the two seat pans but turned out to also be a major structural member. It looked like Swiss cheese with nearly thirty drilled holes of various sizes. The purpose of these was obviously for pop-rivets.  The reason for so many or what they secured was a mystery. The notebook absorbed another “replace” line. The diagonal frame tubes were next. The left member was in fairly decent shape, the only negative being that someone had used it for an electrical conduit for the engine instrument and kill wires. They were jammed tight at the rear opening, which meant that replacing a broken or shorted wire would be a major headache. They would be removed and a better scheme employed. The right tube looked as though it had been beaten with a bicycle chain. The numerous dents and scars started at the seat and went all the way to the aft end. More scars were found on the right side engine support tube whose open end had been badly distorted by the violence. More notebook entries appeared with the now familiar “replace” word.

 

            My homebuilder friend pulled back from the from the disaster that was now my ultralight and smiled, lifting his eyebrows in one of those “Man it sucks to be you” expressions that only he can muster. I gave it no notice, being too busy rhythmically beating my head against the pilot’s cage. What I needed was a plan of action. After a dozen or so brain numbing impacts, I had come up with two options. The first I exercised immediately and picked up the phone. The scrap metal dealer at the other end assured me that the value of 254 lbs of aluminum at the current market price was nowhere near the investment I’d made, even if I did have a trailer to bring to the yard. I hung up and shifted to option number two which involved more thought, this time wearing a hockey helmet.

            Okay, so I wouldn’t have this thing in the air by next weekend. Spring of ’02 or maybe ’06 looked more likely. After all, the guy standing next to me had spent the last four years honchoing a kitplane in his garage that probably wouldn’t fly for another two. I was not a complete idiot. After all, I had a whole airplane, fully assembled right in front of me, just sort of bent and broken in places. I’d restored cars, guns and bicycles. Why not an airplane? Besides, I think it needed me. End of discussion. The gauntlet was down and the challenge accepted. Mitchell Wing #275 would be better than new.

Episode I pictures

Episode II   Episode III   Episode IV   Episode V   Episode VI   Episode VII   Episode VIII   Episode IX   Episode X

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