March 2018The exhaust shroud is one of those projects I've worked on sporadically so have had a lot of time to "stew" over the challenge. The part that caused me the most pause, and perhaps procrastination, is welding thin aluminium. I've mentioned before I was a fair welder at one time but unlike riding a bike it's a use it or loose it skill. Yes I may have been able to find someone to do a good job for me but where's the fun in that!
Anyway, welding aluminium is kinda like herding two cats. The metal at the joint tends to want to bead away from the joint rather than flow into it. There's a dance between the foot rheostat, torch, "puddle" and filler rod that needs to happen smoothly and evenly to get a good result. Too much current and you have a hole, not enough and you have insufficient penetration.
In preparation for the big event I spent some time last summer in the Miller welding truck at Oshkosh. With the knowledge gained I purchased a smaller torch and a "gas lens" for my little hobby TIG welder. This is sort of like putting an aerator on a faucet, straightening and smoothing gas flow. It seems to make a difference.
There are two things that make a big difference in terms of making the welding easier. 1: The accuracy of the joint (I tried to eliminate any gaps) and 2: the cleanliness of the material, electrode and rod. I cleaned the aluminium (6061-T6 and T0) and rod with Alumiprep, and put a new tip on the electrode (I used 2% Lanthanated) prior to each section.
I tackled the easy part first, tacking in a couple of places before welding the seam of the tailpipe section.
Ok it's not beautiful but it's solid and I have the option of orienting the pipe so the weld is at the back!
(difficult to see in this photo unless you zoom in... click on the image, then right click on the image that opens and click open image in new tab. Then you can zoom!)
Also tinnerman nutplates are installed inside the lower shroud half to fasten the tailpipe section.
Back to the hangar...
Tacked in three places,
I bent a welding rod to the basic shape first, then filled the tube with sand and cold formed it around a plywood form.
Then installed on the engine, hopefully for the final time.
I used Scat duct for the cabin heat and carb heat connections, because of relatively high heat tolerance and no inner liner. The inner liners have been known to collapse which could cause a carb heat restriction.
The last (I think) task of tricky fabrication is the exhaust heat shroud. I have the original style exhaust and as far as I can find there is currently no supplier of shrouds for this style of exhaust. Every so often one comes up for sale on the used market but there's a reason they're being replaced right?
Anyway it's a relatively simple design but not so simple to fabricate.
Laminating 3/4" plywood
Then I had some problems locating someone in town with a slip roller small enough to roll the tailpipe shroud so I started working on the wings.
Eventually I got a couple rolled so now I need to practice my welding!
Once the crank case baffle was out of the way, I had a look at the rest of the engine baffles which were in tough shape. I figured fabricating new ones would require multiple installation and removals of the nose cowl and other parts, so I again put off painting. I could see myself dinging or dropping a finished part so why risk it. Thense started the upper baffles project.
The male form and part go into a plywood frame.
And then I carried on forming by hammer.
many trials and eventually a reasonable result was obtained.
Glad that's done.... now what's next.
The original baffle support rods were corroded and worn as much as 50% diameter in places. I purchased some stainless 3/16' rod to make new ones.
As always I leave extra material for final trimming later to allow for any errors in the original parts.
I drew lines 3/4" apart and used a copper wire and measurements from the bench surface to get 15 profiles.
A modicum of hand forming was required and then....
Anyway the first attempt at the left baffle yielded more material for the scrap bin.
Felt was used at the factory, held in place by steel staples and proved to be not a great solution. Anyway engine cooling is obviously important so worthy of 1980s technology !
I mention this because mine is not the only Taylorcraft I've seen with the seals installed in a way that renders them useless.
Another much bigger than expected task completed.
Happy to say it all fit together very well,
I was to and fro on installing the fasteners before or after paint. In the end choose not to risk damaging the paint.
Before long I'm going to need the boot cowl, nose bowl and cowlings finished and top-coated so I can pull lines for the accent colours. Ideally the accent colours should go on within 72 hours of the base coat so I realized I had a bunch of work to do yet before the fuselage could be painted.
So the task list includes repairs to the nosebowl and fabricating a new lower cowling.
I didn't get very far before I realized one of the upper cowling mounts was missing.
Not sure how these were configured originally but the captive nuts will aid in removal and installation, and have to be better than the giant PK screws that were there when I got the airplane.
That thought quickly evaporated with the sticker shock associated with the price of a new aftermarket part (remember I currently pay a 30% premium due to our soft Looney).
In the end I'm glad I didn't opt for a new replacement as I think I would have had to add a doubler or two anyway. Here's why:
From the factory, the BC12D had no airfilter. The carb air intake was a 2" diameter "snorkle" that extended through the lower cowling.
Many/most aircraft were later retrofitted with a filter assembly which meant cutting out the lower part of the nosebowl. The nosebowl is made of very soft aluminium and the filter cutout, which includes a cutout in the front edge of the lower cowling, basically eliminates any structural integrity. So the question is when, not if, the nosebowl will fail.
The joggle tool I made is put to use once more.
Unfortunately I didn't get very far. The original top cowlings that I had hoped to keep, turned out to be different lengths.
So I had to order 2024 T3 025". In the meantime I turned my attention to the interior door panels.
On the boot cowl,
Note this doubler doesn't pick up any of the three lower cowl fasteners. Basically a waste of time and material. Anyhow.....
When the 2024 T3 025" arrived, I used the left upper cowling as the pattern for both left and right, leaving about 3/16" extra material for final trimming.
I break the inboard edge and cleco to the centre hinges.
Then install and check for initial fit.
As one invests more and more time in a task like this, the penalty for making an error gets larger and larger!
So an increasing amount of time is spent thinking, visualizing and measuring before cutting or drilling.
And measurements taken from the tailpipe to lower center camloc mount and to the air box outlet.
and then the tailpipe position is "plotted," double checked, triple checked, and triple-dog-dare me checked before the hole is cut (not to the finished dimension at this point).
From this point on, the cowling needs to be supported while off the aircraft or the flange will kink.
Note the masking tape creates a 2" reference line for locating the camlocs and trimming the edge.
Plywood is good for this kind of job because the laminations help as visual references.
Note; to anneal 2024 requires an oven that can reach over 600 degrees, and several hours. Normalizing is done in this case by torch and soap!
That was a lot of work, but I'm pleased with the result.
The aircraft never had one since I've owned it (and never encountered any cooling issues), but it's supposed to be there so......
The holes that pick up on the intake manifold studs need to be dimpled.