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Firewall Forward

March 2018

The 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.

My hands are shaky so its important to hold the work such that I can rest my hands on something solid for better control of the torch and filler rod.

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!

 I formed the upper and lower halves from .040" 6061-0 last summer (see July 2017 below) and back in my basement shop where it's warm, with the exhaust fastened to the bench, I started the task of trimming and fitting.
 I used my home made joggle tool again and formed a joggle on the lower half so that the top overlaps the bottom.

(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!)
More trimming and some shrinking and forming to complete the fit.
I made 5 of these stand-offs from old breeze clamp material to support the shroud halves and centre the tail pipe section.
Back in the hangar I cut strips of 2024 T3 .032" and joggled each on the brake, then formed and cut four pieces to both complete the joint, and provide added strength and durability to the shroud.
Fitting the joggles.
Fitting and trimming the tailpipe section.
Rather than have soft aluminium against steel, I used woven baffle seal material to make a lap joint
The right side is awkward as the joint is actually in the radius, so the baffle seal material is a good option I believe.

 To connect the tailpipe section to the shroud a flange is needed.


 Thinking ahead to the welding challenge, I spent a lot of time trimming the joint.
 The outlets are also fitted in preparation for welding,
and the joggles are riveted in place.

Also tinnerman nutplates are installed inside the lower shroud half to fasten the tailpipe section.

Back to the hangar...
 Check to make sure everything fits.
 Then we're off herding cats!

Tacked in three places,
 then work my way around, extending each tack an inch or so at a time to reduce the potential for distortion (at least that was my thinking).
 Carb heat outlet next,

and starting to get the hang of it!
 Done....
 Next up is a new engine breather.

I bent a welding rod to the basic shape first, then filled the tube with sand and cold formed it around a plywood form.
 I made a Rube Goldberg tube beader from my oil filter cutter.
It was just adequate enough to make one bead!
 The exhaust gets a couple of coats of ceramic paint and "cooked" for a couple of hours by two heat guns.

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.
Started test fitting some other parts as well and I think the "lump" is ready for installation.

















July 2017
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

 After cutting and sanding, ready to give it a try.
 You guessed it...  My apple press to the rescue again.

 6061-0 .040" is used.
 Success! enough to finish by hand anyway.
 Getting it off the form was the most difficult part.
 Edges are marked and trimmed.
  And some additional forming and shrinking.
 And preliminary fitting to the exhaust.
 Also made the Cabin heat and Carb heat flanges.


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!


















August 2016
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 front baffles I have are original Taylorcraft as far as I can tell (riveted together with steel rivets), but they were in tough shape.
I deriveted the parts and flattened the two front panels to use as patterns.  The cylinder baffles however presented a much bigger challenge.




 I used 3/4" plywood to make male forms.
 Basically cutting routing and sanding until the old part (straightened as best I could) sits snugly on the form.
 The original was 040" soft aluminium and given the extreme forming required I used similar, 6061-0 .040".
 I bent a 90 degree flange on the leading edge as a reference of sorts and also drilled and cut the material to aid the forming.

The male form and part go into a plywood frame.
 On the EAA website there's a video of a poor-man's hydro-forming process that uses a press and old inner-tube so I tried to emulate that process using my apple press and bottle jack.
 I think if I could have pressed another few tons it would have worked!
 As it was it got the basic shape started....


 And then I carried on forming by hammer.














many trials and eventually a reasonable result was obtained.
 Repeat for the right side
 Trying to get a snug fit.  We want the air to pass through the cooling fins, and not bypass next to the crankcase/non-finned areas.














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.

 Stainless is hard on cutting tools so my dye was about finished by the time all 8 threads were cut.

As always I leave extra material for final trimming later to allow for any errors in the original parts.
 This is the reject pile, clearly not thinking straight that day!
Once the front panel is formed its clecoed to the cylinder baffle.
 To get the positioning of the front baffle, the side baffle is needed.  The ones I had were not original and did not fit very well so I made templates from craft card.
 and fabricated new ones from 2024 T3 .032"
 To get the position of the inboard end of the front baffle, the crankcase baffle needs to be in place.  So I alodined it,  riveted it together, and
 installed it (hopefully for good).
 Left side baffle installed
 I used the cowling doublers to ensure the nosecowl was in the right position.
 and worked the front, and side baffles to get 1/4" - 3/8" clearance.
 A little shrinking of the lower flange is needed.
 The right side has an attach angle.  I formed one in a solid piece in place of what was there.
 For ease of maintenance, and to help with fabrication/fitting,  I installed nut-plates at all the joint locations.
 Right side and front baffles installed for positioning and fitting.
 ..... worked to get good clearances.






 Next up, the rear baffles.  I believe mine were original and they were repairable, but I decided to "go for broke" with the goal of all "new" baffles.

I drew lines 3/4" apart and used a copper wire and measurements from the bench surface to get 15 profiles.
 Repeat for the right side.
 3/4" plywood is cut for each profile.
 Glued and through-bolted with threaded rod.
 Then a considerable amount of time was spent sanding to get radii and a snug fit for both the male and female forms.
 Kind of a waffle press.
 Apple press has made more Taylorcraft parts than cider!
 Wow! it worked!

A modicum of hand forming was required and then....
the fitting process begins.















 The right side went so well that perhaps I was over-confident.

Anyway the first attempt at the left baffle yielded more material for the scrap bin.
 So for the second effort,  I did some manual stretching/forming first.
 Then into the apple press, and back on the bench for hand forming and trimming.
 Success!

Top cowlings installed for final trimming of the rear baffles.
 I used my lightning hole roller tool to form flanges for the baffle seals, and dressed out all the tooling marks.
 I deviated from the original design and fabricated a centre panel.  This just makes more sense to me and facilitates removal and installation, and access to the engine mount bolts.  It's secured with 4 No 8 machine screws and nut-plates.
 The rear baffle set.
 More fitting and also doublers installed at the support rod locations.
 The carb and cabin heat ducts are cleaned and straightened, and doublers made to address the multiple and oversize holes on the flanges.
 Baffle seals are made.  I used 3/32 silicone rubber to be held in place by doublers and 3/32 solid rivets.

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 !

 By pulling the seal around the fasteners, some shape can be realized that will aid in achieving a good seal against the cowlings.
 Note the front seals extend up and forward such that air pressure will force them against the nose cowl for a good seal.

I mention this because mine is not the only Taylorcraft I've seen with the seals installed in a way that renders them useless.
Remembering that the intercylinder baffles and lower crankcase baffle are already installed on the engine,  The baffle "kit" is complete.




Oh and I made some spring fasteners for the inter-cylinder baffles.


 Rather than paint the baffles I decided to use an alodine conversion coating.
 I think from the factory they were bare so we'll call this in the spirit of original :)
 Hard on the fingers pushing all those 3/32 rivets through the seal material but anyway a good result.
 Front baffles riveted including the heat ducts
 Ready for installation...
I believe that was the last of the work for which I need the engine installed, so back on the stand it goes and the baffles installed.






Another much bigger than expected task completed.
 Next I had a marathon day of etching, conversion coating and preparing for primer all the other parts I've made over the past couple of months.
 Then in two batches, epoxy primer was applied.

 Then another marathon of several days, riveting everything together.

Happy to say it all fit together very well,
 and I managed to avoid causing any damage with the rivet gun,  perhaps because the stakes were so high, one slip of the rivet gun and many many hours of work might be toast.
 I mentioned earlier that the lower cowling can be damaged relatively easily while off the aircraft.  I had been using a bungee cord to help.  I decided to add a permanent feature in the form of two small stainless tabs.  Eventually I'll have a length of paracord or similar with a couple of hooks as part of CF-CLR's fly-away kit.
 The cowling doublers and intake shroud get pro-seal.
 And a bead to get a nice joint.
 The joggle in the side doublers is about 45 degrees which means a small "trough" was left between the doubler and skin.  Rather than have debris collect in there for the lifetime of the part, I ran a bead of pro-seal there too.
 Last but not least the Dzus fasteners are installed.
 The lower centre fastener is too far from the edge for the clamp, so I used my rivet gun and bucking bar to swage the grommet.  It actually worked better than the C clamp.  Now I know!

I was to and fro on installing the fasteners before or after paint.  In the end choose not to risk damaging the paint.
So with that the cowlings are finished save for topcoat.

Phew!!












June 2016

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.



 Using various bits and bolts I made a new one
I also welded nuts to the underside.

 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.

 The nosebowl was in tough shape, worse than I thought, and at one point I decided to just replace it.

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).


 It's a little confusing that someone would take the time to bondo over universal head rivets rather than just using flush rivets.  Anyway all part of the hard life of a 70 year old Tcraft.
Once the paint and bondo is off a better assessment can be made.

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.

 Basic doubler layout picks up the two lower camlock locations and aims to transfer loads between the two around the air filter opening.

The joggle tool I made is put to use once more.
 The material is 2024 T3 which is considerably stronger than the base material.


 I use my shrinker around the lower edges and inside radii so the doubler takes up the compound curves of the cowl
 2024 T3 is much harder to form, but it will be worth it in the end, achieving a light and sturdy result. (lighter than the bondo anyway!)
 and a new crankcase intake duct is fabricated.
all tooling marks are dressed out.
Technically universal head rivets could be used ie consistent with the original rivets in the area, but given the high visibility of the nosebowl, all  but the duct will be flush riveted...which means a lot of dimpling.




The top cowling attachment point was also cracked so a repair doubler is fabricated.  I also spent many hours with my nylon hammer and English wheel removing dents and returning something resembling the original shape.
 Made new (slightly lighter)  chromoly camloc reinforcements  although with 2024 T3 they're not really needed, except for originality.


 Both upper camloc locations were repaired
 Dimpling again...
 Flush patch in place of the bondo.
 Oh and another satisfying thing about repair instead of replace is I get to keep the TAC stamp.
After a very full week the nosebowl is ready for etching and primer.

 Also primed a few inspection plates.
 Ready to make some noise.
 New Camloc springs as well.
 Top mount repair.
 I left enough space for for a tiny vinyl ester (epoxy) filler.
Phew!

 On to making the lower cowling.  First I need to hang the engine.





Unfortunately I didn't get very far.  The original top cowlings that I had hoped to keep, turned out to be different lengths.
I can only conclude that at some point the right cowling was trimmed for some reason but it was over 1/2" shorter than the left which would have had the nosecowl cocked to the right.  Too bad I wasted time and expensive paint on the old parts but....

So I had to order 2024 T3 025".  In the meantime I turned my attention to the interior door panels.




 Then applied the anti-chafe cowlseals,


On the boot cowl,
 and the nose bowl.
 I pulled out the old lower cowling in the hopes I might be able to use it to provide some preliminary positioning info, but it was basically useless.

Note this doubler doesn't pick up any of the three lower cowl fasteners. Basically a waste of time and material.  Anyhow.....
 With the lower cowling installed the nose bowl was not square... consistent with the short right upper cowling.
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.




 I rigged up different diameter ABS pipes to form the top cowlings.
 a little at a time until a good fit is achieved.
Good enough for now....


 The upper cowl stiffeners are duplicated and upper latch positions located and confirmed by the lower cowling.
 I used the carb air box exhaust as a starting point, and marked a ceterline for reference purposes.
 After taking the plunge and cutting out the oversize blank,
 I pull it into place to start the fabrication process.

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.
 Once I'm content with the nose bowl alignment I drill for the two lower camlocs.
 I need the aft center camloc to hold the cowling at this stage,
 so a new mount is fabricated.
With the mount clecoed to the firewall, I mark the hole location, and as it turned out it was right on the centerline.

 Upper cowlings installed once again to mark the preliminary top edge.
 Engine R&I to install exhaust.

And measurements taken from the tailpipe to lower center camloc mount and to the air box outlet.
 More hand forming of the lower radii....


 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).

 With the cowling installed again, I eyeball and mark the approximate location of the lower flange.
 A length of coper electrical wire is formed to the shape of the lower cowl near the aft edge.
 Contour is transferred to cardboard and then to a piece of old shipping crate.
 The angle of the cut changes from shallow at the outside to steeper toward the center.  I eyeballed it with a jig saw and was mostly lucky.
The lower flange is formed and trimmed.

From this point on, the cowling needs to be supported while off the aircraft or the flange will kink.


 The signature vents need to be formed. The forward pair are slightly larger than the aft.


 I made plywood forms
 and did several practice runs, refining the forms by sanding between each attempt.
 Once satisfied I could get consistant results (and before wearing out the forms)....
 I laid out the locations, drilled the holes, cut the slots and deburred everything.
 Result....
 Next I located and drilled the holes for the remaining camlocs.

Note the masking tape creates a 2" reference line for locating the camlocs and trimming the edge.
 I also use masking tape to mark the cut edge.
After trimming, the lower leading edge needed a little shrinking.  This gives a little bit of a compound curve to the cowling and gets the edge nice and tight against the chafe strip.


 Top cowlings reinstalled so the lower cowling top edge can be marked and trimmed to size.
 Lay out for the lower cowling stiffeners.
 Clamped in place before drilling.
 Adding the latches.
 And the right side



I used a bar of nylon to pull a 3/16th flange to finish off the edges.
 Starting to look better....
Also fabricated the doublers for the lower aft corners, the camloc locations including an oversize one for the center mount.

 Next the question of what to do about the air filter.  The nicest solutions I've seen all have some form of shroud, so I set up to make one.
 First decide on the basic shape including adaquate clearances.
 Then make the blank for the form.  I cut out 4 frames plus a base from 3/4" plywood and glued them together.

 I used the copper house wire again to transfer the basic contour to the form and then used the jig saw with the base unlocked and eyeballed the various angles.
 With a plastic trash bag taped to the lower cowling, epoxy filler is applied to the form and it's clamped in position.
 I now have the exact contour on the lower half and use it to dublicate the contours on the upper half.
 The plug is made from the pieces cut out of the frame.  I cut the basic angles on the table saw so the plug was symmetrical before I started sanding.

Plywood is good for this kind of job because the laminations help as visual references.
 032"  2024-0 material was my choice.  It work hardens quickly so the forming operation was done in several stages, removing the part from the form to normalize it at each stage.

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!
 After the first two stages I thought it wasn't going to work, as some wrinkles were developing and the frame was deforming.  I was ready to trash it.
 After sleeping on it, I kept going, a little at a time....
 and after much pressing, forming, hammering, shrinking and trimming...
 it started to look like something.
 Finally took the plunge and drilled pilot holes.
 To provide a little additional stiffness and durability, a doubler is added in the camloc area.
 I'll use 1/4' camlocs which will be more than adequate, but need a little less space.

Also made some silicone baffle seals to surround the airfilter.  This will ensure the pressure delta between above and below the engine is maintained (also provide a little ram pressure for the A65! :)






That was a lot of work, but I'm pleased with the result.

 Finally a doubler is made for the exhaust opening, and the entire cowling is deburred.
 While the engine is up, I decided to go ahead and make the crankcase baffle.

The aircraft never had one since I've owned it (and never encountered any cooling issues), but it's supposed to be there so......
 Actually there really isn't enough room for this part which probably explains why they're usually so beaten up.

The holes that pick up on the intake manifold studs need to be dimpled.
 I also had to form the baffle in the area of each intake to provide room for the intake hoses
 Then the front baffle that connects with the nose bowl "scoop" is laid out.

 Trimming involves Multiple nose bowl R&I.
 Ready for alumiprep and alodine.

11 comments:

  1. Wow! Amazing workmanship. Really beautiful.

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  2. Your attention to detail and workmanship was a pleasure to see. I'm about to start making new baffles for a BC12D I'm maintaining for a friend. You give me some great ideas to form the rear baffle. Beautiful job!

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    Replies
    1. Thanks Robert, appreciate your kind comments. Glad you found the blog useful and happy to help.

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  3. You are creating a piece of art. I love your attention to detail and your skills (I'm envious). Anyhow, I am in the process of restoring a 1941 Taylorcraft BC-D12. I purchased the plane last April after the previous owner had ground looped it. Twisted the Fuse, (I'm still working on straightening that), also, with the ground loop, he had busted both wing tips (how he did that is beyond me). Furthermore, somehow, the engine had been pushed into the firewall, I do not know if there was a prop strike or not (the engine, needless to say is going to an AP/IA for a major overhaul. The sad thing is the engine just had a major 500 hours ago. Oh well, I do have a lot of work ahead of me but I know it will all be worth it when I'm done. You have given me much needed motivation, as well as some ideas on how to "rebuild" parts (firewall forward) that I cannot locate. Thanks and keep up the great work.
    Jim Dethardt, Little Chute, WI (30 miles north of the EAA in Oshkosh).

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    Replies
    1. Thanks Jim for your kind comments and support. Be.tween you and me I think it's more perseverance than skill. I do things over and over again until I get a reasonable result.

      Glad to hear another Taylorcraft will get a renewed lease on life.

      Sounds like you have at least three of the four corners involved in damage. I would tend to assume the damage goes further than first impressions, at least then you can be pleasantly surprised!

      As for the engine, there is no need to overhaul assuming the engine was up to date before the incident. if the engine was running , inspection of the crank is needed, but the engine can be dismantled and reassembled using "in service" limits. If the compressions are good the cylinders can even be removed without pulling the Pistons from the bores (so no cylinder work required). Anyway a reputable shop will advise you on this.

      Best of luck with your project and don't forget to post your progress on the Taylorcraft forum.

      Delete
  4. Could you tell me the diameter of the cabin heat outlet you made?

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  5. What a great job! Thanks so much. I think I will progress with my new heat muff. Could you give me the o.d. of the carb heat connection on your muff.

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    Replies
    1. Thanks Jack. The Carb heat is 2" (same as carb air box) and the Cabin heat outlet is 1.5". I obtained the position for both by installing the exhaust and muff on the engine and then choosing the best locations and orientations. The stock location for the carb heat created unnecessary tight bends, but that may have been just the one I had. I used Scat duct on both which is not original, but more practical IMO.

      Thanks again and good luck with your fabrication!

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  6. Hi again, at the bottom end of the carb heat shroud , tailpipe section, it looks like it should have a steel end piece to block the air from going out the bottom. How did you make this piece? I have no old exhaust shroud to reference. Thanks.

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  7. HI Jack; Thanks for your question, its a good one!
    I did some research on this and confirmed that the tailpipe end is open.

    This makes sense when you think about what happens when you're airborne. Air (ram) pressure in front of the front baffle causes air to flow into the exhaust shroud and out of the shroud at the tailpipe end, and the bottom of the carb air box. This airflow takes a lot of the heat from the exhaust out of the under cowling area during climb and cruise (cabin air system does the same on the left side through the cabin heat box under the boot cowl).

    When carb heat is on, the engine draws warm air from the shroud, and less air flows out the tailpipe. I wondered if, on the ground, the carb might draw air from the tailpipe more than the shroud but I think even on the ground there's enough flow generated by the prop, and even if not, there's lots of heat around the tailpipe while on the ground.

    The 2" duct for the carb heat is significantly larger than the carb venturi and remember, carb heat is needed most at low power settings where the manifold flow rate is much lower. At higher power settings, flow increases but there's also a lot more heat in the exhaust.

    Hope this make sense.

    ReplyDelete