2.64" Madcow Bomarc

[Tobor]

This will be a somewhat detailed build thread of my Madcow 2.64" Bomarc, with lots of photos .
I purchased it from Madcow during their 20' Black Friday sale, with the help of a $40 gift certificate I received from my 2019 Secret Santa. TY SS .

Note that I deviated significantly from Madcow's instructions, both in order and method of assembly, as well as in the parts used in the construction of this kit.

Planning for this project began back in April of this year (TRF post). Printing parts for the Bomarc began on Aug. 14th (TRF post). And, building commenced on the 28th.


Design of the MC Bomarc's Dorsal Conduit/Tunnel




Construction begins with edge gluing the two plywood wing sections. The plywood is later laminated with balsa sheeting.



Completion of printing dorsal conduit parts, along with laminated wing. PETG was used in all the prints for this project.



More 3D printed parts designed in Fusion 360. Red parts will be integrated into the model, while silver parts are alignment and drill hole jigs.



Installation of motor mount tube (MMT) forward centering ring (CR). The backside of the CR is aligned with the forward end of vertical stabilizer fin slot. BSI 15 minute epoxy was used to secure the CR.



Drill jig for aft airframe section. It was used to locate wing mount screws holes. Red parts in pic are the wing hold-down "anchors". These parts retain the nuts for 6-32 x 0.5" SS flat-head machine screws that will secure the wing to the airframe. The screws replace the wooden dowels that are used in the stock build of the Bomarc.



Next up is the drill jig for the wing. Note that the jig is mounted to the bottom, plywood side of the wing. This prevents the drill bit from splintering the plywood if I had drilled from the top side.



This pic is a dry-fit of the tail section alignment jig. The eyebolt installed in the jig will allow easy removal of said jig



BSI 30 minute epoxy was used to glue the vert. & hor. tail sections together. Jig keeps the tail section square while the epoxy cures.



Continued in next post...

 

[Tobor]

The next step was to locate and drill the holes for the rail buttons and epoxy their associated anchors in place. Sorry, no pics of that. That was followed by epoxying the Ramjet pylons to the aft airframe section, using a 3D printed alignment. Again, no pics of install or jig.... .

However, I did remember to snap a shot of the pylon's inside fillets.... Btw, my favorite tool for forming small radius fillets on my LPR and MPR rockets, is the back end of the handle on my #11 X-Acto knife. After the pylon fillets set up, I completed the Vert. Stab's internal fillets and aft fillet of the fwd. CR. And, for the 3rd time, no pics....... sigh



Installing the large wing hold-down anchor was a bit of a challenge as two of the 6-32 nuts fell out of the anchor while screwing down the wing. As I had chosen to use 15 minute epoxy to secure the anchor, I did not have a lot of working time to correct the problem. (Note to self: Use ample CA when affixing steel bits to plastic pieces). After some vitriol cursing, followed by some breathing meditation, I figured out a way to save my sanity. I turned the model upside down, put the nuts back in place, realigned the anchor and SLOWLY started each screw. Once all three screws had successfully grabbed threads, I righted the model and finished tightening the screws.



Buttoning up the business end of the Bomarc involved lots of Aeropoxy on the inside. Some around the I.D. of the airframe for the aft CR. Some on the MMT O.D. towards it's front end for the fwd CR and more along the MMT's length for the Vert. Stab fin tab. I also applied epoxy to the bottom edge of the Vert. Stab's fin tab. then I carefully slid the MMT (With it's aft CR pre-attached earlier in the build) into the fwd CR at a slight angle to avoid scraping all the epoxy off the fin tab. I then aligned the aft CR with the airframe and finished sliding the MMT forward until it butted up against the back end of the fin tab.

Once all that epoxy was well on it's way to a full cure, I fabbed an airframe doubler out of some scrap 2.6 LOC tubing. This doubler was epoxied to the aft end of the airframe to add reinforcement for hard landings. after that was complete, I installed the Aero Pack 29mm Motor Retainer.



In another deviation from Madcow's instructions, I changed the location and method for attaching the Kevlar shock cord. The stock Bomarc uses a 2.64" x 29mm CR glued to the aft end of the tube coupler, to act as a shelf for the recovery gear.


In my rendition of this kit, I repurposed the CR shelf to be used as a mount for the shock cord anchor. I 3D printed a bracket that straddles the CR and receives a 1/4-20 x 1" stainless steel hex bolt that threads into a 1/4-20 Eyebolt on the other side of the bracket. The bracket was epoxied to a scrap piece of 6mm plywood. Then I used my Dremel tool to trim the plywood to match the bracket's shape. Next, I enlarged the CR's I.D. to about 2". The increased I.D. will reduce the restriction to the ejection charge. The anchor bracket was then epoxied to the CR. Later this anchor assembly was installed in the fwd. airframe section, approx. 9.5" ahead of the CR shelf's stock location. After the epoxy for the recovery anchor had cured, I epoxied the the anchor for the 4th wing hold-down screw below the screw hole I had located earlier (undocumented) in the build.



The two airframe sections are epoxied together using the 4th wing hold-down screw to help line everything up.



And here is the dry-fit of the Dorsal Conduit pieces.



More to come....

 

[Tobor]

Yesterday I epoxied the wing to the airframe. The hex nuts in the hold-down bracket cooperated and remained in place.



Today's task, getting the dorsal conduit glued in place while keeping it all straight, was much more difficult. This morning I started at the aft section due to a slight alignment issue. I used BSI 30 minute epoxy which decided it wanted to behave like 10 minute epoxy... But, I did manage to get things squared before the epoxy took a hard set.



Early this evening I tackled the forward section. I used AeroPoxy which gave me plenty of working time but, when bonding PETG, it tends to act like lubricant. After much futzing around, I got everything to settle down while doing my best to keep the pieces aligned.

 

[Tobor]

Wednesday morning I noticed that Madcow's design truncates the front end of the wing and hor. stab.



To me, it is an odd design choice. So I decided to fix it. I whipped up some extensions in Fusion 360 and printed them.



Getting the wing extensions right took a couple of tries, as I descovered the wing is off center a tad less then 2mm...
I do not think the error will effect the Bomarc's flight performance, but it is annoying just knowing it is there.

Anyhoot today I will filling the spirals on the ramjet tubes and assembling their balsa cones.

 

[Tobor]

Work is progressing slowly atm. Over the past week I been working on the ramjet inlet cowl/inlet spike assemblies (aka nosecones). As they were formed from balsa stock, I decided to use one pump of West System's 105/206 with two table spoons of micro balloons to fill the grain. next day I sanded the filler down with 220 grit first and then 320 grit and finished with 400 grit sandpaper.

Left-filled, Right-filled and sanded


Next I applied a liberal coat of thinned Carpenters Wood Filler (CWF) and sanded that down once it was thoroughly dry.


I then I sprayed the assemblies with Rust-Oleum 2in1 sandable primer and sanded that down the next morning. I next filled remaining gain marks, let dry and sanded again.


After repeating the spraying, sanding, filling, sanding twice more I called it finished. Last Saturday morning I applied two coats of gloss white. And this morning I started masking off the the front of the first assembly for the black coat of paint when life rudely interrupted. Nothing major, just annoying....

More photos soon.

Also wanted to add these to my post...



And my OR sim file

 

[sriegel]

Nice build. It's a fun bird, definitely eye catching. I did my L1 with mine.

 

[Tobor]

I am posting most of the STL files used for the prints in this build ---> Bomarc STLs