MadCow DX3 Massive...Headache

[Handeman]

This is more sled than what is needed, but basic strength and integirty is what I was going for...oh, and functional.

View attachment 549049

Note, for all 3D printed articles, I am using PetG materials @ 100% fill.

I've never owned or used a 3D printer so I'm not sure what PetG and 100% fill does for you, but I was wondering about the placement of the threaded rods. They seem pretty far apart.
I usually place them in as close to the U-bolts in the end caps as I can, one on each side of the sled. The U-bolts come with a flat metal strap which I put on the inside of the cap. I try to put the threaded rods with a large SST washer close enough to the u-bolt that the washers on the outside overlaps the metal strap on the inside of the cap. I figured that would pretty much eliminate the structural properties of the end cap from being a factor in any failure that might occur because of an abnormally strong shock load on the av-bay structure.

I'm probably just over-building.

 

[thzero]

PETG is a type of filament, it has a bit better properties than standard PLA around head resistance (i.e. out in the sun all day). Have no idea why he was printing at 100% infil. Infill is how much interior space is taken up by printing as opposed to just the walls. 100% infill is sorta like a solid piece of plastic, its not but close. But its also a lot of weight especially for something that isn't taking a lot of stress.

 

[rharshberger]

PETG is a type of filament, it has a bit better properties than standard PLA around head resistance (i.e. out in the sun all day). Have no idea why he was printing at 100% infil. Infill is how much interior space is taken up by printing as opposed to just the walls. 100% infill is sorta like a solid piece of plastic, its not but close. But its also a lot of weight especially for something that isn't taking a lot of stress.

Typically I print 100% infill on electronics sleds to get better resistance to crushing from threaded fasteners going through the sled, and the sleds usually are not thick enough in any dimension to warrant using various types of infill patterns. As for the weight, I save that by placing cut-outs so I can route wiring or whatever around the bay through the sled if needed, the difference in weight between a 50% infill and 100% infill is probably less than 28grams on any of my sleds ( the smaller the sled the less the differnce, 29 and 38mm sleds less infill probably only saves a few grams). PETG while a bit more work to print than PLA is my preferred material (ABS might be better, but I can't print it in my current printers location).

 

[DRAGON64]

For proto types and static displays, I print at 15 to 20% infill. For flight articles, I print 100%. Some will call it a waste of material, but I can print 10 or more of these sleds for the price of the reel of PetG, which was $20 at the time. 100% gives me peace of mind when mounting press in threaded inserts and thread forming screws. Also, this bare sled weighs right at or less than many of the sleds designed with G10, or plywood. Actually this kit came with a very heavy sheet of G10 to make the sled from.

I've never owned or used a 3D printer so I'm not sure what PetG and 100% fill does for you, but I was wondering about the placement of the threaded rods. They seem pretty far apart.
I usually place them in as close to the U-bolts in the end caps as I can, one on each side of the sled. The U-bolts come with a flat metal strap which I put on the inside of the cap. I try to put the threaded rods with a large SST washer close enough to the u-bolt that the washers on the outside overlaps the metal strap on the inside of the cap. I figured that would pretty much eliminate the structural properties of the end cap from being a factor in any failure that might occur because of an abnormally strong shock load on the av-bay structure.

I'm probably just over-building.

I spread the rods apart, as I had the room in the bay, and I am using u-bolts mounted 90 degrees from the all thread. The thought was to spread the load of the recovery process across a wider area of the plates, and translate it to the plates on the opposite end of the bay. Because I am flying a long 98mm hybrid, the weight of the motor alone is right at 29# not including the remnants of a spent fuel grain. The nice thing about 3D printing, is I can change the design easily enough and print in a completely different direction... simply put, if I change my mind, I can redesign on the fly.

Looking forward to working on this portion of the project, as I tend to struggle with avionic bays... at least I can get my elbows in there and turn some wrenches.

 

[DRAGON64]

This post is a little wordy, but if you have built large airframes, nothing moves a build along faster, then finishing the longest pole in the tent, and that is motor mount building and fin installation/alignment. So bare with me please.

I mentioned earlier in this thread, that the fin slots are tight, so tight that the fins would NOT go in... not one iota (eye-o-ta). I used a rat tail file to try and widen each side of the slot to get the fins to slide in, and eventually I was able to get the in, but again, the tolerances were too tight. Did I mention for a hobby I like a little (a lot) of tolerance stack.

I wrestled with ideas on how to put the motor mount together, and at what point in the build process do I mount the fins(?) Much dilemma, much deliberation. I saw only two ways; one, build the mount, mount it in the airframe, and then slide the fins in... drill holes in the frame and inject epoxy to make internal fillets etc etc etc.

...or, build the fincan, mount the fins, add fin fillets, then open the end of the airframe at the fin slots, slide motor mount/fin assembly in, and then batten down the hatches.

I am going w/ option two for better control of fin reinforcement throught filleting... and a little help from a 3D printer.

Thinking ahead, I wanted to make sure that my fins mount perpendicular to the motor tube. Rather than 3D printing a large fin jig, or a multi-part fin jig, I designed a simple fin jig that remains permanently mounted to the motor tube.

Printed at 100% fill, with small fillets as seen in the above image, on either side of the fin slots. I printed two of these for fin alignemnet and reinforcement.



The holes act as JB Weld rivets when mounted in place onto the kit supplied G10 centering rings.



The slots are tall, to make sure the fins stay mounted perpendicular to the motor tube, and deep enough so to not change the finslot length any more then necessary. The slot will need to lengthend roughly an 1/8 of an inch, but nobdy will be the wiser.

 

[DRAGON64]

The printed fin jig is merely glued on top of the aft ring/retainer assembly with JB Weld and allowed to cure. You can see the adhive squeezed/oozed up through the holes in the jig mount. Once closer to curing, I wicked in thin CA around the leading edge of the jig to ensure all contact surfaces were adhered. The pencil line will align the two jigs.





After a day of curing, the fins were inserted to help get the top fin jig aligned.

With the top jig mounted, I taped the leading edge of the fins in place, then used thin CA to light tack the fin jig in place. The CA was given at least half-a-day to cure.



It took a while to formulate a plan, but the project is moving faster now that I am finally implementing a plan. The remainder of the build will take a number of days to complete, for simply waiting for JB Weld to cure etc.

 

[DRAGON64]

Shifting gears a bit, there was a little bit of minor bulkhead prep to be done; Just like the couplers not fitting well into the airframe, well, the same could be said of both the airframe mounted bulkheads, and the coupler mounted bulkheads. A method that has worked well for to quickly bring the bulkheads back into a lower tolerance spec is to mount them up in a cordless, and run them across sand paper.







And yes, a mask and full goggles was required, as the dust really flew.

 

[DRAGON64]

Over the course of the last 3-days, I mounted the fins permanently to the motor tube assembly. JB Weld was used on the root and at each end of the fin where they mount into each of the permanently installed fin alignment rings. In this first image you can see where the upper alignment ring was tacked into place with the alignment drawn on the length of the motor tube.




The upper fin alignment was merely tacked in place with 3 small wicked locations as described in an earlier post, pictured here as a small 'drool' looking spot peeking out from under the ring.



The first fin mounted from this past Saturday.

 

[Hobie1dog]

your fin jig is super

 

[KenECoyote]

Beautiful build!

 

[DRAGON64]

Sunday and Monday were the same, fin mount, wait for epoxy to cure etc.

Sunday fin #2:



Monday fin #3:



A view down the motor tube shows complete fin root adhesion for all three fins:

 

[DRAGON64]

your fin jig is super

Beautiful build!

Thanks guys, the build is finally starting to pick up a little pace here.

 

[DabCat]

Thank you for starting a build thread! I've also got one with the intent of flying contrail 98mm bolted hybrids. I'm planning to start my build something next year, maybe around early summer months.

 

[DRAGON64]

That is awesome! I talked with Tom at Contrail Hybrids the week of Thanksgiving, and he states he has tested an "N" reload for the 98mm system, and may see about getting it certified. I will also be sending back my injector manifold for him to modify for upgrades he is making to add more efficient injectors.

 

[Hobie1dog]

I keep going back to drool over those fin alignment jigs you made. I'm watching this thread with great interest.

 

[DRAGON64]

To finish off the 2nd finbalignment ring mount, the 2nd centering ring was mounted to it using JB Weld.



And the filleted.



Next up will be adding fillets of epoxy to the fin root-to-motor tube unions.

 

[DRAGON64]

Pouring fillets is a nominal process, so I will just post the couple images, as the others will be the same. But here we have 2oz of a 3:1 Mr Fiberglass brand epoxy: (note: Mr Fiberglass brand epoxy I believe is the same as US Composites 635 epoxy)

http://www.uscomposites.com/epoxy.html


...and roughly two 1oz epoxy fillets.

 

[DRAGON64]

On the subject of filleting, weight becomes are real consideration when working with an airframe that needs to support hybrid flight hardware and mainstain flight stability. The Contrail 98mm system, w/ its bolted design (no snap rings), is a heavy motor. Here are some images of the motor, and the DX3-M in its early days.



Note the bolt holes in the cases above, and in the nozzle carrier below.



So weight becomes a concern, and when I started filleting the fins, it dawned on me that I should be using a filler to keep the fillets light. So, for the fillets to be used on the airframe, I will be adding West Systems 405. I really wish this thought had occured to me prior to filleting internally, but hind sight being what it is...

 

[DRAGON64]

Since I was printing a couple parts for another project, I thought I would go ahead and print another AV sled with battery covers. Keeping with a color theme of black.



My printer is printing very stringy today... It may need an upgrade... like say maybe a Prusa(?)

 

[DRAGON64]

Not my cleanest fillet ever, but I am one fillet shy of completeing the motor mount/fin can assembly.



Still functional, and once installed, the fillet will be out-of-sight-out-of-mind.