1/32 Scale TBD Devastator Scratch-build - RC Groups
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Jul 23, 2017, 11:30 AM
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cybermike's Avatar
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1/32 Scale TBD Devastator Scratch-build


After a couple of months of playing with the big toys (modding ARFs, repairing damage, assembling a 3D printer and printing a Stearman PT-17, etc.), I thought it was time for another micro build, especially with NEAT coming up. I really enjoyed building and flying the 1/24 scale Peck Zero from the "Flying Models of WW2" book, so I decided to do another build in that vein to give the Zero an adversary. My passion has always been the planes of the US Navy from WW2, especially the Battle of Midway since it was the old Charlton Heston movie from the 70's that sparked my interest in WW2 history. I've already got a Wildcat and a Dauntless, so logically, it was time for a Douglas TBD Devastator torpedo bomber. While the Dauntless gets the lion's share of the credit for the victory at Midway, it was the sacrifice of the brave TBD pilots that kept the Zero's occupied on the deck while the SBDs dropped in from altitude to send four Japanese carriers to the bottom of the sea.

Perusing the plans at Outerzone and Aerofred led me to this plan by Pres Bruning:

which seems to be the best one out there. It is listed as 1/32 scale, with a 20 3/4" wingspan, but my math says 1/32 scale for a 50' wingspan should be 19 18 3/4" wingspan [Thanks Gene!]. So, I altered it to that size which is a perfect match for a 1S setup with the Racerstar BR1104 4000Kv motor and 1AS ESC. I plan on mating that with an AS6410L brick (with AS3X) for rudder/elevator and a "Orlandoo Hunter" 1.7g servo from Banggood for the ailerons. My target weight is under 50g, depending on how much scale detail I decided to add (a droppable torpedo would be cool!).

The plan has a fair bit of incidence on the main wing, causing the LE of the wing to be raised up a bit in the fuselage. I like a removable wing, so I'm altering the plan to zero-zero incidence with the wing flush with the bottom of the fuselage. I'm also designing the wing fillets into the formers and stringers from the start instead of as an afterthought, which should make it easy to make the wing removable.

I like to try something new for each of my builds, so for this one, I am going to 3D print a plug for vacuforming the canopy instead of making a plug from balsa or foam, and if my weight budget allows, I may do the same for some of the other details. I'm also doing the tail feathers with an actual airfoil instead of flat stick-based construction, and am engineering in the wing fillets as part of the formers and stringers on the fuselage. And, while not new, I plan on using nitrate dope for this one instead of glue-stick and Krylon.

I want to cut the parts with my laser, and as much as possible use engineered slot-and-tab construction vs. sticks, so I fired up the devCad suite and got to work on the design as shown in the pictures below. I still have a bit of fettling on the fuselage and wing (magnetic wing mounts, motor mount, Rx mount, etc.), but it's almost done. When complete, I will post the plans, print-wood parts layout, and if anyone wants to laser cut one, PM me and I can send you the CAD files.

On to the build....
Last edited by cybermike; Jul 26, 2017 at 05:01 PM. Reason: Math is hard!
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Jul 23, 2017, 11:45 AM
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cybermike's Avatar

Tail feathers


I started the build with the tail feathers because I wasn't 100% sure the shaped-spar slot-and-tab construction would work well at this scale. Turns out, it works great! I'm using 1/16" balsa throughout to avoid constant breakage of 1/32" sheets. I was able to assemble the tail surfaces with only a couple of handling cracks at the thinnest parts of the spars.

Construction was first gluing together the outline pieces flat on the building board. While that dried, I dry-fit all of the ribs and spars together - this required only a minimal amount of sanding/filing to get everything to fit together tight and square. Note that the leading edge of the rudder/elevator and trailing edge of the fin/h-stab are all shaped spars, so the fit is very tight and straight. For the most part, all of the pieces inside of the outline held together as a unit without any glue.

To finish up, for the vertical stab, I just inserted the "guts" into the outline and held it in my hand while I applied drops of CA to the joints with a bit of finger pressure to hold it tight. For the horizontal stab, I "blocked" up the TE with a thin ruler while I glued the TE outline in place, and blocked up the LE with a couple of magnets and glued it all together on the board. I am happy to say that everything came out super straight, and very strong - definitely less flex than with a 1/16" stick framework. Total weight for all of the tail parts was 1.3 grams - not bad. I cut some gussets for the H-stab, but I'm not sure I will even need them.

Once dry, I separated the movable surfaces by sliding a razor saw in-between the shaped spars and sawing through the ribs. I was light on the glue around the hinge line until they were separated, then applied a 2nd bead to all of the joints. An eagle-eyed viewer may note the rudder/fin hinge line spars are too short and don't reach the top outline piece. I've already fixed that in the CAD files, but for this build I just patched in a bit of extra wood.


Next will be the wing...hope to finish the last of the CAD work and get the wing parts cut today...
Jul 23, 2017, 12:09 PM
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Nice subject and nice work!

How did you design the tail surfaces? Were they done with DevWing? The newest versions of DevWing and DevFus are far more versatile.

If you can do a droppable Ohka, a torpedo should be no problem.

IMO, Pres Bruning plans should be printed out and framed as art.
Jul 23, 2017, 12:27 PM
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Thx! And yes, DevWingCam for the tail and wing, DevFusCam for the fuse, DevCadCam for misc. parts and wood layout, and DevFusMold for the canopy. I'm getting better with all of the programs, but there are still a few things I wish they did better - like extending shaped spars all the way to the wing tips, automatic gusset generation, smarter stringer layouts, better handling of partial ribs, STL export of the 3D preview, etc. But overall, I really like the suite. This is the first time I've done the tail parts with DevWing - I had to do some gimmicks to work around how DevWing parameterizes the design, but nothing too bad.

The real problem with the torpedo is that the fins stick up into the wing so far that they would protude through the top of the wing, not to mention mess up the shape of the bottom keel, but I haven't given up yet . It'd be nice to rig a drop so that full-up elevator triggers it to save the weight of a servo. Unfortunately, I don't think an actuator based dropper will have enough torque.
Jul 23, 2017, 01:27 PM
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Ah! DevFusMold for the canopy. I had been playing with Blender to make some canopies. Blender is HARD to learn compared to these programs. Now that Stefano has developed STL Tools, I can use it, instead. Despite the shortcomings you mentioned, these programs are fantastic. I don't think you could have done the fillets with the previous DevFus, or the tail surfaces with the earlier DevWing. He'll come out with another version in the future.

I'm getting better with these programs, too. When I first got them, I had NO CAD experience. CAD is very difficult to learn on your own. However, I have since played with some other CAD software which has shown me how good DevCAD is.
Jul 23, 2017, 02:03 PM
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Yeah, every CAD program is world unto itself. You either "grok" it, or else it's useless. Before DevCad I was using TurboCAD which isn't too bad, but lacks some features. Prior to that (going back to the turn of the century) I used ModelCAD. A really, really long time ago (like mid-1980's) I messed around with AutoCAD on an Apple II but found it to be completely incomprehensible.

I found the DevCad family pretty easy to grok vs. other ones I've used in the past so it didn't take me long to become productive. 3D has been a real nightmare though - so far TinkerCAD is my favorite, but is pretty limited. Fusion360 might as well be in Mandarin for me. I've started using TurboCADs 3D features and had some success since I at least have a bit of a leg up with it already. Didn't like Sketchup - no native STL support, and took me too long to figure out how to do things. I installed Blender, looked at it for 10 minutes, and uninstalled it. I haven't played with DevCad's STL tools yet, but I still need to add framing to the canopy, so I will give it a shot.

The best part of the DevCad family is that they all work the same, so it's essentially only one program to learn. I bought the license for all of the software - a bit pricey, but worth it IMHO, plus having been a programmer for most of my life, I know how much work it is, and have no problem compensating Stefano fairly.

Good news - looks like the torpedo will fit fairly easily without compromising the wing structure or interfering with the torque rods. Now to figure out a release mechanism...
Jul 23, 2017, 04:32 PM
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DevCad seems pricey until you check the price of other drawing software. It also comes with the best support available. What other software allows you to ask the developer for help?

Maybe I should try Fusion360. My Windows OS is already in Chinese.

How about a tiny pager with a drum to wind a string, attached to a pin, for a torpedo release? You can rig a contact on the elevator servo.
Jul 23, 2017, 05:22 PM
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Quote:
Originally Posted by reylf_gnijieB
How about a tiny pager with a drum to wind a string, attached to a pin, for a torpedo release?
I was thinking the exact same thing. That, or since elevator will be a linear servo on the brick, just a simple string/pulley/pin arrangement with the length set such that full elevator pulls the pin just enough to clear the shackle. BTW-isn't it 4:00AM over there?

Just finished the mods to the wing for the torpedo bay and magnetic mount, so I should be cutting the wing tonite after dinner. I'm cutting with rib alignment tabs, so theoretically I will be able to build the whole wing on the board including dihedral and washout in one continuous panel!
Jul 23, 2017, 05:35 PM
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Well, it's 5:30AM, now. I'm watching a download of the Tour de France. Three weeks every summer I spend 4-6 hours every day watching the race. My wife doesn't like it, but I keep pretty strange schedules the rest of the time, too.

I like your idea of using the servo better.
Jul 23, 2017, 09:03 PM
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I'm a mountain biker but several years ago watched a bit of the Tour, now I'm a full time fan... those guys are machines...130 miles some days, and at average speeds I can't reach at all even on my roadie machine. Week ends can be pretty tough to get sleep when I have the Tour, AMA motocross and a F1 race to watch... most of the time I'm not even into TV but as they say, when it rains, it pours
Congrats to Froome and fingers crossed for Hamilton to make it 6 wins at Austria

Quote:
Originally Posted by reylf_gnijieB
Well, it's 5:30AM, now. I'm watching a download of the Tour de France. Three weeks every summer I spend 4-6 hours every day watching the race. My wife doesn't like it, but I keep pretty strange schedules the rest of the time, too.

I like your idea of using the servo better.
Jul 23, 2017, 10:00 PM
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wow, must be geezing to not know/remember you had a laser. I finally receive my new mechanics to upgrade my 40w unit to 1400x900mm cut area. I only ordered a 1280x800 but the guy sent a larger unit to make sure I'd be able to cut a full 48". Now I have to make an even larger than anticipated home for it....and probably use up a bunch of floor space to give access to 2 sides...many problems to solve but I'm on my way to laser bliss I hope.
Thats a wing panel for a 105" Extra looking small in the picture to give you an idea of the scale of the laser
Jul 23, 2017, 10:09 PM
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back on topic, very nice build and so very true your comments on CAD software. My approach so far has been to avoid from scratch designing and to use and modify others work. But thats surely no where near as satisfying as what you've done, so perhaps this winter with more time to sit in front of that big flat thing....maybe I can learn something beyond superficial use. I'm envious of your build, looking forward to the finished plane
Jul 24, 2017, 10:42 AM
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Jul 24, 2017, 10:53 AM
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Looking forward to this build, I've always liked the Devastator's looks. Another thing that should make this successful, it has a LOT of wing area! If you wanted to add a torpedo and drop mechanism, it should be able to handle that.
Jul 24, 2017, 05:44 PM
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Hey Crossup - good to see you back. Guess you've been playing with the big toys too, eh? I have laser envy

Angelo - the big thick wing definitely factored into my decision making. I want to be able to fly this comfortably in the Downsville Gym at NEAT this year, and it is *not* a big gym. If I can keep it under 50 grams, it should be a delightful floater. I plan on putting functional landing gear on it too. I had originally considered going for retracts or flaps but decided not to push my luck.

Anyway, things always take longer than expected, but finally - the wing parts are all cut and the wing plan is done. The 3D rendering below shows the building tabs (and a few extra "phantom" ribs that I forgot to uncheck in the preview) that should hopefully make it easier to assemble. I'm slightly worried that they will just snap right off, or that I overdid it with the lightening holes, but we shall see...

(Really, no comments on the pilot?


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