TVCs F6F Hellcat featuring CNC & Hotwire - Page 3 - RC Groups
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Nov 12, 2012, 11:52 AM
just Some Useless Geek

Now I'm gonna start thinking about getting back into the kitting of models I've designed. With a CNC rig like that I can churn out kits without having to shell out for laser cutting. The profits could pay for the machinery in the first year. Thinking...
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Feb 16, 2013, 03:59 PM
Jer. 29:11
jeffsch's Avatar

Build questions

Erik, thanks for the kit. The wings look terrific -- I especially like the precision of the cut between the inner and outer wing sections.

I'm reading through the F4U thread but have a couple of questions before I begin on this build.

* How do you keep the fuse sections from drifting relative to each other? I'm a bit concerned that it will turn out like my cardstock models do -- looking a bit like a chinese parade dragon.

I would feel more comfortable if each of the bulkheads had a pair of holes for a dowel crutch to hold everything in alignment.

* What do you recommend for a spar?

Thanks again for a terrific looking kit!
Feb 16, 2013, 05:50 PM
Erik with a K
tvcasualty's Avatar
Hey Jeff,
What I like to do is to build each section a piece at a time, dry fitting each former piece before it is glued together. The model is designed so that if you line up the seam at the bottom it should be very close to lining up, if not perfect.

Sometimes what you are describing does happen with this build style however. I have found the best solution to be examine the model after it is constructed and then take a sharp hobby knife and simply cut the section at the seam where the twist is. Only cut half and if it (the tail) slants one way, cut that side, remove a tiny amount of foam from the cut and then glue it back together. The first time I had to do this it seemed like a big deal, but then after I did it, it was not. Foam is good for that I guess.

I'm not opposed to designing in a center hole in the formers or, to make some sort of shape to hold a true line, but I'm honestly challenged to build like that, and it's easier for me to simply make slight adjustments as I go. I've attached a file with the formers drawn to allow some spars for alinement, check it out see if you think it would help. btw, if you are wondering why the formers do not look like they fit properly, especially towards the middle, it's because they were adjusted after they we drawn to make them fit in their proper spot better. May not make much sense, and it does confuse me to be honest, but the parts do work.

We are working on a new CNC setup to cut wood for making better plugs, so don't think I forgot about getting you a new cowl.
Feb 16, 2013, 07:07 PM
Jer. 29:11
jeffsch's Avatar
Erik, thank you for the quick (and helpful) reply!

What do you recommend for a _wing_ spar? (Sorry I wasn't clear above.)

P.S. I was sure you hadn't forgotten about the cowl.
Feb 17, 2013, 04:26 PM
Jer. 29:11
jeffsch's Avatar

First impressions -- Great!

The first impressions of the kit are very good.

* Is the sheet foam Model Plane Foam? It forms to curves nicely and is smooth on both sides.

* There's wonderful attention to detail in the cutting of the sheet foam parts. They have part numbers engraved on them; that helps minimize confusion. Some have additional reference marks -- these are very helpful -- it would be even more wonderful if every bulkhead and every skin had a mark to indicate center top and center bottom. And the locator tabs that hold pieces in place are already partly cut through.

Here are some pictures from the first couple of hours of assembly.

* I'm using the many-little-strips-of-tape method for clamping; in this case, using packing tape. (The tape comes off after the glue cures.)

* I transferred the hole and center top / bottom, and the horizontal lines (opening for wing) from the updated bulkhead diagrams to help line up the parts.

* The fit between the bulkheads and the skin is fantastic. At one point I was sure that Bulkhead A was too large, but as I glued / taped the skin it fit perfectly!
Feb 19, 2013, 05:50 PM
Erik with a K
tvcasualty's Avatar
It is mpf, as far as the spar for the main wing, I do not use one. I typically fiberglass the bottom with west systems epoxy and very light cloth. Like 1/2 to 3/4 ounce stuff. Looks good so far, thanks for posting the pics!
Feb 19, 2013, 09:49 PM
Registered User
Erik, incredible machine. It looks like one could lose themselves with that thing and not be found for months.

Thank you for sharing.

Feb 23, 2013, 11:26 PM
Jer. 29:11
jeffsch's Avatar
Just a quick build update. One of the bulkheads was a bit big but was easy to mark using the skins on either side.

I glued the wing roots together and used a pair of bamboo skewers to secure them. They were easy to press into the foam and glue in place. (Not a spar exactly, but will sure help the joint deal with stress until the wing is covered.)

The sanded fuse is light -- 62 grams or 2.2 oz -- and sure looks pretty. Erik, it's a thing of beauty. Thank you!

The last picture is a spackling of lightweight filler and Durham's Rock Hard Water Putty. I'll post a weight after it dries and is sanded out.
Feb 23, 2013, 11:32 PM
Erik with a K
tvcasualty's Avatar
Nice job!!
Feb 27, 2013, 12:23 AM
Jer. 29:11
jeffsch's Avatar
Erik, thanks for the encouragment.

After the first round of spackle was sanded off, the fuse weighted 67 grams or 2.4 oz. I'm doing a second round of spackle on the upper half of the fuse for that "I can't believe it's foam" look.

I'm going with aileron pushrods in sleeves and have cut slots in the top of the wing. (See picture.) The exits to the pushrods will exit inside the fuse. I'll make a hole for the aileron servo in the top of the wing when it arrives.

The the outer wing panels are glued to the inner. I love the precision of the dihedral cut -- it's fabulous! I cut some ailerons from scrap MPF and have hinged them. (I'll wait to glue the hinges in until after the wing is covered.)

I've got the cowl cut out and will cut out a hole in the center once the motor arrives. I'm planning a backing plate that will be glued to the cowl and magnet to the firewall. It sure makes it easy to deal with thrust line adjustments and whatnot if you can get the cowl off without too much ceremony.

Now for the big question: what shall I do about a motor mount? I've got this sinking feeling that I should have thought of this _before_ I glued the first former onto the fuse... If I have to open it up the nose with a hot wire, so be it (but I hope not).
Mar 02, 2013, 10:00 AM
Erik with a K
tvcasualty's Avatar
If it makes you feel any better, I almost never think about it until after the model is built for the first time, that's why my P38 still doesn’t have any electrics in it!

Take a look at the attached picture, what I have done is replaced the first former with another former made from something more rigid. 3/16 ply works well, but very recently through my plastics supplier I've been using an "expanded PVC" that is 1/4 inch thick. The stuff is lightweight and easy to glue to. It's soft enough for self taping screws to "bite" into it. Post crash, if it breaks, it accepts super glue very well and allows for easy repairs because it does not splinter. Best of all it machines nicely.
Mar 02, 2013, 11:06 AM
Jer. 29:11
jeffsch's Avatar
Replacing the first former makes sense; I'll use ply (though the expanded PVC sounds interesting).

I been thinking about a 480-sized motor; is that what you would recommend?
Mar 05, 2013, 06:47 PM
Erik with a K
tvcasualty's Avatar
480 sounds just about right to me,
Here are a few pics of what I'm running now, what I've run before, and what I'm going to try and run next...

What I have now is a BH 2217/9

That motor works pretty good with a 10x6 prop, the open cowl keeps it cool, but it could run hotter without the airflow (I used the same motor with ok results in a P51 but it did get a little warmer then I wanted if I remember correctly).

I'm not sure what the little red one is anymore, but I used that motor originally, it worked, but it ran hot in the F6F and eventually broke down in the p51 (not the one pictured but another). You can see it is smaller then the L2215J (200W) motor it's sitting next to.

The one I will be installing next is a 200W hobbyking special (L2215J) I just ordered, and I hope it works as well as the 2217/9.
Last edited by tvcasualty; Mar 05, 2013 at 07:23 PM.
Mar 06, 2013, 04:34 PM
Erik with a K
tvcasualty's Avatar
I've been getting a lot of questions on "harder" materials lately and how they are cut with the 6 Axis machine, here is a video of expanded PVC (p51 firewall) being milled with an 1/8" endmill... Sorry for the rotation, next one I do I will hold the phone sideways...

Milling Expanded PVC (2 min 19 sec)
Mar 12, 2013, 12:10 AM
Jer. 29:11
jeffsch's Avatar
Here's the motor mount with ad hoc lightening holes.

Before mounting it to the fuse, I cut a set of reinforcement strips from the scrap foam with the kit -- a 1" strip to lay along the inside of the fuse skin, and a 1/2" strip to lay overtop of that. Together they'll provide a 1/2" of backing around the circumference of the motor mount.

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