Dumod Infinite II

[Tench745]

This is a thread for a project I'm still in planning stages for. I don't know how far it will get but It's too much of an oddball for me to pass up.

What is it?

Many people are familiar with the Beech Model 18, or Twin Beech. In WWII the Twin Beech was pressed into service as a cargo aircraft, the C-45. After the war when surplus aircraft flooded the market some manufacturers took advantage of these cheap aircraft and began modifying them for new uses.

Modifications included things such as:
-A Panoramic Windshield, void of the old uprights that blocked a pilot's view.
-Volpar (brand) Tricycle Gear, requiring an elongated nose to fit retracts.
-Modified wingtips
-Single, tail
-Triple tail
-Lengthened fuselage
-Larger passenger door
-Aluminum or Fiberglass control surfaces to replace the stock fabric ones.
-A turbo-prop conversion

Of these, no single aircraft made use of more of them than the Dumod Infinite II, also known as the Dumod Liner.
The Dumod company offered a conversion that was fitted with the elongated nose, panoramic windshield, 6'-3" fuselage stretch, Volpar tri-gear, squared wingtips, large passenger door with stairs, fiberglass control surfaces, and a triple tail. To my knowledge only 2 of these complete conversions were sold; N492DM and N445DM (DM for Dumod). Only the factory demonstrator, 445DM remains in flying condition; if you google Dumod that is the aircraft you will find pictures of.

Why do I care?
As some of you may know, my father is an airline pilot, currently employed by American Airlines. But when his career was just beginning he found himself as a captain for Commuter Airlines, a little airline based out of Binghamton, NY. Commuter Airlines owned both 492DM and 445DM, and these 15-passenger aircraft were "the big iron" that my father flew. One fellow pilot recalls "Some call it a baby Constellation, I called it a dog. Fun to fly though. I remember how rain water would come through the radio stack and turning the squelch up to hear the fuel valves operate."
My dad recalls that at night you could lean the mixture by watching the color of the flames shooting out the exhausts on the radial engines.

It's such an odd duck, and something about it being part of my dad's early career draws me to this airplane. I want to do what I can to raise awareness of this unique relic from the 6o's.


The Plan

I'm thinking a 1/12 scale model will be about right, putting this at approximately a 47' wingspan.

I collected a bunch of 1" XPS foam scraps from work and have been looking for a project to make use of them in. This may be the ticket.

 

[Grifflyer]

This will be cool!! I always love the look of triple tails.

 

[Tench745]

Attached is the start of the CAD drawings. Pretty much everything from the wing aft is the same shape-wise.
The Dark Blue line is the nose of a standard Beech 18.
The Magenta line is a standard nose moved to represent the 6'3" fuselage stretch.
The Teal outline is the extended nose of the Dumod.

 

[Tench745]

An update:
Last weekend I tacked up a few layers of foam for the long cockpit/nose portion of the fuselage and cut them to rough shape to get a sense of size of this fuselage. It's quite large for a 48" wingspan...

But, since I had it cut out I decided to carve it to shape. What a weird beak-like thing...

This weekend I turned my attention to the rest of the fuselage. The shape for top and side cutouts were laid out on 1" foam and cut out.

Then, taking a page from Keith Sparks' book, I made up a hotwire bow and a sheet cutting jig.
The bow is pretty simple, some piano wire, nicrome wire, and pvc pipe.
The sheet cutting jig is simply some sheet foam with 1/2" spacers pinned on top and little brads which hold the cutting wire from sliding off.

This is my first time doing this, but it came out very well.
The seams in top and bottom and sides are offset to prevent any notable weak points in the fuselage.
The slabs are glued end-to-end with simple carpenter's glue, taped on both sides, and left to dry much like balsa sheeting would be.

The next step is to cut some foam triangle stock to reinforce the upper fuselage corners where a lot of material will be sanded away. If I'd used 1" foam for the top and sides this step wouldn't be necessary. Of course, that extra foam equates to about 5oz of weight I'm more than happy to leave on the ground.

 

[Tench745]

Now is probably a good time to warn everyone something I already knew but didn't want to be true: PVA glues like white glue and carpenters glue need air to cure. XPS is a CLOSED CELL foam, meaning no air can get in to dry the glue. When I got home only the very edges of the panels were dry and the interior of the joint was wet, so I cleaned off the old glue. Luckily, I grabbed a new bottle of white gorilla glue on the way home from work and was able to re-glue the panels with that.

I also took the time to carve out excess foam from the nose. As a recap, I used a few little dabs of gorilla glue to tack the foam together to make the block the nose was carved from.

To split these blocks back apart I pulled a bit of spider wire through the seams to break the glue bonds.


Next, on the small side of each piece, I marked 3/4" in from the edges of each and cut out the resulting middle piece.

Here are the off-cuts.


And the actual pieces. The top and bottom piece were, obviously, not cut out.I took this opportunity to get a tracing of each piece for patterns in case I ever need to make a new nose.

The bonding surfaces of nose pieces were then roughed up and glued together with some 30 minute epoxy.

Today I begin assembling the panels into a fuselage shape. The sides are glued to the top then pinned, and taped to hold them square while they dry.

The next step will be to add foam triangle stock to the inner fuselage corners so I have enough meat to carve them into shape later.
After that the bottom will get glued on, then when that's dry the tail will get pulled together and glued. I plan to get all that finished by the end of today.

 

[mach1 rc]

Looking good 👍👍👍

 

[Tench745]

As all things go when you're making them up on the spot, order of operations changed. The top was glued in, then the tail brought together so that I could glue in the foam triangle stock while I still had access to the interior of the fuselage.
Now I have to decide whether to leave it open or not until I've figured out servos and pushrods, which means I get to wrap my brain around a CAD file some more.
Edit: here's a picture of the tail pulled together and drying. Triangle stock is not yet installed.

 

[Tench745]

Lots of little things have been coming together this week and a few bigger ones.

I made templates for the empennage, cut them from 1" foam, and then cut the finished shapes to the correct thicknesses.


Because I plan on running this plane with differential thrust I think I can save some weight and ignore movable rudders. This feels wrong to my scale sensibilities, but correct to my RC pilot side. Input is welcomed, as always.
Which means, I can direct my attention towards the elevator. The foam blank was split at the elevator hinge line and trimmed back to add balsa leading and trailing edges. The balsa serves a couple purposes: When the slab was cut to thickness it induced a rather severe curve to the panel that the balsa would help straighten. The balsa also reinforces the otherwise soft foam for dent resistance and hardware installation.

For the elevator trailing edge I used a hacksaw blade to carve a shallow notch on the panel centerline into which I epoxied a strip of 3/32" balsa strip. When the epoxy was cured this strip was cut flush to the trailing edge of the elevator, and then the elevator sanded to its correct bevel.

The little nubs on either side of the elevator on a beech 18 are actually fixed to the h-stab, so these will be cut off and glued in place on the stab. before the leading edge of the elevator is rounded and fitted for hinges.
View attachment

At this stage I just had to set the fuselage up and look at it.
Yup, looks pretty goofy.



And finally, today I had a chance to try cutting my first wing cores. Aluminum templates were made, foam blanks were laminated up with Super 77 spray adhesive, and the first cuts were made.
The center section, which fits into the (eventual) wing saddle in the fuselage, was tackled first since it was a simple constant-chord panel. This went so well that I moved on to the first of the tapered panels. These transition down from the root chord to the area the nacelles will mount. Left and right tapered cores were cut pretty successfully, so I laminated the last two pieces of foam in my stock to try the first of the hard cuts; the outer wing panels.
The outer wing panels are what go from an 8.5" long 1.5" thick airfoil at the nacelles down to a 3.5" x .5" airfoil at the wingtip, all over a span of 17.5"
This last cut did not go as well as the first three. I need to adjust how I do my templates and maybe up the wire tension on my bow because I had the trailing edge undercut at the wingtip.


That uses up all the scrap XPS I grabbed from work before. There's a little more waiting to go out in the trash, so hopefully I can snag that soon and try cutting the outer panels again.

As a side note, when I did my weight estimates for each piece I had accidentally weighed a slightly denser piece of foam. As it sits I'm almost 3oz lighter than my original estimates. Some of this will be used up in control rods and whatever I go with for covering, but I think I'm on track to hit my 30-36 oz target weight.

 

[speedbirdted]

Looks really good so far - Safe to say when one thinks of a twin Beech this is not what comes to mind but that's not a bad thing. Kind of looks like a Connie with dwarfism or something.

What are you going to do as for painting it? From my (limited) experiences paint really, really hates adhering to foam and chips off very easily. Like even normal flight loading which causes very small amounts of bending on painted surfaces eventually causes little bits to chip off. I thought about using some sort of primer but just apply a lot of it but never got around to trying. Maybe you could cover it in some sort of thin balsa sheeting (like 1/32) and then coat that in something?

 

[Tench745]

Looks really good so far - Safe to say when one thinks of a twin Beech this is not what comes to mind but that's not a bad thing. Kind of looks like a Connie with dwarfism or something.

What are you going to do as for painting it? From my (limited) experiences paint really, really hates adhering to foam and chips off very easily. Like even normal flight loading which causes very small amounts of bending on painted surfaces eventually causes little bits to chip off. I thought about using some sort of primer but just apply a lot of it but never got around to trying. Maybe you could cover it in some sort of thin balsa sheeting (like 1/32) and then coat that in something?

It's amazing to look at surviving Beech 18's and see how almost every single one has been modified somehow. You'd think they weren't good airplanes or something... but there were a lot of them and they were cheap after the war, ripe for experimentation/modification.

There are a couple different options that I'm toying with for painting.
- I may fiberglass the finished model with 1/2oz or 3/4oz cloth, I forget which one I have around, and then paint normally.
- Water based polyurethane (WBPU) also holds pretty well on foam and makes a good sanding sealer/primer coat.
-Something like 3M Fastbond contact adhesive is foam safe and makes a pretty good foam primer/sealer if you can spread it thin enough to avoid brush strokes and runs. Sometimes we'll do this in theatre for carved foam pieces but I've never tried it on a plane.

What methods have you tried? I don't want to repeat mistakes when I can go make new ones.

 

[speedbirdted]

So far the only experiences I have with painting this type of foam was when I was trying to make a fake FT explorer but out of a bunch of disks of hotwired insulation foam glued together and with EDF power. It ended up being a huge fail since it was too underpowered but I did learn some valuable lessons in assembling foam this way. Initially I tried bonding the foam together with 3M super 77 which ended up having nowhere near the required strength (as in the fuselage cleanly broke into 4 pieces the first time I landed it) but this was solved by just using gorilla glue. I didn't really have any need to paint it and actually did consider going over it with fiberglass or maybe some kind of thick primer but since it was a crappy plane to start with I didn't bother and ended up just going over the thing with two coats of spray paint. This didn't work at all, because literally every nick the fuselage got caused the paint to either chip off or it would make a dent in the foam and expose the foam underneath so eventually the plane ended up having a bunch of bright green scratches all over it, and this would certainly cause nightmares for anyone trying to do a scale build.

 

[Tench745]

So far the only experiences I have with painting this type of foam was when I was trying to make a fake FT explorer but out of a bunch of disks of hotwired insulation foam glued together and with EDF power. It ended up being a huge fail since it was too underpowered but I did learn some valuable lessons in assembling foam this way. Initially I tried bonding the foam together with 3M super 77 which ended up having nowhere near the required strength (as in the fuselage cleanly broke into 4 pieces the first time I landed it) but this was solved by just using gorilla glue. I didn't really have any need to paint it and actually did consider going over it with fiberglass or maybe some kind of thick primer but since it was a crappy plane to start with I didn't bother and ended up just going over the thing with two coats of spray paint. This didn't work at all, because literally every nick the fuselage got caused the paint to either chip off or it would make a dent in the foam and expose the foam underneath so eventually the plane ended up having a bunch of bright green scratches all over it, and this would certainly cause nightmares for anyone trying to do a scale build.

Honestly, I'm a little surprised the spray paint didn't just melt the fuselage...
From what I understand it's important to seal the foam with something to make a more stable surface for paint to adhere to. That said, acrylic latex house paint is supposed to hold up pretty well because it has some flex to i so it can move with the foam instead of chipping and cracking.

 

[Tench745]

Okay, where did I leave off....
-sorts through the modeling detritus on my table-
I've been doing a little here and there but haven't had the presence of mind to document any of it. That's always the way of it; as I get into the more complex puttering I get lost in the work.
Ah yes, here we are, cutting a bad wing core.

So, Friday last week I bit the bullet and drove back out to the city to grab another stack of scrap foam from work. I quickly laminated up two more wing blanks, cut them to rough shape, and after a little tweaking to my (bad) technique I cut the cores for both outer wing panels.

The leading edge of the elevator was shaped and then it got hinges in the scale locations. This was my first time using Robart hinge points and there was a bit of a learning curve to climb. It was a pretty quick climb though, and I think they turned out fine. Today the elevator control horn was added.


I used the leftover foam bits to laminate up blanks for nacelles and turned my attention to them for that week.
The side and top profiles were cut in and then a round former was put on the front to cut the bulk of the excess foam away and get me a round shape to work from.

Then I split the nacelles down the not-quite middle where they fit over the inner to outer wing panel joint. This gave me a flat surface to put a wing template on and hotwire out the opening for the wing. The inner nacelle piece then got sanded to fit the tapering inner panel and the outer nacelle piece will have bits added back in to properly fit the tapering outer wing panel. It will make more sense when we get there.


Tuesday I decided to cut the dihedral in and glue the outer wing panels to the center section. Unfortunately, when I looked back at the drawings I realized that the dihedral wasn't cut in at the junction of the inner and outer wing panels; it was where the inner panels met the fuselage. This meant I had to glue the outer wing panel square to the inner panels and then cut apart the epoxy joint I had already made to the center section. Thus began the three day saga of cutting, beveling, gluing, recutting and re-gluing wing panels.
To summarize:
Tuesday:
-Outer panels were glued in place
Wednedsay:
-Wire cut the inner panel-to-center joints apart.
-Full wing panels were propped up to the correct dihedral and wire cut.
-Realized the port, outer wing panel wasn't glued well
-Cut the port, outer panel off and sanded the end square
-Re-epoxied outer wing panel on
Thursday:
-Realized the inner panels were not symmetrical, because of the bad joint when initially cutting the dihedral
-Wire cut port inner panel from the center section
-Re-blocked and re-cut dihedral
-re-epoxied wing to center section
Friday:
-Realized port, outer wing panel still too low
-Cut apart port, outer to inner panel joint
-Re-re-epoxied outer panel in place. Looks good now.


Once the epoxy was set from that final fix I set about cutting slots in the wing for some 3/8" x 1/8" poplar wing spars. A carbide burr in a dremel tool, a Dremel router base, and a straight edge made the process mostly painless. There was some wandering of the bit and I switched about half way through to a thicker straight-edge which the dremel base would have a harder time accidentally jumping over.



I returned to work on the fuselage a little today. A slot was cut and a plate was glued in for the wing-mounting dowels. When it's time, the wing will be held into the (as yet uncut) wing saddle and holes for the dowels will be drilled through the bulkhead holes into the wing.


While I was working on the fuselage I cut the tail to eventually receive the H-stab assembly. More shaping will be necessary on that portion before the h-stab gets installed permanently. The elevator servo will also need to be sussed out before too long. The plan right now is to mount it somewhere I can reach from the open wing saddle and run some ny-rod back to the control horn.

 

[Grifflyer]

This is looking really good!

 

[BATTLEAXE]

Honestly, I'm a little surprised the spray paint didn't just melt the fuselage...
From what I understand it's important to seal the foam with something to make a more stable surface for paint to adhere to. That said, acrylic latex house paint is supposed to hold up pretty well because it has some flex to i so it can move with the foam instead of chipping and cracking.

The first plane I ever built was made of the same kind of foam you are using, except you are doing a way better job on the details of it then I did. I also wanted to seal in the plane but failed and gave up on the sealing idea. After the fact I found out what I did wrong, so I will let you know what I should have done and you can experiment with it.

I did the same thing as what you said and used water based polyurethane and torn pieces of craft paper to paper mache the outside of the foam instead of fibre glass cloth, not as much weight. what happened was that the water base soaked into the paper and expanded it to where it just wrinkled the paper, still wet i had to tear it all off and figure something else out. later i found out i should have used oil based PU, doesn't expand the paper. Once you got the paper on, dried and cured, sand off the high spots. Mix in a separate container the same PU and baby powder 1:1 to milky texture and brush that on over the smoothed paper shell, then sand till smooth. Repeat the application while progressively decreasing the grit of sandpaper till you get the desired finish. this will turn out to be a scratch free smooth shell like finish which adds strength, scratch resistance, and a good bond for paint.

 

[Tench745]

A little more progress yesterday. I got the elevator control rod run and the elevator servo mounted. That should be all I need to do internally before I can install the lower fuselage skin and then cut out the wing saddle, depending on how I'm going to do the mounting plate for the wing bolt. In the picture you can see the blue control rod tube and standoffs being clamped in place with pieces of paperless DTFB. Just at the lower left of the picture is the elevator servo mount.

Since I was working at the tail I decided it was high time to finish shaping the triple tails.
I looked through a number of reference photos, these two in particular,

I determined that the vertical stabilizer portion had a curved leading edge with a relatively flat center area and curved tips. The Rudders seem to have a straight taper from the hinge line aft except the tips where they match the curves of the v-stab.

So, I marked out the approximate size and shape of the flat center area and a minimum width was marked along the centerline of the panel. The curved sections were all sanded the same way; about half the thickness to be removed was sanded with straight taper from the edge of the panel to the center area, then the rest of the material was sanded away at roughly a 45° angle, The sharp transition between these two angles was then smoothed away until the shape looked reasonable. The trailing edge was just sanded in a straight taper from the hinge line to the edge of the panel.
Here's the process from marking out to final sand on the center tail. The lower portion of the leading edge isn't sanded here as it will eventually meet up with a dorsal fin on the fuselage.

Tonight I'm hoping to play around with some aerodynamic equations and see if I made a mistake using the scale airfoil on a model this size. Hopefully I can figure out if I need to adjust the angle of incidence on the wing or tail as well. This is basically the first design I'm using an actual tested airfoil for so I get to play with charts and equations.

 

[Tench745]

Looking over the polars for the NACA23018 root airfoil and NACA 23012 tip airfoil, I'm going to want to avoid flying too slow.
Even with 4° of washout the wingtips will stall first, and rather abruptly. I may add a turbulator strip or something to the leading edge of the wing root to prevent that.
Sometime when I have a clearer mind I'll try calculating how much incidence the h-stab might need. I have 0 now, the scale incidence is -2° I think I may need more than that. We'll see.

 

[Tench745]

I'm pretty excited. I got my cowlings today. Parkflyer Plastics had a cowl with radial engine for the now-defunct Fly-2-Build 48" Beech 18 that looks like it will fit my build perfectly.

 

[Tench745]

A quick update:
Right now I'm spending time with family and have been working with my elder brother to build some of his strange flying machines; he's looking to get into RC so I gave him a set of electronics and my old DX6i to get started. I'll probably share some pictures of that in the "what did you do RC today" thread at some point.

With the holidays I haven't been able to make much progress on this project, but work has been done. The pictures I have of progress thus far will be included here and when I get home I'll take a couple more to supplement this post.

Grooves were routed into the wings for poplar spars which I have fitted temporarily.


I placed an order for some RC parts; motors, ESCs, retracts, wheels, etc; and waited for them to come in.
While the postal system was busy whisking parts my way I added a shim to the H-stab saddle to increase the incidence, then coped the ends of the h-stab to fit the vertical fins.

I also cut the wing saddle out of the fuselage and sanded it to final fit with the wing.
With that done I was able to start carving and sanding the fuselage to final shape. (Pictures added)


At about this same time I started fitting cowlings to the nacelles. The cowlings were much shorter than the ones I was going to make for myself, so the nacelles had to be extended forward. This was done quickly by adding two circles of foam to the front of each nacelle. The cowling slides over the first of these pieces. The cowling pieces from Parkflyer Plastics are just slightly larger than the scale nacelle outlines in the PDFs I'm working from so the parts of the nacelle I had made already were a little too small and the lines didn't flow smoothly from cowling to nacelle. So I cut some 1/8" foam shims, epoxied them into place, then sanded them to shape.


When my parts order came in I was able to start in on a bunch of projects.
I was also able to start soldering up motors, ESCs, and battery leads. I spent about 4 hours trying to solder these together while accomplishing basically nothing except getting more and more frustrated with my equipment. The next day I picked up a new 40 watt Weller soldering station and some 60:40 electrical solder, then finished the job in about 30 minutes.

With that done I used a piece of brass tubing to auger holes through the wing to get motor wires into the center section.

Then CAD work to figure out how or if retracts would fit in nacelles. I ended up devising a plywood box which will act as motor mount, retract mount, and spar carry-though. (dark red box) I'll be starting in on these next year when I get home from visiting family.


I think that just about catches everyone up with where the project is right now. As always, questions, comments, critiques, etc are welcomed.

 

[Grifflyer]

That cowling looks really good!