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Old Apr 25, 2011, 08:40 AM
Now in TN!
J Morgan's Avatar
Chapel Hill, TN USA
Joined Apr 2001
23,603 Posts
Build Log
A-26K Counter Invader

Thought I would take a break from the pusher jets and do another twin. The Douglas Invader was arguably the best medium bomber in WW II. It went on to serve in Korea and Vietnam.
In 1962 the US gave On Mark Engineering a contract to modify the 26 for larger engines, larger rudder, strengthened wings, more hardpoints for ordnance, dual flight controls, tip tanks, and other mods to adapt it for the new mission. this version was called the A-26K Counter Invader. Only about 40 aircraft were allotted to this modification.
Wing Mfg makes a short kit that from all accounts that I've read is an excellent flier. I bought the short kit for the wing cores, plastic, and plans only. Since it was designed as a slimer the rest of the contents in the box will be discarded.
I don't often work up kits but prefer to scratchbuild. It just seemed like a good idea to use what was available to bash up the Counter Invader.
The US needed an aircraft with good loiter time and one that could haul a lot of ordnance and deliver it on target. The main target for the Invader was the Ho Chi Minh trail and mainly at night to prevent the North Vietnamese from supplying their forces in the south.
The kit is a pretty stand off scale model and I want to scale it up quite a bit. I like a model that looks much like the real thing.
My buddy Wade also has the kit and I'm pestering him to build one up too so we can have 2 of them in formation at Vegas this fall. Wade has a kit (forgot which one...Savex?) of a L-39 that had a unique type fuse construction. It has a laminated fuse of glass/balsa/glass and I'm trying to duplicate that in this build. It will make a very light fuse that is also very strong. I always enjoy trying new methods and this one will be exactly that. Not sure if I've ever seen this before. So I'm making two fuses, one for me and one for Wade. Also, my buddy Stack drew up some more scale drawings to change it to more of a scale look. I'll be working off those for the fuse and tail area, use the wing cores from the kit.
Wade is also making the nacelles and tip tanks so you can see I'm getting a lot of help here.
Now before everyone gets excited about maiden pics and video, there might not be any until Vegas do to the fact my grass field will not allow me to do it here at home. But hopefully in the fall we will get that at the Vegas event. So Nut, keep your thumbs limbered up.
To start the fuse I needed a fuse plug. I blew Stack's drawings up on the overhead projector and made templates of the top and side view of the fuse. Then the process involved the grunt work of carving and shaping the fuse blank. I used white foam because I have a supply of 2'x4'x4" white foam. But white foam is messy as heck and most I did outside in the spring winds.
Once it was sanded to final shape I gave it two cots of poly to seal and give a good surface for packing tape. The packing tape was applied and it was ready for a coat of wax and then three thin coats of PVA sprayed on.
Once that was dry I could start building up the fuse.
First step was to give the plug a single layer of 3/4 oz glass with West Systems epoxy. I let it cure overnight and then started skinning the fuse with 1/16" contest grade balsa. I've done tons of foam fuses skinned with balsa so this wasn't anything new and actually I'm getting quite fast at it. My method is to dampen the balsa with a damp rag and then give the balsa a SKIM coat of Gorilla glue(brown type). Once that is placed on the fuse I use masking tape to suck the balsa flat against the plug. Start with the largest sections possible and wait about 90 minutes for the glue to set. You are limited to how much you can do because the overlapping tape gets in the way. Once you have all the large sections done, it's just a matter of cutting pieces to fit the areas left. Sort of like making a jigsaw puzzle. Occasionally some glue will expand from under the balsa edges. these need to be carefully cut and picked off the plug so that the next piece will be able to fit up snug.
Anyone that has followed my threads have seen this process on quite a few on my builds.
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Old Apr 25, 2011, 08:55 AM
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Beautiful Choice J. I'm on with you.
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Old Apr 25, 2011, 09:05 AM
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Old Apr 25, 2011, 09:55 AM
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Subscribed as well

John if you need info on the A-26 I can get it from my father who flew them as water bombers for about 5 or so years for Conair.

Cheers,

John
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Old Apr 25, 2011, 10:26 AM
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New Bern, North Carolina, United States
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Dang you work fast!! I havent even started the SEFF repairs yet!!
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Old Apr 25, 2011, 10:42 AM
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stacker's Avatar
Salt Lake City Utah
Joined Mar 2004
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John,
Progress looks great. Got to love the clean lines of the A-26. I've been a sucker for the A-26 ever since I built several of the old Comet rubber powered ones as a kid. Man, makes my fingers hurt thinking of hand cutting all those printed wood parts out.

Keep up the super work -- Dave
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Old Apr 25, 2011, 11:01 AM
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United States, WI, Fond du Lac
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Cool Project! I just saw a real one at National Museum of the Air Force in Dayton a few weeks ago. Check out the virtual tour: http://www.nmusafvirtualtour.com. They have 2: An invader in the Korean war gallery and a counter invader in the Southeast Asia gallery.

I think you'll find the sandwich construction of the fuselage to be surprisingly light and rigid. It's analogous to what I've been doing with formed FFF foam.

Pat
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Old Apr 25, 2011, 11:04 AM
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eatond's Avatar
United States, MO, St Charles
Joined Nov 2002
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Hi John,
Dang it. When you first mentioned doing the A-26 and using the Wing short kit I got all excited because I have a partially completed one hanging here in my basement. I thought I'd get to do a build along with you.

I'll just have to be satisfied with watching you try out this interesting new technique and stealing every good idea I can for my bird.

Really looking forward to this build.

Dan Eaton
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Old Apr 25, 2011, 12:29 PM
ARFs make me BARF
mrittinger's Avatar
United States, MI, Roseville
Joined Dec 2000
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Looks nice. I still fly my s400 powered 42" bird. Very efficient wing being long and slender...


Mark
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Old Apr 25, 2011, 01:34 PM
Now in TN!
J Morgan's Avatar
Chapel Hill, TN USA
Joined Apr 2001
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Thanks guys

John,
Thanks for the offer. If I hit a roadblock I will certainly call upon you. Fortunately, there is quite a bit of info out there.

Ge to fixing that Vamp Bryan!

This will be my second Invader Stack, I built one of those about 45 yrs ago too.

Yes Pat, it does turn out very light and very strong. I started about a week ago and have one fuse done. But I get ahead of the build log.

Thanks Dan,
You should pull it down and build along.

Mark,
I've seen yours in video and it does fly nice. I've heard the ARFS do too so I imagine it is a good sound planform for an aircraft. It certainly looks like it.
The Wings wing cores are a bit oversize but I'm just going to go along with that.

So where were we?
After getting the fuse all skinned up,I used a sanding block to smooth it all down and then lightweight spackle to fill all the cracks and blemishes. Remember,in this process, spackle is your friend. another sanding and the fuse got another layer of glass/epoxy.
After curing, I lightly sanded the glass and then made cuts from front to rear the full length of the fuse. No way I could make one cut and pry it off. Too stiff. Before I cut it I made lineup marks along so that I could glue it back together accurately.
The PVA comes off with the shells so I had to clean it out with water. Then tacked glued the halves together and used 2.3oz cloth strip along the inside seams and 3/4 oz on the outside that I can feather out easily.
The 53.5" long fuse weighs 7.5 oz and is extremely strong requiring very few bulkheads.
So that's where #1 is and I have #2 almost skinned up.
Now I have to get back to skinning.

J
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Old Apr 25, 2011, 01:57 PM
ARFs make me BARF
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United States, MI, Roseville
Joined Dec 2000
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Hollow formed balsa fuselages were the way my Dad did things back in the 70's.
He would make a foam mold and layer on some bondo, sanded smooth.Then he'd use 1/16 contest stock , wet and form around the mold and let dry. A simple framework of two side stringers of 3/16sq and a top and bottom "keel" of 3/16sq was it. A firewall, a former at the LE and TE, and one at the stab LE was enough. He would also mold up a 1/16 balsa doubler around the wing saddle. VERY light, very strong. The whole molding thing takes away the woods natural tendency to want to "pop back" to being flat , removing the potential energy and making a stronger unit.....

Nice work, as usual. The long narrow wing might seem like there isn't much there, but it does lift well. My little one is 28 oz RTF and has barely 220 sq in....yet it flies great.
Tip-tanks do not seem to hurt flying either, so dont be afraid to add them.

Mark
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Old Apr 25, 2011, 01:59 PM
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Crystal River Fl.
Joined Jul 2008
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Looking good John! I'm anxous to see how that glass balsa glass fuse turns out for you. From what I seen of your samples it should be really strong and light and look really good! w.
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Old Apr 25, 2011, 03:40 PM
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Salt Lake City Utah
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The fuse shell looks cool. Reminds me of a TA154 and how they formed the wood shell.

Take care -- Stacker
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Old Apr 25, 2011, 03:50 PM
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Skunk Water, Rhode Island
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woo..hooo...!!

Fuzz
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Old Apr 25, 2011, 03:58 PM
Now in TN!
J Morgan's Avatar
Chapel Hill, TN USA
Joined Apr 2001
23,603 Posts
I remember seeing molded balsa fuselages back in the day but didn't know how they built them. Interesting Mark, thanks for the info.
This one will have bulkheads at the wing LE & TE, maybe another halfway or so back to the tail and of course one at the nose to attach the FG nose I have planned.
Tip tanks..... actually I've found that all the planes I have with tip tanks helped the flight characteristics. Tamed them down, esp the F-104. They do reduce the roll rate but the 26 wasn't know for rolling anyway.

Thanks Walt,
The test samples really convinced me to do it this way.

Hey Stack,
Funny you should mention that, the Moskito is on my build list too.

Here are the test samples Walt was talking about. I took them to SEFF to show the boys. They are 1 sq ft in area, I did one with single layer of 3/4 oz on bottom and one with two layers. The single weighed 1.7 oz and the double was 2.1 oz. Decided the single layer was strong enough.

Just about done skinning #2



J
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