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Mar 23, 2020, 09:08 PM
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Build Log

Douglas A-20 Havoc bomber


Many years ago I built a small twin motor model of the A-20 Havoc (also called the Boston by the Brits). That one was 48" wingspan and built entirely of pink foam sheet.
Latest blog entry: Assembling a Spandau kit.
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Mar 23, 2020, 09:14 PM
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I was inspired to build another A-20 when I saw this thread... https://www.rcgroups.com/forums/show...-in-1-12-scale

This one will be 70" wingspan and will incorporate many of the 3d printed parts provided by "coolmodelguy" in his build log. I began this project some years ago by loading a 3 view of the plane into Turbocad and drawing up some basic outline and cross sections that I can use to print off full size part templates.

I started by cutting out a fuselage profile out of 1/8" plywood. This will be used to cut foam sheets to the shape of the fuselage and eventually will become the backbone of the fuselage.
Latest blog entry: Assembling a Spandau kit.
Mar 23, 2020, 09:16 PM
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At the same time I also began printing out some of the plastic parts that I will need. The first parts are the fuselage nose section and the cockpit canopy frame. These are printed from PLA filament and I scaled them to fit my slightly larger model. Eventually, I will printed the engine cowls and some other small parts.
Latest blog entry: Assembling a Spandau kit.
Mar 23, 2020, 09:18 PM
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I am making the tail surfaces from 3/4" pink foam sheet. They are sanded to shape and covered with .75 oz fiberglass cloth and water based polyurethane coating.
Latest blog entry: Assembling a Spandau kit.
Mar 24, 2020, 12:26 AM
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OliverW's Avatar
This is going to be awesome! The havoc is one of my many favorites..
Mar 24, 2020, 07:34 AM
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J Morgan's Avatar
Yep, going to be a cool project. I have a Havoc plan and just never seem to get around to it.

J
Mar 24, 2020, 03:10 PM
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Slow day today. I finished glassing the tail surfaces and drew up some airfoil cutting templates. I have a good cutting bow and a big power supply that outputs 24 v / 20 amps. But, I need a better way to control the power to the bow. Right now with what I'm using the wire is either too hot or not hot enough. I ordered a big variable rheostat that should do the job. Once that comes I can start cutting foam.
Latest blog entry: Assembling a Spandau kit.
Mar 24, 2020, 03:28 PM
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J Morgan's Avatar
Quote:
Originally Posted by radfordc
Slow day today. I finished glassing the tail surfaces and drew up some airfoil cutting templates. I have a good cutting bow and a big power supply that outputs 24 v / 20 amps. But, I need a better way to control the power to the bow. Right now with what I'm using the wire is either too hot or not hot enough. I ordered a big variable rheostat that should do the job. Once that comes I can start cutting foam.
That will do it. You need to be able to change the heat for diff lengths wire with diff bows. But you already know that.
I have a power supply built off plans from RCG many, many years ago, uses a simple light dimmer switch and has been flawless
all through those years. And I do a lot of cutting.

J
Mar 25, 2020, 11:45 AM
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Nice, i look forward to follow your build.

// Mats W
Mar 25, 2020, 04:09 PM
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Cut the wing cores today. I built them with 2 degrees of washout due to the extreme taper of the wing planform. I did cheat and increased the chord of the tips by a couple of inches. Cutting highly tapered cores by hand is difficult. The hot wire burns grooves in the tip because it travels so slow in comparison to the root. The cores do have these grooves but with spackel and sanding it should work. The wings will not have balsa sheeting....rather I'm going to use 1.4 oz fiberglass and WBPU. I will include balsa LE and TE and a hardwood spar.
Latest blog entry: Assembling a Spandau kit.
Mar 26, 2020, 10:44 AM
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After thinking some more I've decided I am going to sheet the wings with 1/16 balsa. The slight weight gain should be worth the strength and durability in the long run. I ordered a pair of 3536-910 motors and escs. These give 350 watts each and should turn a 11 in 3 blade prop using 3s packs.
Latest blog entry: Assembling a Spandau kit.
Mar 28, 2020, 05:56 AM
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J Morgan's Avatar
Is that blue foam that you making wings out of? I like to use 1 lb virgin white, esp if you are sheeting with balsa. Makes a very light and strong wing.
Sheeting the wing is a good decision, it will really strengthen it and give a great surface to finish. Plus I find that the sheeted wing with a balsa LE doesn't need add'l spars for strength. Just my 2 cents.

J
Mar 28, 2020, 01:20 PM
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John, you're right about the white foam being better. I had the blue stuff and didn't want to bother tracking down the good stuff. It is hard to find good foam around here.

I've been working on the project but haven't kept up with the photos. I'll try to get some posted today.
Latest blog entry: Assembling a Spandau kit.
Mar 28, 2020, 01:36 PM
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Yesterday I printed out the cowls and started the engine nacelles. Getting the shapes and angles right so that the nacelles fit the wing is a little tricky but I think it came out OK. A friend gave me a set of landing gear units from a 78" span B-24 ARF that should work out well.
Latest blog entry: Assembling a Spandau kit.
Mar 28, 2020, 04:23 PM
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Fuselage coming together today.
Latest blog entry: Assembling a Spandau kit.


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