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Old Feb 11, 2010, 09:34 AM
Registered User
Beijing, China
Joined Nov 2006
1,223 Posts
Build Log
1/72 B-17G Flying Fortress

This is my first build thread on RCG. I felt it was about time to show some of my work. I'm a two-fingered typist, so don't expect me to write too much.

The original idea was to use depron, with 4 4mm pager motors, and PZ brick. The motors didn't produce enough thrust, though. (see prototype #1) However, I did work out CG and thrust lines.

Prototype #2 has a profile fuselage, and became the test bed for a lot of different motors. I really wanted to get the lightest 6mm motors I could, since any extra weight is multiplied by four. I flew it with 2 6x14mm and 2 4x12mm motors, amd had plenty of power. So, I am using two 6x14mm, and two very light 6x10mm motors. The next issue was that with four motors I was pushing the limit of the 1A PZ speed control.

Which has brought me to #3. After weighing the depron, I felt I could produce a lighter plane, with a better wing airfoil, and a full fuse for less weight. After building the wing, I decided it was too flat for rudder/elevator control. So, I went to a 4ch Plantraco Rx with in-hinge coils all around. This gives me full house controls at about the same weight as the PZ brick, while eliminating the weight of pushrods. It also gave me 2A to work with for the motors. I will use a Monolith programmed on a Polk's Tracker III for control. Since the ailerons are large, I really want a dual-rate setting, to be safe. Overall weight should be less than 30g, including gun turrets and battery. We'll see...
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Old Feb 11, 2010, 10:08 AM
Fly, dangit!!!
Led Zepplin's Avatar
Little Rock, AR USA
Joined Jan 2007
1,050 Posts
Richard - wow, looks like a pretty ambitious project, and a scale one to boot! I'm signing on for the ride, good luck! 30g sound like it'll be a tough target, but I'm betting you can do it!

p.s. What type of covering will you use?

p.p.s. What's your WS?

J. B.
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Old Feb 11, 2010, 10:26 AM
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wparsons's Avatar
Canada, ON, Markham
Joined Aug 2008
8,841 Posts
What's the WS like at this scale? Looks good so far!
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Old Feb 11, 2010, 10:42 AM
Registered User
Beijing, China
Joined Nov 2006
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James,

This project has been a chore. I just finished four months of too much work, and we moved on one of my 5 days off. I am just getting back to building, since I have a three week break. It's hard to find everything after moving.

Wingspan is 17.3"/440mm; true 1/72 scale; that's what makes it tough. Covering is silver Esaki, which is why it has to be a "G" model. I figured one color covering was easier, but it will have a red tail.

I just put all the parts I have (fuse and wing half covered, empanage covered), with Rx, coils, and props on a scale: 20g. Battery is about 4g, so it leaves 6g for the rest of the covering and details. Even at 35g it should fly fairly realistically. The real weight question is the four nacelles.

If it looks half as good as your planes, I'll be happy. I consider this a prototype because the wingtip shape is wrong, and the fuse is too slender in the rear. I just want to see it fly, then plan on improving it. The next one will have an OD/grey printed tissue. I have the artwork already.
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Old Feb 13, 2010, 12:43 AM
Like a Boss
Heliman420's Avatar
Canada, BC, Whistler
Joined Jun 2006
2,130 Posts
I somehow missed this thread... Very nice!!! ive been waiting for someone to do a 1/72 multi engine bomber.
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Old Feb 13, 2010, 04:13 AM
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Beijing, China
Joined Nov 2006
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wparsons and heliman,

Thanks for the kind words.
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Old Feb 13, 2010, 11:36 AM
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Joined Jan 2005
489 Posts
Wow a 1/72 B-17!!! I will follow this very closely. I have wanted to build one out of .025 and .018 DUROBATICS so I'll be interested in the rc gear that you use. Great work!!!!
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Old Feb 14, 2010, 02:08 AM
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Beijing, China
Joined Nov 2006
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Bob,

I'm using a Plantraco Micro 9 4ch Rx with 5.5mm OD, 3.8mm ID 60 ohm coils.

I am now recovering the wing. I tried to cover dry, since I was afraid of warping the wing covering one side at a time. It was a horrid wrinkled mess, even after repeated shrinking.

The wing seems to be plenty strong, and the wet applied tissue looks so much better. Silver Esaki is much less user friendly than the white or other colors. It shrinks much less, too. Live and learn.
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Last edited by reylf_gnijieB; Mar 07, 2010 at 09:10 AM. Reason: Coil size was wrong
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Old Feb 14, 2010, 04:40 AM
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Beijing, China
Joined Nov 2006
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I'm having a heck of a time with the covering. I wet the tissue, and it is dry before I can get it worked smooth. Beijing is extremely dry in the winter, and I think this is causing most of my trouble. Very frustrating.

I think the frosty atmosphere at home hasn't been helping, either. My wife just told me it's Valentine's Day. OOPS! So that's why she's been grumpy all day. Hey, this is China, and that's a western holiday, it shouldn't matter. No more building today.
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Old Feb 14, 2010, 10:21 AM
Beware the Axis of Weasel.
thewildweasel's Avatar
Plymouth, Devon
Joined May 2007
2,464 Posts
Do you have radiators? If so try putting bowls of hot water on them when they're switched on, this will get some moisture into the atmosphere. Or for an extreme fix you could use a wallpaper steamer...
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Old Feb 14, 2010, 01:53 PM
Hals und Beinbruch !!!!
lw_hughes's Avatar
Joined Aug 2002
730 Posts
Watching with interest, the silver tissue is hard to work with. I would suggest solite as you can deal with warps so easily. lw
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Old Feb 15, 2010, 11:58 AM
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Beijing, China
Joined Nov 2006
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Here's a shot of the wiring. The actuator extensions are litz wire, which I made using a variation of Graham Stabler's "Whrlitzer." All the motor connections are staggered, so no insulation is needed. The motor sticks are 2x2mm pine. They are the only non-balsa in the entire plane.

Thanks for the covering suggestion, guys. We have an aquarium which loses a 1/4" of water a day, so that is the most humid room. It was my wife's idea to cover in the living room to take advantage of it. I am now completely soaking the tissue on the cutting mat, and having a bit better luck. I am also getting used to using glue sticks instead of dope.

Motors are held on with just a bit of epoxy. I used small rubber bands (orthodontic) to align and hold them on while the glue dried. Worked pretty well.

One camera card isn't talking to my computer, so I can't show some of the intermediate steps.
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Old Feb 15, 2010, 12:14 PM
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Beijing, China
Joined Nov 2006
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Here are some shots of the assembled plane, partly covered. The trailing edges of the aileron openings are too soft, so it just isn't possible to get the tissue tight there. I can live with the covering imperfections, since they won't keep it from flying. These are some of the problems I'll work out on the next iteration.

Now that the wing is covered, I have to start on the nacelles. I expect them to be a bit fiddly. I've got a rough idea how they'll be done, but will basically be figuring it out as I go.

The rear of the fuselage can't be closed up until the tail and controls are finished. None of the control surfaces are covered yet; I still have to mount the actuator magnets. They each have two 2.5x0.8mm magnets, mounted on a small piece of 1/64" ply. I have to get the coils mounted and everything lined up well before putting them in place. Then, I'll cover.

A lot of what makes this complex is having to plan several steps ahead so that I can get everything in place before another part is built or covered.
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Old Feb 15, 2010, 12:31 PM
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Beijing, China
Joined Nov 2006
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The bottom of the wing has some pretty ugly wrinkles. They got worse when I covered the top. I'm going to try shrinking a few more times, but don't really have high hopes.

The hatch is for battery and Rx access. I won't mount them until the end, since I will use their position for balance. I'm hoping to be able to put the battery on the CG, or close, so I can change battery size once I get it flying. The hatch will be held on with magnets.
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Old Feb 15, 2010, 12:35 PM
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Tim Cullip's Avatar
USA, NC, Apex
Joined Aug 2003
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Quote:
Originally Posted by reylf_gnijieB View Post
I'm having a heck of a time with the covering. I wet the tissue, and it is dry before I can get it worked smooth. Beijing is extremely dry in the winter, and I think this is causing most of my trouble. Very frustrating.
How about a spray bottle of water (one that sprays a fine mist) that you can repeatedly spray the tissue as you are applying/gluing/adjusting it. That way you should be able to keep it wet right up until you are happy with the placement. I've found that using glue stick with wet tissue allows you to easily adjust the position all you want (whereas glue stick on dry tissue is much harder to adjust after initial placement).

I've also found silver tissue to be harder to work with than regular, but if I keep it wet while I'm working with it then it works out well.

Tim
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