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Old Apr 02, 2006, 11:49 AM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
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Scratch-built Guillow's B-17

Anyone interested in following my build of the Guillow's B-17? ESB did a great build thread on his a few years back, and he got me dreaming. I got the plans and vac formed parts from Guillows, and started the build a couple of weeks ago.

Mine will be modified a bit more than ESB's was (I just can't seem to leave things be!) and will have retracts (including steerable tailwheel if I can manage it), full sheeting and four 280 motors. Weight management will be my biggest challenge, as I want to keep her around 35 ounces AUW. I've already decided the tail surfaces will have pink foam on balsa frames, covered in silkspan to keep the "Big-A**ed Bird" from becoming too tail-heavy.

Since this is my first multi-engined model, I'll be looking for advice on power systems and wiring and probably a lot more besides.
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Old Apr 02, 2006, 12:21 PM
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Hootersville, NC
Joined Jan 2003
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I'm tuned in!
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Old Apr 02, 2006, 01:19 PM
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Ann Arbor, Michigan, United States
Joined Sep 2003
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sounds awesome steve85. i was inspired by esb's conversion too! i bought the kit and enough wood for another build, but never got started. please post pics and details of your conversion.
-beanie
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Old Apr 02, 2006, 03:30 PM
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Arizona
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I'm watchin'..........sounds cool!!!
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Old Apr 02, 2006, 04:24 PM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
2,895 Posts
Okay, here goes. First, a disclaimer. I NEVER build anything exactly according to plan. I like to modify stuff as I go along, so this Fort will probably be more inspired by the Guillows kit than attempt to duplicate it. I often draw up my mods (ruler and pencil - I gotta teach myself to CAD one of these days) so if anyone wants to try to do what I do, I'll be happy to post or e-mail scans of anything I actually put down on paper.

I started with the wings. The Guillows B-17 wing seems like a well-designed, light structure (it was designed for display and MAYBE control line flying with two Cox .049s). It uses two 1/16" sheet spars, two 1/4 x 3/8 wing joiners and fifteen 1/16" sheet ribs per side. According to the plan, everything locks together for what must end up being a rigid, light structure. I say "must be" because I couldn't duplicate it by scratch-building! I thought I cut out everything very carefully according to the templates on the plan, but when it came to assembling, I just couldn't get everything together without having to twist the structure back and forth, ending up with a highly stressed, warped structure. I'm sure this isn't the case with the kit, but cutting everything from scratch to the tolerances required for the structure to lock in place the way it was designed to was just beyond my ability. After a week's spare-time work, I gave up, tossed the parts into my scrap bin, had a good stiff scotch and redesigned the wing structure.

Steve
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Old Apr 03, 2006, 04:50 PM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
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Retract Design

After my trials and tribulations with the wing structure, I decided I'd take a break from building and do a little "puzzling" over the retract design. I've never done retracts before, so if anyone has comments or suggestions, chime in while it's still not too late to make changes!

I'm using the Superlight .10 size retracts from Hobby Lobby with 3/32" wire. Since they normally retract laterally into a wing (think Hurricane or Spitfire), I'll rotate them 90 degrees so they'll retract forward into the nacelle of the B-17. Unfortunately, this puts the actuator arm in front of the retract unit, requiring a fore-and-aft movement to raise or lower the gear. I've designed a linkage using 90 degree bellcranks to produce the required motion from a single servo (see the top view) but is this reasonable? My wheels are Du-Bro Superlights and weigh only 4 grams each, so the torque required shouldn't be excessive. Anyone got any thoughts on which servo I should use for this?

The plan positions for ribs F-2 and F-3 weren't ideal for mounting the retract unit, so I moved them around a bit until they were. Drawing the slightly modified airfoil sections for the new ribs made me wish I could use CAD, probably not for the last time.

As far as I can tell, the real B-17's gear retracted over an arc less than 90 degrees, so I had to fudge the down position gear angle a little to make sure everything would fit (see the side view). Obviously I'm going to have to re-design the nacelle structure to accomodate the retracted wheel and provide a solid motor mount, but that will have to wait for another day...

Steve
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Old Apr 03, 2006, 06:21 PM
Pusher jets rule!
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Mesa, AZ
Joined Jan 2006
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Steve,

I just bought the full kit a few weeks ago from my LHS. My recent GWS C-130 build has me wanting more multi's (I got a Guillows B-29 off eBay also)!

I want to use as much of the included wood as possible, but will probably substitute basswood stringers. I will also remove as much unneccessary balsa from the fuse and wings as possible to save on weight, so I can use the smallest motors I can get away with.

I'm on the fence regarding retracts and flaps, and will watch this thread with interest.

Pat
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Old Apr 04, 2006, 06:51 AM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
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Pat,

Building from a plan, I get to pick my own wood , so I've gone for 4-6 lb contest grade wherever possible. To keep things as light as possible, and given that I'll be sheeting the whole thing, I've decided to use only every second rib in the wing, no trailing edge and only the wing stringers I think I need to ensure good bonding of the wing skins (1 mm sheet). I'm going to plank the fuselage, so the same goes for the fuselage stringers. For tail surfaces, I'm going to try pink foam over balsa frames, covered with silkspan and thinned carpenter's glue.

If you're going to be covering with silkspan or one of the heat-shrink coverings, I'm not sure bass stringers in the fuselage will be worth the extra weight. The plan sure has a lot of them, so I'll bet the fuse will be plenty stiff with the kit-supplied balsa stringers. Just my opinion, though

Steve
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Old Apr 04, 2006, 04:27 PM
IAMCANADIAN
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Canada, BC, Squamish
Joined Jul 2002
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Cool. Scratch-built Guillow's is an apt moniker for the technique. I built a Spirit of St. Louis and Guillow's inspired is about what I did as well. Turned out pretty good though.

Here's the thread.
http://www.rcgroups.com/forums/showthread.php?t=190091
I did my fuse like you've done your wing. Build it, chuck it, redesign/engineer using plan as a 3-view and build again.

Subscribing
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Last edited by Grejen; Apr 04, 2006 at 04:38 PM.
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Old Apr 04, 2006, 05:14 PM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
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Greg,

That's really nice work on your Spirit of St Louis. I had been following your thread, but lost track of it when I moved to Norway last summer. Doesn't take long to get behind on the Zone!

I am Canadian (too)!

Steve
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Old Apr 04, 2006, 07:13 PM
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Oakland, CA
Joined Nov 2002
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Hey Steve:

I like the project. Very ambitious. I think you are on the right track with your plan to cut down on all of the stringers and ribs while sheeting. I sheeted a guillows dauntless with 1/32 and only had a few stringers and found it to be very strong. Keep up the posting as you progress and good luck.

Alex
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Old Apr 05, 2006, 05:15 PM
Callsign: CornDog
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Milpitas, CA
Joined May 2003
5,364 Posts
Signing in! The B-17 has been one of my fovorite bombers ever since the 4th grade! I don't know why I haven't considered building one yet. Steve, I think you may have motivated me to do just that; I'll add it to my other 234,093,249 planes I want to build!
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Old Apr 05, 2006, 05:53 PM
Kansas is windy.
pburress's Avatar
Wichita, KS
Joined Jun 2003
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Sounds great!

If you are definitely planning on staying with brushed motors, consider the j250 (aka dcm-189) instead of the 280. The j250 is lighter than a 280 and puts out a lot more power- closer to a speed 300. Prop/gear it for 6 amps or less for reasonably long motor life. If fits right in a 280 gearbox. Plus, these motors are cheap!

Phil
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Old Apr 10, 2006, 06:11 PM
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Milpitas, CA
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Has there been any progress on this???
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Old Apr 15, 2006, 05:40 PM
The "pro" in procrastination
Steve85's Avatar
Canada, ON, Kingston
Joined Mar 2004
2,895 Posts
Sorry for the gap in posting, but work got busy and then I took off on a week-long holiday, so there wasn't much time to post. Phil, I'm using the 280 sized motors basically because they worked for ESB when he did his conversion a few years ago. Mine are GWS cans, and they only cost $3.75 apiece at Tower Hobbies, so cost isn't really an issue. I'll be running Cox 5 inch props on them direct drive, drawing around 2-3 amps each. The J-250s sound like they would draw quite a bit more, requiring a significantly larger battery. As it is, I figure I'll need about a 2000 mAh pack to get decent flight times with the 280s, but haven't yet decided on NiMh or Lipos. I may have to trade off AUW against balancing at the correct C of G, but I won't know for sure until it's nearly finished.

Next post will show some progress.

Steve
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