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Old Jan 15, 2013, 07:15 PM
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United States, MA, Newton
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A twin twin build: Ivan Pettigrew's Twin Otter 480

My dad (DaveSy) and I are each embarking on a Twotter build, starting from two sets of Ivan's plans, and two short kits from Manzano Laser. He's in London, Ont, Canada, and I'm in Boston, MA. Hoping to be flying when we get together this summer, maybe even with floats! Since one of us still works for a living, we'll see how that goes...

Based on initial calculations, and other builds, planned motor setup is for 2x Turnigy 2217 20turn 860kv 22A Outrunner each w/ 18Amp ESC, wired in parallel to central 3S LiPos, and 3-blade 9x7 master airscrew props. When all the bits arrive we'll try some bench testing and to see if that's realistic. Planning to include the optional flaps, with in-wing servos (2 x ail, 2 x flap). I'm sure we will get into some wiring discussions later on - already reading heated debates about extending motor vs battery leads.

Hoping this thread will be useful as a joint notebook for us, so we can each make half the mistakes :-)

I will follow up shortly with some initial pictures.
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Old Jan 15, 2013, 07:33 PM
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And we're off...

Build "the elder" has started with the stab and elevator:

1) Two main spars were sanded as advised before construction. Used 3/16" by 1/2" rather than by 5/8" as ribs [from short kit] are only 1/2" deep. Used 3/32" shim at LE and 3/16" at TE. will use 1/4" square for LE as I have it rather than 1/4" by 3/16" . Will gently notch TE for ribs, maybe 1/32". And will probably add 1/32" cap strips to ribs akin to previous builder, but lighter. Ruler and sanding bar used as wights to keep ribs down at free ends. Don't forget to sand off the residual laser cutting dimples to ensure good gluing contact.

2) Stab and elevator ready to remove from the board. Note additional 3/32" gussets to stiffen corners a little. TE will be cut later. The elevator spar was quite hard so am going to use it as a joiner rather than wire.

3) There is also a 1/32" ply reinforcement. It has a V-notch in either end partially hidden by gussets. The items to the left are the sanded fin and rudder spars, ready for the next part of the project.

So far we've discovered that the plans and build notes (w/ pix) are good guidance but still leave lots of "exercises for the student". For example:

I just looked at the build notes (to check out the fin and rudder), and he has little gussets in almost identical places to mine on the stab and elevator, but they're not on the plans! Actually, I lied a bit: the plan does show two small gussets at the TE of the elevator, but the picture shows more.
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Old Jan 15, 2013, 07:49 PM
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For my part, build "the younger" has started with the fuse.

Built two side frames, first over the plan, second over the first w/ clingfilm between. My favorite tool, the machined square 1-2-3 gauge block, gives nearly perfect alignment of the two. The only difficulty was to decide where the bottom longeron is intended to bend, and where it should be spliced with straight sections. I ended up using three splices, two in the nose section, and one just aft of wing. Also, the doubler for the landing gear seems to measure 1/2" x 3/16" on the plans, but labelled as 5/8" x 3/16". I decided on the former.

After a bit of sanding and CA on reverse sides, I've started framing it up, with the center section that's parallel. The stations from the side view don't seem to completely align with the top view, so I tried to pick a reference point that let the tail end join at the right spot, but I don't suppose it matters much. I've added a few corner gussets along with the specified diagonal braces (tho I mounted them within the stretcher frame rather than beside them). The gusset weight gets paid for by a curved cutout in the wing mount brace (which again, seems oddly labelled on the plan as 5 1/4" wide but the fuse is only 5 3/16" wide and the brace appears to fit between the longerons?).
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Old Jan 15, 2013, 08:18 PM
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Cool idea and a great subject. Subscribed.
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Old Jan 16, 2013, 07:37 AM
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Joined Aug 2004
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There's another Father/Son team building in Norway, but I dn't think they have a thread. Arvid (SASflyer) and Patrick Havsgard are building two of Ivan's Mosquito 480s (if you see this Arvid, how about some pictures?).

The Twotter is a very good flier and can be built very light, judging by the ones I've see.
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Old Jan 16, 2013, 09:34 AM
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Some motor chitchat - all hypothetical until we actually receive them!

D: On review of motor specs, I was initially taken aback a bit that the 2217 is listed as a 22A motor (I hadn't paid much attention to this), since we have 18A ESCs, but looking at its detailed specs, they say operating current is 6-17A and peak current is 19A. Whereas the ESC is rated as 18A continuous and 22A burst.

All this is pretty consistent and all we have to do is ensure our props draw less than 18A. Even 15A will give us 330W. Should be lots so long as we meet the 4lb target.

Just checking.

P: I'd been looking at the performance data quoted at HK, which claims that 3S w/ 10x7 prop draws 16A w/ 970g thrust. So if you make the 4lb target it could just about fly vertical And 9x7 three-blade should be similar to 2 blade 10x7 from what I've read.

For slow flying we're probably still over-powered. We'll see!

There was another thread that talked about this plane w/ twin 3530 Turnigy motors, which sounds like it could climb on one engine. I wonder how that worked out?
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Old Jan 16, 2013, 12:29 PM
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Nice project and hope the parts work out OK.

I really like the little 2217 motors. A lot of power in a small package. I used one in a motor glider (72" wingspan) with an 11x7 prop and 3s2200. It would draw over 300W on a climb to altitude and only get a little warm (I had a temp sensor behind it). I did have a 30A speed control on it with an airscoop for the ESC.
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Old Jan 16, 2013, 01:35 PM
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The elder has to make some progress before disappearing for nearly 2 months at the end of next week. The fin and rudder seem to have a variety of approaches to build. The notes suggest that the fin can be built in the air directly on the fuzz. My choice is the conventional approach, at least for the outlines as shown on the picture. The main spars are constant width over the bottom 5" so the upper parts float at increasing height over the board. 1/4" shim works well at the LE and front spar; a little less behind the spar. So the top joins were partly made by eye. That's where CA is handy, although I prefer yellow wood glue, especially the little bottle shown with its neat dispenser. It can be easily refilled. I'm then going to remove the fin and rudder from the board and put in the remaining ribs in the air in order to center them more easily (except for F2A which goes in after the stab is in place). The black items are plastic bags full of lead shot which are handy to hold things down.

The gussets are a bit heavier than the plans show. The bottom member of the fin was unspecified and not provided with the laser cut parts so I glued 2 by 1/4" sq. together and carved from there. I again notched the TE about 1/16". I note on both the fin and the elevator that the TE is specified as 1/2" wide but is only shown as 7/16" which nicely allows for the notch.
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Old Jan 16, 2013, 02:24 PM
London, Canada
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Quote:
Originally Posted by portablevcb View Post
Nice project and hope the parts work out OK.

I really like the little 2217 motors. A lot of power in a small package. I used one in a motor glider (72" wingspan) with an 11x7 prop and 3s2200. It would draw over 300W on a climb to altitude and only get a little warm (I had a temp sensor behind it). I did have a 30A speed control on it with an airscoop for the ESC.
Yes, although I've never run them that hard. A club member used one on an EasyGlider with a 12 x 6.5 prop and a 30A ESC and it really performs at well over 200W. I also use one in a 6' span motor glider, a derivative of an Olympic III at 33ozs with an 11 x 6 prop and a sedate 130W but it still moves out well. Both on 3S batteries.
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Old Jan 16, 2013, 10:26 PM
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Hi guys, just thought I'd chime in and say that yes, I did notice this thread, and added a link to it in the sticky. Some may think it gets boring to go into all the build details, but if anyone is ever to build one of these models in the future, all that will be very valuable - as I know you've already found yourselves from reading the other threads. So keep it up - even if you don't get a lot of chit-chat in the meantime (I am the worst about being absent, I know).

It's nice to see a father and son do this hobby together. Interestingly I don't see it very often. Seems to me as if sons often like to do things different from their dads out of principle.

Anyway, you picked a great model to build and hopefully you two have some fun bantering back and forth about it and who knows, maybe you'll have some fun mid-airs someday too.


Luke
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Old Jan 17, 2013, 12:48 PM
build like there is no 2moz
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United Kingdom, England, Bristol
Joined Apr 2009
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Good Choice, and looking good

I love the idea...

You are both progressing well, from the looks of it.
I built a twin otter 480 myself, with the optional flaps, and they add a new dimension to the flying. (Like flying backwards!)
I also built and kitted out a set of floats (which were great fun)

Good choice on re-inforcing the rudder (i suggest stiffening the fin up too) Mine was always a source of trouble (Hanger rash mainly!), especially the rear spar, where it leaves the fuselage (i added bass wood side stiffeners)

I look forward to seeing the build,s and the flights

Scott
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Old Jan 17, 2013, 12:58 PM
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I'd be interested in hearing more about the floats. Were they from a kit or plans we could get hold of? Or at least basic dimensions of the setup - thinking of doing the same.
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Old Jan 17, 2013, 01:36 PM
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DaveSy: With the aid of a little masking tape and some weight, we get a crude mockup of all the tail feathers, rough sanded and finished for the time being. The ribs in each of the four parts has been cap-stripped with 1/32" by 3/16" strip. I think it looks better that way, improves stiffness, especially at the thin TE's, helps covering, and is very therapeutic, but it does take a while. I did quite a bit of hand trimming of the ribs to allow for the caps. The final step was to cut the TE away from the elevator in the middle section.

On to the wing! Then we might have a complete Otter between us.
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Old Jan 17, 2013, 02:26 PM
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Far off yet, but thinking about an RCAF colour scheme. Variants on skis and floats, and certainly easy to see!

There are lots of nice pictures if you google CC138. There's also an official RCAF page for the twin otter, which includes a picture archive.

What's the deal with the larger 'rib' mid-wing?
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Last edited by patricksurry; Jan 17, 2013 at 02:36 PM. Reason: add pix
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Old Jan 17, 2013, 04:12 PM
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Quote:
Originally Posted by patricksurry View Post
What's the deal with the larger 'rib' mid-wing?
They are called "wing fences." In this case, they are likely used to stop the spanwise flow from the root of the wing during high AOA attitudes common of STOL aircraft from influencing the hopefully un-stalled tip of the wing.

In general, they stop any spanwise flow from one portion of the wing from interfering with another portion of the wing. They are also common on swept wing aircraft.
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