Sagitta 900 (build and update for ALES) - RC Groups
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Dec 28, 2016, 03:20 PM
ʇsol
vulturetec's Avatar
Build Log

Sagitta 900 (build and update for ALES)


With the talk about the Sagitta over here, I thought it's about time to start the build log on my Sagitta project.

For the most part I'm keeping things generally "originally", with small exceptions. I've taken note of the suggestions to lengthen the fuselage, delete the balance tab on the rudder, use an airfoil on the stabilator, and a couple of other things. I was going to update the airfoil but opted to stick with the original E205 for now - I think the S3021 outline would be an easy replacement/update but the job's already been done. I can build another wing later if I want. This is also going to be electric. Hopefully it will be a halfway decent conversion. With the modern radio equipment, trading rx batteries for a BEC, etc, the weight gain isn't horrible.

Another large part of the project are personal goals. Earlier this year I decided to learn how to do some more advanced CAD work and built a CNC machine for cutting parts. I opted for Fusion 360 and a Shapeoko machine. Without any real CAD experience (apart from Coreldraw and some minor Autocad stuff years ago) and having never touched CNC stuff I thought putting the pieces together would be a huge challenge. Happily everything I've done has been relatively easy and has worked out really well. After cutting parts for a few wings and a Fokker trimotor kit, one of the goals with the Sagitta is to put all the pieces together and hopefully have a halfway decent sailplane for ALES.

On with the build...

First - it's electric. A minor redesign of the front end of the fuselage had to be done to squeeze a decent motor into the nose. I've been using a Gliderdrive SK3 hybrid in my ElectraLite with a lot of success. Crunching the numbers, it seems like it should do well with the Sagitta. Installation will be virtually the same as the ElectraLite, with the motor slipped into a nose-block that gets blended into the rest of the fuse. There are more streamlined ways of doing this but this is a simple solution.

Instead of just boring a hole in a block I decided to do some 3D design in Fusion for block to fit the motor fairly tightly with the front of the opening beveled to match the motor. Part of this is to help streamline the nose over what I ended up with on the ElectraLite. Cooling hasn't been a problem on that model, hopefully it will be ok here as well.

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I split the block into two halves rather than attempt to machine it in one shot (I don't have any tools long enough to do that anyway). Part of the 3D modeling includes the shape of the fuselage, so this was really the only way I could do it. Here's an idea of what one of the halves should look like:

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Rough machining out of balsa:
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After the final smoothing passes and removing from the wasteboard - it actually came out pretty nice.
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Dec 28, 2016, 04:03 PM
DLG Bug Bit Me
Tim Harbour's Avatar
Sweet project and will be watching with interest.
Thank you for posting this.
Dec 28, 2016, 09:34 PM
ʇsol
vulturetec's Avatar
Quote:
Originally Posted by Tim Harbour
Sweet project and will be watching with interest.
Thank you for posting this.
Thanks Tim
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Dec 28, 2016, 10:10 PM
ʇsol
vulturetec's Avatar
Good or bad, I'm keeping the overall wing design original. Even so, after working on a couple of wing projects, I've found that generating the ribs on the computer is more accurate and easier than tracing and digitizing the plans. For the Sagitta I created the wing in DevWing using measurements off the original Sagitta plans.

The final E205 rib outlines were a fairly close match to the original plans. Eventually I "built" the wing and it's major components in Fusion 360.

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...and a nice overview of what it all looks like put together - mostly, anyway:

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Dec 28, 2016, 10:31 PM
ʇsol
vulturetec's Avatar
Spoilers

Pondered on the spoiler-actuation for a bit. I've always used pull-lines (as indicated on the plans). After doing some comparisons I found the total weight of two feather-light servos was something like 4g greater than the "traditional" method when the nyrod housings and magnets (etc) are figured in.

Did some more work in Fusion to model the way the whole thing will work - seems pretty straight forward. This was my first (and so far only) effort doing any motion-analysis in Fusion. Neat stuff.

Spoiler control concept (0 min 34 sec)


With everything positioned "just right" I got the spoiler-servo rib designed:

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Before cutting the rib I laminated 1/64" ply to each side of the 3/32" balsa stock where the servo will be mounted. The CNC cutting came out great, predrilled the mounting holes and everything.

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Dec 28, 2016, 10:56 PM
ʇsol
vulturetec's Avatar
Wing rod/tube block

More neat Fusion stuff... doing the wing "build" in Fusion makes creating additional parts fairly simple and well fitting (hopefully).

Here's how the tube block finished up with the rest of the wing design:

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The render of the final block design looks promising:
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With the based block design finished I made a mirror image for the opposite wing and used Fusion's CAM functions to generate gcode for the machining. The original plans call for pine, but I found the thin areas of the blocks to be really soft and flimsy. In the end I machined the blocks from poplar.

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Fairly happy with the final results. Fusion render on the left, finished product on the right.

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Dec 28, 2016, 11:33 PM
ʇsol
vulturetec's Avatar
Ribs

With all the minor alterations done to the ribs in Fusion (spoiler servos, wing-tube blocks, leading-edges, etc) it's time to cut ribs!

When I first started on the idea of doing CNC parts cutting I looked for a simple CAM program to generate gcode from simple 2D vector drawings. "Cut2D" fit the bill nicely. In Cut2D I set up two sets of ribs per sheet, one rib for each wing-side with the thought that cutting the duplicate ribs from the same wood should roughly also duplicate the weight of the wood on each panel. That's probably going optimistically overboard, but it was easy to do anyway. I also weighed the wood ahead of time, putting the lighter wood progressively toward the wing tips.

Here's what the rib layup in Cut2D looks like. Each sheet can be cut individually, but it's quicker to set up several sheet at a time. I cut 4 sheets (8 ribs) at once:

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...and the finished cuts
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Doing the virtual-build in Fusion let me create the outlines for the webs too. I ended up CNCing the individual webs out of the various sizes of wood called for on the original plans. The smaller holes in the ribs are for the building jig I use, the jig and the precut webs meant no additional plans or measurements are needed when assembling the wing. That's the idea at least.

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Dec 30, 2016, 05:54 PM
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3810's Avatar
Fantastic project, completely above my head with all this CNC stuff, but love your work !
Jan 01, 2017, 10:09 AM
ʇsol
vulturetec's Avatar
Quote:
Originally Posted by 3810
Fantastic project, completely above my head with all this CNC stuff, but love your work !
Thanks!
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Jan 01, 2017, 11:19 AM
ʇsol
vulturetec's Avatar
Leading Edges

The wing's leading edge was a challenge - for me, anyway. I liked the original hardwood leading edge for durability:



Shaping it by hand (planing/sanding) didn't seem too precise to me, and without any equipment to do the work I decided to try CNCing it. So - back to fusion, again.

Using the wing rib layout from DevWing I came up with a triangle shape that seemed to make gluing up the LE sheets fairly simple (no sharp bends), and could later to shaped to the right outline. This also seemed like a good shape to CNC:



After machining a couple of small test pieces in pine, I built a 2-rib wing section from a couple of inboard E205 ribs. Surprisingly it fit perfectly. Now on to producing the leading edges! With the basic design done in Fusion it wasn't too difficult to repeat the LE design to mill out a plank of wood. Here's a render of what the final plank of leading-edge parts looked like from Fusion 360:

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The parts were milled out of a plank of basswood. This process was very challenging for me and probably overkill, but it seemed like a good idea at the time. A better machine would probably have done a better job, but the final parts turned out pretty well. Most of the parts were very smooth but some had some noticeable ridges from the machining. Initially I also had problems getting everything zeroed so the final dimensions fit the ribs properly. I'm attributing most of the little annoyances to my lack of experience and asking a lot of precision from a belt driven machine that I could have assembled better. It worked though...no pictures of the machining process but here's one the planks with leftover LE material on it:

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A couple of pieces had to be lightly sanded for the ridges or because they were slightly oversized, but for the most part it was a drop-in fit to the ribs:

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Jan 01, 2017, 11:50 AM
ʇsol
vulturetec's Avatar
Inboard wing (part 1)

Since I'm using building jig I needed to assemble some of the "flat" parts away from the jig (I think some of this is similar to what's in the "Legend" instructions too). The lower skins and spars were glued up against a heavy straight-edge. Initially I did this on a wood building surface then switched to glass. It was easier to keep the outside surfaces of the spar and skins flush on the glass. This is a great idea if the cutting is precise and if the wood dimensions are precise, otherwise there's still sanding to do.

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Now for a dry test-fit of all the parts before I started gluing it together. Pretty much everything fit tightly when in the jig.

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The joiner-block, ply doublers, and inboard balsa webs were all assembled on the inboard skin/spar assembly. I can' t remember the exact order I used to put the pieces together, but it all went together pretty well. The ply was all designed in Fusion 360 and the cuts were precise so it all jigged itself with no additional sanding/shaping.

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Now more dry-fitting of the rest of the wing. The webs are all pre-cut so no measuring or plans were really needed. I assembled the wing rib-by-rib using the pre-cut webs as a jig for each preceding rib. The idea (I hoped) was to keep each rib vertical and spaced correctly. Seemed to work. I ended up drawing reference lines on the inside of the skins to help me keep the ribs square with the spars

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When all the ribs were glued in place on the spar I glued in the top-spar in one-shot, clamping the top and bottom spars together between the webs. Seemed to work ok. Also added the leading-edge to the front of the ribs then glued the lower skin to the bottom of the ribs and leading edge.

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Jan 08, 2017, 04:39 PM
ʇsol
vulturetec's Avatar
A view of a (mostly) finished tip panel

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I worked on the fuselage outline last year when I worked out the changes for the motor modifications, but hadn't done much more than the outline. So...spent some time coming up with the fuselage formers. My plan is to put the servos under the wing so I eliminated the extra notches for the pushrod casings, and made new holes at the correct location in the appropriate formers. Based on my experience with my Electra Lite and Spectra, I suspect the first former (F2) won't be needed. We'll see.

I created the formers in CorelDraw first, then QCed the results in Fusion.

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Jan 16, 2017, 07:48 PM
ʇsol
vulturetec's Avatar
The topic of motors was raised in another thread. An alternative and better fitting motor appears to be a 24mm "inrunner" motor with a gearbox. Great Planes' Electrifly "Ammo" 24-45-3790 with their in-line 4.3:1 gearbox works out to be a decent fit. With an 11x5 prop the calculated rate of climb is quite good with a reasonably low current draw. Installation works out better too. Except for some filler-balsa, a nose-block isn't really required, just a ply front mounting plate and maybe a support-former in the back.

The inrunner+gearbox is slightly heavier than the gliderdrive, but with the balsa nose-block required of the gliderdrive it's not a big difference. On the plus-side the inrunner installation results in a slightly narrower front-end on the fuselage.

In the end I've opted to stick with the hybrid outrunner Gliderdrive motor, mostly because I've got all the equipment already - and my experience with the Electrifly stuff hasn't been great. There are far better quality yet similar inrunner options available , but at a price.

With that settled, back to the build: did some minor re-drawing of the fuselage sides, double checking the hole positions for the joiner and alignment rods, and servo cable hole, etc. Then I went ahead and finalized the former designs to fit the fuselage sides precisely. During that step I also added alignment tabs to the formers, and made recessed pockets for the tabs on the fuselage. The recessed pockets are only part way through the first ply but still work to self-jig the fuselage sides and formers. True interlocking tab-and-slot design would have put holes in the fuselage, which I wanted to avoid.

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Jan 16, 2017, 07:57 PM
ʇsol
vulturetec's Avatar
From the outset I knew cutting the fuselage sides was going to be a challenge. Sure, a band-saw and scrollsaw would work fine...stack the two blanks, cut them, drill the holes, etc. I really wanted to use CNC on this to make everything fit cleanly from the outset. After some research and fiddling I figured out a technique that lets me cut virtually any length of material on my desktop CNC without too much effort. This fuselage was my first attempt at it and it worked amazingly well. This really opens up some doors for future projects!

After cutting the blanks with the band-saw the fuselage side-cuts took about an hour to do, total. With a proper CNC machine it wouldn't have taken very long but as a hobbiest I can't complain.

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The camera adds some distortion here. I stuck the two sides together and they're a perfect match.

Next...cut the formers and start on the tail.
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Jan 16, 2017, 08:22 PM
Registered User
Beautiful work, I think Lee Renault would approve. When this is done we gotta get you to build some vintage scale sailplanes.
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