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Old 10-10-2007, 04:37 AM   #1
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Default Build Thread: Scratch Built Semi-symmetrical wing for Ultrafly Super Decathlon

As my flying skills progress, and my desire to do more and better aerobatic maneuvers increases, I have found that I'm beginning to realize the limitations of the Decathlon's stock Clarke-Y airfoil.

I got together with PD1 this afternoon, and in a few hours, we sketched out a new wing for the Decathlon using a Semi-symmetrical airfoil. Specifically a NACA 25014 design.

The idea is for this wing to be a direct fit. This way I can swap in either wing as the mood strikes me.

This is my first balsa scratch-build. This thread will chronicle the fabrication of the wing as I go through it, as well as the eventual maiden flight and my impressions.

Please be patient if I am slow to update, as I will have to shoe-horn this build between family and work obligations.

The wing design is identical in span and chord to the stock wing. The wing thickness is just a little thicker. Pretty much the difference in thickness is the addition of the semi-symmetrical profile to the underside. Another difference is the wingtips will be rounded to more accurately reflect the 1:1 version of our namesake.

The Specs:
Span: 43"
Chord: 7"
Max thickness (at the spar): 7/8"

I've done my best to scan in the plan. Included in the attachments below are the full scale rib profiles, a down-sized gif showing the overall plan for the wing.


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Old 10-10-2007, 05:11 AM   #2
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Old 10-10-2007, 03:08 PM   #3
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Hey Gnascher!

I'll be following along on this one too. I can hardly wait to see your results. It should give this SD a whole new personality.

Cheers! BH

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Old 10-10-2007, 03:33 PM   #4
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Thanks guys. I'll try not to disappoint!

I'm going to try and do some preliminary work tonight and see if I can create some templates for those wing ribs in aluminum. That'll help me get nice consistent results when fabricating the over 20 ribs this build requires.

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Old 10-11-2007, 02:50 AM   #5
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Ok. I created an aluminum template from the rib template PD1 sent me.

It's probably overkill to make a metal template, but now I've got it, and it'll help with consistency.

First, I printed the rib template and rough cut it out. I then glued it onto a piece of heavy cardboard.

The cardboard template was then used to transfer the drawing to the aluminum stock.

I then rough cut it and did the final shaping with a sander. Once I was close, I then scanned the metal rib into the computer and super-imposed the original image over the rib. I'm confident of the accuracy of my reproduction to within about 0.5mm or better.

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One issue did present itself. It appears the spar notches on the paper template don't match the spar position of the drawn plan. The drawn plan looks more correct to my eye, but we'll see what PD1 has to say.

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Old 10-11-2007, 03:50 AM   #6
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Ok, then I was not seeing things earlier. I thought the rib pattern would not match the spar locations. Still this should not be a real problem just modify the build. Also an old build trick I used to use was to make two rib patterns out of metal then stack balsa between them and sand the stack this way I cut all ribs the same, even works for tapered wings.

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Old 10-11-2007, 04:07 AM   #7
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Originally Posted by oimmuk View Post
Ok, then I was not seeing things earlier. I thought the rib pattern would not match the spar locations. Still this should not be a real problem just modify the build.
Yeah ... I'm just not sure whether to modify the template or the plan. I'm thinking the plan's position is better.

Also an old build trick I used to use was to make two rib patterns out of metal then stack balsa between them and sand the stack this way I cut all ribs the same, even works for tapered wings.
That's pretty much my plan ... just using one metal template though. With the tools I have on hand, it's a little bit of a pain to make the metal templates. I really need to get myself a little band saw.

The holes I drilled in the template will be for a dowel. I'm going to put together a stack of rib blanks, drill down through the the stack and slide the dowel in to hold it all in place, then rough-cut and sand to shape.

The holes will also permit feeding the servo wires through, since I'm planning on using two servos mounted in the wing for the ailerons. I guess they also save a little weight, but that's only a side benefit.

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Old 10-11-2007, 03:51 PM   #8
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Gnasher, The plans position is correct. The difference is where I measured from when drawing the rib.
On the rib I measured back from the front of the leading edge. On the plan I measured from the rear of the leading edge.
There should be 1/4 in difference between the two.

I should have drawn the rib at the same time as the plan.
On the plus side, All the fascia on my house is now wrapped in aluminum.

You know I have a lot of that thin aluminum left over. Good for cowls, gear doors?
Need any?

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Old 10-11-2007, 04:00 PM   #9
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Gnasher, The plans position is correct. The difference is where I measured from when drawing the rib.
On the rib I measured back from the front of the leading edge. On the plan I measured from the rear of the leading edge.
There should be 1/4 in difference between the two.
I figured as much, because it is just about 1/4" off.

You know I have a lot of that thin aluminum left over. Good for cowls, gear doors?
Need any?
Not at present, but I'll keep it in mind.

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Old 10-13-2007, 11:13 PM   #10
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Default Ribs and formers cut

I got my ribs and wing tip formers cut today. They are cut from 3/32" balsa with the grain running along the long axis. The two wing-tip formers are 1/8" balsa so they'll be a little more robust in case I ever cartwheel the plane or impact the wing tip in any way.

First I cut the rib blanks, and used my metal template to locate and drill holes through all of the blanks.
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I then created a jig from a couple of dowels the same diameter as my holes, and a piece of hardwood I had lying around.
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The Jig allows me to consistently locate and hold my template steady for cutting.
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Perfect fit!
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I then created a template out of cardboard for the spar notches and to trim for the LE and TE.
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...here they all are spread out on the plan.
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I haven't trimmed the center-section ribs or the outer-most ribs as I'm not 100% sure how I'll go about those construction details yet, and I want to leave my options open. I'll trim them to fit as necessary when the time comes.



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Old 10-14-2007, 02:46 PM   #11
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Hi Gnasher, Wow, those are large pictures.

Run the leading edge all the way from center section to wing tip.
After the wing is framed you can cut away the unwanted section in the front center.

By having the leading edge pieces join in the center, however temporary, it helps to align the wing and prevent unwanted twists while building.

Looks good.

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Old 10-14-2007, 04:08 PM   #12
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Originally Posted by pd1 View Post
Hi Gnasher, Wow, those are large pictures.

Run the leading edge all the way from center section to wing tip.
After the wing is framed you can cut away the unwanted section in the front center.

By having the leading edge pieces join in the center, however temporary, it helps to align the wing and prevent unwanted twists while building.

Looks good.

Paul
Thanks Paul. That sounds like a good idea.

How do you like my cutting jig? I could only effectively do one rib at a time, but since I could usually do the cut in one pass it went pretty quickly.

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Old 10-14-2007, 06:58 PM   #13
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Gnasher, The jig looked good and worked for you.
I'm too lazy to make a jig for all the ribs, I just pin the paper and rough cut the rib.
Then sand to final size.

But with your method, you can make more ribs faster than I can.

The jig is all ready for your next build.

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Old 10-16-2007, 01:47 AM   #14
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Progress report:

I cut my spars and leading and trailing edges out of 1/4" balsa stock tonight (no pics ... probably nothing more boring than some pics of sticks). I also picked up the last of the materials I'm going to need, including yellow and white econocote (I already have some black in stock for trim).

I've also dry-fitted the right wing, and it looks like everything's going to go together real nice.

I'm at a point where I'm going to need to make decisions about how certain pieces will go together before I start cutting things to length, so I'm probably not going to get to touch this again until the weekend unless I can finagle a free evening this week. Possibly Wednesday night when the Sox aren't playing.

Specifically I'm thinking about the joinery at the first and last ribs where the LE and TE meet, as well as how best to brace up the tip formers. I've got a couple of different approaches that I'm tossing about. I'm also going to ruminate a bit over how much dihedral to put in the wing, and how to establish that dihedral when I join the wing halves.

Finally, since this wing has a different bottom profile than my stock wing, I'm going to have to think about how to fit it to the wing saddle.

Option #1 is to modify the wing saddle to fit the wing. I'm shying away from this option since it would require modifying and re-covering the fuselage, as well as fabricating some kind of "adapter" if I find I ever want to fly the flat-bottom wing again. I think I might when I'm float-flying or doing some AP.

Option #2 is to modify the shape of the ribs in center section of the wing to match the stock profile. I'm leaning toward this option as it requires no changes to existing hardware, and shouldn't really negatively impact the performance of the new wing. Also, if I go forward with building a new fuse, it's easier to build a flat wing saddle than a profiled one.

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Old 10-18-2007, 04:14 AM   #15
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Default Some progress today

I've made a little progress today. I got most of the right wing constructed.

I still have to install and brace the wing tip former. I'll probably do that last ... I'd likely snap it off while I'm handling the thing.

I also have to still add some strengthening gussets here and there, and come up with a decent servo mount.

I've also still got the center section to build, which is why I haven't trimmed the inner ends of the wing spars yet. The inner end of the squared-off TE is going to get drilled for a dowel which will allow everything to mate up strong with the shaped trailing edge / aileron stock that I have.

Looking at the way the wing mates up to the plane, I think I'm not going to flatten the center-section ribs. Instead I'm going to probably build up the leading and trailing sections just a bit. It seems that the overall profile of this wing will mate up better that way. I mocked up a rib with a flattened bottom, and the wing actually ended up sitting too low in the saddle.

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Old 10-18-2007, 03:14 PM   #16
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Gnasher, On the wing tips, add a gusset between the end rib and the leading edge and the trailing edge.
!/8 sheet or doubled 3/32. 3/4 to 1 inch sides.
If you use the 3/32 sheet, alternate the grain.
I forgot to draw them in.

Before you go too much further, you can join the wing halves.
About 1 to 1 1/2 inch blocks under the wing tip ribs will give good dihedral.
You can go more or less if you want.

Tie the spars and trailing edges with 1/32 to 1/16 ply strips.

The piece of wood that goes between the front ribs, the one behind the windshield.
I'd cut it to length then place it behind the former in the fuselage and drill the holes to match.

Then I'd check incidence and glue the piece in place.
I'd probably make that piece out of two 3/32 pieces doubled up with a single 1/16 or double 1/32 ply front and back to hold the dowels.

Looks good so far.

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Old 10-18-2007, 03:32 PM   #17
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Originally Posted by pd1 View Post
Gnasher, On the wing tips, add a gusset between the end rib and the leading edge and the trailing edge.
!/8 sheet or doubled 3/32. 3/4 to 1 inch sides.
If you use the 3/32 sheet, alternate the grain.
I forgot to draw them in.
I was definitely planning on the gussets. I remember we discussed them even though they weren't drawn in. I have some 1/8 stock that I'll use. It's also what I used for the wing tip former.

Before you go too much further, you can join the wing halves.
About 1 to 1 1/2 inch blocks under the wing tip ribs will give good dihedral.
You can go more or less if you want.
Gotta build the other half first.

I'm thinking minimal dihedral ... like we discussed, just enough to prevent the droopy look. Should I worry about filling the gap at the bottom? I'm planing on having the central rib where the two wing-halves join be a double.


Tie the spars and trailing edges with 1/32 to 1/16 ply strips.

The piece of wood that goes between the front ribs, the one behind the windshield.
I'd cut it to length then place it behind the former in the fuselage and drill the holes to match.

Then I'd check incidence and glue the piece in place.
I'd probably make that piece out of two 3/32 pieces doubled up with a single 1/16 or double 1/32 ply front and back to hold the dowels.
All good advice. I don't have an incidence meter ... any good rules of thumb on establishing AOI? How much should I shoot for?

Looks good so far.

Paul
Thanks! It's time consuming, but not as difficult as I had anticipated.

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Old 10-18-2007, 03:50 PM   #18
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Incidence, Take the drawing of the rib, the one with the leading edge . Draw a line from the front of the rib to the trailing edge.
That's your chord line.
Transfer this line to a rib.

You now need a reference line on the fuselage.

Take a yardstick, place it under the stabilizer and continue a line to go under the wing saddle.
This line should be parallel with the stabilizer.

Measure up equal amounts from the reference line to the wing saddle and draw a line connecting the two points.
Again this should be parallel to the stabilizer.

Place the rib with the chord line on the newly drawn line and and cut the wing saddle to match the rib.

The rib, and the wing should now fit and be parallel to the stabilizer.

Space between the center ribs, don't worry about it. If the ribs are doubled up, there won't be any real space.
When you sheet the bottom of the wing, you butt join the sheets.

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Old 10-18-2007, 05:42 PM   #19
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Place the rib with the chord line on the newly drawn line and and cut the wing saddle to match the rib.
Clear as a bell, thanks. However it looks like if I cut the wing saddle to the wing profile, the wing will end up sitting too low. Apparently there enough of a difference in chord-line relationship between these two wings.

I mocked up the new wing on the fuselage and with the approximate AOI, and the wing spar resting on the saddle, it looks like everything lines up perfectly in terms of the top of the windshield matching the top of the new wing, and the TE and the rear mount.

I think the approach I'm going to need to take is to build up a bit in the wing-saddle area of the wing to match the existing flat saddle. Basically to flatten the wing over the saddle by addition, rather than fit the saddle to the wing by subtraction.

I'm only going to need to add a little bit in the leading and trailing sections. I think the sheeting will conform to this just fine and then blend into the flying part of the wing smoothly.

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Old 10-18-2007, 07:21 PM   #20
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Gnasher, Building up the fuselage will also work just as well.

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Old 10-18-2007, 07:48 PM   #21
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Originally Posted by pd1 View Post
Gnasher, Building up the fuselage will also work just as well.

Paul
Agreed. But then I lose compatibility with the old wing. Maybe I'll never want to fly that one again ... who knows.

Perhaps I'll build an adapter that I can slide in there ... that should be easy enough.

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Old 10-18-2007, 07:54 PM   #22
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Of course you could build a second fuse as well.
If you have an untimely end with this one, you will still have the old one to fall back on.

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Old 10-18-2007, 08:04 PM   #23
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Originally Posted by pd1 View Post
Of course you could build a second fuse as well.
If you have an untimely end with this one, you will still have the old one to fall back on.

Paul
Yes I could. I'll wanna fly this wing soon though. Don't know when I'll have time to put together a fuse plan, get my hands on the stock and build it in the immediate future. I'm having enough trouble carving time out to build this one.

But, that is an argument in favor of not modding this wing to fit the saddle of my existing fuse.

I can fly the wing, see if I like it and then build a new fuse to suit. I'm sure we could build a lighter version than the one I have now too.

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Old 10-23-2007, 06:16 AM   #24
Gnascher
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Default The end is in sight!

I've made some more progress.

I assembled the left wing-half over the weekend, and joined the wing-halves tonight.

I created a box-section at the wing spars using 1/16 ply. I also backed up the joint of the TE with the same material. This has resulted in a very strong joint and I still have the forward mount and central ribs to install. If this wing fails, it's not going to be in the middle!
(The TE looks bowed in the 2nd photo, but it's not. I used a 3' piece of aluminum square channel along the TE for reference when doing the join)
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The halves freshly joined and sized up on the plane to figure out where the forward wing mount should end up.
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Here's a head on view. The cross-beam for the forward wing mount has been located and cemented in place. I created the cross-beam by sandwiching a piece of 1/4" balsa between two pieces of 1/16 ply. This should provide plenty of strength for the wing mounting dowels.
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I haven't drilled for the dowels yet because I still have to get my angle of attack dialed in. I also need to come up with my scheme for the wing saddle. I'm starting to think I may do nothing besides fabricate a shim for the TE to set my angle of attack, and not worry about the fact that there will be some small gaps at the top of the fuse for now rather than come up with some elaborate scheme to mod the wing saddle or wing for a seamless fit. As long as my shim goes full width, between that and the dowels up front, I'm confident the wing will be sufficiently supported.

I still need to install some gussets at the corners, sheet the center section, and install and brace the tip formers. I'm thinking I may also sheet the top of the wing from the LE to the spar. This is how the original wing is done and it's real strong like that..



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Old 10-23-2007, 01:43 PM   #25
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Gnascher,

I've been following along on the progress. Man, that wing is going to look great on your SD! I can hardly wait to see it finished.

When you're ready for those 'finishing touches', just holler and I'll see if Santa can make an early delivery, LOL!

Again, it's looking great! Cheers! BH

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