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Author Topic: Need recommendations for rib (planform) layout for an A Electric  (Read 1240 times)
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wmazz
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« on: August 05, 2017, 11:06:08 PM »

My own design for A Electric, so anything is possible. Never built anything larger than a 1/2a (except for a Witch Doctor 800
that I never flew, so it doesn't count Sad) I am trying to decide between full geodesic, or half geodesic. My wing is 7.5" x 60"

I don't have the practical experience to know if half geodesic is sufficient or when a full geodesic is necessary?

It seems to me that smaller planes would be fine with the "half geodesic" and a few sub ribs along the leading edge? And a larger
plane should have the full geodesic? I don't have enough experience, I think it is easy to draw up a design, and another skill set
to build a reliable design that won't go boom a few seconds later Sad

Perhaps a fast plane needs the extra strength, but I am not planing on building a hot plane, for now, but I would rather make a
set of airfoil patterns that I can use for more than 1 wing.


Bill M.
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danberry
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« Reply #1 on: August 06, 2017, 10:17:47 AM »

Are you willing to sheet a D-box?
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wmazz
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« Reply #2 on: August 06, 2017, 03:39:21 PM »

Are you willing to sheet a D-box?

Yes, Sad
I assume along with the D-Box you would recommend a 1/2 geodesic for the rear ~66% of the ribs?

I have been using the method described in the 2002 NFFS Sympo: Jigs, Tools and Techniques for Balsa wood
Construction, Chuck Markos. It (D-Box) would mean more work initially making jigs.

Eventually I would like to get the ribs laser cut, but that doesn't seem practical until I have a design that has
been well developed.

Or would it be faster to have rib blanks with the D-Box cut by laser, then add notches for spars myself and
cut them to fit ??  

Is Profili capable of supplying the right file types, or is that something I need to do in Acad?

Thanks

Bill M.


Regarding jigs, I have problems with the the printer paper getting fuzzy at the out line of the rib.
Is there any suggestions on improving my method of cutting jigs. Print the paper, glue to 1/16 ply
cut and sand?

Has anyone tried to print ribs to clear plastic overhead projector sheets?
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wmazz
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« Reply #3 on: August 06, 2017, 03:45:36 PM »

oops Smiley
« Last Edit: August 06, 2017, 04:14:32 PM by wmazz » Logged

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danberry
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« Reply #4 on: August 06, 2017, 04:01:07 PM »

OK then. A D-box with diagonals behind the spar is sufficient.
Carbon caps on the ribs make it extra rigid.
You don't need a lot of jigs. Glue the ribs and spars to the bottom of the D-box sheeting while flat on the board. Join the panels. Sheet the top of the panels one at a time while shimming the panel appropriately. I do the left main first since it stays flat. Next night I do the right main with washin jacked into place. Then the tips. Pretty easy stuff.

Laser cut stuff goes faster if it fits.

What is the aspect ratio of the planned build?
Strength and stiffness are not the same thing. Fast planes need a rigid wing. The D-box plus CF caps will usually make it rigid enough.
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wmazz
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« Reply #5 on: August 06, 2017, 04:30:57 PM »

What is the aspect ratio of the planned build?

7.5 x 64, 8.5 : 1 AR

Modified AG 03 with 2.5% camber and 9% thick.

I am not interested in building a thinner wing until I have more experience.

Strength and stiffness are not the same thing. Fast planes need a rigid wing. The D-box plus CF caps will usually make it rigid enough.

From what I have read, stiffness implies that wing is resistant to twisting or
flutter while under power.


Any recommendations for the trailing edge?


Bill M.

What do you recommend for the D-Box leading edge and top spar? A leading edge
that the D-Box sheet overlaps or butts into the LE.
« Last Edit: August 06, 2017, 04:41:14 PM by wmazz » Logged

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« Reply #6 on: August 06, 2017, 06:45:35 PM »

I'd use 1/16" sheet for a D-box on that wing.
1/8 x 3/8 TE would do fine.
I have a D-box wing where the sheeting meets the spar and the leading. I have several wings that the sheeting glues onto the spar and LE.
The butt-joined one was the first D-box I built. Draw your conclusions about which one I like to build........

Stiffness or rigidity is as you describe. If you're flying fast you cannot have a wing that will twist. The center mains are where the action is.
If it flutters you have real big problems.
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« Reply #7 on: August 06, 2017, 09:11:33 PM »

Is this an undercambered section?
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USch
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« Reply #8 on: August 07, 2017, 04:25:46 PM »

Just my 2 pence worth.
Here my preferred way to build D-boxes in balsa. The false LE gives good support to the top box-sheet and you can fit a small hardwood strip to protect the precious nose radius (where you hit first, at least around here  Wink )
Dont mind the section in the scetch, I had a tailplane section around, so I used it rather than drawing a new section, its to hot here to do much work, except some model building.

Urs
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wmazz
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« Reply #9 on: August 08, 2017, 12:23:52 AM »

Is this an undercambered section?

No, it is a flat bottom airfoil with a raised leading edge. I modified the original to fit
my needs, and be forgiving.


Just my 2 pence worth.
Here my preferred way to build D-boxes in balsa. The false LE gives good support to the top box-sheet and you can fit a small hardwood strip to protect the precious nose radius (where you hit first, at least around here  Wink )
Dont mind the section in the scetch, I had a tailplane section around, so I used it rather than drawing a new section, its to hot here to do much work, except some model building.

Thank you for taking the time to draw an example.

Question? Why does your example show a vertical spar with horizontal grain? Wouldn't a
vertical shear web be better? or is your example a good I-beam?

It looks like I would build the wings in two pieces. Build the front 33%, D box first, then the
rear 66% last?

I have tried making my own templates for "D" boxes before, but that has proved difficult
and time consuming. I stated that I used the recommendations in the 2002 Sympo, but
when I reread it, I remembered that I had problems with the plastic. I am not sure where
I got the idea for my rib patterns I use, but I am certain that I am doing it the hard way.

I normally use 1/16th plywood to make the patterns, but the paper kurff (fuzzy edge)
creates problems with seeing the actual pattern line.


Bill M.
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mike
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« Reply #10 on: August 08, 2017, 03:49:32 AM »

There's no mention of covering material above, that I could find.  This will have an impact on torsional stiffness.  Plastic films add little torsional stiffness.
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« Reply #11 on: August 08, 2017, 06:40:07 AM »

Bill, in regard to my fast scetch. The fill-in has nothing to do with wood grain, it was just to highlight the different members of the structure: spar, sheeting and LE. Of course vertical grain in the spar web is the way to do it. And yes that's a I or double T spar

The AG-03 is exactly the type of section to build with a double falsa LE as the "pronounced philips entry" scetch. This gives you some meat to sand the entry without weakening to much the lower sheeting.

I've never seen the 2002 Sympo report, so I dont know why they ask for templates. The only templates I see to do would be for the ribs, nothing else. Eventually one could water slightly the top sheeting and keep it till dry bent over the ribs to avoid stresses build in, I never did anything like that and my wings are normally straight  Cheesy

Urs
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wmazz
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« Reply #12 on: August 10, 2017, 12:51:16 AM »

I've never seen the 2002 Sympo report, so I don't know why they ask for templates. The only templates I see to do would be for the ribs, nothing else.

This is an example of how I cut ribs. After making those 3 jigs, then I would fab 3 to 4 more jigs to
cut the D tube, spars, LE, & TE.

I have used single templates before, and they are not bad for large ribs. But small ribs are a pain.

Eventually one could water slightly the top sheeting and keep it till dry bent over the ribs to avoid stresses build in

That is a good idea.

There's no mention of covering material above, that I could find.  This will have an impact on torsional stiffness.  Plastic films add little torsional stiffness.

I have mostly used silk span for my FF models, I have tried a double layer of Jap-tissue once, and
Monocote & Ultracote on my RC gliders.

I was thinking about trying Poly Span. Does anyone have a better alternative?

I am concerned my first few models having a fast decent after DT, and knocking off wing keys. Do
wooden dowels do a good job of stay attached to poly span?


Bill M.
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« Reply #13 on: August 10, 2017, 10:15:43 AM »

Thanks Bill for the pictures.

It's true, you never finish to learn  Wink
It's a lifetime I believed that jigs should a) speed up the job whenever you have to do things more than once, b) increase accuracy, c) replicate pieces long after the build is done.
Now I see that it might also help passing a rainy Sunday with nothing to do  Grin

Well, being a simple person my jigs are simple as well. Here some of my rib templates. I use fibreglass board to do them because it is harder than plywood and you cannot chip off pieces if you hold the scalpel the wrong way. Then there are two pins glued to it to hold your balsa strip firmly down. I use to work not directly on the working table but on a small piece of plywood underneath, in this way I can turn around the template and balsa once cut the top curve. The template contains also all the cuts for spars. As preparation I cut small rectangles from the right balsa sheet, slightly larger than the actual rib.

Urs

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« Reply #14 on: August 10, 2017, 10:48:16 PM »

Well, being a simple person my jigs are simple as well. Here some of my rib templates.

I have tried copper clad pcb material and the first thing I noticed is the all the
square edges your patterns have. Mine (copper clad pcb material) patterns had
rounded lines and spars that I cut to deep. The pattern (rib template) was a similar
size goe 182, with a D box, and my pattern broke in half at the upper & lower main
spars.

What tools do you use? electric disk, belt, oscillating sanders?

I used my Foredom rotary tools

http://www.foredom.net/rotarytools.aspx


What thickness fiberglass do you use? 1mm, 1.5mm?

Thanks

Bill M.
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« Reply #15 on: August 12, 2017, 04:56:08 AM »

Today I normally draw the section with a CAD program like DrawSight because I dont have Profili (but that's another story). Then print it and glue it with Scotch M77 to the fiberglass board. Cutting is done oldfashioned with a jigsaw about 0.5mm outside the trace. and without any cut-out for spars. Finishing with sanding blocks from about 320 grain to 400 and compairing it with a second print to check accuracy. A caliper also helps to get the right thickness. Only after finishing the outline I cut the spar recesses.

Material: if I have to design directly to the board I use to copper cladded board with the standard 1.5mm thickness. This can allow for soldering the hold-down pins directely to the pattern without gluing. The copper clad allows for very precise scratiching with a toolmaker scriber (?). But nowadays I have some fiberglass board without copper, thickness 1.5mm which I use mostly. To my humble opinion fiberglass boards are exellent materials for al sort of jigs, beeing termally stable and not influenced by humidity and other. So my stock contains boards from 0.5-3mm for all occasions.

Urs
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wmazz
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« Reply #16 on: August 13, 2017, 02:28:19 PM »

Finishing with sanding blocks from about 320 grain to 400 and comparing it with a second print to check accuracy.

A caliper also helps to get the right thickness. Only after finishing the outline I cut the spar recesses.


To Everyone, Thank you for sharing Smiley

I learned a lot!


Bill M.
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wmazz
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« Reply #17 on: August 17, 2017, 12:55:21 AM »

Any suggestions on where to find a fiberglass reinforced board for making rib patterns like Urs used?

I tried McMaster Carr and the fiberglass reinforced board they had was used as an electrical insulation.
It was the only 1/16" board I could find, it was red colored and about as flexible as a 24" long piece of
plywood.

Or any other suggestions for making rib patterns? I also purchased a small pcb board, but I would like
to save it for another project.


Bill M.

Electrical-Insulating Fiberglass Sheets and Strips
https://www.mcmaster.com/#standard-fiberglass-sheets/=18z3cw0
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danberry
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« Reply #18 on: August 17, 2017, 01:25:04 AM »

1/16 ply.
Harden the edges with thin CyA.
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« Reply #19 on: August 17, 2017, 04:20:03 AM »

Come to think of, another nice material for pattern is called FORMICA, a phenol resin board used to protect chipboard in kitchen furniture. Avaiable in different thicknesses as the fiberglass boards. We used it in the pre CNC days to make the cutting patterns for styrofoam wings, it resists well the heat of the cutting wire and is not so abrasive as glass boards.

Urs
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