Building the prototype XCBD - Page 3 - RC Groups
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Feb 08, 2009, 07:52 PM
Stealth Plane Works
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Quote:
Originally Posted by will_newton
looks to me like the layers taper off. there are only three layers of carbon on the top skin root. i think anker is showing what is underneath each shape as well. which makes for some initial confusion.
Anker, I noticed the leading edge on the top skin is not labeled at the tip, but I assume it is 3.7 carbon?
There are two layers of 3.7 oz carbon under the two triangular pieces in the center panel.

Yes, the LE at the tip is also 3.7 oz carbon.
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Feb 09, 2009, 12:43 AM
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Regarding the question of fabric quality/performances over time:

- For the glass sizing: this is the coating on top of the tread to facilitate “wetting” of the fabric during layup. It is true that it will lose some of its characteristics over time especially if not stored properly (i.e. out of direct light, out of dust, no humidity, etc). What is more critical is to get the right sizing for the resin used.
- All materials used in fabric manufacturing for composite work will attract humidity. Kevlar is by far the worst. Utmost precaution shall be taken for storage as to remove humidity once in the fabric is NOT easy.
- Humidity has two main effects: to reduce the mechanical proprieties of the molecule structure (Kevlar is once again very bad) and reduce the quality of the composite structure as the interaction fabric/resin is hampered. Not speaking of… added mass!!!

I have a question about the heavy use of carbon especially in the top skin. Carbon is not the best material to use when it comes to compression loads, but works extremely well in traction and for stiffness. Wouldn’t it be preferable to replace some of the carbon layers by “S” glass? I did some test coupons few years back and for top skins the best results were with an “S” glass structure. If extra stiffness in torsion was required (“S” glass is not bad in this department) just adding 45º carbon would do the trick.
Feb 09, 2009, 07:48 AM
Stealth Plane Works
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Quote:
Originally Posted by fnev
Regarding the question of fabric quality/performances over time:

- For the glass sizing: this is the coating on top of the tread to facilitate “wetting” of the fabric during layup. It is true that it will lose some of its characteristics over time especially if not stored properly (i.e. out of direct light, out of dust, no humidity, etc). What is more critical is to get the right sizing for the resin used.
- All materials used in fabric manufacturing for composite work will attract humidity. Kevlar is by far the worst. Utmost precaution shall be taken for storage as to remove humidity once in the fabric is NOT easy.
- Humidity has two main effects: to reduce the mechanical proprieties of the molecule structure (Kevlar is once again very bad) and reduce the quality of the composite structure as the interaction fabric/resin is hampered. Not speaking of… added mass!!!

I have a question about the heavy use of carbon especially in the top skin. Carbon is not the best material to use when it comes to compression loads, but works extremely well in traction and for stiffness. Wouldn’t it be preferable to replace some of the carbon layers by “S” glass? I did some test coupons few years back and for top skins the best results were with an “S” glass structure. If extra stiffness in torsion was required (“S” glass is not bad in this department) just adding 45º carbon would do the trick.
I let Mark Drela do the structural analysis and recommend layups.

I believe carbon is the best available material for dealing with compression load in the top skins. But I'll let the structural engineers in this forum get into the detail.

I have never seen the effects of humidity on Kevlar/resin that you describe. But maybe the dryness of my basement and the fact that I usually own a plane for about 3 years before retiring it, are factors.

Anker
Feb 09, 2009, 09:16 PM
Registered User
citing my composite professor's handbook...
Kosmatka, J.B. "Advanced Composite Materials," Mechanics of Aerospace Structures. 2006.

Unidirectional properties in compression with a fiber volume fraction of 60%:
[Young's modulus, Ultimate Strength, Density]
S2 glass/epoxy-[7.9 Msi, 134 Ksi, .072 lb/in^3]
AS-4 cf/epoxy-[20 Msi, 240 Ksi, .057 lb/in^3]
ultra high modulus graphite/epoxy-[42.23 Msi, 90 Ksi, .062 b/in^3]

Unless you're somehow using the ultra high modulus stuff (then only losing due to ultimate strength), the data seems to say carbon's the way to go.
Feb 10, 2009, 06:28 AM
Registered User
You are absolutely correct: I was comparing with very high strength carbon. I should have gone back to our data base instead of working of my memory. An other consideration is what is used for “design value”. This may differ from what is in the text books as it takes in account the exact material used and the process(es) use(d).

It is of no value to argue here about meaningless numbers for most of the people looking or participating on this forum.

The point I wanted to make is that the use of “S” glass has a lot of advantages as seen below:


BEST-------------------------------------------> WORST

DENSITY Kevlar Graphite S glass E glass

STIFFNESS Graphite Kevlar S glass E glass

TOUGHNESS Kevlar S glass E glass Graphite

IMPACT Kevlar S glass E glass Graphite
RESISTANCE

COST E glass S glass Kevlar Graphite

Therefore you might get for a similar final weight and strength/stiffness a cheaper product. Personally I dislike working with Kevlar and will avoid using it unless it is no other alterative for very specific reasons. Since the use of “S” glass introduces some interesting mechanical proprieties in the layup, I would have a tendency to minimize the use of graphite (as much as possible) as well.

Sorry if I introduced some confusion here. I hope the above helps, even without all the numbers one should attach to it if serious structural analysis is required.
Feb 10, 2009, 03:43 PM
Registered User
Huh, interesting points. I wasn't arguing, I was just thinking performance wise carbon seemed better. What kind of mechanical properties do you speaking of? I haven't had much experience with S-glass, maybe I'll try some out. I remember it being very hard to work with around compound curves...stiff stuff.
Feb 11, 2009, 12:47 AM
Registered User
Yes, “S” glass is pretty stiff. But by choosing the right type of fabric you might alleviate the “contouring” problems. Plus there are basic molding techniques that help as well.

I do have numbers. As I said, they are the results of coupons testing made for a specific application with different fabrics but the same resin system. They are the DESIGN VALUES that are used for the stress analysis of this specific application.

With the “S” glass the value for tensile and compression are very close to the values with two type of carbon selected (not the highest strength). The highest strength carbon gives far better tensile (+50%) and very similar compression values. BEWARE: this is NOT with the same resin/fiber ratio, the reason being that in the real world this not necessary realistically achievable.

Maybe a way to go, as it is becoming more and more accessible, is to use prepreg. I am sure that some of the people involved on this forum have access to leftovers or shelf life expired products. The problem would be to get weights and curing requirements within what would be still reasonable for modeler’s use.
Feb 11, 2009, 07:17 AM
Stealth Plane Works
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Quote:
Originally Posted by fnev
Yes, “S” glass is pretty stiff. But by choosing the right type of fabric you might alleviate the “contouring” problems. Plus there are basic molding techniques that help as well.

I do have numbers. As I said, they are the results of coupons testing made for a specific application with different fabrics but the same resin system. They are the DESIGN VALUES that are used for the stress analysis of this specific application.

With the “S” glass the value for tensile and compression are very close to the values with two type of carbon selected (not the highest strength). The highest strength carbon gives far better tensile (+50%) and very similar compression values. BEWARE: this is NOT with the same resin/fiber ratio, the reason being that in the real world this not necessary realistically achievable.

Maybe a way to go, as it is becoming more and more accessible, is to use prepreg. I am sure that some of the people involved on this forum have access to leftovers or shelf life expired products. The problem would be to get weights and curing requirements within what would be still reasonable for modeler’s use.
You bring up a good point about the state of cloth and resin that may have sat for a long time in the average hobbyist's basement, and also the hobbyists ability to do the layup and curing properly. My initial investment in tooling and supplies was between $6000 and $7000, something my wife has reminded me of several times.

I am already on my second gallon of ProSet part A and will go through several gallons this year, so it has a very short shelf-life.

Selection of the right resin and proper post-curing is very critical.
Feb 21, 2009, 03:06 AM
Proud member of LISF and ESL
Thanks for posting this Anker. I will be following.
Mar 20, 2009, 09:28 AM
Stealth Plane Works
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Finally back to building the XCBD


After catching up with my backlog of Supra builds and core cutting I have time again to continue building the XCBD.

Yesterday's task was making master ribs. These ribs are used as templates for cutting the center and joiner ribs for the wing.

First I enter the airfoils, chords and hole positions to CompuFoil3D and print them out on my laser printer. Then I 3M77 the printouts to the 1/4" ply used for the ribs, rough cut them on the band saw and then fine sand them on my oscillating sander.

This sander is a fantastic tool. It has cylinders of varying diameters and a belt. It also has a table with adjustable angle settings. When sanding, the belt or cylinder oscillates up and down so it isn't the same small area of the paper that gets worn. Finally it has a vacuum attachment to cut down on the dust.

Its not necessary to get the airfoil exact, it just needs to be close and never inside the lines. What is important is to drill the holes in exactly the right positions. I do this on my drill press.

Once the holes are drilled I remove the printouts and mark the ribs as master ribs with a Sharpie.
Last edited by Anker; Mar 21, 2009 at 08:55 AM.
Mar 20, 2009, 05:44 PM
Pompano Hill Flyers
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......
Mar 20, 2009, 08:15 PM
Stealth Plane Works
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Ribs


Now that the master ribs are done I can cut and add ribs to the cores.

The first step is to mark on a plywood sheet where the tube spar holes are to be drilled.





I use the ribs to ensure that the holes are properly spaced, but I don't mark the rib outlines yet, that's done later.

The holes are then drilled on a drill press. They don't have to be drilled exactly where they are marked on the sheet, as long as they are close.





Once the holes are drilled I use a piece of carbon tube and the master ribs to draw a rib outline that has the hole in the exact position it should be in. Because the master ribs are a bit oversize and the way I draw the outline I get a slightly oversize rib.



The ribs are then cut out on a band saw. The kerf is kept outside or just touching the outline, and doesn't have to be right on. Getting it close cuts down on work.





Major straying from the outline is removed on a disk sander. I just have to make sure I don't remove so much that I get inside the outline, the rib must be kept slightly oversize.



The next step is to bond the ribs to the cores. I place the cores in the top beds and weigh them down to ensure that they have the correct shape. I then insert a piece of carbon tube in the tube cutout, smear a thin layer of foam safe CA on the end of the core, spray the rib with kicker, and then slide the rib onto the tube and the core, ensuring that there is overlap all the way round. This method works really great. The bond is instant and I can move right on to the next rib.







The last step is to sand the ribs flush with the foam. I first sand the bottoms by placing the cores on the top beds, weighing them down and then placing a strip of blue masking tape over the foam and folding it over the leading and trailing edges. I then use a Dremel drum sander to remove the excess. The masking tape protects the foam and I can see when the sander touches the tape. The final sanding is done with a metal strip sander. The result is that the rib and foam match up perfectly.











I then turn the core over, lay it in the bottom beds and do the same to the top of the rib.
Mar 20, 2009, 08:35 PM
wishes this caption was longe
will_newton's Avatar
Quote:
Originally Posted by Anker
I first sand the bottoms by placing the cores on the top beds, weighing them down and then placing a strip of blue masking tape over the foam and folding it over the leading and trailing edges. I then use a Dremel drum sander to remove the excess. The masking tape protects the foam and I can see when the sander touches the tape.
That is a good idea. It's almost a shame that folks may miss all this good info you are posting about wing building since it is under the XCBD heading. This would make a great companion to Phil Barnes's vac bagging video with all these detail shots.
Last edited by will_newton; Mar 20, 2009 at 08:41 PM. Reason: can't spell
Mar 20, 2009, 08:36 PM
Yep, Naza-controlled Tricopter
tonyestep's Avatar
Very pretty work, Anker. Thanks for these excellent photos.
Mar 21, 2009, 08:14 AM
Stealth Plane Works
Anker's Avatar
Quote:
Originally Posted by will_newton
That is a good idea. It's almost a shame that folks may miss all this good info you are posting about wing building since it is under the XCBD heading. This would make a great companion to Phil Barnes's vac bagging video with all these detail shots.
Will,

I have seriously considered doing a writeup and a video to complement Phil's video tape. I have found that some of the things Phil does requires superhuman skills that I simply don't have, and have figured out alternative approaches that work for me.

Anker


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