SuperGee II continued (thread #3) - Page 50 - RC Groups
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Apr 19, 2006, 10:40 AM
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Frank

You don't need to cut the main glass skins to an exact size. You can just lay the two mylars on the bench and cover them with oversize pieces of glass. After wetting everything out you can just trim the glass to the edge of the mylars. There is no problem extending the glass all the way to the edge of the mylar because glass can be easily trimmed off the LE after bagging.

For all the doublers I would be tempted to attach two layers of the glass together with 3M77 to make a single piece of 1.5oz fabric.

1) Spread the glass out on the bench and smooth it out with a drafting brush.
2) Spray a piece of wax paper (kitchen paper in the UK, I think) with a light spray of 3M77.
3) lay the wax paper on the fiberglass at a 45 degree angle to the factory edge. I would use a long piece of wax paper that would be at least long enough to do the flaperon doublers. Laying the wax paper on at a 45 degree angle to the fiberglass fibers will make it easier to plot out the doublers on the fabric later.
4) Use scissors to cut out the area of fiberglass with the wax paper attached.
5) Repeat this to make a second piece of fiberglass/wax paper.
6) Spray both pieces of fiberglass (with wax paper backing) with a slightly heavier coat of 3M77 and stick them together. You should then have two pieces of glass stuck together with wax paper on both sides.
7) Draw out your doublers. Fine point permanent markers work well to draw on wax paper.
8) Use scissors or a knife and straight edge to cut out the doublers.
9) When doing the layup you can peel the wax paper off of one side or the other depending on whether the piece is a left or a right. Then stick it in place on the layup and peel the other piece of wax paper off.

This will be easier in terms of having far fewer pieces to cut. It will be more difficult until you learn the correct amount of 3M77 to use. For sticking the wax paper to the glass you want to use just one quick spray. Maybe use two quick passes over the glass when sticking the two pieces of glass together. You don't want to get a lot of 3M77 into the glass, just enough to hold the glass together. Maybe Mark's post-it glue method could be used for part of this, I'm not sure.

strong versus weak fibers

Most fiberglass is not totally balanced. That means that there are different numbers or different strengths of fibers in the warp direction (fibers that run for the length of the roll) versus the fill direction (the fibers that run across the width of the fabric. When doing bias layups with unbalanced fabrics you always want to match the fibers on the top versus the bottom of the wing such that the warp and fill fibers are always parallel. If the stronger fibers on the top of the wing are perpendicular to the stronger fibers on the botom of the wing then the wing can warp either immediately when taken out of the bag or over time as the epoxy continues to cure or with temperature changes.

Most fiberglass in the .75oz range that I've seen is almost completely balanced but not quite perfectly balanced. Most fiberglass in the 1.5oz range is highly unbalanced with the warp direction having far greater strength than the fill direction. Highly unbalanced fabrics are good for using on a 0-90 layup with the high stength direction running spanwise. This type of layup would be used on a wing with no spar or a spar that needs help, the skin would be used for bending strength in this case. Unidirectional carbon is an extreme example of a highly unbalanced fabric. The old WACO kits (Magic, merlin, etc.) used multiple layers of 1.4oz fiberglass (highly unbalanced fabric). Those wings had as many as seven layers of the glass at the root and tepered to two layers at the tip. They relied heavily on the glass for bending stiffness/strength.

Back to the hand launch wing: The .75oz glass, used on a bias works well here. the wing gets it's bending strength/stiffness from a carbon spar (carbon rods in Frank's wing). The glass is used for ding resistance and torsional stiffness. The .75oz glass is probably almost perfectly balanced so you might not need to worry about which way the warp and fill fibers are aligned. There have been reports from guys who have built balsa HL tails and failed to align the warp and fill fibers top and bottom so the tails developed a bit of a warp. The wing might not have that problem but it will be easy to avoid that problem with a little forethought. The simplest way to avoid the problem and have a completely balanced layup in this case will be to reverse the direction of the warp and fill fibers between the two layers of glass being used for each piece of the layup. see the attached image:

The following image shows how to make a single piece of fabric from two pieces of .75oz fabric. This piece of fabric then becomes a completely balanced piece of 1.5oz fabric. You could stick these two pieces of fabric together as described in the text above to use as a stock of balanced fabric for making doublers. Or you could simply use the two pieces of fabric as shown in the diagram as the main wing skin covering the entire mylar. This same double layer fabric would go on both sides of the wing. It is completely balanced as shown in the second diagram and can be treated as a single perfectly balanced fabric.
Last edited by Phil Barnes; Apr 19, 2006 at 05:20 PM.
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Apr 19, 2006, 12:35 PM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
thanks Phil for the info.

I was planning originally to do it the way you suggested (after learning it from your video and Aerospace's tape). The reason why I didn't was that I wanted to use the glass as a way to practice using the paper carriers in preparation for working with kevlar. This way I already got double the practice while only using a single core. I want to do all I can not to waste the kevlar. Glass is relatively cheap.

The method you sugggest was the way I layed up a V tail for a replacement fuselage for my Gentle Lady. I converted it to a pod and boom and made the pod by the lost foam method. Lacked some knowledge then, it came out a little heavier than the original but I digress....

I was thinking of altering the paper carriers orientation on the cloth such that if one was up and to the left, its mate (remember each layer of kevlar gets 2 layers of glass) would go up and to the right. Because the both can go down in only one way, the strong fibers would be perpendicular to its mate evenly distributing the strength. I label the paper carriers such that I know its orientation before use even if it is obvious.

I also was thinking of making, as you call it, a composite fabric (sort of) but instead spray glue as you described in the video the glass to a second layer of glass but oriented perpendicular to the first to balance the final fabric. I did not do this because I did not know how well West Systems epoxy would wet out two layers of glass. Granted .75 is rather loose but I didn't want to develop a bad habit and try it on heavier cloth someday only to find out it won't work.

Thanks again. Keep the advice coming.

Frank
Apr 19, 2006, 01:23 PM
Registered User
Back in the days when I made WACO Magic wings with as many as seven layers of 1.4oz glass, the method was to lay out all the glass on the mylars (all seven layers) and then roll the epoxy into the glass. I used West Systems epoxy back then also and there was no problem saturating all the way down to the mylars.
Apr 19, 2006, 03:14 PM
Registered User
Phil's second sketch might be misleading. It shows how the pieces look after being cut out on the table, NOT what they look like on the wing.

Once all the pieces are on the wing...

Red fibers on the top and bottom surfaces are parallel
Blue fibers on the top and bottom surfaces are parallel
Apr 19, 2006, 05:31 PM
Chuck 'Em and Chase 'Em
Fly2High's Avatar
Mark,
wouldn't it be better if the top panel had two layers and the bottom panel had two layers where each set of two layers had the blue perpendicular so as to balance the top and balance the bottom?

If there were only two layer, one on top and one on the bottom I understand that you do not want to have the strong fibers perpendicular to each other b/c it will twist over time while drying.

Isn't the point to get as balance a cloth as possible? My suggestion is to orient 2 layers of a lighter fabric to get the same weight (probably more strength) but balance the cloth by orientation.

Frank
Apr 19, 2006, 05:48 PM
Registered User
Frank gets it. I went back and added some text to my previous post to make the point more clear (I hope).

I have added a new sketch below to show the fiber orientation for a layup using a single layer of unbalanced fabric on top and on bottom of the wing.

The following sketch shows the proper orientation of the fabric during a layup using unbalanced fabric. The top and bottom mylars are laying face up on the bench. A single layer of unbalanced fabric is draped over each myalr. The red arrows represent the strong fiber direction. The blue arrrows represent the weak fiber direction. The core will be placed between these two mylars. The top mylar gets flipped over when it gets placed on top of the core so that when the skins are in place on the core, the strong fibers on top of the wing are parallel to the strong fibers on the bottom of the wing.
Apr 19, 2006, 05:58 PM
Registered User
Here is another sketch to show the common mistake that people make when using unbalanced fabric. The top and bottom mylars are on the layup bench again. This time a single piece of unbalanced bias fabric is laid over both mylars. When these mylars are placed over the core, the top mylar will be inverted and the the top strong fibers will be perpendicular to the bottom strong fibers. This situation happens a lot when people use mylars that are hinged at the trailing edge and then they naturally cover the mylars with a single piece of fabric.

Note: This is the WRONG way to do a bias skin layup
Apr 19, 2006, 06:09 PM
Registered User
One more sketch. This time the sketch shows the easiest way to lay up small parts like tails while making certain that the top and bottom strong fibers end up being parallel to each other. The bottom mylars are placed on the bench all in the natural position, with the trailing edges parallel to the bench edge, for instance. The top mylars are all placed on the bench such that they are perpendicular to the bottom mylars. A single piece of bias cut fabric drapes over all mylars. When these mylars are placed over the cores, the top mylars get inverted and all of the strong fibers end up parallel.
Apr 19, 2006, 09:19 PM
I speak Texan
Jason Courville's Avatar
Phil,

Does this need to be done with S-glass?

Jason
Apr 19, 2006, 09:36 PM
Registered User
All of that stuff in the last few posts was just an attempt to explain how to use fiberglass in layups in a way that keeps the fibers aligned so that the parts being made won't have any tendency to warp. Many guys are simply not aware that many fabrics (especially fiberglass) are not balanced and that laying up parts with unbalanced fabrics can cause parts to warp when the fibers are not aligned properly. That was the entire point of all those posts.

The concept is the same for any fabric. Mark has told us that S-glass is 15% stronger (stiffer?) than E-glass. So if you use the same weight of S-glass in place of E-glass to make a part then the part will be 15% stiffer/stronger than the E-glass part. You only "need" to use S-glass if you need the extra 15% strength/stiffness.

All of the fiber alignment stuff is a completely separate issue from any talk about material type.
Apr 19, 2006, 09:49 PM
I speak Texan
Jason Courville's Avatar
Thanks Phil,

Until I read your last post, I had no idea this needed to be done.
Thanks for your hlep.

Jason
Apr 20, 2006, 06:55 AM
Registered User
I misunderstood Jason and answered the wrong question in my last post. Jason's actual question is a good one. First, the short answer; It might be a good idea to pay attention to the fabric orientation of S-glass if the S-glass fabric you are using is unbalanced. You need to look at the specifications for the fabric you are using to see if it is balanced.

Now, the long answer:

Most places that sell fabrics will have the technical data for the fabrics that they sell. You may have to dig around a little to find it but people will usually ask them questions about this data so they are bound to end up posting the data somewhere on their web site. I found the data for the fiberglass and Kevlar (aramid) that CST sells and I attached an image of those pages below. Look at the columns of data; the first column of interest is the one listing fiber count in each direction, the second is breaking strength in each direction. Notice that the .72oz fiberglass shows the same thread count in each direction but the breaking strength is a little different. Most fiberglass in that weight range is like that; almost balanced but not quite. Look at the 1.38oz fiberglass. Notice that the thread count is different and the breaking strength is very different in the two directions. Most fiberglass in the weight range around 1.5oz is like that; highly unbalanced.

Now look at the data for Kevlar (aramid) fabric. Note that the thread count and breaking strength is exactly the same in both directions of all the aramid fabrics that they sell. This is typical of the aramid or kevlar fabrics that we use and that is why you never hear anybody talking about fabric balance or worrying about strong versus weak fiber direction for these fabrics.
Apr 20, 2006, 07:12 AM
Registered User

Does fabric balance matter?


You can be blissfully ignorant of this issue of fabric balance for a long time and never encounter any problems. A part that is laid up with unbalanced fabric, used on a bias, with the strong and weak fiber directions misaligned may or may not warp. Unbalanced twisting forces will likey develop in such a part but if the unbalanced forces are not strong enough to cause a warp then nothing will happen. Fiberglass that is almost perfectly balanced like the .72oz stuff above will probably not cause a wing to warp even if it is applied incorrectly, but it might. I first learned about this issue when I bagged a bunch of balsa HL tails with bias 3/4oz glass. All of those tails would come out of the bag with a slight twist and the twist would allways be in the same direction. A discussion about this issue in the SALglider group yaers ago finally woke me up to this question of fabric balance. I had been laying out all of the tail mylars on the layup bench with their trailing edges facing the same way. I then covered all of the tail mylars with a single piece of bias cut fabric. This meant that every tail piece ended up with the strong fibers on top perpendicular to the strong fibers on the bottom of the part. The parts were heated a bit during the cure. When they came out of the bag and cooled a bit, they twisted because the contracting forces along the diagonals of the part were unbalanced.
Apr 20, 2006, 07:20 AM
Registered User

I don't have all the answers


Maybe someone else can answer some questions for me and advance my knowledge a bit. Looking at the fiberglass data above: The .72oz fiberglass has the same thread count in both directions. Presumably the fibers are the same type in both directions. So why is the breaking strength different in the two directions?

The thing that matters for twisting forces and warping is actually fabric stiffness rather than fabric breaking strength. So why do they list breaking strength? How would find out the stiffness of the fabric as used in a layup? Is the breaking strength number a good approximation for what the stiffness of the fabric will be in a layup?

Why do they make fiberglass such that it is almost always unbalanced? Does fabric balance just not matter to the vast majority of fiberglass users?

Kevlar/aramid fabrics appear to always be balanced when they have the same fiber count in each direction. Fiberglass fabrics may not be balanced even when they have the same fiber count in both directions. Why the difference?

Gosh, the learning just never stops.
Apr 20, 2006, 08:21 AM
ASK
ASK
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Answers to some of Phil's Questions


Learning never stops, indeed. Thanks to Mark and Phil for all the contributions. A lot of us are playing with laminating fiberglass, some of us for the first time. I've learned a lot!

My personal background include a lot of exposure to fabrics when I was a kid, growing up in the family window coverings business. We had a drapery workroom, and I chipped in along with the rest of the family. One of the fabrics we worked a lot with was fiberglass, which had/has fireproofing capability of value in commercial buildings.

I'll have a go at some of the questions, but only to contribute a piece of the puzzle.

Quote:
Originally Posted by Phil Barnes
Maybe someone else can answer some questions for me and advance my knowledge a bit. Looking at the fiberglass data above: The .72oz fiberglass has the same thread count in both directions. Presumably the fibers are the same type in both directions. So why is the breaking strength different in the two directions?
The yarns used in fabrics are often different materials.

The yarn that is attached to the loom is called the warp, and the woof or weft is woven through it. Therefore warp is lengthwise to the roll, and woof goes across it.

One thread direction, typically the woof, will often have loose and fuzzy fibers twisted into the yarn. This helps fill the fabric, making it more opaque, and gives it a texture. This texture is part of why sellers of fiberglass cloth call each type a "style".

For lamination, the fuzzy fibers absorb more resin. Is this intentional? Dunno. I do know that fiberglass fabric has long had applications other than lamination. This probably accounts for some of what we see.

Quote:
Originally Posted by Phil Barnes
The thing that matters for twisting forces and warping is actually fabric stiffness rather than fabric breaking strength. So why do they list breaking strength? How would find out the stiffness of the fabric as used in a layup? Is the breaking strength number a good approximation for what the stiffness of the fabric will be in a layup?
Stiffness is a property of the fabric in resin, and resin differs. A manufacturer can only tell us what they know--breaking strength. Stiffness correlates to breaking strength, but I don't know if it is a linear thing that can be easily calculated. Variables in the yarn as well as resin would play here.

Quote:
Originally Posted by Phil Barnes
Why do they make fiberglass such that it is almost always unbalanced? Does fabric balance just not matter to the vast majority of fiberglass users?
I'm guessing that it has to do with filling the fabric, mostly for other applications, including laminations not for aircraft.

Quote:
Originally Posted by Phil Barnes
Kevlar/aramid fabrics appear to always be balanced when they have the same fiber count in each direction. Fiberglass fabrics may not be balanced even when they have the same fiber count in both directions. Why the difference?
More hypotheses here--people with direct knowledge please chip in!

Fiberglass was commercialized for many uses other than lamination, and at one time those uses probably exceeded lamination. This could have had a significant effect on the styles we see today.

The properties gained by making different strength and weight yarns seem to be the driver here. Yarn (thread) can be made in widely different manner, with the number of running fibers and fill fibers, the type of fiber in each, the method and type of twisting, all part of the design.

If you have unbalanced glass in the shop, take a close (magnified) look at the fabric. You might see a repeating or random pattern of loose fiber in one yarn direction, twisted into the yarn itself. In heavily textured fabrics we called these "tufts". I doubt that's a technical term, but it's what we called it in the workroom.

I stopped helping make draperies before I entered college, and the entire family left draperies sometime later for other businesses. Never thought I'd call that stuff up again.

Aradhana Singh


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