Jul 18, 2012, 10:37 AM
Registered User
Way back, Mark D said hoop strength is important to maintaining stiffness. It's somewhere in the "Stiffer Booms" thread, after a bunch of people were saying HS wasn't important.

I would have thought the kevlar should be in the middle of the cf layers, as it's going to be thicker than the cf and will move the inner and outer layers farther apart, which again should improve hoop strength. I've always visualized it as a sandwich. If laminating balsa with cf, the sequence is cf/balsa/cf. But I can't point to any numbers to support this.

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Jul 18, 2012, 10:56 AM
Registered User
Does everyone agree that hoop strength is really important to the stiffness of the boom?
This post by Mark D talks about some of that :

There is some other useful info as well as points of view in that thread.

This is my opinion, no science involved If kevlar is used in the layup, it will work better in the center of the layup, thus: cf/k/cf as a sandwich. This supports the kevlar on both sides, to keep it from buckling and cracking it's epoxy layer. Also, for a given weight, it's slightly thicker, so the cf is farther apart. It's similar to any kind of sandwich construction, cf/filler/cf, not cf/cf/filler, etc.

I know Adam will run tests.

Jul 18, 2012, 11:06 AM
Father of Fr3aK, DLG Pilot
tom43004's Avatar

I run kevlar on the inside. Part of this is to keep the frayable uni carbon between two layers of woven cloth. I understand that having your bending load carrying fibers further apart helps, but we're only talking about 8 or 10 thousandths of a difference.

For me it's about the available materials, the manufacturing process, and nothing more. Most folks agree that my stuff is probably overbuilt back there but it's not particularly heavy so I don't see a compelling reason to change. I've been rough on my airplanes and this is one area that I simply never worry about. Geralds fuse is the only one I've ever seen that I believe to be tougher than mine.
Jul 18, 2012, 11:19 AM
Aurora Builder
Very good debate here, waiting to see the results of testing

My take on Geralds original question is a 42-45g beefy fuselage is definitely worth building and flying. Not worrying about your aircraft structure is very important when in the middle of a contest in windy conditions.

I think there is something to be said for a nice 33-37g fuselage. It might not be as tough as what you are building now, but for gentler conditions with the right wing and tails it has a place. Lately it seems our contests have been in wind so an ultra light plane is not nearly as important but that all depends on the wing. A 42g fuselage that fits 4 servos in the pod might be preferable to a lighter, less rugged fuselage.
Jul 18, 2012, 11:48 AM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
I've built all the way down to 35g and don't trust them. There's only so far you can go while maintaining a suitably strong boom to withstand launch forces. Tails have advanced and create more force on launch. Launch technique has improved across the board.

Most of my fuses are between 40 and 42g with canopy and hard points. Going much lighter than that means removing the safety margin from the forward half of the fuse which leads to more nose weight and a comparable AUW with less structure. That's hardly progress.

Until we find a way to make the boom section of the one piece fuse weigh less than 12g consistently, I think 40g is a great target.
Jul 18, 2012, 12:24 PM
Registered User
I'm not sure what the current weights are but how are the XXLite fuse's down to the mid 20 gram region? They seem to be holding up in competition. I know they are designed for 4 servos in the pod but i am still curious because everything else I hear about is in the low 40's.

Jul 18, 2012, 01:22 PM
Registered User
The XXLite has an advantage that it is a shorter fuselage, swinging a lighter wing, and therefore the tail loads also do not need to be as high. So it doesn't need to be as beefy as a normal length fuselage for a normal aspect ratio wing. But I doubt your numbers are correct. It will be 2g or so just for a decent hatch.

The fuselage I provide is, if I recall correctly, about 1.5" longer in the back than I use it. But that of course would depend on the tails; it is just based on my Edge spec. Some people will want the length so I don't cut it off, but leave the fuselage full length as it comes from the mold. It is usable right to the rear. With that segment removed, the fuselage of course gets a little lighter and the CG may then be in front of the rear wing bolt instead of roughly at it.

As for Kevlar sandwiched between carbon layers, it is not something I'd recommend. The biggest reasons are the failure mode and the structural properties.

When one makes a structure carbon/Kevlar/carbon and subjects it to load until failure, a primary failure mode will be matrix failure at the carbon/Kevlar boundary followed by delamination. Kevlar boundaries are known to be weak compared to other fabrics. I've run across that info in various literature. I've actually tested this, when working on my second version Spring-V stabilator mount. When samples were broken that had an inner Kevlar layer, inter-layer delamination started the failure process, followed by buckling and shattering of a carbon layer. Without that inner Kevlar layer, the resultant part was rather notably stronger.

Structural properties...

The primary bending loads our fuselages take are lateral loads from the vertical tail. The wider the carbon is separated (that goes front-to-back) the greater benefit in stiffness one obtains.

Here are some not-so-accurate numbers. Assume the boom is 0.4" wide. Assume the Kevlar layer is 0.003" thick in a laminate. Assume for the sake of simplicity that we can misuse the I-beam stiffness comparison formula to get an idea of what happens as we move the layers around. Let's also assume that since the modulus of the Kevlar is quite a bit less than the modulus of the carbon, that we can neglect its contribution to bending stiffness. That isn't accurate of course, since the Kevlar will contribute something, but it will only be a small contribution.

Also for simplicity assume that all the carbon involved has the same modulus and each layer is 0.005" thick. Faulty assumptions of course, but it makes things simpler.

In my layup, the layers are carbon/carbon/carbon/Kevlar. The alternative layup is carbon/Kevlar/carbon/carbon. Rough order of magnitude, how do they compare?

For the current layup, we get separations of roughly 0.4" carbon, 0.4"-2x0.005" carbon, 0.4"-4x0.005"carbon, 0.4"-6x0.005" Kevlar. I was lazy and just used outside positions to keep things simpler. The difference really won't matter in the comparison.

Stiffness-wise, as mentioned before, neglect the Kevlar. So we have carbon layers separated out as 0.4", 0.39", 0.38".

For the alternative layup, we get separations of roughly 0.4" carbon, 0.4"-2x0.003" Kevlar, 0.4"-2x0.003"-2x0.005" carbon, 0.4"-2x0.003"-4x0.005" carbon. Again, neglecting the Kevlar, we have carbon layers separated out as 0.4", 0.384", 0.374".

So how do these compare?

For a universal beam structure, with thin caps, the stiffness is proportional to the square of the separation of the caps. So let's sum the squares and see what we get:

Original layup -> 0.4565
Alternative layup with Kevlar in an inner layer -> 0.447332

So, the alternative layup would be roughly 2% less stiff in bending.

Now, how about hoop stiffness?

Let's zoom into a small patch of the boom. We can now look at it as a sheet. The stiffer this sheet, in the direction perpendicular to the long axis of the boom, the stiffer the boom in the hoop direction and therefore the more it resists deformation.

In the current layup, there is 0-90 carbon on the outer surface. So half of that layer's carbon is correctly oriented to support this stiffness direction as a surface layer on that little patch of boom.

With the 1oz Kevlar 0-90 on the inner surface of the boom, then half of that layer's Kevlar is correctly oriented to support this stiffness direction as a surface layer on that little patch of boom.

Now Kevlar is far from ideal; it is weak under compression. It is pretty good under tension, comparable to S-glass. So how much it helps depends on whether that side of the little patch is under compression or tension at the moment. If under tension, the result will be stiffer than if under compression (tension -> bending towards the carbon; compression -> bending towards the Kevlar). But in any event the stiffness contribution under compression from the Kevlar side will be no worse than raw epoxy and should be better.

If instead a unicarbon laminate were on this inner side, it would be oriented along the axis of the boom and therefore be perpendicular to the hoop load direction. The fibers would contribute essentially nothing over the raw matrix (epoxy) of the same thickness. So it would fail sooner. Epoxy that can take greater deformation without structural damage, such as MGS, would do better than other epoxy choices in such a layup.

Jul 18, 2012, 03:34 PM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
Don't forget Gerald that in addition to having a higher aspect ratio wing on the XXLite, it also has shorter tails and considerably SMALLER tails as well. All of these things lead to less force generation by the tails on launch, which is our critical design point.

Stobels are probably the lightest fuselages out there that I'm aware of for full sized DLGs. They're commonly between 35 and 38 grams from what I've seen. They're also somewhat fragile in the grand scheme of things, partially because of the lack of glass or kevlar up front, and also because of the size. Making a fuselage 2.4 friendly has a weight penalty in the fabric transition and choice. Making a fuselage all carbon up front tends to make it lighter and stiff but carbon likes to shatter.
Jul 18, 2012, 05:10 PM
Registered User
Interesting discussion. Just a quick correction: The XXlite uses the Helios vertical which is quite big. The horizontal is smaller, though.

Jul 18, 2012, 05:20 PM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
Ah. Thanks for the correction Jan. I knew that you were modifying the horizontal for the XXLite... thought that you were modifying the vertical as well.

Out of curiosity, what is your fuse weight typically?
Jul 18, 2012, 05:23 PM
Registered User
No idea about weights, Tom. I don't make the XXlite, I just happened to know about the vertical size. Krikkens is the builder.
Jul 18, 2012, 05:45 PM
Registered User

I need to go back and re-read your post a couple of times to make certain I understand it.

Yup, misunderstood some things.

On weight issues: To me, the weight of the pod is not as critical as the weight of the boom. After all, most of us are carrying nose weight. So the if the pod needs weight, no big deal.
If hatches are to save weight, they do so at the expense of access to equipment, and wouldn't a good nose cone be stronger than a hatch anyhow? Most damage on my fuse's, other than lawn darting them, is damage to the nose cone in the form of stress to the kevlar or sometimes damage to cf reinforcements.

Just opinions.

Last edited by GaryO; Jul 19, 2012 at 10:50 PM.
Jul 18, 2012, 05:57 PM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
Originally Posted by Jan_B View Post
No idea about weights, Tom. I don't make the XXlite, I just happened to know about the vertical size. Krikkens is the builder.
Oh good grief. I didn't even look at your screen name and assumed you were Kristof. Sorry for the mistake (again)

Can you tell I've been staring at the computer for 12 hours straight?
Jul 18, 2012, 05:57 PM
Jim C
ShadowFalken's Avatar
Gary, 0-90 means fabric where one run of fiber is down the boom line and one is in hoop around the diameter of the boom. Gerald uses both fabric and unidirectional carbon in his schedule. The hoop strength is ..... Notable.

I thought 0-90 and bias were the common terms for that. 0-90=down the boom and around the boom. Bias is at angles off perpendicular.
Jul 18, 2012, 06:06 PM
Just fly it!
wyowindworks's Avatar
Originally Posted by ShadowFalken View Post
I thought 0-90 and bias were the common terms for that. 0-90=down the boom and around the boom. Bias is at angles off perpendicular.
If you want to get really persnickey, bias would refer to the way that the fabric is cut from the roll while +/-45 would refer to the orientation to some stated reference line.

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