I didn't get any responses to my question on tailbooms. I can remove the Kevlar from the tailboom without costing appreciably in stiffness, and of course that will take weight out of the back - probably about a gram. One could then probably lose a couple grams up front with the same balance. The cost is that the tailboom would no longer be as stiff or strong in the hoop direction, so a little strength would be lost, mostly in the vertical direction. If it broke, it would shatter like most tailbooms rather than crack.

What do you think of the tradeoffs? Which would you rather have?

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I had a balloon fail on me tonight. Thankfully it failed before I walked away. But I had to pull the mold apart, reset the fabric, add back some epoxy, and do it over. I'm interested in seeing how this one comes out - whether the surface defects are larger and/or more numerous, and what happens to the weight. I'm running the pressure higher and longer with the mold not fully closed to evacuate more epoxy to compensate.

Hopefully this won't be a second, but it probably will be. Hopefully it will be at least a second, rather than a trashcan sample.

In the past with bladders I've been able to recover, by (1) ignoring it since the leaks tend to be quite small and self-seal as the epoxy gels (2) if bad enough, insert a new bladder inside the old one from the back, or pull the old one out the back and insert a new one.

Balloons fail worse than bladders when they fail.

Gerald

Best of luck with failed balloon. I've never had a useable part after a balloon failure (painted in the mold pylon planes).

I've had lots of good parts after balloon failures, but then again I've probably had more balloon failures than most people have pulled fuselages.

I'm glad this one happened while you were there. It's very frustrating to make scrap. I have about 15 downstairs in my "bones" pile that I just refuse to toss out but eventually I will. Bladders fail and stay put but just leak. Balloons fail and snap back, often pulling a layer of fabric with them depending on how sticky the layup is at that point. I've been successful at saving them with blown bladders / balloons as much as an hour after closure, which is about 1:45 after epoxy mix in my case. You have to work quickly, and the parting plane will be full of sticky epoxy pulling PVA everywhere, but you can save them. This is how I know that my Corian molds will release with Partall #2 wax alone. In a few cases, the PVA was completely torn off the parting plane while opening the mold to replace a blown bladder. No major sticking resulted thankfully.

Either way, good luck. I always sleep better when the compressor doesn't run, ever. I have a small tank on mine so if there is any significant leaking at all, it runs frequently enough to really annoy me.

I have managed a few times to pull the whole filler tube and dead balloon out and replace .
I draw it back out of the tail without opening the mould. Happened right after inflation so things werent tacky yet.

If the kevlar is producing the hoop strength, leave it in, even if there is a weight penalty. A broken boom carries a large fiscal penalty, sometimes

But I don't understand why kevlar is used for hoop strength. Please explain why it's used, rather than something else. I always thought kevlar had poor bending strength, with most of that coming from the epoxy in the matrix? I would have thought the optimum would be cf at 90.

Gary

Gary,

I've been one of the guys pushing kevlar use in the boom. It just makes sense. If all you were worried about was stiffness then you'd use HM carbon everywhere and call it a day... but we want them to survive light midairs, the car ride to the flying field, TSA, stairwells from the basement, etc. I just think kevlar makes a lot of sense back there.

If you've ever been involved in a midair where the airplane is almost cut in two because the boom was all carbon and glass, you'd know why I like that kevlar. It keeps the pieces in relative alignment while you land the airframe and repairs are simple. With carbon and glass you're generally looking at a complete rekit. My stuff is pretty tough, but Gerald's has been tougher in my opinion. He's got a bit more of an eye for perfection and less on mass production. I design stuff to be mass produced as quickly as possible, which means more compromise.

The layup does get a bit annoying when there are lots of pieces to put lots of places to get the structure you want. I can certainly see the draw of simplifying things!

The tailboom portion of the layup on this "bunker buster" fuselage is an asymmetric sandwich structure in this version (version 1A, which is first run, primary layup pattern). The outer layer is 1K carbon, oriented 0-90. Inside that is 100gsm HM Uni. These first layers are a sandwich which overlap at the seam on the sides, so they are doubled there. Inside that are strips of 100gsm HM uni that run down the sides of the boom full length, and extend up to the rear wing bolt. Inside that are narrower strips of 100gsm HM Uni that go on the sides of the boom in the more forward region, and extend much of the way to the front wing bolt. These are staggered relative to each other to graduate the stiffness falloff. Inside that is a layer of 1oz Kevlar oriented 0-90. This layer is not full width; the Kevlar on the top does not touch the Kevlar on the bottom, but both layers overlap the HM Uni side reinforcements.

This gives a structure which is quite a bit thicker at the sides than the top and bottom. It also gives a structure that has fiber direction oriented perpendicular to the boom on the outside of the layup and on the inside, turning the wall into a rigid structure. That is where some of the strength of this boom comes from. When you bend a tube, initially it is quite stiff. But as one increases the bend, the tube starts to thin at the highest load area. As it thins, it gets less stiff, and therefore concentrates the bend in that region. The process continues in this direction until the tube fails at that high stress region. If a tube strongly resists deformation, it can take quite a bit more load before failure, and maintains stiffness better through large deflections relative to a tube which is more squishy.

Kevlar also holds things together nicely when things go wrong, as Tom pointed out. But 1oz Kevlar, particularly in relatively narrow strips, pretty much requires wax paper backing for cutting and transfer or the short threads will get out of control. Kevlar going random directions in a fuselage is something to be strongly avoided!

Note though that this tailboom is stiffer and stronger in the horizontal direction than in the vertical direction.

I tried an alternative layup using 0.25oz Kevlar which handles easier (version 1B). At that time, I removed the fuselages by brute leverage from the tailboom until the threaded inserts released from the alignment pins in the mold. It takes enough force that pretty much any other fuselage would break... Well, this weaker version broke. The boom snapped cleanly off, though it did take a bit of force before failing. There wasn't enough Kevlar to prevent a total failure. So although it was perhaps a gram lighter and certainly easier, I use the more robust layup for the fuselages I'm delivering.

I also changed my method of removing fuselages!

Gerald

My outer and inner fabric is STYLE 104 glass 0.5oz.
Outside it helps to reduce pin holes and inside it insulates carbon
Yuri..

I figured that was what the kevlar was for, but don't you have to add either more resin or something, to get adequate hoop strength?

Hmm, Craig Roberson is molding some booms with kevlar in them, maybe I can get one to do some destructive testing

Gary

Why put woven material on the outside?

I did a batch of strength and stiffness tests on tubes to determine which fabric ordering resulted in the most performance for the money/labor. The sample groups that I tested were:
+/-45 woven outside, UD inside
UD outside, +/45 woven inside
0/90 woven outside, UD inside
UD outside, 0/90 woven inside

I tested the stiffness, strength, and impact in the middle of the sample while the sample underwent a bending load.

Putting the UD on the outside with a 0/90 on the inside performed the best in every one of my tests.

Perhaps we're just overbuilding things a bit.

Mainly because I have woven material on the outside and the inside. This forms a structure where the fibers are oriented to improve crush resistance. When fibers are oriented in the load carrying direction on only one side of a laminate, one is relying on the matrix properties on the other, and that becomes the limiting factor.

Additionally, the outer layer is not HM. The inner carbon uni is. A structure which has the higher modulus materials towards the inside distributes stresses better through the thickness of the material. The reverse concentrates stresses into the more brittle fibers at the surface.

Gerald

Originally Posted by tom43004

Perhaps we're just overbuilding things a bit.

+1
Yuri.

Tom, my tests were pulls from my fuselage mold with the oval boom.

Gerald, have you tested those theories? I'm not trying to be facetious which my questioning. I enjoy testing layups so I do. I've learned that you cannot have everything. Put bias material on the outside and you improve the torsional properties but reduce the longitudinal and hoop properties. Put the HM or UD materials on the outside and the longitudinal strength and stiffness goes up but the hoop properties decrease. Just like aerodynamics you can't have everything.

I was curious as to why you were making the decisions you did. I like to learn. I'm going to have to do some tests on the woven, UD, woven layup as well as putting HM inside verses outside. For me the question isn't about overbuilding but about the effects of the material location in the laminate stack.

Originally Posted by tom43004

Perhaps we're just overbuilding things a bit.

I think we are Tom but then again I prefer a light plane built to a minimum. I've had little issues with durability and most mid-airs are survivable (might have to switch planes in a round and repair).

Gerald, are you suggesting that you would remove the kevlar and not replace it with FG or some other material for hoop strength? That doesnt sound smart but maybe your layup is thick enough?

James