Composite Tube Fabrication Methods

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mstull

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Billski,

That all sounds good. You could just slide an aluminum or steel tube inside the paper tube to keep it round. It can be wrapped in several places with masking tape to get the perfect sliding fit. Do you really care if the finished part is perfectly round?

Also, I wouldn't bother with vaccuum bagging at all. With this type of layup, you can wrap your cloth quite tightly as you brush on the epoxy. So you can get a pretty darn lean layup. You'd only save a few grams by vaccuum bagging.

You could even just spiral wrap your layup with the perforated plastic. If you wrap it tightly, it will have the same effect as the vaccuum.
 

wsimpso1

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Mark,

Well, roundness is pretty important because these are control push-pull tubes, and the failure mode in compression is column collapse, so roundness defects matter a fair bit. It is also a lot easier to shove foam ribs into it and glue them in if the tube is round.

I could just use aluminum if I were not worried about all of that metal messing with my internal antennas - an advantage of fiberglass structures is that you can use internal antennas. Thus, fiberglass (not carbon) tubes. I know, properly applied, aluminum tubes won't mess much with the antennas, but I figured that I would rather be ahead of that curve than behind it.

Billski
 

mstull

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I meant to just put the aluminum tube inside the paper tube temporarily until your layup cures. You must be planning to make the fiberglass tubes really thin walled, to be worried about collapse. I'm not sure it's a legitimate concern though. I've made dozens of fiberglass tubes, and they are very resistant to collapse... even if you bend them over your knee. They'll usually crumble on the compression side, before they collapse. And any little out of roundness won't weaken them at all.

I don't think almuminum tubes would interfere with radio communications at all either. People use hand held radios inside metal cockpits all the time. I'm always amazed seeing people talk on their cell phones in Wal Mart... nearly sealed in an all metal building. Enough signal must come in through the glass doors.
 

Bob Kelly

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wsimso1 :
Sounds to me Your following in the foot steps of a fella that wished to heck he hadn't made a STELTH AIRCRAFT ! the radars could not pick him up ! at all ...
he finally resorted in adding a sort of wire along the leading edge and trailing edge ( Magnetic tape i think ) just so other craft could see him
... this was a while back in a Vari-ease
of Burt Rutton design... thats about all I remember of it ... but you might consider the idea ...too see through for radio waves may not be a good thing !

Just a thought !

and Mark ! Good pic of you standing on that tube ! IMPRESSIVE !

Your still the only guy I know that flys arround ALOT on 25hp ! simply amazeing !
Good work !

Bob........
 

wsimpso1

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Two points on the current state of radar:

ATC radar is almost exclusively based upon Mode C Transponder response to interrogation and they run at pretty low power just to prevent cluttering up the returns with hits off of buildings, radio towers, road and construction vehicles, etc. Some radar facilities still have the capability of turning up their power in order to get a skin paint, but their numbers are decreasing;

If the FAA or DoD wants to get a skin paint off of my airplane, I will still have an engine and oil cooler (and probably glychol cooler) and propeller and fuel lines and wiring for lights and rudder cables all still serving as radar reflectors. The airplane will be plenty visible to radar.

The VariEZ story has been around... I think that VariEZ plans became available before Transponders and Mode C became widespread and the VariEZ is a very small airplane. The VariEZ was also intended to be a hand prop start/minimal electrical system aircraft, so its owners were probably not carrying transponders if they did not have to. Maybe 30 years ago ATC had trouble seeing plastic airplanes, but with a transponder in the bird, no problem.

Tube collapse - I was not referring to wall collapse, but full length elastic buckling from compression, which pays attention to EI, the section stiffness in bending and to the length of the column. This is taught in Mechanical Engineering Design textbooks (Shigley, Shigley and Mitchell, and I think Timoshencko and Gere), and is even the topic of government issued design bulletins. Yeah, there are government issued charts showing the allowable load vs length for all of the standard sized 4130 steel and aluminum alloy tubes for use in design of welded tube structures and control systems. With short tubes, the ultimate load is the yield strength of the metal times the tube cross sectional area, and the allowable laod is the ultimate load divided by 1.5. At some length, the allowable load starts dropping. There are factors for both ends fixed, one end fixed/one end free, and both ends free. For controls, both ends are free and that gives the lowest allowables. Euler worked out one rule, and a guy named J. B. Johnson worked out some more, and then there is the further refinement of Engesser, blah, blah, blah.

For local buckling resistence, simple foam sections glued in at intervals is insurance, although we use thinner aluminum tubes in the same places without any local reinforcement against buckling. Yes I will be testing one of my tubes to see if it is as strong as I want it to be.

I have been making vacuum bagged samples with cloth table rolled onto flourescent tube and paper tube, and they are nice. Full scale tubes using braided socks will untimately be made. Details a little later...

Billski
 
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orion

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You might have mis-typed in one point: With short tubes the ultimate load is the yield strength of the metal times the tube's cross-sectional area ....
 

Bob Kelly

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Msimpso1 :
thanks fer setting me stright on that transponder thing.... yah it was a long time ago .... heck I was a KID ! hehehehhe
anyway .. good info thanks !
keep at it , you'll figger it out !
it don't even sound like its got a fighting chance of defeetin' ya !

haing in there !

Bob......

how about a pic of your bird so far ????
 

AeroER

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Market made composite tubes using aluminum tubes for the mandrel, then blowing hot air through the tube to expand the aluminum (and aid the cure). The temperature needs to be held until the epoxy is cured, then when everything works, the mandrel can be pulled out.

I don't know whether Marske put the tubes under tension to keep them straight, I don't recall that he mentioned that detail. I would.

Washing aluminum out with acid ought to be reserved for special applications. Such as a manpowered aircraft.

I like the idea of sacrificial fluorescent light tubes. I'd add a good coat of release.
 

terke

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Billski;

I made carbon fibre tubes, by slitting PVC tube. Opened the slit using two layers of tongue depressors every 6-8"(two layers was the thickness of the blade I used to cut the pipe). The tube was then wrapped in packing tape. You want the tongue depressors to stick a long way in to the pipe (at least an inch), that's important. Layup your composite in whatever is your favourite way and let cure (I wrapped with a long strip of peel-ply for compression and because I needed to bond the tube to something else later). Once cured use a stick or anything else long enough to knock the tongue depressors out of the gap. Once they are all out, the pipe slides right out. Tested method.
 

AeroER

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I found your 2009 thread here -



I'm working my way through each page in reverse. Still a relevant topic.
 

wsimpso1

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I found your 2009 thread here -



I'm working my way through each page in reverse. Still a relevant topic.
I am glad you are finding them useful. The base thread here is 16 years old, the other one is 12 years old. Looking around my shop, I can tell you that the knowledge is just as good now as it was when we were busily putting those posts on.
 

Victor Bravo

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I wasn't on this forum when the original discussion started, but the solution to the manufacture of composite push-pull tubes looks a lot simpler to me than most of the stuff I have seen written.

Get a thick wall metal tube or pipe. Thick enough wall that it will hold its shape under vacuum. Put two thicknesses of clear plastic "painter's cloth" or "painter's plastic" on the tube. Roll the composite laminations around all of this in whatever fiver orientation you want. Vacuum it, heat cure it, or whatever you want to do. If you need to hang it vertically to make it perfectly straight, then hang it from a tree or lean it against your house... whatever.

When the thing is cured, pull the mandrel out, it would be easy because you have two layers of plastic that will slide easily on each other. If the last layer of plastic wants to spend eternity with the inside of the composite tube, let it... it weighs nearly nothing.
 

Hephaestus

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While playing with the J3/J6ish project talked to a couple suppliers of composite tubes... Wow... Just wow. Tailboom was going to be more expensive than the motor.

Ended up finding a xwinder 4axis - used. This seemed a much better investment, and customizing.

Should be able to make the tailboom for a much more reasonable price. Plus wind tubes for controls too.
 

Victor Bravo

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Ended up finding a xwinder 4axis - used. This seemed a much better investment, and customizing.

Should be able to make the tailboom for a much more reasonable price. Plus wind tubes for controls too.
Congratulations on being elected (with a unanimous vote!) to your new official position as the worldwide HBA go-to guy for composite tubes... :)

I have an idea that may be well worth you looking into.

Instead of winding a typical round tube for a light aircraft wing spar, you could wind a D-shaped tube or half-tube. Put lengthwise fibers (or pulltrusions) at the upper and lower "corners". Then glue two of the finished D-shaped sections together, to create a round spar with a shear web in the middle. You would have a very torsionally efficient spar that doesn't suffer from the typical inefficiency (buckling) of all other tube spars.

With this kind of arrangement, you can make some very interesting parts for very light, UL, and LSA aircraft that result in higher structural efficiency, lower weight, greater torsional rigidity, etc. And be able to offer this at a reasonable cost.
 

Hephaestus

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Congratulations on being elected (with a unanimous vote!) to your new official position as the worldwide HBA go-to guy for composite tubes... :)
ROFL - wait until I manage to make something useful first.

Finding the 4axis was pretty cool bonus though, has me contemplating whether I could do flaps/ailerons so they're almost a black aluminum version (think modern tailwind builds).

For those who haven't a clue:
 
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rv7charlie

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So, during 15 years of effort to minimize RF interference by using Fglass instead of aluminum for pushrods, did anyone consult an expert in RF propagation to see whether the interference issue is actually an issue at aviation-related frequencies?

;-)
 

wsimpso1

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Jim Wier at RST has been doing buried antennas for over 40 years, and he gave us practical rules for antennas and metal that interferes with them.
So, during 15 years of effort to minimize RF interference by using Fglass instead of aluminum for pushrods, did anyone consult an expert in RF propagation to see whether the interference issue is actually an issue at aviation-related frequencies?
Yes. Jim Wier at RST Engineering started doing buried antennas more than 40 years ago, and came by a set of rules that work really well.


In various places in his text, Jim tells us:
  • Metal airplanes includes "steel tube fuselages, airplanes with wooden wings with metal flying wires, aileron cables, and other large ironmongery";
  • "If there is a piece of metal more than an eighth of a wavelength long within a quarter wavelength of the 'plastic plane' antenna, the antenna performance will be degraded";
  • 12 inches of 18 AWG wire is as bad as a 12" square of aluminum;
  • Carbon fiber structures and humans are also conductors that detune antennas;
  • Note that quarter wave dipole antenna routinely straddle our metal vertical tails and single element quarter wave antenna mount on metal surfaces or with a ground plane. Metal in the middle of a dipole antenna has almost no effect at all, and the ground plane is necessary to single element quarter wave antenna. So, he modifies the second comment above to mean within a quarter wave of the tips of the antenna, with suggestions that placing the tips of dipoles is important and bending the antenna around to achieve that changes performance little;
  • Part of the designs are ferrite bead baluns;
  • He also discusses running any airplane you can fly to Grass Valley California over his range and plot out your antenna performance;
  • Antennas buried inside structures with carbon fiber completely "fraps" these antennas.
Sounds like he understands the topic with both theory and experience. He also confesses that there have been some notably inappropriate designs that still work fairly well.

My aileron and elevator pushrods and my rudder cables are way over 1/8 wavelength (ca 12") and well within 1/4 wavelength (24") of my antenna tips. If these control elements were continuous, Jim tells us that we will have degraded performance... I did not want to build my bird with metal tubes and then find out in flight that my scheme was unacceptable - I did some simple and entertaining experiments in fiberglass fabrication that I have shared.

Now I will confess to knowing a PhD electrical engineer who's thesis and professional life since has been antennas, and he has designed stuff that flies in space. I shall reach out to him sometime on the topic of how important my fiberglass tubes are to allowing my VHF dipoles to work well vs aluminum tubes. The time to do this will not be tomorrow, nor maybe even yet this year, but will occur when I can avoid looking like a bonehead looking for free medical advice...

Billski
 
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rv7charlie

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"If there is a piece of metal more than an eighth of a wavelength long within a quarter wavelength of the 'plastic plane' antenna, the antenna performance will be degraded";
To pick one item for brevity, the above is quite certainly true. But 'degraded' is not quantified. People have been burying quarter-wave comm antennas inside steel tube fuselages for much longer than Jim (who's work I respect) has been writing about them, and while the antenna characteristics are in fact 'degraded', the degradation in virtually all cases does not rise to the level of actually mattering to the operational performance of the antenna. Measurable on an antenna range? Certainly (well, likely). Detectable by the humans on either end of the communications operation to/from that aircraft? Extremely unlikely.

Another example:
This:
echouat tip1.JPG
is one of the recommended installation methods for RVs & other metal a/c for the uAvionix EchoUAT ADSB out/in transceiver. The flanges to the right in the pic attach to the outermost rib in the wing, with the assembly inside the fiberglass wingtip. The wifi signal from the ADSB unit is completely 'shadowed' by the wing rib. The transponder 'spike' antenna (connector seen center top and the ball on the end seen below center) is above the bottom level of the wing. Yet this installation method is flying successfully on multiple RVs, and was recommended to me by the uAvionix engineer (the pic is one he sent me).

I suppose a corollary to this 'degraded' question would be: Does the intake system and the typical exhaust system on a Lycoming engine degrade its performance (vs optimum)? I think we all know the answer, yet we seem to fly the engine more or less successfully.

Compromises.....
 
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wktaylor

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I presume You are looking for high strength precision structural tubes for push-pull and/or torque-tube or driveshaft applications? no particular order/endorsement... simply what is available, already...

Phenolic Compostie Epoxy Sheets, Plates, Rods and Tubes
DragonPlate | Engineered Carbon Fiber Composite Sheets, Tubes and Structural Components | Made in USA
Advanced Tubing - Polygon Composites Technology
CompoTech - Carbon Composite Components Desgin and Manufacturing

and for extra giggles...

NOTE. In my relatively 'young' mid-west neighborhood [1995-and-on] ALL of the lighting poles are made of wound-fiberglass + resin... and have stood-up against howling straight-line winds [+90MPH] and -25F up-to +115F ambient temps... barely showing signs of age... except for one that was run-over by a teenager. They are designed to be tough/durable, but to fail-safely in shear and local crushing on impact... not like a wooden telephone pole that can be deadly in a direct-on crash.
 
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