Carbon Fiber Fabric Body

Hi all,

I'm looking at making a hexacopter drone body out of carbon fiber fabric, and am wondering if anyone has any expertise in this area and wouldn't mind helping me out a little. In your opinion, what is the best weave and weight for a very complex body? Also, where can I get it the cheapest, as I will be using vacuum resin infusion and have to use multiple layers?

Thanks to any and all who reply,

Ryan

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  • I have some experience with CF lay-up and component design. This is only because I've done a little of it for drone-building. I would not say that I have expertise.

    My project was several years ago. I was designing props for heavy-lift multi-copters as there were no efficient static-thrust props available at the time.

    What I learned:

    Choice of weave depends on the contours you need to conform to, the desired strength and rigidity of the part, the allowable weight, and aesthetics. Remember that the strength of all fiber reinforcing fabrics is entirely due to the fiber's tensile strength. Carbon has terrible sheer strength, and even worse abrasion-resistance. It's important to keep this in mind and make sure that you have the best fabrics in the correct orientation to get the working properties that you need. For most of the parts that would be found in a typical MAV, a 3K plain weave should be a good compromise. It is the most common, versatile, and most economical weave. The fiber bundles are evenly distributed on perpendicular axes, giving good strength in both. 3K twill weave will give about the same performance, with a better appearance. It also costs more. 1K plain weave will conform better to small and complex forms. 10K weaves will be much stronger, with weight and aesthetic penalties. You would want to use this as a "utility" fabric and use it as an internal layer, keeping a finer and more attractive weave on the outside layer.

    There are many weaves available. They vary in strength, thickness, weight, and appearance. Some even blend in other fibers. A common blend is CF/Kevlar. This is normally in a twill. The advantages are increased toughness, abrasion-resistance, and bold appearance. The trade-offs are a little loss of rigidity and increase in weight.

    The real beauty of CF is how little you can use to get the desired characteristics. For instance, if you were looking to keep weight to an absolute minimum, you could get by with only 2-3 layers of 3K fabric (maybe .75mm) in many cases. Light-weight laminating materials can also be used to keep weight low while keeping rigidity very high. This method uses a hard or soft core material, say balsa, and puts a layer or two of CF fabric on each side. This structure is very light and extremely rigid. In tubular structures you can often reduce weight and/or gain strength by increasing the diameter and decreasing wall thickness, to a point, of course. In the end, the number of layers should be determined by the strength and weight requirements.

    The choice of resin is just as important as the fabric choice. To take advantage of the CF's rigidity, you might want a very hard-curing resin. The limiting factor here is brittleness. For impact-resistance, you might want something with a small amount of elasticity to it. I settled on isophthalic polyester resin as the best compromise. It's also in the middle of the road in price. Some of the fancier resins get really expensive. Another factor in resign choice is curing requirements. Some require curing schedules that are difficult to replicate outside of a dedicated manufacturing environment. Infusion method also has an impact on resin choice, as the resin needs to have the right viscosity and sufficient pot life to fully saturate the fabric and allow draw-off of the excess.

    You'll also want to do your research on lay-up methods and techniques. It's really important for weight and strength to get the right fiber orientation and the right distribution of the right volume of resin in the fabric. Not enough or too much will both yield undesirable results. If you're using vacuum-infusion, be absolutely sure that your mold is sealed and will hold a vacuum for a while. Sucking air can waste a lot of time and money. 

    And if you're doing this by hand, as I did...

    Get a Tyvec suit and nitrile gloves. Carbon fiber bits are far more annoying than fiberglass. A respirator is a must. If you use your household scissors to cut the fabric, plan on tossing them out afterward. I've never seen anything dull scissors faster. And the best hint of all? Spray the fabric with with a "workable fixative" or spray-adhesive to gently bond the fibers before cutting. Otherwise the cut pieces start to fall apart pretty quickly. There are application-specific adhesives for this, that double as a bond between fabric layers. This keeps your fabric in place in the mold prior to infusion. Do a test to make sure that the bonding agent does not interfere with resin curing.

    I can't speak to price. I am fortunate enough to have a retailer that will sell the small quantities that I needed. I paid a premium for it too. :-) 

    A little verbose, but I hope that helps.

    • Chris,

      Thanks for the advice, it sounds like you have a far better understanding of the field than I do. I really appreciate your response, and it helps clear up most of my questions, but I do have one more: Have you ever measured the thermal expansion of a cured work piece? I'm thinking of making a band that I would heat, and then fit onto a motor housing, and then let cool/shrink onto the housing to produce a screw-less joint between the motor and the arm. Do you think such a thing is possible?

      Anyways, thanks again,

      Ryan

      • Hi Ryan,

        I ask a lot of basic questions on various forums and consume a fair amount of other's time. It was nice to be able to contribute.

        In answer to your question, I don't think that a synthetic composite material will respond the same way to temperature as metals do. In any case, epoxies, polyesters, and vinyl esters have finite working temperature ranges. The upper threshold is usually pretty low, and if exceeded, the resin will start to break down pretty quickly.

        It's a pretty intriguing idea for saving weight. How would you replace the motor bearings?

        If it were my design, I would most likely use aluminum, titanium, or nylon screws. Still screws, but a little lighter than stainless.

        Cheers,

        Chris

        • Chris,

          I just googled the coefficient of thermal expansion of just the fiber itself, and unfortunately it looks like it is basically zero( a cte of  <2, compared to aluminum's 13). So i've switched gears a little bit and am probably just going to make an aluminum joint, i think I can trim the weight down to about 20 grams per mount. By the way, do you have any idea how much vibration a motor will transmit down a carbon fiber tube that's a little over a foot long? I'm trying to decide how much vibration isolation I will need for my electronics.

          Thanks,

          Ryan

          • Ryan,

            Yeah I think you'd run into issues with press-fit. The resin will also have a very different cte than the fiber.

            I have no way to quantify how much a carbon tube will transmit vibration. I think any transmission and resonance characteristics would be unique to your design. The only way I know of to get that information prior to building a proto is 3D modeling and simulation.

            My approach would be to pay a great deal of attention to prop/motor balancing, and then worry about isolation if you actually have a problem. Balancing may add little to no weight, where vibration isolation or damping will invariably add complexity, and probably weight. While I have little experience with vibration problems, I hear that some people that have trouble balancing motors can correct the problem by replacing the bearings with better quality parts. Same for props. I think ultimately, if you need ultra-low vibe, your best bet is to start with quality stuff.

            I was thinking about your design challenge yesterday. I don't know what your airframe will look like, nor what motors you plan to use. However if you use the typical brushless outrunner with a mounting bracket on the base, you could build threaded inserts into your composite parts. I've seen metal inserts that are purpose-built to go into glass fiber an carbon fiber composites. If you can get them positioned precisely enough in your mold, these could be in an inexpensive and light weight motor-attach method. 

            Cheers,

            Chris

            • Chris,

              Since I'm trying to cut down on weight, I've decided to make a frame composed of a "+" of 4 carbon fiber tubes with a metal connecter in the middle, and then a shaft that comes down through the center as a mast of sorts. I'm going to use piano wire or some braided fishing line to put a significant amount of tension from four direction on each motor mount. However, I'm still going to make a carbon fiber shroud of sorts for the electronics in the center, but at this point it's not going to be structural. Thanks for your suggestion in regards to the motor mounts, I think that's probably the direction I'm going to go. E.g. the bracket will be pressed, and then probably glued in to the end of the 1/4 inch tube.

              Thanks,

              Ryan

              • By the way, I think that after this project I may make a completely carbon fiber flying wing (somewhat reminiscent of a german Horten HO 229 Gotha), so I was wondering if you've ever experienced interference with your electronics from the cf, since I've read conflicting reports. It'd be nice to only have my antennas sticking out, not my gps, etc.

                Thanks,

                Ryan

              • I always find it amazing how much a design can change after it is thoroughly thought through:)

  • Just to clarify, right now I'm thinking of a thickness of about 1-1.3 mm, so about 3 or 4 sheets of this: 

    5.7 oz Carbon Fabric: 50" Wide 2x2 Twill

    https://www.acpsales.com/OnlineStore.php?cat=4981

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