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Carbon Fiber Skin vs Fabric Skin

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  • Carbon Fiber Skin vs Fabric Skin

    Full disclosure I'M NEW at all this, have some background in building vehicles and such and hold my PPL. Fell in love with the BearHawk and am currently in the learning and research phase of all this. No I'm not made of money but came across this while looking into stuff for my other non pilot life. feel free to scold me anytime I spout nonsense! I went down this rabbit hole purely out of curiosity (seen a squirrel and chased it).

    I was curious (minus cost prohibition) if anyone has tried to replace a traditional fabric (like polly fiber fabric) with a carbon fiber composite? I did some comparisons of very thin 1mm Carbon Fiber (CF) sheeting to fabric in the area of durability, toughness, pressures and so on. I am NOT suggesting the use of CF for anything structural, just skins. I know there has been some concerns out there on blogs and such about when CF fails its catastrophic. I talked with my aunt who is a composites engineer (not in aviation) about some of those concerns. Her thought was the CF sheeting could be (in theory) safely used as skins, even on control surfaces because the loads are so low. as for concern about failures, anything could happen but the typical rock pelting from prop-wash on the horizontal stabilizer shouldn't cause catastrophic failure and would likely be highly resistant to that kind of use in a backcountry setting. I found some sheets pretty close to 4'x8'x1mm for pretty reasonable (as far as CF prices go) for about $380 a sheet. I have no idea how much would be needed to skin the Bearhawk as a direct replacement for fabric.

    I called a local manufacture (in the USA) for some Q&A about CF sheets and found the following:


    Q: What is the weight, strength of the 1mm CF sheeting?

    A: 3.1 oz per square yard. 1mm" thickness, Strength 33,000 psi

    Q: What is the cost of a "sheet"

    A: Sheet price $795 (4’ x 8’ sheet Gloss finish) $755 (4'x8" sheet Matt finish) (prices as of 12/2020) for the USA Manufacture found an overseas for $380 a sheet same size.

    Q: What type of CF are the sheets?

    A: 0 / 90 degree Twill, two layers of CF with epoxy. Sheets are made via the pre-preg CF material and compressed and heated to prescribed temperatures for curing (for good quality control and strength)

    Q: What would a sheet failure look like for the material? (I explained the sheets would be used in theory for skin of an aircraft, not required to be structural).

    A: This was a bit of a longer conversation back and forth and of course they wouldn't commit to anything officially but they have been creating CF stuff for a long time. uses have been in motor sports and off-roading applications. It boils down to this. if the CF is being used as a skin (even control surfaces skins) and not relied upon for structural support. there's no reason the sheeting wouldn't be sufficient. Even if used on a bush plane where there would be potential for rock chips and debris from prop wash, pelting the CF; it wouldn't likely cause the catastrophic failures that scares everyone about CF, simply because there's not a significant enough "load" on the panels. so even the "mega rock chip" on say the bottom side of the horizontal stabilizer that is experiencing some "loads" associated with airflow, it wouldn't be enough to cause the explosive failures you see in structural CF testing. It would most likely cause a pitting or "tear" in the CF but not be catastrophic to the surrounding portions of the panel.

    Q: How flexible are the sheets to work with?

    A: as a sheet goes its pretty flexible and as thin as that sheet is would likely be shipped rolled up.

    Q: How well do the panels hold up to UV light?

    A: Very well, over time you may notice some yellowing of the epoxy resin on the surface but it can be cleaned away with denatured alcohol. they haven't noticed any other property changes. from UV exposure. Additionally you can spray clear coat UV protection over it if you are overly concerned. it can also be painted with auto/aviation paints as well.

    So how does this compare to a heavy fabric on aircraft? To be clear its defiantly not cheaper. but from a physics perspective (my very limited understanding). Aircraft Spruce is listing the following specifications for Poly-Fiber heavy fabric.
    • Weight – 3.4 oz/sq yd
    • Breaking Strength – Avg 113.5 lbs/in
    • Bursting Strength – 279 psi
    • Tear Strength – Warp 108 lbs, Fill 119 lbs

    I'm still interested in what gotcha's might be out there by folks with some experience (obviously cost is the biggest gotcha). Any thoughts?

    Below are the links for reference.

    Dragon Plate CF manufacture (USA) https://dragonplate.com

    Aircraft Spruce's heavy fabric listing https://www.aircraftspruce.com/catal...fiberheavy.php

  • #2
    • How do you attach it to the airframe?
    • How would you handle the compound curves around the tail and vertical fin?
    • How would you handle the joint attachments or sheet overlap? Traditional fabric since shrinkable you can fit around compound curves where fitting a semi stiff sheet may be hard to make it conform.

    I think its a cool idea in theory but... I'm afraid it will look like Frankenstein's face with all of the skin sections lapped or fastened together.

    Chuck

    Comment


    • #3
      Have you seen Mike Patey's Scrappy build on YouTube? He is doing something similar with a cub. He lays up the carbon on the airframe rather than using sheets to fit the curves. You should check it out.
      I'll admit I have tossed the idea around in my head as well, and it is a fun idea to toy with. However, there are a lot of hurdles one would have to clear. I for one absolutely think it's doable, but I also think build time would go way up and weight a little up. It would also probably necessitate the use of a 540 to counter the extra weight in back.
      4 Place Bravo
      #1624
      Currently making form blocks

      Comment


      • #4
        I went down the same train of thought a couple of years ago. It would not be that difficult to do, there are several ways to do it but I really don't like using a male mold for CF. If it was female molded, no problem but I am not going to go to the effort to make a female mold for a one off.

        For a male mold, you could fill in the space between the stringers with foam, and use that as a mold. Easier would be to cover the fuselage with heat shrink plastic or Dacron and use that as the mold. I did that on the base of the tail and made a CF tail fairing. Very easy.

        If someone had a female mold and sold the parts i might have bought them. In the end,dacron fabric is very tough stuff. I used Oratex which is pre-finished Dacron similar to something called Coverite on model airplanes. I haven't looked back on covering with CF. I don't really see a benefit other than maybe cool factor and you could remove it to work on the back of the airplane.

        Comment


        • #5
          Originally posted by chuckkemp View Post
          • How do you attach it to the airframe?
          • How would you handle the compound curves around the tail and vertical fin?
          • How would you handle the joint attachments or sheet overlap? Traditional fabric since shrinkable you can fit around compound curves where fitting a semi stiff sheet may be hard to make it conform.

          I think its a cool idea in theory but... I'm afraid it will look like Frankenstein's face with all of the skin sections lapped or fastened together.

          Chuck
          All great questions and I had some ideas on it, similar to what svyolo posted last.

          As for the attaching to the airframe I was simply thinking of brazing some sheetmetal tabs or of the likes along the frame to create a panel situation for easy removal and such. I did see the Scrappy build and thought it looked very nice! its kind of what inspired the idea in some respects. But I likely would paint sections for effect. as for corners and joints I would do a CF wetting epoxy around the panels to "form it". as for tail feathers and stabilizers. My thought would be to use PVC pipe to essentially use for molding the edges to the precut flat sheets that way I could avoid all the foam cutting in-between the ribs.... that way the only real work comes in to edge work for the whole project since the panels are large enough to cut to size and the 1mm sheets are flexible enough to lay over the ribs.

          I really appreciate you guys responding. I frankly was expecting a harsher "your either crazy, rich and unemployed, or all of that" response.

          Comment


          • #6
            From my experience working with formula cars, some of which were carbon fiber construction, I would not consider using carbon fiber or any similar material in place of the traditional fabric covering.

            Reason 1: is that the tube fuselage is quite elastic relative to the carbon fiber. In my tube frame race cars, keeping ridged panels, such as carbon fiber and even aluminum attached to a tube frame is an on going maintenance problem. The attachment points tend to wear and fret and need attention.

            Reason 2: in an accident or even when there is minor damage, the damaged carbon fiber is really hazardous to handle. It is like reaching into a box of razor blades. One solution to this issue is to use the carbon Kevlar weave fabric.

            Comment


            • alan317
              alan317 commented
              Editing a comment
              Thanks for the info! I was hoping someone with first hand experience would chime in on it.

          • #7
            I went down this same exact line of thinking as well. I ended up abandoning it but do plan to use composites for the floors and doors and maybe the seats as well.

            Comment


            • svyolo
              svyolo commented
              Editing a comment
              I did interior panels, 1 layer of Kevlar on the inside, one layer of CF facing outboard. I am still planning on floorboards but my first attempt didn't work out and the kit supplied parts can be replaced later. I was planning on CF doors. I will probably still do CF cargo doors, but the front doors I think I will use 3/4 in square aluminum tube. I get most of the weight savings of CF but much easier and cheaper to fabricate. I also used a single layer of kevlar as a rear cargo bulkhead.

              Doors and floorboards are easy to replace somewhere down the road.

              I also made a CF fairing for the base of the vertical fin. So far so good but isn't flying yet. I liked the idea.

          • #8
            Composites are great. But you don't wrap steel in CF. Design a new air frame that takes advantage of the properties of the composite. Scrappy will be this one off one trick semi-worthless pony. And when that's your design goal, which it is in that case, sure, but most people that build an airplane aren't going to have 10-15 other airplanes to choose from for their mission that day.

            You want a heavy bearhawk, replace the fabric with CF. And then you'll have a very non durable worthless aircraft with incredible galvanic corrosion problems and probably a pronounced cg issue due to where you added all that weight. You're not going to be able to just screw sheets onto tabs.

            The guys over at dark aero are doing it right. Clean sheet.
            Last edited by zkelley2; 12-15-2020, 12:32 AM.

            Comment


            • Battson
              Battson commented
              Editing a comment
              My thoughts exactly.

          • #9
            I can't imagine carbon being able to take the hits like fabric does. It's a fairly brittle substance.

            Comment


            • chuckkemp
              chuckkemp commented
              Editing a comment
              As several mfr's are offering CF engine cowlings, wing tips etc. there proves to be acceptance and usability of CF. The MD900 helicopter's fuselage is comprised of a large portion of CF, though any damage of a structural nature had to be sent back to the factory for repair.

              In reality if Alan317 could pull this off I wouldn't think the weight penalty wouldn't be any worse than taking a fabric covered Stinson 108 and metallizing it as was done numerous times. I could see using a molded shape attached similar to what Mike Patey did with Scrappy as a workable solution but trying to wrap with CF sheets seems the effort wouldn't be worth it. Nor would a molded shape unless you plan to sell additional units to cover the cost of the molds and effort to fabricate.

              Perhaps a CF panel covered belly and boot cowl could be worth investigating… Don't ever expect to get your cost back when you sell and not many others will appreciate your effort or expense.

              The boys at DarkAero are impressive young lads.
              Last edited by chuckkemp; 12-15-2020, 10:36 PM.

            • zkelley2
              zkelley2 commented
              Editing a comment
              Metalizing a stinson or any old fabric airplane comes as a massive weight penalty. There's a good reason you see very few. It takes a good airplane and kills the useful. You almost can't sell them. At least to someone that intends on doing more than looking at their airplane.
              Last edited by zkelley2; 12-16-2020, 12:02 AM.

            • rodsmith
              rodsmith commented
              Editing a comment
              I never understood metalizing a fabric airplane, when if you wanted a metal airplane you could just buy a Cessna.

          • #10
            No disagreement that CF has a place in areas which are out of the way - wings / tips, cowling, canopy, etc - even a pod which is between the gear and easily removed and repaired.

            Under the belly and empennage, those areas take a lot of stone damage on a plane like the Bearhawk. Similarly, there is a reason why carbon props have a Nickel leading edge.

            Comment


            • #11
              After considering covering the fuselage with composite, the only way I saw it as superior to Dacron is that it is removable. Honestly, that was the only benefit I could think of. It wouldn't have to be that much heavier, maybe 5-20 lbs. But it is heavier. It is not that hard to repair, but it is harder to repair than Dacron, and the CF repair will be visible and unsightly. It is not that expensive, but it is more expensive than Dacron.

              In my mind, it did not win the comparison for that particular application.

              Mike Patey is kind of the monster truck guy of aviation. He has people in the background doing lots of work. I not sure I would use his work as something to emulate in a one man shop trying to get an airplane flying. And, I don't have or need a monster truck.

              Comment


              • #12
                I really put the idea out to see what kind of knowledge was already out there about it and if anyone was doing it. I guess, my thoughts were to use the 1mm pre-made panels on the fuselage, simply because the CF was non-structural and relatively resistant to trauma, UV and the elements. panels could be cut to size with relative ease I wasn't particularly concerned about the stiffness issue of fixing the panels at the anchor points with screws simply because plexiglass has similar issues if its not drummled out instead of drilled out and micro tractors lead to major cracks. The thought to mitigate that was, to over drill the holes in the CF a small amount to allow for the "wiggle room" that would be needed to account for flight characteristics of airframe shift in flight. But to be honest it all sounds far more trouble than it's worth (not to mention expense). I've never been opposed to fabric, I was just experimenting with different ideas and getting thoughts.

                Comment


                • zkelley2
                  zkelley2 commented
                  Editing a comment
                  You'd need to oversize drill and put a fiberglass bushing in to prevent making a battery. You need to do this anywhere carbon and metal meet.
                  The dark aero guys do a good job explaining why even if you do a fiberglass on top of carbon layup, as soon as you put a hole and metal bolt through it, you've got a battery. They went to pretty good length to not have this.

                • svyolo
                  svyolo commented
                  Editing a comment
                  I don't believe the reactivity is that much worse than aluminum fasteners in steel. The aluminum corrodes. In the case of CF, the CF is more noble so the steel corrodes.

                  The general rule is you can use small cathodic bits in a large anode, but not the other way around. I have no experience with CF in this regard but it should be no different than an aluminum boot cowl being attached mechanically and electrically to the steel fuselage. There is enough aluminum to sacrifice that the aluminum lasts a long time, or at least long enough. The airplane will be retired or crashed before the boot cowl turns into white powder.
                  But if you just put a small aluminum screw in a large steel structure, the life of the screw can be measured in months. I actually did that when I was 18, not intentionally of course. It cost me a half a day of work to drill another 100 holes and put in SS screws.
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