Kevlar for covering

[svyolo]

Bob designed a great, light steel fuselage and aluminum wing. I wouldn't change a thing. He also seems to have fallen for composites. Bob also likes to "roll his own". How about female molded kevlar skins for the BH. Kevlar is 1/3 the price of Oratex. Once the molds are made, the labor to make each one is very low.

Oratex, unpainted is about 5 oz/sq yard. The lightest kevlar I can find is 1.7 oz (2.6 finished with epoxy) and 5 oz (7.5 oz). You could use the light stuff in areas not likely to get damaged, the heavier in areas of heavy abrasion.

Kevlar needs to be female molded. Sanding makes a mess of kevlar. But it is almost unbreakable. It is so difficult to cut, as dry cloth, you need special scissors.

I would love to do it myself, but my building mission is to get it built. Not create side projects. I would be customer #1 to buy a set of fuselage skins.

I am thinking about the 1.7 oz clothe on control surfaces. It would save a little weight, but would save 2 or 3 times as much in lead you wouldn't have put in the control surfaces. And, it is 1/3 the price of Oratex.

I would cover 1/2 the fuselage with Monokote, and heat shrink adhesive backed covering for model airplanes, then I would cut 1" foam to wedge between the stringers as a backing for the monokote to keep it from sagging between stringers. Then I would use 5 oz fiberglass over the monokote, let it cure, then another layer of heavier cloth. Then some foam of some type as a "core", then a last layer of medium cloth to make a light but rigid mold. Remove it from the airframe, and surface the inside of the mold.

[whee]

Interesting idea. It would be pretty cool to see it done. I don't know anything about kevlar but I find it difficult to believe that you'd only have .9oz of paint per square yard. Traditional covering systems have something like 12oz/sqyd of coatings on top of the fabric. How much of that is primer and paint I don't know. I do recall a guy that build a EAB cub; he left the covering off the fuselage and talked about covering it with kevlar but he never did.

Oratex is expensive so I think a better comparison would be with traditional coverings. Stewart Systems cost me $1913.

[Archer39J]

I plan on using Oratex despite the significant cost, hear $6k or so for a 4-place? A completed surface with no post finishing is my primary motivation, weight decrease is the second. I take it you would still need quite a bit of fill and fair, as well as primer and paint, with Kevlar?

[svyolo]

You can use a UV pigment (West recommends a maximum of 20% pigment) plus aluminum powder in the epoxy. Roll it into the mold as a gelcoat. The fabric acts as a filter for the aluminum powder and it is trapped on the outside surface. Kevlar doesn't do UV well. But with pigment/aluminum powder, no paint required, or just a whisper thin coating of single stage paint. I have done lots of fiberglass work, both fabrication and repair. Even 1.7 oz kevlar is vastly tougher than any traditional covering. A big rock off the main mounts might cause a point fracture of the epoxy, but the cloth will hold. I have only used kevlar a bit. It is tough to work with. Female mold only. But it might be the ultimate bush plane fabric. And in a strange twist of fate, it is cheaper than any other covering/finishing material used for fabric covered aircraft.

Carbon Fiber is 30% stronger, but "toughness" is more important than strength for covering.

I won't be the female mold maker, but I might make a 3 ft fuselage section and send it to Avipro and see if they want to approve it, and do it. I would buy the finished product in a heartbeat. It would be better/ easier/ less labor, and depending on paint, as light or lighter than anything else. Oratex is so light already, we are definitely into the "law of diminishing returns". But kevlar is also 1/3 the price.

[Chris In Milwaukee]

We're talking about a "crunchy outer shell" here, and not just substituting Kevlar for Dacron? Interesting. Sure could make a slippery airplane like that, I suspect. Any comparison to a "traditional" aircraft coating, weight-wise? As long as we're just hypothesizing, of course.

[Archer39J]

Hmm, that's intriguing. What about repairs? I know very little about covering products and procedures but if we're talking rigid panels with Kevlar would that be more difficult than existing products?

Given the apparent cost benefit I suspect there may be a good reason it isn't widely used that may not be immediately apparent. It's certainly worth investigating though.

[svyolo]

Repair would be a little harder than Oratex, but a lot easier and faster than traditional fabric. Other than a little hole and a crease, I don't think much could damage kevlar. Probably the underlying structure (leading edge of tail, or fuselage stringer) would get damaged before the Kevlar got torn.

Surfacing Kevlar isn't easy, unless you are willing to pile on a lot of weight. You can't sand into the fabric. It is much better to female mold it, and make sure the surface turns out good. it does need to be protected from UV.

I am going to make the floor boards and interior panels out of kevlar sandwich. I will "female mold" the flat panels on a sheet of glass. The light kevlar is very flexible as a single sheet. I will experiment with covering the control surfaces with the flat sheets.

[Archer39J]

Nice, what do you plan to use for the core?

I have enough 1/4" 2-ply nomex sandwich to do the floors and bulkhead, but I'm kind of debating whether I still want to or not. Bolting everything straight through aluminum floor boards is appealing and it's going to be kind of a pain to notch out every little thing and still have it look good. That and I'm concerned about the wear of unfinished fiberglass on the floor. Still thinking it through.

[svyolo]

I was going to use 1/4" nomex as well. Also the cargo bulkhead, and interior side panels. The side panels will just be "skeletal" sandwiched. Just a 1" wide strip of core every 6" or so to stiffen it. The rest of the side panels will just be 1 layer of 1.7 oz kevlar.

[svyolo]

The more I think about (for a couple of weeks) the more I think I might just male mold the fuselage in kevlar and accept the fabric print through. A very light coat of paint. I am building a bush plane, not a show plane. I will run in past the local FAA inspector beforehand.

[svyolo]

I have read the AC 43 regs on fabric covering. I am not sure how applicable they are, but I believe 1.7 oz/yd2 passes those rules. Bur I believe epoxy coated kevlar might be different. I believe it behaves more as a stressed skin. I have never tried to contact Bob, but I might in this case. Just as polyester cloth was a quantum leap over cotton, so is aramid fiber a quantum leap over polyester. I probably have 9 months before I cover.

[Bdflies]

Interesting concept here! I'm curious about something though. When a surface is covered with dacron cloth, after attachment to the structure, the cloth is heat shrunk. It gets VERY tight. After that, coatings stiffen the cloth considerably. I know the Oratex isn't usually painted after shrinking, but it is heat shrunk as well. The tightness of the fabric is critical to resist billowing and buffeting in prop blast.
Will Kevlar shrink similarly? Will the Kevlar be as tight as dacron, before impregnation of the resin system? Will a single layer of impregnated Kevlar be stiff enough to resist billowing in the prop blast and on control surfaces?
I understand the inherent strength of the Kevlar fabric, but it's my impression that the stiffness I'm asking about would be provided by the resin substrate. I built a composite plane, years back. Granted it was fiberglass, not Kevlar, but the rule was minimum 3 layer laminates for anything that would see the airstream.
Just wondering what your thoughts are about stiffness of a single Kevlar wrap.

Bill

[bergy]

I too have wondered how/where I could use Kevlar in my Patrol project. I have owned an 18.5' Kevlar canoe for years, paddled it hundreds of miles thru the Ontario wilderness. The stuff is slippery; in a stiff crosswind it will go as fast sideways as forward. And it is light; the 18.5' canoe with aluminum gunwales and thwarts goes about 48# (I think). I did run into a rock in a river one time and seriously scratched off a steak of gel-coat. The double-layered bottom held up fine, but the thin layer of foam-filler on the bottom compressed a bit and did not expand back to it's original contour 100%. I also had it stored for a few years in the woods behind my garden. The balsam-fir trees were so dense that sunlight never reached it. when not in use it developed a patina of mold/lichens or something. I found that the resin was weathering away and the raw fabric was soon exposed on almost the entire bottom. I scrubbed it well and dropped it off at a friend's autobody shop. Told him to empty his spray-gun whenever he had some catalyzing clear-coat left over at clean up time. Two or three coats seem to have done the trick. It looks almost good as new.

[Bcone1381]

I made a kayak that has a foundation of aircraft grad fabric sealed with flexible epoxy product from West System, then a protective layer of fiberglass on the outer layer. I don't wee why a layer of Kevlar in place of the fiberglass would not be stronger than my fiberglass layup, but I cant see that it would be lighter than Oratex, or polyfiber.

[svyolo]

Bdflies;
Yeah I have been scratching my head on this for a couple of weeks. All shrink fabric is under tension when installed. The dope (or other coatings) then shrink it some more. Some structures require this for structural support. The BH fuselage does not. The control surfaces, I am not sure of, and will have to ask Bob. While they are not "primary" structure, they are critical obviously.
The Kevlar used as a covering material would be under zero stress at rest. It will be under tension when loaded by the airflow, just like regular fabric. It is considerable stiffer in tension than other aircraft fabrics.

On the fuseage, it is just covering. The control surfaces, it might be a combo of covering, and structure. A kevlar cover would act as a better "stressed skin" than fabric.

AC 43 has a fabric covering section, and a different section on composite repair. The two don't mix. I would guess kevlar would have to meet the strength test for fabric, which is loading a finished 1 inch strip with a certain amount of weight. I haven't tried it, but I am pretty sure 1.7 oz would pass. I believe it would hold 100-150 lbs.

I am pretty sure I am using Oratex on the plane. Just thinking aloud if there isn't something better, and 1/3 the price. I would love to use Kevlar on the control surfaces, and reduce the amount of lead weight. Somebody will have to approve that before I try it.