inexpensive but good alternatives to graphlite carbon pultrusions?

[wsimpso1]

Billski, IIRC, in a recent post you mentioned that you'd used straight epoxy to seal the foam in prep for laminating, rather than using epoxy/micro slurry, and that you'd seen little/no weight increase. Do you suppose there's any reason to be concerned that straight epoxy might not give the same foam/laminate bond strength immediately and/or long term? I don't have any reason to doubt the ability of straight epoxy to do the job, but I also have very limited tech knowledge of these materials.

Hmm. Starting from the beginning, we need to fill all of the broken cells in the foam surface or the resin we add to the cloth will end up filling it while displacing the air bubbles into the glass, which reduces the interlaminar strengths. So, we could fill with neat epoxy, but using a 50/50 mix of resin and microballoons and squeegee off all of the excess is lighter and works just as well, so that is what we do with open wet layups. The slurry is weaker than neat epoxy, but it only needs to be stronger than the foam to work at attaching foam to the glass-epoxy skins.

When I went to vacuum bagging, my experimental parts with slurry on the foam had the micro migrate into the glass cloth when the vacuum was applied, and that is BAD for interlaminar strengths. So I wet my foam with neat epoxy and let it be heavier than if microslurry was used. I believe that I conveyed that my vacuum bagged parts were plenty light already, not that they couldn't be lighter with micro. My apologies if I mislead on this.

If you were really after every ounce, and zealously seek less dense surface under your fiber-resin layer, you can do what is called hardshelling. You seal the foam with a somewhat wet micro-resin, squeegeeing excess from the part, let it cure, then sand it with 200 grit just before vacuum bagging on the facing. This is usually done where the part has a lot of shape or curvature to it. You laminate the first facing against the mold and then the foam, bag it and let it cure. Then you hard shell the foam and apply the other facing. This scheme only works for one side, but if you had the foam in net shape (curved surfaces and edges) I suppose you could hard shell both sides.

Billski

 

[lr27]

Irrelevant ?
Aluminum skin 2024 t3 .020" is $2.56 per square foot at Aircraft Spruce.
Carbon sheet .020" is $17.56 per square foot here: http://www.carbonfiberglass.com/Carbon-Fiber-Sheets-Composite-Panels-Fibre

That's an expensive way to get it. 4 layers of 4.1 oz unidirectional fabric, each layer at right angles to the previous, would be stiffer, stronger and lighter than the aluminum. Cost, if you did at least 25 square feet, would be $7.16 per square foot. Of course you'd need some epoxy. About 0.006 gallons per square foot. If your epoxy is $100/gallon, that's 60 cents per square foot more. Two layers of 6 oz. twill, which is supposed to come out at .009" thick, comes to $4.24 per square foot plus maybe about the same amount of epoxy.
http://www.sollercomposites.com/composites/carbon fiber uni-fabric.html
http://www.solarcomposites.com/composites/compositecarbon.html#12K

BTW, anyone know if this epoxy with nanotubes is a fad or something useful? Do you die if you sand it?
http://www.sollercomposites.com/composites/Epoxy.html
And when do we get our nanotube cloth? ;-)

 

[rtfm]

Do any of the composite books cover solid fiberglass wing skin? Like the Jupiter 1.
My interest is, and I think the topic of this thread is inexpensive alternatives ideas....<snip>
So my logic leads to simple fiberglass sheet skin. It could be homemade skin.
Even better would be factory made skin.
But this is all new, I haven't ever seen any simple sheet fiberglass skinned aircraft other than Dyke Delta. Which is non-structural skin according to Mr.Dyke, if I remember correctly.

So of course there will be guessing until tests are made. For me this is all just conversation for the future. Most likely I will stick with fabric skin. But who knows.

Hi,
I made a number of test panels to verify the weight, stiffness and transparency of solid fibreglass sheets. You can find the full story here: http://rtfm-aero.com/category/build-log/fuselage/covering-the-airframe/

In short, however,
I made two test pieces – a single layer of 270g/m^2 cloth (8oz) - designated "A1", and two layers of cloth (designated "A2"). The single layer was less opaque, but lacked stiffness. The two-layer test piece was more opaque, but also stiffer.
Weight: single layer, 250mm x 250mm = 33g Two-layer 65g





3rd attempt: Piece C1
1x ply of 135g/m^2 (4oz), 2x ply of 270g (8oz), 1x ply of 135g (4oz). Clamped with 10 clamps between two 1″ thick MDF sheets. This is test piece C1. Weight: 68g, stiffness OK, transparency not too bad, but not uniform.

Final test piece (D1)
This time I pulled out the big guns. I fired up my trusty vacuum pump, and sandwiched a new C1 test piece between Mylar sheets under as much pressure as I can pull. This is probably what I should have done in the first place. Almost the same weight, but much better translucency. Stiffness quite acceptable I think.

Verdict: too heavy, and not translucent enough.

 

[BBerson]

Wow that Soller composites prices are low, maybe half of Aircraft Spruce.
Might as well forget about fiberglass, I guess.

How about a solid three ply wing skin of carbon/Kevlar/carbon? The Kevlar being a sort of lighter core and maybe prevent shatter.

 

[wsimpso1]

My Grob is 230 pounds heavier than a very similar metal Vivat motorglider with same engine and same wing span.

If they have the same payload and the same g-rating, then the Grob is 230 pounds heavier not because they used sandwich, but because they designed the whole ship heavier. Pretty standard stuff is 4 pound per cubic foot PVC foam. Makes sturdy substantial parts. Quarter inch cores made of this stuff weigh 0.083 lb/ft^2. If you have 350 ft^2 of sandwich skin, you have 29 pounds of foam in the airplane. You will never take out much skin weight by going coreless.

Some examples- cores are 1/4" 4 pound foam, which weighs 0.083 lb/ft^2:

This is sturdy enough for my bird with a 240+ knot Vne;
Open wet layup - Glass with Core - 3 UNI wet layup on the outside plus 2 BID on the inside is .619 and about 217 lbs;
Vacuum bagged wet layup - Glass with Core - 22 oz TRIAX on the outside plus 15 oz BIAX on the inside is 0.494 lb/ft^2 and about 173 lbs;
Open wet layup - Glass only - 3 UNI wet layup on the outside plus 3 BID is 0.658 lb/ft^2 and about 230 lbs plus more internal structure;
Vacuum bagged wet layup - Carbon cloth with Core - 2 ply 5.8oz carbon on each side is 0.358 lb/ft^2 and about 125 lbs;

Let's go down in Vne and accept higher risk of handling damage;
Open wet layup - Glass with Core - 2 UNI wet layup on the outside plus 2 BID on the inside is 0.522 lb/ft^2 and about 183 lbs;
Vacuum bagged wet layup - Glass with Core - 15 oz TRIAX on the outside plus 12 oz BIAX on the inside is 0.416 lb/ft^2 with core and about 146 lbs;
Open wet layup - Glass only - 2 UNI plus 2 BID is 0.439 lb/ft^2 for fiber/resin and about 154 lbs plus more internal structure;
Vacuum bagged wet layup - Carbon cloth with Core - 2 ply 4oz carbon on each side is 0.283 lb/ft^2 and about 99 lbs;

Skipping the cores saves 29 pounds of foam but results in more cloth and resin and more internal structure. Going carbon saves 40 to 50 pounds on skins.

I also did costs with cores, the 3UNI/2BID with cores is about $7400, 22 ozTRIAX/18 oz BIAX with cores is about $6300 plus vacuum bag supplies (peel ply, perf ply, blanket, film, and mastic), 4 plies 5.5 oz carbon with cores is $10800 plus the same vacuum bagging supplies, and 3BID/3UNI without cores is about $8100.

So, you can build lighter with cores and vacuum bagging, and lighter still by going carbon/cores/vacuum bagging. Going from glass/coreless to carbon/cores while vacuum bagging both is a 33% price hike reduces your empty weight by 100 pounds. $27 per pound for reducing empty weight is a bargain worth thinking about.


Billski

 

[pictsidhe]

Glass cloth has about 1/3 the modulus of Al, to get the same stiffness, it needs to be about 45% thicker. By which time it's about the same weight.
Non flat sheet form will change everything.

 

[BBerson]

Billski,
The Grob repair manual lists the wing sandwich from outside inward. (Number in parentheses are my estimates/conversion)
Outer ply= 79g/qm linen (2.3oz)--------------------- estimated weight (.040lb/ft with resin)
Second ply= 276/qm double twill (8.1oz)---------------------------------(.110lb/ft with resin)
Conticell 60 8mm weight 60 kg/m2 (.32" 4.3/cu.ft.)---------------------(.120lb/ft)
Single inner ply= 276qm double twill (8.1oz)-----------------------------(.110lb/ft with resin)
_________
Sandwich skin total .380lb/sq.ft not including gelcoat.

The wing area is 440sq.ft.
That might support your argument that sandwich is reasonably light considering no ribs.
So I don't know why each Grob wing seems to weigh around 300 pounds. The spar must be very heavy. Or the gelcoat is very thick. I sanded some cracked gelcoat areas that were 1/8" thick. It has two layers of gelcoat, one from the mold and another after assembly, I assume.
The empty weight is 1320 lbs. And the Limbach engine is only about 160 lbs.

I am not sure why it's so heavy. It doesn't look like they used too much glass on the wing skins.

 

[Vigilant1]

I think there's a math problem, or maybe I just misinterpreted what you were doing. My calcs are in bold. 1 oz = 28.3 grams. 1 sq meter = 10.76 sq ft

Billski,
The Grob repair manual lists the wing sandwich from outside inward. (Number in parentheses are my estimates/conversion)
Outer ply= 79g/qm linen (2.3oz) (2.8 oz) --------------------- estimated weight (.040lb/ft) (.26 oz/sq ft = .0163 lb/sq ft)
Second ply= 276/qm double twill (8.1oz) (9.75 oz)---------------------------------(.110lb/ft) .906 oz/sq ft = .0566 lb/sq ft
Conticell 60 8mm weight 60 kg/m2 (.32" 4.3/cu.ft.) = 480 g for a 1sqm piece 8mm thick/10.76 = 44.61 g/ft = (.120lb/ft) = .0985 lb/sq ft
Single inner ply= 276qm double twill (8.1oz)-----------------------------(.110lb/ft) .0566 lb/sq ft
_________
Sandwich skin total .380lb/sq.ft .228 lb/sq ft not including gelcoat or resin.

Maybe you've just got some darn heavy gelcoat. That would explain the resistance to falling trees.

 

[wsimpso1]

Billski,
The Grob repair manual lists the wing sandwich from outside inward. (Number in parentheses are my estimates/conversion)
Outer ply= 79g/qm linen (2.3oz)--------------------- estimated weight (.040lb/ft)
Second ply= 276/qm double twill (8.1oz)---------------------------------(.110lb/ft)
Conticell 60 8mm weight 60 kg/m2 (.32" 4.3/cu.ft.)---------------------(.120lb/ft)
Single inner ply= 276qm double twill (8.1oz)-----------------------------(.110lb/ft)
_________
Sandwich skin total .380lb/sq.ft not including gelcoat or resin. With resin and gelcoat that might be .70 total

The wing area is 440sq.ft. X .70 = 308 pounds or 154 per wing.
That might support your argument that sandwich is reasonably light considering no ribs.
So I don't know why each Grob wing seems to weigh around 300 pounds. The spar must be very heavy. Or the gelcoat is very thick. I sanded some cracked gelcoat areas that were 1/8" thick. It has two layers of gelcoat, one from the mold and another after assembly, I assume.
The empty weight is 1320 lbs. And the Limbach engine is only about 160 lbs.

I am not sure why it's so heavy. It doesn't look like they used too much glass on the wing skins.

Our weight estimates are pretty close IF the cloth is carrying an equal weight of with resin. Your number is only slightly higher than mine for the foam, the total number is about OK with laminating resin, but without gelcoat. But if you have as much at 1/8" of gelcoat in places, you may have landed on a biggie.

440 square feet of gelcoat 0.010" thick will be somewhere around 27 pounds, 0.100" thick will be 270 pounds. Gel coat could certainly be a culprit. I know that is in a centerboard sailboats. Some guys will profile sand their hulls, taking out hollows 1/16" or more deep and still not go through the gelcoat, then find that their boats are on the edge of legal because they lost quite a few pounds in the hull during the sanding. That is a lesson for you guys if you want to increase your useable load ... That gelcoat does nothing for strength, but it sure adds weight. A lesson for building...

Billski

 

[BBerson]

I think there's a math problem, or maybe I just misinterpreted what you were doing. My calcs are in bold. 1 oz = 28.3 grams. 1 sq meter = 10.76 sq ft


Maybe you've just got some darn heavy gelcoat. That would explain the resistance to falling trees.

I just copied my notes from 15 years ago.
I rechecked my notes and now I see I did include resin in the fabric weight. I had simply doubled the fabric weight assuming 50% fabric/resin ratio for hand layup.
So post #127 is now edited.

 

[BBerson]

I seem to recall that molded gelcoat needs to be .020" thick. My early attempts at molding a canoe resulted in the gelcoat alligatoring. Then they said it should be .020" thick to prevent alligatoring.
I kind of lost interest in molding after that.

 

[wsimpso1]

Glass cloth has about 1/3 the modulus of Al, to get the same stiffness, it needs to be about 45% thicker. By which time it's about the same weight.
Non flat sheet form will change everything.

I agree with your conclusion but not the numbers in between. S-Glass bidirectional laminates are about 1/3 of E for aluminum, but the much more commonly used E-Glass is more like 1/5th. The density of laminated E- and S-Glass are about 2/3 of aluminum. Details follow:

E-glass E11 = E22 = 2.2Mpsi, S-Glass is 3.4 Mpsi, aluminum is 10.5 Mpsi

E-Glass will have to be 168% of aluminum thickness to equal stiffness of aluminum, which will make it about 107% of the weight of aluminum;
S-Glass will have to be 145% of aluminum thickness to equal stiffness of aluminum, which will make it about 93% of the weight of aluminum.

Yeah, solid laminates of fiberglass are about on par with aluminum for weight at a given bending stiffness, which is the biggest part of how we size skins...

Now carbon fiber can be thinner and do the same job, as well as its being much lower density than aluminum, so we have considerable possibility for weight savings in carbon fiber.

Billski

 

[lr27]

I seem to recall that molded gelcoat needs to be .020" thick. My early attempts at molding a canoe resulted in the gelcoat alligatoring. Then they said it should be .020" thick to prevent alligatoring.
I kind of lost interest in molding after that.

Paint the inside of the mold instead. I don't know the details, but I know the paint on molded RC sailplane wings is very thin. I don't know how they keep it even when sprayed on top of mold release, but they do.

 

[Laromin]

Has anyone had a chance to try the Toray pultrusions:
https://www.toray-cfe.com/en/composites/products/pultruded-profiles/121-plates.html

The strength and modulus values are even better than the graphlite,
and going by the prices shown at Suter Kunststoffe,they are reasonable competitive costwise.

 

[pictsidhe]

Bill, this is the page I usually browse to for rough comparisons: http://www.performance-composites.com/carbonfibre/mechanicalproperties_2.asp

 

[berridos]

The nice thing about the Toray pultrusions is that they are T700 and T800S. Thats very high modulus and have a standard TG of 110ºC. T700-165Gpa and T800S - 210Gpa.

 

[autoreply]

The nice thing about the Toray pultrusions is that they are T700 and T800S. Thats very high modulus and have a standard TG of 110ºC. T700-165Gpa and T800S - 210Gpa.

Those are fiber properties. Your final laminate will not even come close to those values!


Heavy gelcoat is not uncommon. Plenty of sailplanes with more weight in gelcoat as yours truly...

 

[lr27]

Those are fiber properties. Your final laminate will not even come close to those values!


Heavy gelcoat is not uncommon. Plenty of sailplanes with more weight in gelcoat as yours truly...

I don't think the modulus figures are for just fibers. Toray's spec sheet gives a value of 230 Gpa for the fiber's elastic modulus. 165 Gpa for the rods is 71 percent of that, which is almost believable in a pultrusion, isn't it?
https://www.toray-cfe.com/en/composites/products/pultruded-profiles/121-plates.html

 

[Vigilant1]

It was interesting to look at the surface of Toray's "RS" series rods, which apparently are designed to be embedded in mortar or grout and which appear to be extremely rough compared to the rods designed for embedding in an epoxy matrix. The RS series sure look like they'd have a lot of "bite", and that there would be no chance they'd be slipping around if used in an epoxy bed. But, I guess the ones actually designed for epoxy are better, or at least good enough.

I didn't see any signs of a US dealer.

 

[wsimpso1]

Those are fiber properties. Your final laminate will not even come close to those values!


Heavy gelcoat is not uncommon. Plenty of sailplanes with more weight in gelcoat as yours truly...

I looked at that website, and I agree that the data does look like way too high modulii for laminates, more like fiber characteristics.

Billski