inexpensive but good alternatives to graphlite carbon pultrusions?

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wsimpso1

Super Moderator
Staff member
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

Well-Known Member
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

Billski

pictsidhe

Well-Known Member
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

Light Plane Philosopher
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.

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Vigilant1

Well-Known Member
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.

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wsimpso1

Super Moderator
Staff member
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

Light Plane Philosopher
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

Light Plane Philosopher
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

Super Moderator
Staff member
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

Well-Known Member
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.

berridos

Well-Known Member
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.

Super Moderator
Staff member
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

Well-Known Member
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

Well-Known Member
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

Super Moderator
Staff member
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