weight-saving techniques

[kennyrayandersen]

Kenny, Glad to see your back, I was wondering where your were.
It seems to me your your first method might be the lightest. Four layers of 6 oz. glass on both sides of the foam would require a lot of heavy epoxy to wet out and fill. Bagging would reduce the epoxy, but not everybody can do that. Also the perforations in the foam would add more epoxy weight.

1/8" plywood is only 10# per sheet. A thin tight weave cloth wouldn't add much weight. This is the method I am using, but I am using 1/8" Cedar strips with light glass on both sides of the strips instead of using 1/8" plywood. I hope my skins will be strong enough. What do you think?

I know your goal is 250#, mine is 500# (600# on the high side) A little heavier sandwich wall won't be as big an issue for me as it will for you. However, I am counting ounces every step of the way.



Larry C

Larry,
I get kinda of busy sometimes. The Koreans sent me to France and my family has joined me now and my access from work is limited so I haven't been on too much lately.

I tend to think that you are right on the weight and so was suggesting that the plywood might be a good choice (though I read it's about 12# per sheets -- not enough to quibble about). For the little lightweight one 'm trying to do I think I can go 1-2 plies inside and 2-3 out as needed. Poking 2 mm holes 3 mm deep ever 50 mm really won't add much weight though. However, once you start hitting 4 plies per side plus all of the resin, you may not be too far from the plywood weight and I think it probably easier to do.

They also make 1/16 plywood, but it's aircraft grade and expensive, but it would certainly be enough for the walls and roof. The 1/8 is fairly available though, so I think that it make more sense to use that.

Still, you will want to cover the plywood with either glass or aluminum for weatherproofing.

[angib]

It is also worth remembering that the reason the foam is in the middle of the sandwich is to provide a shear connection between the two skins. This can also be done with intermittent connections. One rule of thumb we used in warship structures was that there is '50t' of effective plating on a stiffener - that's a width 50 times the skin thickness. Sometimes 30t was used, but it's that sort of number. 50 times 1/8" plywood skin gives you about a 6" stiffener spacing, so an all-wood 'sandwich' can be built like this:



If you want a lightweight, structurally efficient panel, the stiffeners should run in one direction only - whichever is the shortest span. The stiffeners can be fantastically thin before they're too weak (if you could build them out of 1/8" ply, they would be strong enough) so a nominal 1x2 is probably as small as is still easy to build.

And, yes, I know that everyone's Dad told them that two-direction stiffeners, also called egg-boxing, is best but they were all wrong.

[angib]

They also make 1/16 plywood, but it's aircraft grade and expensive, but it would certainly be enough for the walls and roof. The 1/8 is fairly available though, so I think that it make more sense to use that.

And this is where reality creeps in - 1/16 plywood skins will have enough strength overall, but they won't pass the 'can you poke your finger through it?' test. OK, finger is an exaggeration, but 1/16 ply is going to be very fragile unless it's bonded onto a super-strong structural (read expensive) foam core.

A former boss had a "ball-pein hammer test" - all structures need to be strong enough to be hit with a regular ball-pein hammer. Not hit hard, but maybe just a little tap? In North America, isn't resistance to some degree of hail going to be a design criterion? In lots of sandwich structures, skin durability needs to determine skin thickness, not overall strength.

[kennyrayandersen]

They also make 1/16 plywood, but it's aircraft grade and expensive, but it would certainly be enough for the walls and roof. The 1/8 is fairly available though, so I think that it make more sense to use that.

And this is where reality creeps in - 1/16 plywood skins will have enough strength overall, but they won't pass the 'can you poke your finger through it?' test. OK, finger is an exaggeration, but 1/16 ply is going to be very fragile unless it's bonded onto a super-strong structural (read expensive) foam core.

A former boss had a "ball-pein hammer test" - all structures need to be strong enough to be hit with a regular ball-pein hammer. Not hit hard, but maybe just a little tap? In North America, isn't resistance to some degree of hail going to be a design criterion? In lots of sandwich structures, skin durability needs to determine skin thickness, not overall strength.

There is the theoretical, and then there is the practical to be sure. Notice I didn't even THINK about using that on the floor! I agree that it's proably more practical to use the 1/8 anyway, and it not THAT heavy. I'm not sure what the minimum number for plies it takes not to poke your knee into it, but my guess is two to three, which will be substantiated by test (always a good way to confirm the calculations!).

[kennyrayandersen]

Kenny, Glad to see your back, I was wondering where your were.
It seems to me your your first method might be the lightest. Four layers of 6 oz. glass on both sides of the foam would require a lot of heavy epoxy to wet out and fill. Bagging would reduce the epoxy, but not everybody can do that. Also the perforations in the foam would add more epoxy weight.

1/8" plywood is only 10# per sheet. A thin tight weave cloth wouldn't add much weight. This is the method I am using, but I am using 1/8" Cedar strips with light glass on both sides of the strips instead of using 1/8" plywood. I hope my skins will be strong enough. What do you think?

I know your goal is 250#, mine is 500# (600# on the high side) A little heavier sandwich wall won't be as big an issue for me as it will for you. However, I am counting ounces every step of the way.

The only issue is that the wood doesn't have so much strength accross the short ditection... I would test a small piece before I commited entirely to the method. It may be that you want to put some cheap 1/8 inch strips the other way, before you lay your pretty wood down... That way you are kind of making you own 'structural' plywood, which can carry load in more than just one direction.

[GPW]

You guys ever take apart a hollow core door ? I did ... Surprise .. the door consisted of two thin skins of plywood and on the inside was what seemed to be random strips ,loose coils of corrugated cardboard cut into strips and glued (tubes/corrugations perpendicular to the skins) between the skins ... Nothing else .. Having slept for many years on a bed made from a hollow core door , I can attest to its strength ... 240#
Very lightweight !!
Don't see why this couldn't be yet another lightweight TT construction method ...just a consideration... ?
Having built many model airplanes over the past 50 years , with wings/ structures skinned with Balsa, plywood, fiberglass, carbon fiber, Kevlar, cardboard, Mylar , etc. etc.
The Skins are the Strength of the structure.. and the further apart they are , the stronger the structure is ... as long as the "spacers" , ribs, foam , cardboard? .. are utilized according to proper engineering procedures ... as so aptly put previously ^ by Angib ..
As applies to trailer making ... just making the walls, floor , Thicker would increase the strength tremendously (most thoroughly crash tested)... Say we wanted a STRONG trailer wall. Instead of using the usual 3/4"-1" foam as a spacer , use 2" , or even 4" ... better insulation too ... twofold benefit ! Still Light ...

[kennyrayandersen]

You guys ever take apart a hollow core door ? I did ... Surprise .. the door consisted of two thin skins of plywood and on the inside was what seemed to be random strips ,loose coils of corrugated cardboard cut into strips and glued (tubes/corrugations perpendicular to the skins) between the skins ... Nothing else .. Having slept for many years on a bed made from a hollow core door , I can attest to its strength ... 240#
Very lightweight !!
Don't see why this couldn't be yet another lightweight TT construction method ...just a consideration... ?
Having built many model airplanes over the past 50 years , with wings/ structures skinned with Balsa, plywood, fiberglass, carbon fiber, Kevlar, cardboard, Mylar , etc. etc.
The Skins are the Strength of the structure.. and the further apart they are , the stronger the structure is ... as long as the "spacers" , ribs, foam , cardboard? .. are utilized according to proper engineering procedures ... as so aptly put previously ^ by Angib ..
As applies to trailer making ... just making the walls, floor , Thicker would increase the strength tremendously (most thoroughly crash tested)... Say we wanted a STRONG trailer wall. Instead of using the usual 3/4"-1" foam as a spacer , use 2" , or even 4" ... better insulation too ... twofold benefit ! Still Light ...

That's whay I was advocating the use of 2 inch core for the tear floor! The doors that you are speaking about generally have the 1/8 in thick plywood as skins. Couple that with the home depot foam and you have a hum-dinger (1 inch foam on the roof walls and 2 inch on the floor should do it). I think that;s easier than trying to come up with some odd thing for the interior of the sandwich...

[angib]

One more thing to look at, for walls at least, would be a single skin of 1/4" ply with maybe 1"x1" stiffeners at 12-16" spacing. The 1/4" is probably a tougher skin than anything being suggested here and the occasional 1x1s may weigh less than a 12-16" wide strip of foam.

It couldn't withstand the 200mph winds that a decent 1/8" ply-foam sandwich could, but then the trailer would have blown away, so what's the benefit? I'm guessing it gets as good a ball-pein rating as anything else.

Oh, and it's easy.

[cappy208]

One more thing to look at, for walls at least, would be a single skin of 1/4" ply with maybe 1"x1" stiffeners at 12-16" spacing. The 1/4" is probably a tougher skin than anything being suggested here and the occasional 1x1s may weigh less than a 12-16" wide strip of foam......


Oh, and it's easy.

That is probably the BEST suggestion I have heard/seen.read yet! TYVM

Now I have to rethink my wall choices. Would an exterior of Al skin be acceptable in lieu of the outer plywood? I was thinking of finishing the outside edge of the stiffener with epoxy to give the 3M tape something to grab onto. Anyone got experience using 3M tape to fasten Al directly onto wood? I forgot who said it, but I didnt realize that the accumulation of foam, glass, and resin would almost equal a plywood wall, Sort of takes the emphasis off the hi tech foam and glass thought.

[kennyrayandersen]

One more thing to look at, for walls at least, would be a single skin of 1/4" ply with maybe 1"x1" stiffeners at 12-16" spacing. The 1/4" is probably a tougher skin than anything being suggested here and the occasional 1x1s may weigh less than a 12-16" wide strip of foam.

It couldn't withstand the 200mph winds that a decent 1/8" ply-foam sandwich could, but then the trailer would have blown away, so what's the benefit? I'm guessing it gets as good a ball-pein rating as anything else.

Oh, and it's easy.

This would be one of the few times that I would venture to disagree with the trailer-master (Andrew). The advantage of the 1/8 plywood-foam core-1/8plywood is that is weighs the same as the 1/4 inch, but it's MUCH stiffer and stronger. I analyze a lot of aerospace structures and composites specifically and when done right really spank the solid walls. The other BIG advantage is you get the insulation value as a freebie when using the foam as your core. Personally, I wouldn't even consider any other method -- its just down to a choice of facesheet skins, which will be determined with some testing.

[kennyrayandersen]

One more thing to look at, for walls at least, would be a single skin of 1/4" ply with maybe 1"x1" stiffeners at 12-16" spacing. The 1/4" is probably a tougher skin than anything being suggested here and the occasional 1x1s may weigh less than a 12-16" wide strip of foam......


Oh, and it's easy.

That is probably the BEST suggestion I have heard/seen.read yet! TYVM

Now I have to rethink my wall choices. Would an exterior of Al skin be acceptable in lieu of the outer plywood? I was thinking of finishing the outside edge of the stiffener with epoxy to give the 3M tape something to grab onto. Anyone got experience using 3M tape to fasten Al directly onto wood? I forgot who said it, but I didnt realize that the accumulation of foam, glass, and resin would almost equal a plywood wall, Sort of takes the emphasis off the hi tech foam and glass thought.

I'm thinking you would want at least the 1/8 inch plywood on the exterior to give you something to mount the aluminum to, but Mad jack and some others just staple the aluminum on anyway (around the edges). So if you framed the foam in wood, you might get away with it. However, the foam doesn't weigh much, and is therefore pretty cheap insurance.

[Larry C]

One more thing to look at, for walls at least, would be a single skin of 1/4" ply with maybe 1"x1" stiffeners at 12-16" spacing. The 1/4" is probably a tougher skin than anything being suggested here and the occasional 1x1s may weigh less than a 12-16" wide strip of foam.

It couldn't withstand the 200mph winds that a decent 1/8" ply-foam sandwich could, but then the trailer would have blown away, so what's the benefit? I'm guessing it gets as good a ball-pein rating as anything else.

Oh, and it's easy.

This would be one of the few times that I would venture to disagree with the trailer-master (Andrew). The advantage of the 1/8 plywood-foam core-1/8plywood is that is weighs the same as the 1/4 inch, but it's MUCH stiffer and stronger. I analyze a lot of aerospace structures and composites specifically and when done right really spank the solid walls. The other BIG advantage is you get the insulation value as a freebie when using the foam as your core. Personally, I wouldn't even consider any other method -- its just down to a choice of facesheet skins, which will be determined with some testing.

My floor is done with 1" stringers on 12" spacing with foam in between. the foam was thickness planed to the exact thickness of the stringers. The extra blocking at the ends, down the center and the corners was not really needed, but I have reasons for all of it. The skins are 1/8" plywood.



The complete sandwich after assembly with epoxy glue was very stiff and strong. However, I could lift a corner and the panel would flex in the middle. I added a layer of thin glass/epoxy to each skin. This dramatically stiffened the whole panel. I can now lift a corner without any flexing.

I am convinced the addition of a layer of fiberglass to the 1/8" ply skins is definitely worth the slight added weight. I will be doing the same on my walls. Another thing that I think helps is using a thickness planer to plane the foam to the exact thickness of any framing. Also the addition of the fiberglass is why my floor passed the knee test. My floor is 1-1/4" total thickness 5x8, 60 #

[S. Heisley]

The following quote is from Pete42. I copied it from MyAway build thread because I thought it might be pertinent to this discussion thread:

Pete42 wrote:

…I built an all fiberglass airplane we carved everything from foam using a hot wire and templates.
when it came time to apply the fiberglass cloth we mixed the resin and used small 1 inch brushes and "stippled" the resin into the foam removing the excess with a plastic squggie
the resin does not add strength only weight which on an airplane the lighter the better.
stippling is nothing more than holding the brush like you did the spatula and in an up and down motion dab the resin onto the cloth.
I often wonder why other than the all cardboard one no one has built an all fiberglass tear.
ol' pete