Here, 1:1 modelling in an eggshell:

http://users.skynet.be/BAMRS/dh103/f...nstruction.htm


Photograph 1, turtle deck of the Simla under construction. Hanna didn't want to see it, she doubted whether this would turn out well.
The turtle deck consists of a double layer with the grain at an angle of 90 degrees to each other.
Photograph 2, result.
3, Molding the canopy was okay for her......

TF

Some of the latest fai aerobatic models are also sandwiches done the same way
I repaired a crushed one for My son
Tricker than making the original
Done both
Some builders use foam some use balsa some use both
Some vacuum bag I designed a large press to use when vacuum could not be used
For example the inner support can be collapsed when bagging the outer shell
Easy to distort the shells.

Originally Posted by CGordon

Here's an indoor plane I recently got at a swap meet. The wing is just adequately strong with foam that is 2.5mm thick, at 400mm span. Now if I sized it up 10x, to a 4 meter plane, that foam would now be 25mm ( 1 inch) thick and I actually doubt it could even support it's own weight. If it was made of medium balsa, I think it might, but with spruce, I think it would be a decent match for the foam indoor properties.

Originally Posted by CGordon

Wondering if this makes sense from the standpoint of scale theory.

Here's a page that gives the strength properties of EPS foam of various densities:

https://www.foambymail.com/polystyrene-foam-sheet.html

Let's assume your swap meet plane was made of the lightest grade foam in the chart, with a density of 1.02 lb/cu-ft. The chart gives compressive, flexural, tensile, and shear strengths of 12, 27, 18, 20 lb/sq-in respectively. These are properties of the material itself. It doesn't matter whether it's an inch thick or a mile, this is the amount of stress a 1 square inch section can support.

Now let's scale up your plane by a factor of 10 and see what happens. If we make it out of the same stuff (constant density) and keep all proportions the same, the weight will increase by a factor of 10 cubed; i.e. 1000 times the original weight. The lift required to hold the plane up is equal to the weight; so the lift will also increase by a factor of 1000. The various loads imposed on the structure are proportional to the lift - so they will also increase by a factor of 1000.

But the cross-sectional area of all the structural members will only increase by a factor of 100. This means that all the stresses (which are loads divided by cross sectional areas) will increase by a factor of 10. But the foam's material properties do not change.

This means that if the original model was "just adequately strong", the 10 times size version made of identical materials will be about 10 times too weak . As you said, it might not even support its own weight.

This is an example of the "square-cube law" and is the actual reason higher strength materials are required for larger structures. Variation in aerodynamic loads with Reynolds Number is important, but strength of materials is much more important.

If you use solid pieces of stronger materials the weight penalty will be extreme. This is why hollow shapes, like tubes and boxes, are used for high strength, light weight applications. Or I-beam shapes that put the "meat" right where the highest loads are concentrated.

Or
You can use the chicken egg approach
Use solid one lb foam blocks and skin it with glass and epoxy
This works very well for much of an airframe
The caveat is you need to curve each price as much as possible
Even extremely gentle. curves, make a big difference
The techniques for super light ,strong models now include lots of foam composite
Years back I did custom building including scale kit from box to completed.
Some Scale kits were terrific
Lou Proctors for example
Others were wretched mish mashes of wood bits which although heavy, were structually weak,

Originally Posted by Coupez

Here's a page that gives the strength properties of EPS foam of various densities:

https://www.foambymail.com/polystyrene-foam-sheet.html

Let's assume your swap meet plane was made of the lightest grade foam in the chart, with a density of 1.02 lb/cu-ft. The chart gives compressive, flexural, tensile, and shear strengths of 12, 27, 18, 20 lb/sq-in respectively. These are properties of the material itself. It doesn't matter whether it's an inch thick or a mile, this is the amount of stress a 1 square inch section can support.

Now let's scale up your plane by a factor of 10 and see what happens. If we make it out of the same stuff (constant density) and keep all proportions the same, the weight will increase by a factor of 10 cubed; i.e. 1000 times the original weight. The lift required to hold the plane up is equal to the weight; so the lift will also increase by a factor of 1000. The various loads imposed on the structure are proportional to the lift - so they will also increase by a factor of 1000.

But the cross-sectional area of all the structural members will only increase by a factor of 100. This means that all the stresses (which are loads divided by cross sectional areas) will increase by a factor of 10. But the foam's material properties do not change.

This means that if the original model was "just adequately strong", the 10 times size version made of identical materials will be about 10 times too weak . As you said, it might not even support its own weight.

This is an example of the "square-cube law" and is the actual reason higher strength materials are required for larger structures. Variation in aerodynamic loads with Reynolds Number is important, but strength of materials is much more important.

If you use solid pieces of stronger materials the weight penalty will be extreme. This is why hollow shapes, like tubes and boxes, are used for high strength, light weight applications. Or I-beam shapes that put the "meat" right where the highest loads are concentrated.

Thank you, this is really the meat of what I'm getting at with the question.

I must add though, some amazing construction techniques have shown up on this thread along the way!

Originally Posted by richard hanson

Or
You can use the chicken egg approach
Use solid one lb foam blocks and skin it with glass and epoxy
This works very well for much of an airframe

Not for my body

For a fuselage I have found that it's difficult to beat a fiberglass/balsa composite. The only drawbacks with that is you have to be pretty committed to your design and takes a bit of practice to master. Also requires a bit of equipment investment. The up side is light weight/high strength, faster build cycles, repeatability.

For wings I still go with the balsa sheeted, hot wire cut foam. This method offers great strength to weight as well but is a very versatile method. You can easily play with combinations of airfoils, washout, multiple tapers and other aerodynamic features but you can also add in spars, spar caps, full depth spars etc to suit your strength requirements.

On the skinning over foam idea....

Many years back I flew a fair bit of the local control line combat events. It was all "speed limited" .15 size stuff. Huge fun....

I wanted to play with cheap yet decently durable options to make these disposable models in bulk. Yet to keep them light. I decided to cut foam cores and overlay them with newspaper attached with thinned white glue.

The result was a layered approach for the high load areas and a single skin over much of the rest. I've posted the sketch below a few times when the topic has come up over the years.

When one of the models got chewed up pretty badly and beyond hope of repair from being buzzsawed by a prop I opted to destructo-test the remains with special focus on the engine mount area. This being a single 3/8 balsa half rib to connect the motor mount to the tail boom made from aluminium arrow shaft.

I took an overhand swing into the concrete floor of my shop. A couple of light but firm hits did nothing at all. Took a couple of fairly good swings comparable of how I'd hit a medium size nail. Still no damage. It took two or three full on "sledge hammer" swings before that mount came loose from the foam. It was a shocker for me to see it being so strong and tough. No more fears about engine vibration after that.

So skinning over foam with some manner of composite can sure produce a good sturdy method of structure. And it doesn't need to even be glass or carbon. Paper fibers encased in a bonding and fixing resin (the thinned white glue) can work just fine. And then along came water based polyurethane a year or two after my thinned glue paper mache method which I've still yet to try.

I must say, you may be right. All my whole life I have been housed in skin over flesh body and, I must say, that works well too.

TF

Plus it gives you something to read while the glue dries

Ray.

But will be outdated by a day.

Ok
Here is a super secret technique I used for cowlings
White foam one pound density
Cut n sand to shape
Silkspan light brushed on the foam
Now,
the secret ingredient
White glue mixed with spackle
Brush on
When dry it will sand
This makes a male plug for 4-6 oz glass and resin part which can be finished however you like
The male foam comes, with some digging out cleanly because ofthespackle glue
Or build up the silkspan glue n spacklea few layersthen silkspan the paint
Then carefully chip out the foam
..doing one off designs , I did the cowlings this way
The parts can be quite strong and light

Originally Posted by BMatthews

So skinning over foam with some manner of composite can sure produce a good sturdy method of structure. And it doesn't need to even be glass or carbon. Paper fibers encased in a bonding and fixing resin (the thinned white glue) can work just fine.

Way back in 1977, my first large trainer was a Sterling "Gazariator" - basically a .60 size Fledgling. Conventional balsa and ply structure.

On the first flight, the elevator clevis came apart (I didn't know better and used the kit ones, with snap-in nylon pins). The test pilot tried to maintain altitude with full power, and flew the airplane through a patch of woods. He saved the fuselage, but both wings were sheared off.

Rather than build a new wing framework, I bought one of the aftermarket foam wing kits from "Wing Mfg". It came with cardboard skins, to be applied with contact cement. Nothing in the instructions about spars or dihedral braces, so you can see where this is going.

Things went very well on flight #2 until the test pilot tried a loop. One wing panel went up about 15 degrees. He kept the plane slow after that and managed to land without losing the wing. I bought two 48" 1/4 x 3/8 spruce sticks at the hobby shop, gouged out channels for them in the bottom of the wing, and epoxied them in. No more problems with wing folding!

The plane took a lot of abuse, and I decided to replace the wing. Bought another foam wing kit, decided to skin it with balsa this time. Also decided to put the spars in first. Joined the panels with recommended dihedral, cut the channels, laid in the spruce sticks with plenty of 2 hour epoxy. Put the wing across two metal garbage cans to dry, which didn't quite come all the way to the wing tips.

After the setting time was up, I picked up the wing -- and it had polyhedral! The curing epoxy generated enough heat that the unsupported tips sagged a couple of inches. Fortunately it was symmetrical, so I skinned it and flew it that way.

Bruce and Ray, you two guys got me thinking about different ways to build a stressed skin onto a foam core. Currently I am using the traditional 1lb white foam core with balsa sheeting. In order to keep the weight down I have been buying AAA contest balsa in the 6lb to 8lb range. By the time I have enough acceptable wood to cover 1,300 sq in of wings and 300 sq in of stabs I have about $200 invested. So you can easily see how beneficial a substitute would be. Looking at your pictures and reading your posts pretty much set off a light bulb. What if I did my cores as usual but sheeted them with 1/16 blue foam? For paint prep I glass the balsa with 3/4oz cloth but with the foam I would use 1.4. Going to try and get some sample peices done over the weekend. Thanks guys for the inspiration!

Originally Posted by Coupez

Way back in 1977, my first large trainer was a Sterling "Gazariator" - basically a .60 size Fledgling. Conventional balsa and ply structure.

On the first flight, the elevator clevis came apart (I didn't know better and used the kit ones, with snap-in nylon pins). The test pilot tried to maintain altitude with full power, and flew the airplane through a patch of woods. He saved the fuselage, but both wings were sheared off.

Rather than build a new wing framework, I bought one of the aftermarket foam wing kits from "Wing Mfg". It came with cardboard skins, to be applied with contact cement. Nothing in the instructions about spars or dihedral braces, so you can see where this is going.

Things went very well on flight #2 until the test pilot tried a loop. One wing panel went up about 15 degrees. He kept the plane slow after that and managed to land without losing the wing. I bought two 48" 1/4 x 3/8 spruce sticks at the hobby shop, gouged out channels for them in the bottom of the wing, and epoxied them in. No more problems with wing folding!

The plane took a lot of abuse, and I decided to replace the wing. Bought another foam wing kit, decided to skin it with balsa this time. Also decided to put the spars in first. Joined the panels with recommended dihedral, cut the channels, laid in the spruce sticks with plenty of 2 hour epoxy. Put the wing across two metal garbage cans to dry, which didn't quite come all the way to the wing tips.

After the setting time was up, I picked up the wing -- and it had polyhedral! The curing epoxy generated enough heat that the unsupported tips sagged a couple of inches. Fortunately it was symmetrical, so I skinned it and flew it that way.

Great story, I have a fondness for the Fledgling, the first R/C airplane I ever flew. Powered with an Enya .35 and guided by Kraft. 1974 if memory serves.