Originally Posted by Extreme Sports

Gosh, that's heavy!

This was free disposable coro, so I'm assuming the quality of it accounts for the variance in thickness and weight. It's no wonder most of my coro builds don't fly too well with all the extra weight of this cheap stuff I've been getting. You do get what you pay for.

Originally Posted by Extreme Sports

In my database I have: 345g/m^2 for 2mm, 450g/m^2 for 3mm and 470g/m^2 for 3.5mm. Most of this is measured over many samples - there can be small variability between sheets. No idea what 4mm weighs - it is definitely NOT in my build repertoire!

But this will be of no use to you, given the very strange metric system I use to measure stuff

That looked like Greek at first, ima have to do some cipherin on that

If the 4x8 4mm coro is around 5lbs a sheet(which is around what I've found, without getting into odd densities) I reckon 2mm should be at least 25% lighter than 4mm. The only real difference (for a given density) would be in the hight of flutes and web where verticals meet the sheet.
At 15' (4.5m?) I can live with that "5lbs per sheet" approximation, as I can't see much more than 3 sheets. Add in a bit of Sitka Spruce and foam for structure and I think ill still be around 40lbs. for the airframe. We're looking at a Zenoah G62 for our power plant, as ive been told it'll handle a 40lbs plane "with authority" Unless I can find one used in good shape, it'll have to wait till January or so before the budget will allow for it.

If my calcs are correct, in your extra-terrestrial units, a 4'x8' sheet of our 2mm would have a mass of about 2lbs. The 4mm we get locally seems to be made of much thicker plastic than 2mm, and so is far more rigid - and therefore very difficult to work with at normal correx model size.

Weight considerations aside, I'd probably want to tinker around with a sheet of 4mm and see if I could make the normal folds needed for a correx plane. At a guess, I think 4mm will make a horrid LE if the flutes are run chordwise, but I can't see why slitting flutes and running them spanwise (with ribs to provide shape and an acetate LE cap) won't work with 4mm at a 4.5m span. Fuselage folds will probably require a " 'V' cut and fold" technique - the normal "score and crease" method is more suited to thin correx (if you want a decent appearance). A dremel routing tool or carefully applied soldering iron can be used for this (you want to leave the outer skin intact).

Being a fan of light weight, I'd first want to prove that 3mm can't do the job

I just may have to find a sheet of 2mm somewhere and see what all the fuss is about. The weight per durability factor is appealing. I have done a few hybrids with the 4mm coro and foam but just haven't been all too satisfied with the results. Trying to keep down the weight with the foam and using the strength of the thick coro have still resulted with too heavy of a plane to justify the joining of the two.

I think there are some plastic railing covers that would make a fuselage similar to the downspouts if you don' mind that kind of thing. Some guys are using them on the bigger planes.

No railing is going to hold the ammount of fuel I need for my purpose, unless its absolutely massive, like Paul Bunyon's handrail. I'm looking at about 2-3 US gallons worth of twostroke, placed as close to the CoG as possible.

ES, my question, which I'm afraid can only be answered by experimentation, is how many ribs a 4mm wing would need, at the size I propose, with the flutes running spanwise? How many do you think a 2mm one would need? I'll likely go with the 2mm, as I have to order it online regardless, 4mm is just a bit more available, and sometimes significantly cheaper too.

Agreed - I'd support making the fuse from 4mm correx simply because it will enable you to make whatever shape and size fuse you want. The Telemaster is nice and boxy, so should be easy enough to build. Much easier than the wing.

Some thoughts on ribs and a 2mm spanwise wing:

• Ribs can provide both shape and structural strength...provided they are done right. You will need both.
• My best guess is to have a rib every 200mm (that is 8" or 3/4' in pre-modern era units). 300mm spacing could work, esp if you use 3mm for the skin
• Now the catch: Correx is not a good rib material for the simple reason that for best structural strength you want the correx skin firmly glued to each and every rib (and the spar). There is not enough surface area on the top and bottom of a correx rib for this to work. Also, as you fold the correx over the ribs, you want to 'stretch' it and have it stick immediately to the ribs - that way you get a nice taut skin. What worked well on my large wing was to hot wire cut a section of foam to the rib profile, and then slice it into thick ribs (~10mm or more...I'd use 20mm for your wing). I then glued these to the bottom skin as I built the wing, and finally used foam safe contact adhesive to glue the top skin to the ribs and spar as I closed the wing. This technique is analogous to an aluminium skinned wing construction as used in full sized GA aircraft.
• This was a second reason for building the wing with the slits in the correx on the outside - more surface area to glue to the ribs ahead of the spar to create a strong D-box.
• There might be some other ways to achieve the same effect if foam cutting is not something you want to do....but likely they will be more labour intensive and heavier.
• Ribs also need to be strong in compression, and correx is always weak in 1 dimension (unless you laminate each rib with perpendicular flutes)

I think 2mm correx will be strong and rigid enough for a 4.5m wing (assuming a properly designed spar and thick airfoil section of course), but be warned: You will have to slit a LOT of flutes, and slitting flutes is significantly harder in 2mm correx than 3mm (and there are 50% more). There is a big risk that you will cut through both skins and have to start all over again. So, unless you can develop a slitting tool that makes the job mindlessly easy and prevents cutting both skins by mistake, I'd consider using 3mm for the wing.

Interesting that your 4mm can be cheaper than 2mm. Here 2mm costs about USD 3.50 for a 4'x 8'sheet. 3mm is about double that, and 4mm another big jump up.

Originally Posted by Extreme Sports

Interesting that your 4mm can be cheaper than 2mm. Here 2mm costs about USD 3.50 for a 4'x 8'sheet. 3mm is about double that, and 4mm another big jump up.

Maybe he's obtaining the election signs like I do.....free.

Yep, election signs. None up around here right now, but the pole-cat politicians will start the money grubbing any time.

I'd have to acquire a piece of 2mm before I could consider building a slitting tool. With 4mm, I make an epoxy ball on the end of a razor blade. The ball goes in the flute and glides along while the blade does its work. I reckon I could use the same technique on the 2mm, possibly with exacto blades for a bit more dexterity in the thin stuff. Between that and a straight edge, its almost mindless.

I'm not afraid of hot wire cutting, I'd just have to learn how. Is there any particular kind of foam that you'd recommend?

Here's the video that got me into coro a few years ago. I built one similar and rambled around Arizona for a while.

Nomad bicycle camper (8 min 32 sec)

Sadly the 'free' 3.5mm correx available here is generally about 600mm x 400mm. Useful for odds and ends, but too small to make anything flyable.

I use scrap (read 'free') off-cuts of ceiling insulation boards from the local supplier. They come in thicknesses up to 80mm and are 600mm long. It is essentially the same foam as Depron (XPS) and not the common packaging polystyrene (EPS). I suspect there is a global standard on this sort of thing, so at worst you should be able to find something similar. However, given that the US gets a LOT colder than Southern Africa (i.e. larger market for insulation foams), I'm sure you will be spoiled for choice. Hot wire cutting small sections from scrap foam is pretty easy - much simpler than doing a whole wing.

EPS could work too, but tends to be more flexible - so you might slice the ribs a bit thicker in this case.

Just thinking aloud: If your span is 4.5m, your chord will be just under 600mm. If you use a Clark-Y or equivalent airfoil with a thickness of ~12%, your rib depth will need to be (12% of 600) - (2 x 4mm) = 64mm (or 68mm if you skin with 2mm). So an 80mm thick block of foam should be enough.

That is also the depth of your spar. Bearing in mind that the strength of the spar goes up with the cube of the depth, you might be conservative and go for a relatively thicker airfoil - maybe as much as 80mm (~14%). An 80mm spar built as an I beam along with the spanwise correx flutes should make for a pretty strong and rigid 4.5m wing. You could even run a rear spar part span just to be 100% certain....our models tend to get subjected to a lot more G's than full size. Hell, done carefully, you could even use a ton of yardsticks and stay true to the spirit of SPAD!

Thank all that is holy for people like you sir! Those who have the knowledge and are willing to share it are invaluable!

Been watching videos of hot wire cutting, and I reckon I can manage it. The spar is what I was most concerned with, both design and material. I had considered an I beam but wasn't sure on dimensions, as I know nothing of the construction of aircraft, really. So thank you very much for your math sir, as now I have a workable set of dimensions for my wing, and can scale just about everything else off of that. With the amount of time ive spent doing my home work, I've come to realize that this is going to take awhile with my budget, but most of this stuff should be cheap and easy. We are indeed spoiled for foam, Depron is reasonably priced, ill start shopping for some, along with more homework on a cutting rig I can cobble together with junk ive got laying around. While I'm at it ill hunt up some Sitka Spruce, might as well go the extra mile(1.609 kilometers?) past the yardsticks.

Time for Kaveman to learn the Metric system. I had already resolved to teach my daughter in parallel to our silly standard system. As a carpenter, I work with 16ths, maybe 32nds, yet engineers work in 10ths of inches, go figure.
Thanks again for simplifying a great deal for me with those wing dimensions! Just to push my luck, what would the other dimension on that spar be? 80mm tall but how wide? I'll probably build it at least 4m long. Or is that wide...? Building houses is simpleton stuff apparently.

Haha. What you are getting is some gut feel ideas...not gospel truths

You mentioned carpentry. Do you have access to decent woodworking tools such as a bench saw or similar that could be used to rip Spruce to a defined size?

Reason I ask is that the spar is probably the most crucial part to get right in a plane this size. Both because if it breaks, you will see a LOT of time investment fall from the sky and because you are going to need to be very conservative on the design safety factors. Simple TLAR is too risky for a plane of this size from a safety point of view.

Fortunately calculating the stresses in a wing spar is not rocket science - it is a simple I beam, and the maths for that is a routine 2nd year university problem (fortunately I still have my lecture notes from donkey's years ago). But once we determine the ideal spar dimensions, you can either choose the next largest size of timber (and then design ribs, chord and spar construction around standard sizes), or you can cut the stock size to whatever is optimal - which will give you the lightest build. Also, with decent woodworking tools you can add lightning holes etc.

This is going to be a considerable undertaking regardless of how simple you build. Work goes up by the cube of size and as mentioned before, a plane this size is a safety risk, so the standard of design and construction needs to be a notch or two above the usual SPAD...even if you plan to fly it in an uninhabited desert

I was also wondering it it would make sense to open a build thread in a more visible place. I suspect many people will find this process interesting. I'm not sure where would be best, or even if you want to broadcast this project - so your call.

On the note of woodworking tools, about the only things I don't have ready access to anymore would be certain stationary sanders, like disk, drum, and belt. But as long as school is in session, I can go down to the highschool where the woodshop teacher is happy to allow me the use of his shop.
A tablesaw can be fashioned from a skillsaw bolted to a piece of plywood and plunge cut. While VERY UNSAFE it can at times be more accurate than many actual tablesaws.

While I agree about the build thread, the legality of the project is still very much in question. The FAA can't seem to get it's poop in a group when it comes to small models, let alone big scary things like this. Hence, id rather keep it buried here in the SPAD section for the time being. In fact my wife is wanting me to back off until we know which legal loopholes we have to jump through. From what I'm reading across the web, I'd be irritating folks on grounds of size and weight, then again because it's got a camera. I'll be contacting a lawyer or paralegal before anything else at this point. Now that ive got some rough figures and can approximate size and weight, somebody can wade through the piles of FAA legal jargon and see what I need to do.