hackdroot

Since it's totally dark by the time I get home from work now, I was looking for a fun project to keep me busy during the work week. I've always wanted to build a large foamy and have a bunch of the blue fan fold foam in the garage. The only high wing plane I own is an old Hobbyzone Super cub I had sitting in the corner of the garage. I was wondering if I could simply scale up the dimensions of the airframe. This would be constructed using blucor and other materials, and would scale to approx a 12' wingspan (the original is 4'). I could work on the power system once the airframe weight is known after assembly. I have absolutely no design experience. Am I naive in thinking this simplistic approach will work? (With the appropriate power system, proper CG, thrust angle, and structurally sound airframe).

-Eric

BMatthews

The blue foamcore would work as skins for the wings and other low stress parts. But you'd need to engineer up some spar structures and include other materials in with the build that would take the stresses of flight and ground handling. Figure on the blue core being not much better than a cross between covering material and marshmallow stuff which could be used for light duty shape holding.

So all in all given your admission that you've not done anything like this and know little or nothing about the structures of models in this 12 foot size range it's likely best to lay down until the feeling goes away.

Why not build to sizes that are well known to work with bluecore? I've seen some amazing things done with this stuff. Jet fighters with tail props or EDF units that look REALLY GOOD along with great looking vintage style biplanes. The trick seems to keep the models down under 40 inch span and even then be prepared to use some carbon strip reinforcement in the bigger ones from 34 to 40 inch.

hackdroot

I actually have built models up to 42" with the blucor, but these were from others plans and not my own design. I had planned on using balsa, basswood, and plywood for some of the underlying structure. I wasn't sure how well the blucor would hold up, but after seeing Davids design on this 8 footer I was pretty impressed with how well it help up:

http://www.rcgroups.com/forums/showt...ighlight=giant

BMatthews

If you can use the pictures of the construction details of that 8 footer as a guide but expand on it to produce a somewhat more solid structure then you'd be good to go.

Keep in mind though that these folks used an electric system which produces very little in the way of vibration. If you go with a gas engine that basically shakes like a paint mixer in comparison then you'll need to use some serious amount of thin to medium thickness plywood around the engine area. Aircraft birch for the firewall mount would be wise but you could get away with relatively cheap mahogany door skin plywood for the fuselage doublers. You'll also still need to think of the foam as more of a covering skin more than as a load bearing material. So you'll want to use fuselage stringers of wood with wood diagonals inside behind the foam skin. Formers could be done in FFF but I'd suggest laminating them with file folder card stock front and back to give the formers some added strength. A box of such folders from the big stationary stores is super cheap and it's a good model making material to have around anyway.

I'd suggest that for a 12 foot FFF based model that you should use two main spars much like many full size aircraft. THen go with functional load bearing wing struts with a V format just like the original full size Super Cub. The spars would be located along the wing chord where indicated by the mounts for the outer ends of the V struts.

The wings will be separate and mount to the fuselage. WIth functional load carrying struts you don't need any load bearing joiner rods, tubes or wires, You just need a good solid U into U like shackle. The struts, fuselage and wing panels form locked in place triangles on each side. So even though the wing woudl flap up and down with no effort the struts lock it in place. This obviously means that the struts are holding up all the flight loading. As well the wing spars are doing part of the job as well as being under a fair amount of compression inwards due to how the struts support and transfer the loading in flight.

Note that with load bearing struts of this sort you need to design a load carrying "box" within the cabin area. The top of the box will need to withstand a lot of compression loading so T or tubular anti crush cross members should be built into the top of the box so they line up with the spar locations in the wings. These will also be what you connect the wing roots to. The sides of the box will be under vertical compression loads as well. So again some vertical stringers that form T sections when glued to the box sides will add the stiffness needed to prevent just deflecting and collapsing. Across the bottom of the box you'll need a carry through "beam". This beam will have a LOT of stretching loads in it and as well it has to be where the strut connections mount. So care needs to be taken with this.

All this struts and fuselage load bearing box may sound daunting. But in reality it's the best way to build a reasonably light yet very strong structure. It's why the full size planes of this type so typically use wing struts. It'll take a bit of a mental shift to appreciate the differences from our usual one piece wing that takes all the loads but if you can manage the shift in thinking it makes the model easier to assemble and break down at the field.

Another option would be to use the two main spar design but at the same time use small diameter music wire joiner rods in the same way that model gliders do. The wing struts would still be load bearing as the fairly small joiner wires would be used only for ground assembly ease. Something like 3/16 wires won't hold the wings on their own but they will hold the wings in place well enough to make hooking up the struts easier. You'll still need to have a screw or pin to secure the wing panels at the roots though. If you rely on the slip wires to hold it then you'll see the wings slide off and fly away if you should go inverted. You need something to lock the wings in place after slipping them on over the joiner rods. Or if you opt for the simple U shackles to join the wings then a small "saw horse" or FFF box "jackstand" that fits under the wing tip to hold the wing up will also do the job.

hackdroot

Thank you for taking the time on your detailed input. You brought up some considerations that will surely help the integrity of the airframe. The cardstock laminating the blucor is an excellent low cost solution. I'm purchasing some of the building material today. Some of my concerns still though:

1) I'm trying to figure out how to properly scale up this airframe. Simply tripling the dimensions isn't an appropriate way to accomplish this. I may use it as a baseline and use some of the fundamental ratio's I've read about to adjust the tail size in relation to the wing area.

2) Which wood to use for maximum strength, minimum weight. I do have quite a bit of basswood could use for stringers and have some light ply for the strut attachments on the wing and fuse ends.

3) Airfoil design. I'm taking a crash course on this right now. The HZ Super Cub uses a non-symmetrical, deep camber design which produces stable slow flight with high lift. Not sure how to design the airfoil for this yet, but I am learning a lot about it in the process. Once the fuse is done and most of the wing pieces cut, I will have a better idea of which power system to use and the end result AUW. Maybe I'm just over thinking this and should just get to cutting.

EDIT: After browsing Lowes I decided to do the fuselage in the thicker white foam (I think it was 1"). Although it will add more weight, it will be easier to build and stronger than with the fff. I have a 50-65 350kV outrunner that swings an 18x10 prop at over 1200w which I could use if it gets heavy.

-Eric