Hi, I'm currently drawing plans for a Cessna 140 that I want to model. The wingspan will be 56", and I hope to power it with a .26 or .30 4-stroke. I also want to put a scale interior in it. I am having difficulty figuring out how to attach the wing. Most of the kits I've built have had the wing spars continuing through the fuselage, and have bolted on in the rear. On this particular model, this would get in the way of the scale interior. Also if I put in a scale interior, how would I access the radio gear like the battery and receiver?

with out the spars connecting the two halves togethet you may have to do what the glider guys do.use music wire secured in the fuse with brass tubes in the wing panels and slide them on.you need two one thick wire 1/4 at the main spar locaction and 1/8 just in front of the trailing edge.only other choice would be to make the struts functional other wise the wing is going to fold.

Thanks for the reply. I intend to make the wing struts functional. Please tell me more about the music wire method. Does the wing remove in one piece or two? Is the wire visible through the windshield?
Thanks,
Pat

the wing is two pieces and yes the wire would be seen from the windshield unless you could hide it behind a bulkhead.you will find that drawing scale plans is a bunch of compromises .

Hi, thanks again for the reply. Since I am planning on making the wing struts functional, is there a better way of attaching the wings? I was originally thinking about using two metal rods welded to a 1/8 inch metal bar. The metal bar would then be screwed to the fuselage and the wings would slide on and be secured by a screw into each rod. The fuselage would be a double layer of plywood where the wings attach. My concern with this is whether or not it will be strong enough, and if it will weigh too much. I thought about using aluminum, but I don’t have the tools to weld aluminum.

You may want to look at the method that SIG uses on the 1/5 scale cub. The middle part of the wing is part of the fuselage with tubes that accept the wing tube.

Hi, that's hind of what I'm trying to do, however I don't want the wing tubes to go through the fuselage. They would be visible through the windshield, and get in the way of the scale interior. If I make the wing struts functional, how much leeway do I have for the wing connection?

The connection at the root is going to bear 1/2 of the total force any way you figure it. So lets say worst case at a 50 g load for a 5 LB model you will be dealing with a load of 125 lbs. total or 62.5 lbs. at the root. This will be a combination of compressive force towards the center and shear force up and down so your cabin will have to be able to withstand both the compressive and shear force as far as the wing attachment I would make the root of the wing itself out of 1" thick balsa stock into which you set two or three sockets for the rods that you have attached to a 1/32 thick piece aluminum sheet stock which is fastened to the cabin side. I would look at combining this with the window and doorframe so you can get it down the side of the fuse for maximum shear strength. You can fiberglass the into the top sheathing and fuse sides for maximum bonding and strength. I would also fiberglass the cabin ceiling along with the top of the wing sheathing for maximum strength. A hard point to mount your spar on both the wing and fuselage and your done.

Now keep in mind that the actual amount of stress depends upon your gross weight and the 5lb ref. is only an example.

To better define what I was saying in my earlier post, the point I am trying to make is this. With what you are trying to do you have to look at the whole assembly as a whole.

The top of the cabin is going to have to deal with the total of forces I described each wing root and strut attachment point will have to support 31.25 lbs. of force and that is what you have to try to picture.

Not all of that force will be in one direction at any give time with the exception of your cabin ceiling or roof having to deal with the compressive force most of the time. If you make you cabin roof /wing sheathing over the cabin out of 3/16 to 1/4 inch stock which is fiberglassed on both sides this will have a lot of strength under compression if you run you balsa from side to side. So all you will have to worry about is having your mounting surface deal with the shear force caused by the wing root by making this mount a part of the door window frame you will be distributing this stress down the side of the fuselage

By making the root rib out of 1 inch thick stock all of the load can be taken up by the tubes that you use as sockets that slide over the rods protruding from your fuselage sides and this will give you a lot of area to distribute the load to the Spars and sheathing you use for your wing. So as you can see I would sheath at least from the root rib out to the firs or second set of ribs. You might want to fiberglass the wing root assembly also.

Now to secure the wing to the rods all you have to do is drill through you tubes/sockets and rods in the center of each rib thread the rod to accept a machine screw that you run down from the top of the wing through the socket tube and into the rod and back out again so it extends toward the bottom of the wing place a nut on it for a safety and you wing is secure. You can recess both the heads of the screw and the nut below the surface of the wing for clean airflow.

Thank you, that gives me something to work with. Would you make the wing ribs that the sockets go through (with the exception of the root rib) out of plywood? A 1/32 piece of aluminum is going to be quite flexible, is this just to prevent tear-out of the rods? Would I laminate this to the plywood that the cabin area is being made of, then fiberglass over that? Also, would using fiberglass on the wing roots prevent me from using silkspan and dope on the wing? I was going to try to keep it true to the original with a fabric covered wing.

Yes the aluminum is just to prevent the tear out of the rods, I would laminate this to the cabin sides. As far as the fiberglass that should not be a problem with the silk span, or any fabric covering. I have used silkspan over fiberglass without any problems.

Given time to reflect on what I said above, if you are going to use plywood for the cabin sides I would not even bother with the aluminum. I normaly use balsa for my fuselages and try to keep the use of metal or plywood to a minimum. In your case I would just fiberglass both sides of the ply with lightweight cloth/epoxy and leave it at that. Now as far as the rods 1/4 inch hardwood dowels should suffice. That is how the wings on my Belanca Airbus (flying W) mount, and it has a 120 inch wing and is pulled by a zenoa 35 cc gas engine this plane weigs in a 20 lbs.. Now I fly it scale and do not use it for aerobatics much but have had no problem with the structure thus far. Laminating a thin ply to the root rib might be a good idea to secure the sockets better but might be overkill.
The main goal we have to keep in mind is to keep the airframe as light as possible, that will give you the best performance. I have found it all to easy to over engineer my airframes in the past adding lots of weight in the process.

Thanks, that sounds even better. I was just planning on using the plywood on the cabin from the wing trailing edge forward. The rear will be balsa. I have a tendency to try to over engineer things as well, and I think that comes from building kits. A lot of the kits I've built have been over engineered. However they do stand up well to some not-so scale maneuvers.

For me those "not so scale maneuvers" are just before impact, ahm landing, most of the time....

nightflyer,
I have built a model of the Cessna 120 and had the same problem...how to build the wing carry thru so that I can make a scale cabin. I was building from a plan so I did as it was drawn but I think I'd try to do what was done in the real thing. The main load on the spar carry thru is compression so I suggest using brass (or aluminum) U channel or square section to tie into the wing spars . If the wings need to be removable you could use pins to attach them to the fuselage. Metal is good in compression so a channel or square tube section of fairly small size would work and not interfere with the scale interior. Of course the carry thru parts would have to be securely attached to the cabin structure. In order for this to work the struts need to be functional and a solid mount to the bottom of the fuse is necessary.
Here is a sketch of the idea.
Good luck with your project.
Paul

Make sure that the fuse sides that the wing will be attached to can't flex and twist outwards at the front of the cabin. The wing creates both induced and parasitic drag which will want to pull on the wings in a backward direction in flight, even more so with a fat, flat bottom, under cambered or Clark Y kind of airfoil and constant chord wing. The cabin at the trailing edge of the wing will have to deal with some compressive force, the way i see it anyways.

I agree.
Although I didn't say it I was thinking of a two-spar carry thru ( fore and aft ).
Paul

That is why I was stresing that he glass both the top and bottom and sides of the cabin for strength as well as the inside. It is critical that the cabin structure be rigid as possible for the reasons you have stated.

Yes, if using light ply for the side maybe go with a lamination of glass or even maybe thinner 1/32 or 1/16 birch ply, the more resistant the cabin sides are to twisting and bending the less carry through structure you'll need. I think you could get a rigid structure made even from 1/32 birch ply laminated to both sides of 1/8 balsa center for a small plane like this. I've used that to make a structure for a fuselage that had two big doors on either side that could open, that structure carried the loads around the doors, worked good and didn't weigh a whole lot.
The best thing is to try a keep the plane light as possible, then you won't need as much strength. Although parasitic drag will remain the same , the lighter the plane the less induced drag the wings will develop.

Canadagoose I like that idea of using a balsa core that with 1/32 birch ply for the cabin sides, that should keep it light. That would be much lighter than a single slab of plywood, and should keep the cabin sides rigid and light. I like you float planes by the way that twin looks very interesting....
Regards
Paul

Thanks iron eagle, currently building the twin.
That method of laminating thin ply and balsa ends up actually very strong and rigid, sort of like laminating balsa to foam core for wings.

Hi, thanks for the suggestion. The balsa/ply lamination sounds a lot lighter then a single slab of plywood, or the two I was planning.[X(] It would probably be easier to cut too.

Good luck with the build Nightflyer post some pics when you get it done.
Canadagoose, good luck with that twin looks like it will be a real screamer of a float plane.