View Full Version : Monocoque Fuse - Pucara IA 58

01-02-2006, 03:20 PM
I'm goin' to start buildin' an IA 58 Pucara. I have a plans for this plane, 1.4 mts wingspan for two .25 glow motors, I'm planning converting this to electric.
The plans didn't have a fuse top view, just a side view and includes its inner frames (6) in halves.
How do I have to build this fuse?


Sky Sharkster
01-02-2006, 06:35 PM
To RiverPlane, I'm not familiar with that particular plane but normally you would cut out 2 of each of the bulkheads, number them 1-6, then pin down the top and bottom keel pieces (long wood pieces that form the "spine" of the model, usually called "longerons" or keels) and glue each half-bulkhead in place. When this is dry, remove assembly from board and glue opposite bulkhead (half) pieces in place. Now you will have the upper and lower keels and each bulkhead (complete, both matching halves) in place. There may be a few "extra" pieces to add, like a wing strut doubler, landing gear or stabilizer brace, just add these as you go. When this is done, begin adding "stringers"( usually square balsa wood, there will be slots shown on the bulkheads to indicate size and location) into the slots of each bulkhead, each stringer should go from nose to tail. Put them on in this order; Top left stringer, top right stringer. Bottom left, bottom right. Keep alternating in this way until they're all in place. This method insures you won't warp the fuselage by putting too much tension on one side.
At some point you'll have to add servos (or at least servo trays) before the fuse gets too "closed in", you will figure it out as you go. Same for battery tray, pushrods, ESC and receiver. A couple of hatches may be needed.
Once it's framed up and you've made provisions to get at the controls, electronics and power system you can glue tail surfaces, motor mount, wing attachments, etc in place and cover (or sheet) the fuselage. Some modelers like to cover the tail before gluing in place.
It's a lot of work, "scratch" building but it's also very rewarding! A "one-of-a-kind" model and YOU built it! Good Luck, Ron

01-02-2006, 11:05 PM
sounds like it's meant to be built in halves. Pin the keels to the board, install the formers, then add longerons and planking. Then do the same for the other half and glue the completed halves together.
Having at least 1/8" spacers under the keels makes it easier to plank.
There is less of a risk of building a bent fuse this way. But you must make sure that the 2 halves are identical or they won't join. Easier if you use the same side view turned upside down for the second half.
There are a few pictures of this method on the last page of this: http://members.aol.com/KMyersEFO/shaw3.pdf

Attached is a picture of my FW190 done that way.


01-03-2006, 01:11 PM
Thanks you both for the answers. I post the fuse plans, you can see the formers at the bottom (Numbers 1 to 6). The person that made the drawing insist me that the fuse is build as monocoque. You cannot see longerons or stringers drawed. Besides I think is better to have entire formers than halves, but it just my opinion. Please continue posting if you have any idea.

01-03-2006, 08:41 PM
the half shell method doesn't need longerons. Sometimes I add them where sheets join but you could just use scrap pieces of balsa for that.
The fuselage looks buildable except the bottom corner is kind of sharp. Your sheet may split when going around it. Soft sheets and wetting them (with water or ammonia) may get them around.
The formers only hold the shape and see very little stress after the two halves are joined, wouldn't worry about half formers. I use 1.5mm balsa for all formers that have no other function. Never had one fail.


01-03-2006, 09:00 PM
Does it say to plank in 4x10 balsa strips? That can be done. There is a description of the method here: http://modelairplanenews.com/how_to/strip_plank1.asp
I would try to use sheets (or at least wider strips) on the flat parts of the bottom and sides.
I'd say it's next to impossible to build this straight in one piece; I'd build half shells to keep it straight. Only formers 2 and 3 need to be strong. Add a ply doubler to them after the halves are joined.


Sky Sharkster
01-04-2006, 11:41 AM
To RiverPlane, After seeing the drawing, I think Hans is right. There's no stringers and only a couple of short longerons shown (behind cockpit and wing) so you won't be able to create an "internal structure" then sheet it. I'd suggest attaching the 1/2 bulkheads to your board, raised equally to allow for sheet overhang. The two longerons will help with the rear but in front you're on your own! Then use wet sheet balsa and mold it around the bulkheads. Maybe it could be sheeted with strips, if so you might start with (about) 1" wide strips (full length) starting just below the cockpit and running straight back to the tail. Then add strips above and below this strip. The strips will have to be more narrow around the top where the curve is greater and could be wider at the bottom.
The only other method that comes to mind is pretty work-intensive; Carve a foam mold using the bulkheads as a guide,split the mold vertically, then sheet each half of the mold. Remove mold from sheeting, add bulkheads. If you use this method, it might be worthwhile to fiberglass the structure after it's joined.
Either way, you picked a tough subject! Good Luck, Ron

01-04-2006, 09:59 PM
Chris Golds uses a foam plug to form the sheeting. I think it goes like this: make plug from blue foam, soak sheets in ammonia until really soft, put them on the plug (no glue), hold them there with rubber bands etc until dry. The sheets hold the shape of the plug when dry. Use these preformed sheets to make your fuselage.
All these methods are easier than they look.

The easiest would probably be to modify the fuselage and build it as a box with a round top. Sheets for the flat bottom and the flat part of the sides, triangle stock in the bottom corner, strip plank the round top or use thinner sheet and bend it around the top. This is similar to my FW190 in that picture.


01-05-2006, 03:53 AM
Many thanks for the tips.
I decided to CAD the plans. This plan is very old and is a copy of a copy of a copy... Maybe I can straight one or two lines... This will help to know it better.

I like the half method, but I think I must add a couple of extra formers. Also adding stringers won't hurt. HUL, I like the plank link, I got that tool, is a must. Take a look at this method www.rcuniverse.com/forum/m_2143837/mpage_1/key_/tm.htm

Back to Pucara, looks like behind the cockpit runs a longeron (marked in plan as "Balsa Dura 6x20") but none is seen on former 4. If I am correct, should it be there, right?
I really don't get why former 6 is that tall. It shows the tail's trailing edge?
Thanks you all again

01-05-2006, 04:39 AM
Back to Pucara, looks like behind the cockpit runs a longeron (marked in plan as "Balsa Dura 6x20") but none is seen on former 4. If I am correct, should it be there, right?
I really don't get why former 6 is that tall. It shows the tail's trailing edge?
Thanks you all again I think it should be there. It's what I would call a "keel". Use a keel (3x20 each instead of 6x20, but reduce size to 1.5x15 for electric) for each half, if you build half shells. Like the former for my FW190 shown below.
Former 6 is the inside line (the line closer to the number 6), the outside line is the whole cross section at that point, including all planking and the elevator.

Is the planking meant to be 4mm thick? I would actually use much thinner skins (1.5 or 2mm) and glass it instead. And 2 more formers between 3/4 and 5/6.
The weak spot will be between canopy and leading edge of the wing. Doublers, stringers or glassing the inside in that place wouldn't hurt. Same thing for former 2 where the front wheel attaches.

I have seen that method in a magazine. Don't know for sure, but looks hard to do to me.


01-05-2006, 01:05 PM
If I understand what your "keel" means, it must lay all over the building table when doing half fuse, right? I mean it runs in the middle top & bottom of formers.
I realize former 6 is not hollow. That's all.
The issue with glassing the fuse is the finishing. Don't know if monokote can be applied over glass. I did it over fiberglass on wing union in other projects and looks a little ugly. Maybe first use a thin sheet, glass and sheet again :confused:???
Anyway, is the best method the half fuse. I'll go for it. Our friend at http://www.airfieldmodels.com (build guru) make fuses this way.
I got a book about the 30th aniversary of this plane, not yet in my hands, but on the way. Hope find more info there.
Post a pic of the real fuse.

Regards, Alex

01-05-2006, 09:43 PM
the Pucara fuselage is quite similar to the FW190 fuselage, flat bottom and round top. There are a few more picture of the build of the FW190 fuselage in this thread: http://www.rcgroups.com/forums/showthread.php?t=365211
Have a look and see if they help.

The keels go on the board (on top of the spacers), the half formers on top of the keels. The picture shows it. I drew a stringer where the round top and the flat side meet, but you could leave it off and add strips of balsa between the formers after the round sheet is on (attach with cyano). The spacers allow for sheeting overlap, don't have to cut the sheets to the exact size that way.

Glassing is easy, but you need to paint. Monokote doesn't go over it, I think (never tried). I don't like film, it keeps wrinkling and blistering.
Glassing using 3/4 oz/sqyd (or 25g/m2) cloth using polyurethane paint (oil based, water based may warp your work). Some people use epoxy, others use epoxy thinned with alcohol, I prefer the polyurethane (try on a scrap piece first to see if it dries reasonably quickly and can be sanded, some brands can't be sanded). Some people dope the wood before they put the glass on, to keep the poly or epoxy from soaking into the wood and adding unnecessary weight, I haven't tried this yet.
Lay the glass on dry and paint through it. Sand, primer (automotive grey primer) and paint (water based acrylic from the art shop). It gives a nice stiff surface.