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Building Tips and technical articles. => Building techniques => Topic started by: Tim Wescott on May 29, 2017, 10:00:57 PM
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Thinkin' of building an Ercoupe for scale, with it's nice simply-curved but significantly tapered fuselage. Thinkin' about keeping it light. The fuselage would go from roughly 6" diameter down to 2" or so (I'm planning on 1/6 scale).
Do I want to:
- Mold a bunch of panels individually, then glue them up later -- so I'd have a sort of hybrid between planking and molding
- Splice some straight sheets together into a honkin' big sheet to do the top, and ditto for the bottom -- this would leave the grains of the top and bottom moldings meeting at quite a sharp angle, and result in considerable waste
- Cut the rough taper into a bunch of sheets (i.e., taper from 3" or so down to 1", with as many sheets as needed to make up the circumference), splice edge-to-edge, and mold that
Structurally, I think I like the idea of pre-tapering and splicing before molding, but I'm not sure if the resulting tapered sheet would be flexible enough, or if all I'd do would be to invent a really laborious process to make balsa scrap.
Your thoughts appreciated -- particularly if you've been down this road before, and especially if you found a nice fuselage at the end of it.
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Liking your 3rd option, don't understand the 1st one..
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Your thoughts appreciated -- particularly if you've been down this road before, and especially if you found a nice fuselage at the end of it.
Berkeley makes an Aircoupe kit which is a bit smaller than your 1/6 project.
They do pop up from time to time on ebay and I'm sure plans are available.
With full size Berkeley plans as a start, you could easily enlarge them to your 1/6 scale or a bit larger.
A possibility.
Good luck.
CB
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The two successful fuse's I molded were for my old Sierra design, a Trivial Pursuit based ship, and I did a left and right shell. Slicing the front to achieve the taper I had in it worked very well, and with the formers shaped from sectioning the mold buck for patterns, the fit was quite good.
When installing controls I worked with the outboard half on the formers and then added the inboard side when they were complete. It worked out well that way, and with a 3/32" sheet fuse, was fairly light.
The ship flew very well, suffered an inverted splat from a dust devil, and I built the next fuse the same way, reused the wing and tailfeathers, and had a very good ship again. Wish I had saved the mold buck, but it got lost in the shuffle! I have a Bob Hunt foam wing ready to do a third one.
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the final fuse in rough form and the mold buck
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The front end nearly ready for completion
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Tim I am thinking none of the above.
I have successfully molded shells of that shape quite easily using two layers of 1/16 balsa laminated together. For the tight compound curves you want to splice the wood in the grain direction every 3/8in test before you mold and you are done. Two shells left right side or top bottom layout will do fine.
The way i did it was to make a mold (undersized to compensate the shell thickness) out of anything Blue pink foam whatever. I put the shells in the bath tub to soak up the i pulled one out of the water and stuck it to the side of the tub and smeared white Elmer's glue on it, I placed the second sheet on top of that one, After that i wrapped then with Ace band aid over the mold. Next day i glued a Exacto knife blade on top of a block and carefully cut the parting line off.
After you join the second shell you have a full fuselage.
Will had a good idea but the problem with molding a single layer of wood is that it wants to return to the original shape.
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You could probably get away with a double layer of 1/32.
However, I believe for building a "one up" fuselage or model, molding may not be necessary.
You'll still need a sub structure.
I may have been 13 or 14 when I built the Berkeley Aircoupe. It was a challenge and I never finished the model.
I will tackle the Aircoupe again one day. I think about it quite often.
The Berkeley Aircoupe kit calls for planking the fuselage.
This Aristocraft kit of the Grumman F3F-1 is an example of a model with a sub structure then sheeted or planked.
I stopped working on both of these for different reasons, mostly retracts and airfoil issues.
The sub structure for this Gee Bee R-3 is planked. A method that works well and is quite common in scale model building or models with round tapered fuselages. Floyd Carter used planking on his Lockheed Sirius.
So, there are construction options, but for a "one up" model I personally wouldn't spend the time molding outer fuselage sheeting to go over a sub structure. I look at it as extra work which is not needed. But that's me.
If I build an Aircoupe, most likely I'll plank the fuselage over a sub structure.
CB
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Avaiojet with two skins roughly 1/8in thick why would you need all that internal structure for. The only structure you need with this kind of skins is to mount the motor and some reinforcements for the wing saddle if you do a take apart wing. If you really need reinforcements the add a layer of 93g/M2 carbon fiber inside the fuse shells before you join them and you are done.
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Avaiojet with two skins roughly 1/8in thick why would you need all that internal structure for. The only structure you need with this kind of skins is to mount the motor and some reinforcements for the wing saddle if you do a take apart wing. If you really need reinforcements the add a layer of 93g/M2 carbon fiber inside the fuse shells before you join them and you are done.
Traian,
What two 1/8" skins are you talking about?
CB
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Tho layers of 1/16 balsa laminated together to create the left right shells (or top bottom shells). I have used the term shells and skins for the same thing.
Sorry for the confusion.
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I believe Bobby Hunt has a disc/tape on molding balsa, At Robins View Productions in the vendors corner.
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Tho layers of 1/16 balsa laminated together to create the left right shells (or top bottom shells). I have used the term shells and skins for the same thing.
Sorry for the confusion.
Traian,
I know of the outstanding work you do with contemporary Epoxies and CF. H^^
I'm just an old school hack that doesn't want to put in the time for molding a one-up fuselage.
Besides, the way I build this model may need the over built thing because it's designed for touch n goes on grass.
I'm hoping it flares like an R/C model.
BTW. I have seen photos of your models. Absolutely great work. H^^
Thanks for the interest and the reply.
CB
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Like so many things, there is more than one way. I use a layer of 1/32 and a layer of 1/16 with carbon fiber sandwiched between bonded with thinned aliphatic resin. For compound curves make scarf cuts and vee cuts to stop buckling. In my mind, the hardest part is the buck! Because someone said it was nearly impossible, I even molded a canopy. Patience is required.
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Avaiojet with two skins roughly 1/8in thick why would you need all that internal structure for. The only structure you need with this kind of skins is to mount the motor and some reinforcements for the wing saddle if you do a take apart wing. If you really need reinforcements the add a layer of 93g/M2 carbon fiber inside the fuse shells before you join them and you are done.
Traian,
I have samples of CF cloth.
You suggested adding a layer to the inside. Using Epoxy to laminate it in place?
Charles
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Charles, I have used both epoxy and oderless CA. I prefer the CA. I failed to mention the carbon fiber "formers" in my Sierra fuse; there was no attempt by the 3/32 to return to original shape on either one of my fuse's. I did not completely cover the insides with the CF, only strips every so often.
As far as I know, that plane is still flying somewhere around Nashville.
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The basic techniques have been covered. I have done well making molded parts by first shaping the mold from blue foam. The skins are two layers of 1/32" with a layer of silkspan in the middle. I use Titebond to soak the silkspan before assembling. The thin balsa can be bent somewhat in both axes with minimum "slicing". Thicker balsa resists bending and makes the job harder.
With a final layer of carbon veil after final assembly, this shell is very rigid and light.
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In my mind, the hardest part is the buck!
See me about bucks. Matched R and L parts for Tim should be easy.
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See me about bucks. Matched R and L parts for Tim should be easy.
Were you going to print the bucks or something? Interesting idea -- I was planning on polyurethane foam and a long sanding block.
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Dear Tim,
Think over this:
https://lh4.googleusercontent.com/-IKOLTLNWz10/T29-tQzqxII/AAAAAAAAAao/dwRl5KrOC6MlP0C3z0-bLP-Q9m4bqECWQCL0B/w764-h573-no/20120325202.jpg
The full album:
https://plus.google.com/photos/117790355930193335731/albums/5715088163776972593
Original topic here:
http://stunthanger.com/smf/building-techniques/the-bluefoam-model-or-how-to-build-a-stunt-plane-without-a-bit-of-balsa/
Regards: Istvan :!
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Now I think of it, Al Rabe has some molding stuff on one of his discs.
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Istvan,
Outstanding workmanship. I've said this to you in the past.
CB
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Were you going to print the bucks or something?
Yup.
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I think it'd take me less work to make the bucks up with green foam and a long board than it'd take me to do a decent 3D model of an Ercoupe fuselage. But I'm going to keep this in mind as an option.
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Dear Charles,
Thanks for the nice words: I realized that that is not a tragedy for you, to "switch" the basic technological thinking method onto another rail :)
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Traian,
I have samples of CF cloth.
You suggested adding a layer to the inside. Using Epoxy to laminate it in place?
Charles
That is correct. There is a technique to it to keep the part light. If you place the cloth on the wood then you wet it out you will get a heavy part because the balsa will soak up the resin and it is just unnecessary weight. Instead you can stick the carbon to a piece of plastic (2-3mil poly) with 3M 77 (extremely light application, spray 2ft above on time and let it "rain" down on the plastic), after that wet the carbon on the plastic and blot out the excess resin with paper towels. Now you are ready to apply it to the balsa (Leave the plastic in place it will peel off easily). One more thing here: the resin content is very low at this point and you do not want ant dust on the balsa wood since it will kill the bond so vacuum it the best you can before you apply the carbon.
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Dear Charles,
Thanks for the nice words: I realized that that is not a tragedy for you, to "switch" the basic technological thinking method onto another rail :)
Istvan,
You're kind.
I might just give it a shot.
Do you have plans for this model? Templates? etc.
Charles
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Charles,
I made a good quality 1:1 copy of the original pencil drawing (it has made on brownpaper-back: I use that from my student years, because it is easy to rub & redraw), so the copy is scannable. Some ribs were modified, but all the plywood templates are present, and I am ready to draw them onto paper and scan it. For airfoils I used
http://www.profili2.com/
software.
(Excuse me for long "offtopic", Tim)