abufletcher

Does epoxy or for that matter slow CA have the same (or greater) scale strength as a weld on an original aircraft? Also does anyone have any experience with simulating the steel tube fuse construction of WWI planes with dowels. How strong is the resulting structure?

Flak

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I can't answer the first question. I am not an engineer. However, I can tell you I scratch built a trainer type plane of my own design for my son with all the stringers in the fuse being dowel rods. The plane flew great and was very stout.
"Keep "Em Flying!"
Flak

BykrDan

I can also attest that the dowel-stringer construction produces a nice looking and strong aircraft (not sure you can even tell the difference under covering).

Capgains

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Aren't dowels kind of heavy to build with, or do you use fewer of them as a result of their strength?

Dan

Ducted Fan Dan

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If you look at balsa stringers compared to dowel rods, dowel rods are far and away stronger. They are however heavier.

Jim OHaver

Does epoxy or for that matter slow CA have the same (or greater) scale strength as a weld on an original aircraft? Also does anyone have any experience with simulating the steel tube fuse construction of WWI planes with dowels. How strong is the resulting structure?

I'm not an engineer, but I think I know where you're headed with the query. Maybe I can help; I've been down this road several times. First, I don't think the expression "scale strength" is a particularly meaningful term for what we do. I have done some testing on my own, largely because I didn't believe what I was being told by experienced modelers who happened to be people who had alphabet soup behind their names. Turns out they're right, even though what they say sounds counterintuitive.

One. Forget about the difference between adhesives. The effectiveness of your load bearing structure will not depend significantly on the strength of the adhesive, but rather on the load-spreading ability of the joint. I use plain old aliphatic resin for most purposes. Epoxies do not add to the strength and are incredibly heavy. Cyanos are impossible to sand, cut, or work over. If you think about this for a moment, it will become clear that this is so: think about the last time you examined the post-mortem of a crash or dork (yours or someone else's). If you look closely at the fractures, you will see that it is untypical for a joint to have failed. The wood along the fracture actually shows a thin layer of adhesive to which is still adhered a thin layer of wood fiber that tore off the "other" part. In other words, the joint did not fail, the material did. It is the fibrous nature of balsa that permits this to happen, and it can be stopped through better engineering, by gusseting each joint. Doen like this, old-fashioned "wood cement", ambroid, white glue, whatever, will provide PLENTY of local integrity, since the gussett puts the glue in shear, where its strength can be put to good effect.

Two. Dowels are a losing proposition. If you care to do the research, you will find that the material available to us that has the best strength to weight ratio is not spruce or fiberglass or carbon rod, but plain old balsa. BUT, it's strength has to be correctly applied. It is TREMENDOUSLY strong in grain-wise compression, less so in tension. In torsion, it is relatively weak. Dowels are less strong for a given mass than balsa. It will also prove to be very difficult to get go, straight-grain material, and a stick with grain run-out is a break waiting to occur. Finally, I can't imagine trying to get dowels cut so as to fit against each other well; filing a half-round into the end of a stick so that it will butt snug to the side of another dowel? It would take forever to cut and fit so many that way, and the resulting joint would still be poor. So, use balsa, but get the bits in place so that all parts are being held in compression against each other. This is done by use of diagonals, which create triangular space frames that can not distort out of shape until they fail and will resist shock loads, but spreading such shock loads throughout the structure.

NO STAND-ALONE BUTT-JOINTS!

But your question is part engineering and part cosmetics. It looks to me like you want to duplicate a steel tube structure with something that is round in cross-section. Well, okay, but why? With but a few exceptions, it's going to be covered up, anyway, so build the framework up as diagonally braced truss of balsa, and lap each joint with a small gusset of birch ply. Your framework will weigh as close to nothing as you can get. If you're hell-bent on building some project that had exposed tubing structure (such as some of the WW1 pushers like the DH-2), then different techniques altogether are suggested, and I would go with soft aluminum tube inside which was nested a balsa dowel, the tube being for local hard point to support cable rigging attachments and to provide a smooth and cosmetically correct appearance. (Balsa dowels are made by forcing square strip through prgressively smaller holes in a drill guage until you get the diamter down to the dimension you want. With practice, and with good, straight and close grain balsa, you can get it down to 3/32" diameter. ) Intersections of these joints have to be internally gusseeted with small pins into end grain, through holes drilled in the side walls of the tube at correct locations. I've built a couple of these, and you need everything in a jig to make it come out square. It works, but it's tedious, and requires a pretty good "eye" and steady hand. Not for the timid. I've never even thought of trying to replicate a welded steel tube fuselage that is covered (such as Piper Cub, Fokker D7, or such), as there will be no functional improvement by so doing, when a properly gusseted truss is hard to beat.

Hope this helps. Jim

Capgains

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Great answer!!!

I like your last paragraph. What makes this hobby great is that you can choose materials that "cheat" compared to what full scale application requires, and be so much lighter. You sure don't see any of us spending time trying to find an engine that creates the scale amount of thrust only.

Dan

BobH

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Several people have sucessfully used dowels on their planes. Proctor uses them in the construction of their DVII. Dowels are usefull for some scale applications. Covering a fuse or any other part will not hid the fact that you are using square balsa instead of round stock. Wooden dowels are stronger than balsa per deminsion (not mass) and heavier and more difficult to work with. Gussets are most usefull when working with round stock. If you were truely wanting to duplicate a scale plane in all its aspects I would say.."try it". There is nothing to loose in the experience. It's not necessary but you might enjoy the learning ?? The worst that can happen is you'll find that either you like or dont like it and may learn something along the way.. You can contact Proctor via phone and ask them about their DVII construction. They are super helpfull guys.. BobH.

PT21Flyer

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If you are looking for true scale appearance, why not get 4130 steel tubing and jig weld it with an oxy-acetylene torch just like the real thing. My Dad and I built a 1/4 scale model of his full scale J-2 Cub this way and it came out weighing 15 lbs. which is as light or lighter than most 1/4 scale cubs. It's a whole lot stronger than anything else too.

Jerry

John Cole

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Proctor has a great construction method for building with dowels. They have you use a drill jig, and "cut" the dowel to length with a brad point drill bit. Works pretty well. The joints are then rienforced with brass 'star' fittings which serve as anchors for the bracing wires. But Jim is right, good fitting, gusseted joints will be much stronger and lighter. So why does Proctor and others choose to be impracticle? Scale realism. Alot of these models are never flown, or even covered. Scale realism outweighs function. It`s a tradeoff that scale builders are always faced with.
The point Jim is making, I think, is that there is no substitute for a carefully fitted, gusseted joint. He`s exactly correct. 1/64" ply serves as a great gusset material, and adds a huge amount of strength and integrity to the joint, while adding very little weight. And remember this.. static strength is one thing, but vibration puts structural loads on an airframe that may surprise you.

John

BobH

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The point Jim was making is that you should not use dowels for a lot of reasons. I haven't had the opportunity to try them but I might sometime in the future. There is little doubt that square stock, regardless of materiel, is easier to join than round stock. On the other hand I have seen scale planes with dowel construction that looked and flew very well. Grahm Mears (sp) built a Waco using welded tube construction. The plane is specatular and was recently sold here on rcu. I don't think you could match what he did with traditional methods.. ?

John Cole

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Yep, a junction of dowel joints is really easy to mess up! And a slathering of epoxy really doesn`t help much, and adds weight. This makes me wonder why more full size a/c builders don`t use square tubing. Expense? Weight?
I`ve just finished up plans (finally) for a DH-2, and the booms will be 3/8" thinwall aluminum tubing, using ramin dowel inside at the boom junctions for compression loads. No glue at all, just cables.
For all other glue joints, it`s very important to make the joints as perfect as you can. Don`t rely on the glue to make the joint strong, regardless of what adhesive you use.

John

BobH

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Tubes have the mechanical advantage of an even distribution of load over the entire bearing surface. Squares have weak points at the intersections of the sides. Building a fuse with square or rectangular stock would require a larger cross section to give you the same strength as round stock. So weight becomes a factor for the full scale planes. For our models we get away just fine with square stock even when its not the best choice; leading and trailing edges for example.

Jim Messer

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After Bob Nelitz designed his famous 1/3 scale J-3 Cub fuselage completely from dowels, I thought - "What a great idea". So I designed a 1/3 scale J-3 Kitten using wooden arrowshafs instead of dowels, because they were straight and lighter. Joints were a lot more difficult, but by drilling and pegging with a 1/8 dowel that acted like a nail - the assembly turned out beautiful - absolutely georgeous. Then I covered it with Ceconite, and painted it - and you know what? It didn't look any different completely finished than if I had used square wood with rounded edges. so - never again! Once is enough.

abufletcher

Well, thanks for all the tremendous expertise!!! It's always such a joy to explore such rich experience. In the end I went ahead (I started this post quite a while ago) and used traditional balsa construction -- with a few scale concessions. Here's the fuselage as it turned out. The bracing wires are .25 wire and the "turnbuckles" are just bits of black tape coated with CA. Under antique Solartex they will provide good shadow effects.

However, after just discovering a great source for cheap 1/2" turnbuckles (jewelry clasps at 13 for $2 at the local art supply shop) I'm sorely tempted to remove the wires and replace them with black elastic "wire" (also from the art supply shop). On the other hand the wire cross-bracing (with bits of paper to avoid and metal on metal) actually provides some degree of structural stability.

abufletcher

I think it might be a good idea to add the 1/64" ply gussets though.

BobH

My Feedback: (2)

Your fuse is looking very good. There are a lot of ways to "skin the cat" so usually you can choose what ever method you like.
Jim, you did learn that you won't use dowels again so that at least is usefull !

LesUyeda

I decided to build this Jungmeister out of carbon fibre kite tubing. I used a jewelers file to make a concave end to the tube at the junctures. Destructive testing said that the joints were as strong as anything else I could have used. Since this model would have used at least 1/4 square hard balsa, and I only had to use 1/8 tubing, I believe that there is an advantage. I did use wood in part of it just as an measure of economy. I got tired of paying for the tubing, and in some places the wood was easier. The resulting fuselage was more resistant to torque and bend than anything I had ever built before.

If you are using silk and dope, you can tell the difference between tubing and rounded wood.

Les

John Cole

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Les,
True composite construction! Nice work. What did you bond the tubing with?

John

LesUyeda

Thank you.

Sorry about that. It may be important:-)))))) I used medium CA on all of the joints.

Les

BobH

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Les, Very Kewl looking Bird!! builds very nice with the composit dowels.. Cant wait to see the finished product!

The PIPE

Dear Abufletcher:

The PIPE Here yet AGAIN...and Les Uyeda DID hit upon something I will be trying on my very FIRST RC Giant Scale aircraft construction project, a Dave Platt 1/4th sized Jungmeister to be started next winter!

RCUers MAY have seen the photos that I've posted here at RCU of my small, 42-inch wingspan Gordon Whitehead/RC Modeler magazine plans-built Jungmeister, a "flying dedication" to my favorite singing act of ALL time, Scandinavia's ABBA, and it's a plane that is SO old that it's in need of a WHOLE LOT of oil removal from its fuselage structure before IT will fly again (it hasn't flown since 1984)...and about that time, twenty years ago, I got myself a copy of the Dave Platt Jungmeister RC Giant Scale plans from RC Modeler for the future construction of a "Giant Scale ABBABIPE", to be finished in the EXACT same color scheme as my smaller one has always had.

After a recent experience with working BOTH with 3/16th inch OD hollow carbon fiber tubing for the bellcrank-actuated Frise aileron pushrod linkages on my Balsa USA Swizzle Stick...and with fiberglass tubing to house the push-pull Sullivan "Gold'N'Cable" 1/16th OD cable pushrods going out to the tail of that same "dear ol'Swizzler" of mine, I decided that when I finally get the time to start building up that Platt plans Jungmeister, I would use CST Sales' fiberglass tubing (at https://www.cstsales.com/Fiberglass/...micro_rods.htm )...their T621-4 catalog number item (just over 1/4th inch OD)..for the LONGERONS, and for the verticals, diagonals and crosspieces, Aerospace Composites' WRAPPED carbon fiber tubes...at http://www.acp-composites.com/acp-tube.htm and CLICK ON THE "Carbon Tubes Price List" button to get the WRAPPED carbon tubing prices and lengths) would be the best materials I would consider for building that Jungmeister's fuselage with.

My choice for using FIBERGLASS tube for the longerons comes from the possibility-most likely "remote at best"-of the partially electrically conductive carbon fiber tubing causing radio glitches in a continuous length long enough to be a longeron-while my choice of WRAPPED carbon tube for the verticals, diagonals and crosspieces is because "ordinary" carbon tubing, with ALL the carbon filaments running "end-to-end", would be TOO likely to SPLIT LENGTHWISE just like a piece of hollow bamboo...it DID a couple of times on those pushrods on the Swizzler's aileron linkage while I was building it...and WRAPPED carbon tubes have a layer or two of carbon filaments DIAGONALLY wrapped around it, just like a hockey stick or baseball bat often has done to it with cloth tape.

At each joint, the tubes are best "stuffed" with a balsa "plug" inside the tube where the joint is to be made (and THAT process would take a WHOLE LOT of planning during the construction!!!) and with the ends of the carbon crosspieces all fitted with them as well in each end. The making of what are properly called "fishmouths" at the end of each crosspiece-the semicircular shaped cutouts previously mentioned in this thread...would best be made by GRINDING them into the ends with a Dremel cylindrical grinding stone of the proper outside diameter, that's MATCHED to the size of the longeron tubing being used (just check at http://www.dremel.com/html/products/.../grinding.html to find a proper bit to use with YOUR size choice of composite tubing) so that the fiberglass or wrapped carbon tube being used in construction will NOT get split or otherwise damaged needlessly while making up the pieces for an all-composite tubing fuselage or other structure!

To finally "pin" each joint so it cannot POSSIBLY come apart, using those 0.025 inch diameter bamboo skewers that are available from Proctor Enterprises...they're used for the fake fuselage crossbrace rigging on the rear fuselages of their "Antic" series of scale-like sport aircraft...as pins, with them inserted into the joint lengthwise and right THROUGH the longeron and the balsa plug there, and STRAIGHT into the plug at the end of the tube to "pin" each tube in place...THAT results in a structure that can REALLY resist the wear and tear of most flying needs! Of course, BOTH CST Sales and Aerspace Composites have carbon RODS in that aforementioned small diameter that could be cut apart into pins for the exact same purpose!

For ADHESIVES to use with this style of composite tube fuselage construction I've described here, a SLOW drying CA or epoxy would be best for all of the joints needed...and WITH that fishmouth-ing of the crosspiece ends, the need for a LOT of epoxy REALLY diminishes with tight-fitting joints EVERYWHERE one looks in the structure's appearance-so "weight buildup" should NOT be a serious concern with so much LESS epoxy being used!

I've done the DOWEL thing before to build up a fuselage structure...my SIG 1/6th scale Piper L-4 Grasshopper conversion, and my 30% finished scratchbuilt 1/6th scale Fokker D VII have that form of construction on them...but after seeing what IS available for composite TUBING on the market these days, thoughts of that Platt Jungmeister, the Glen Torrance Fokker Dr I, and even the Proctor Fokker Eindecker (as one of the 5 "M.5K/MG" shoulder-winged production prototype versions built early in 1915) and their BIG Fokker D VII, along with many other scale aircraft subjects too numerous to mention, that ALL had metal tubing fuselage structures in them, being done up with a composite tubing structure INSTEAD, is just TOO TEMPTING to pass up!

Just hope the info and links in THIS reply gets people that have posted in this thread already, and at least thinking about doing it themselves pretty much in the way that I've mentioned it here!

Yours Sincerely,

The PIPE!

abufletcher

Thinking of composite fuselages, I saw a VERY nice model of a DVII in Flying Scale Models where only the section of the fuse that could be seen inside the cockpit had been done with tubing (either dowls or rods, I don't know). This seems like a really smart idea and I might consider doing this on my Eindecker. I could also include a section of dowl/tubing where the handles protrude through the "canvas" skin. As they say the key to scale building is to SIMULATE not REPLICATE.

abufletcher

But one teeny question: How does Solartex (and other coverings) work on composite tubing?

The PIPE

Dear Abufletcher:

The PIPE here yet AGAIN...and I'm ALSO a convert to Hank Iltzsch's method of using REAL METAL TUBING...for tail surface outlines on scale aircraft (especially Pioneer/WW I/Golden Age subjects)...and Balsarite, or perhaps even Sig's "Stix-It" iron-on adhesive (which I have yet to try) SHOULD work just fine on composite structures to get Solartex and other iron-on fabrics to adhere to it, as I've found that it sure DOES do on those metal tubing tail surface outlines!

Just BRUSH it on the places on the composite structure where you want your covering to stick when ironing it in place...and it SHOULD "stay put" with no problems.

Hope THIS little tip helped as well...

Yours Sincerely,

The PIPE!