Carbon fibre tubeing vs stainless steel tubeing

[Brent]

Ok so I have some ideas for this winters build and there's a few things I am still trying to work out before I can put pencil to paper and start and do a little designing Before  making a final plan to build from. I know you can silver solder the stainless steel tubing and it will be strong if done correctly, However creating a good solid T piece with Carbon Can be difficult right?.

Cutting it short I am thinking of framing up a fuselage and want to go the extra mile instead of using balsa and so forth. The full size aircraft uses tubing so wanting to replicate the full sizes structure the same way to add to the detail of the model. might sound like a lot of messing around but I just want to go that extra mile.

 

Any input would be great because I can not silver solder stainless steel and my only other option would be carbon fiber which is good because I used to be a fiberglasser. I am just a little concerned of the strength compared to stainless and also strength of making 45 and 90 degree joints. I do have some ideas as to how to make a form of insert that helps anchor the 2 tubes together internally for the carbon fiber method but a little worried as to the overall strength of the tube itself over the stainless steel tube. As said before any info will be great

Edited By Brent on 07/09/2014 16:14:12

[Erfolg]

I am a little surprised that SS can be brazed. My own experiences with the material is that the properties which make it corrosion resistant make it hard to weld. That is a self healing passive film in an oxidating environment. So from experience, welding was done using a gas, to prevent oxidation. From my experience this was TIG welding using argon as the gas. The other problem was that of the HAZ (heat affected zone) where there was a tendency for both grain growth and precipation issues at the grain boundary regions.

So i would have thought with brazing, the first issue is a flux, that will remove the film, there after not a clue.

As to the issue of relevant strength, I guess it would be based on weight for weight with similar OD geometries? On that basis, it would not surprise me that CF would have it.

[Brent]

I think the braizing with ss is abit too complex to be fair. don't get me wrong I would love to go that route if it can be done. I will have to get hold of my friend who is an engineer by trade and makes one off café racer parts for motor bikes and see what he thinks. downside to that is it will have to wait due to me being in hospital but in all honesty im more familiar with the carbon method. I also agree in regards to weight for weight and was going to go with the same outside diameter but go along with a much smaller inside diameter.

Here's a picture of what I want to achieve though on this models frame there is several members missing and several tubes in the wrong place etc. but this proves it is not impossible. tho model is 1/4 scale which is what I am also wanting to work to in size and scale.

[Brent]

Also the way the wing mounts on has been changed which if anyone reading this that knows there j3's well enough they will also spot this. I want to build a scale wing and use the same mounting method. there's a 1/3rd scale plan set for a cub drawn by bob nelitz and he uses the full size method of attaching the wing and it works great. although he had to build a metal tube frame work for the strength side of things in the center of the fus and attach the rest of the wooden structure to it.

The cub above is stainless steel and then powder coated you have to admit it looks bloody great and was reported to be light and stronger then a built up wooden structure.

[kc]

I think brass tube has been used for a scale fuselage by some modellers. Why use stainless when brass is easily available in many sizes? I guess people used silver solder for the main parts and later used soft solders for the extra bits to avoid meltdown.

[Brent]

brass is more expensive then the carbon tube and harder to find in 1 meter+ lengths, though may be perfect for building a scale under carriage which I intend to replicate the cubs full size one. I don't see point in paying 100 quid plus to robart for there 1/4 version when I could build it my self for half the cost. yes ok it means more work but that's what the hobby is all about. pushing our own skills to the limit and learning new things and id rather use the 50 quid I saved and put it towards some coverall or some solar tex. or some other item I can not make .

This is not an impossible build it just has to be planned very carefully so when it comes to designing I have all my answers and I know the important sizes and weights and factors about what material to use where, it will also help make sure the project keeps going and does not come to a stand still due to a problem halfway though rendering the whole project as a waist of time. I also want the model to be safe when built because the last thing I want is it suffering a mechanical failure due to a design fault of my own and it crashing and hurting someone.

Iv not worked with brass but I do no it can be solderd. is it classed as a soft metal or a hard metal. if its a hard metal then il consider using it for the under carriage.

[Martin Harris - Moderator]

I don't think you'd have any problems with previous joints unsoldering as the silver solder used with stainless needs the parent metal to be at red heat.

Erf, I've used fluxed silver solder rod to make exhaust parts in stainless (and the undercarriage for my own 1/4 scale Cub) and I can assure you that it works very well indeed. The only problem is its considerable cost - but luckily I was given some...

Conventional silver solder and flux paste would work out a little cheaper to use.

P.S. Brent - if you haven't done so already, a little search around the 'net should find the Robart drawings for the undercarriage - very useful!  PM me if you can't find them...

Edited By Martin Harris on 07/09/2014 18:49:21

[Brent]

Thanks for that martin I will have a look shortly. Obviously I will need to learn to silver solder but im in the process of trying to get my friend paul who can do this to teach me when I finally return home. I can solder circuit boards brilliantly because I work in a computer shop and specialize in soldering motherboards etc for them, I can solder piano wire but would love to learn how to silver solder to take my hobby to the next level and be able to push the wood aside for once and use a metal tubing to replicate the full size. call it extreme which some of you may think so but when your building a scale model you try to replicate things the full size would have had or be made from right? so its only taking the realism to the next level and in the process showing your skills and ability's.

I would also love to learn to make my own exhausts so learning how to silver solder is a must .

[kc]

I reckon a trip to the Model Engineer Exhibition ( usually at Sandown Park in December) to have a chat with the always helpful engineers on the SMEE stand about silver soldering etc would be worthwhile. Or contact your local Model Engineering society.

[Ben B]

I'd go CF just for the simple reason that silver soldering a complex piece like that is going to be blinking hard work. With CF you can epoxy tubes and sockets in the safe knowledge that once they're glued they're not going anywhere. With welding the heat remains fairly localised so you're unlikely to knacker existing joints whilst working on the next one. With brazing this is a strong possibility. I did some friction stick brazing in ali and it wasn't fun. Admittidly ali has a much higher conduction of heat than stainless but it was still annoying and still a possibility- the heat would conduct down the ali into my previously perfect joints melting them. I ended up having to jig the entire workpiece to prevent terminal collapse.... CF is the future

[Brent]

looking at the structure I could build the front with stainless steel but where it gets complicated wing backwards I could use some wood for the rest and turn the ends down of the upper and lower longerons and epoxy them into the stainless tubing to join the rear wooden section to the front metal section which would still allow me to keep the main strength in the center of the plane for the wing mounting to be built scale and also be strong and versatile. allot of scale cubs lack strength in the center where the wing mounts due to the scale opening door. this is another reason why I haven't gone out and bought a kit.

The problem I have with CF is it may not work and the vibrations may weaken the joints in the airframe of the fus. its allot of money on CF if it turns out to be a failure, The stainless steel tubing is a working tried and tested method so it is doable just very tricky. the 3rd method iv just mentioned seems the be a better approach but then if I get in touch with my friend who is an engineer by trade and he says its possible to teach me or even build the framework from my design for the right cost I will go down that route. keep your opinions coming guys.

[Malcolm Fisher]

If you are OK with soldering, stainless steel can be soldered using conventional solder. However, it needs a phosphoric acid flux rather than Baker's fluid, resin cored or the fluxes used with plumbing fittings. This was OK for the cabane struts I made from stainless steel spokes for a Tiger Moth but I don't know if it would be suitable for your project.

I have also brazed stainless steel but it is very difficult to clean up the black oxidisation afterwards.

Malcolm

[flight1]

For welding steel you should see **LINK** for proper inspiration ,

i would love to be able to build that way-some day maybe

[Brent]

Flight1 thats where I got my info from and ideas. What puzzles me is he does a 1/3 scale cub with scale wing bolting system etc and scale built wing but did not do it on the older 1/4th scale model. You have to be honnest the finished models look fantastic but il be damned if im paying over ?2000 for the kit alone thats not including postage or import duty and tax and so forth. There is also so much room to add more detail to the cub. I betbthe finished airframe is rock solid though

[flight1]

Well if you by one you have lost all the fun in building one, all things considered if you ad up the materials and labour you would find it good value

I have also toyed with the idea of using carbon tube and Jb weld to sick it all together which is within my skill level, but if you have a curve the carbon rod will be in tension and wwill be trying to straighten out putting this load on the joint-- as for steel you just bend it

mind you... you could lay up your own carbon rod to the shape required

Edited By flight1 on 07/09/2014 21:55:12

[Brent]

Well just had a quick chat with my friend paul at one off welding and engineering and he will be happy to build and weld the fuse and tail for me on condition I do the cuting and bending and shapingvof parts required and he is doing the welding and il watch learn and have a go on scrap material. Cost hasn't been discussed but his level of quality in is work is outstanding so the projects a go ahead with the stainless steel. Im starting to feel slightly excited now haha. I will document the build on here but it will have to after my chemotherapy is completed and once I have bought all materials for the fuselage. Il be able to spend my revovery doing somthing il enjoy whilst I await my op date to come through. So it will be a long build log but I promiss to make it a good one to read there just maybee breaks between parts of the build but im sure paul can email me pictures and il continue to update the thread whilst in hosi.

[Erfolg]

I think it is Martin that has made me think a little, and recognise why we often (that is a as groups) see things differently.

In the world of full scale engineering, SS is more often than not used in circumstances where specific goals are sought. It may be corrosion resistance, or maybe that a product will not be contaminated.

Where a long service life is required, it is pretty obvious, that if the joining material is the same as the parent material, it could well have a similar life expectancy. On that basis, brass or silver could be a weak link, hence the preference for welding using the same material filler.

Whatever appearances may suggest, all common materials have grains and hence grain boundaries. Turbine blades may come close to a single crystal, although not what we buy. For non engineers, it may be tempting to be dismissive of atomic structures, slip planes, millers indices, cooling velocities, loading rates etc. Yet for understanding they are often relevant. It is the difference to understanding a failure and predicting processing and or service performance

If you heat a steel up as in welding, brazing or even heat treatment, you will see grain growth. At a practical level, the material will preferentially bend in these regions, in this case the HAZ is one such area. What ever the amateur metallurgist may suggest, the grain growth is dependant on two aspects, temperature and just as importantly for how long. It is not just dependant on temperature, it is often very dependant on time and often chemistry and environment.

Some of the other detrimental aspects such as precipitation of some elements into the grain boundaries, are also driven by heat and time.

It has been suggested than any old flux will do, I am not convinced. If that were so, SS would almost certainly be less resistant to corrosion than we have come to expect. I would suggest it needs to be aggressive enough to break down the Chrome oxide film to allow the wetting (formation of the intermetalic compound) to be take place.

Perhaps what is pertinent, is any of this relevant to a model aircraft? The duty cycle is probably very low, when compared to a typical full size engineering situation. That is other perhaps in ares where higher loading conditions are concentrated, due to regular duty, or failure occurs due to an exceptional event, like a heavy landing, or a high g pull out.

In short I believe you can braze a frame, you will find that adjacent to the joint will be weaker than the original as processed tube. The joint made, could well be adequate to hold the frame together, although the ultimate strength per unit area will be very much lower than the parent material. Will it matter though, probably not, in most circumstances, other than these are the potential failure sites.

My one cautionary note would be, do not subject a brazed joint or tube to high loads, without over engineering the area, to compensate.

As an aside, if you have very old kitchen knives (in the form of cutlery) where the blade has been welded to the shank, where cleaning has used a washing up liquid, often evidence of chloride attack can be seen at the interface of the welding process due to the disorganised and large grain structure at the grain boundaries.

It is evident that the world of Full scale engineering and model engineering can be worlds apart, in many aspects.

[Jamie sawyer]

I like the idea of carbon rods/ tubes etc and Epoxy. Vibration and fatigue can be minimized by balancing your propeller and mounting the engine mount on rubber pads.

What I am wondering is 2 things, if you do go down the ss tube route how are you going to over come the onboard radio mounting scenario.

Secondly if you do choose carbon fiber will this affect the 2.4 ghz signal ???......or for that matter is all this metal tubing etc going to cause issues with the signal ??

[Brent]

If its an open structure covered with fabric it shouldn't cause any interference. If it was sheeted metal it definatly would cause issues. From what people have said that goes down the route I am they dont have issues only where sheet metal is used rubber pads are a must ti reduce vibration I agree. The only down side ti carbon rod and tube is like you say the bends will be under load all the time :/. I will do range checks and use 2 4ghz I think 35mhz would suffer interference.

[Erfolg]

At present I am suffering from a cold and unable to sleep, with my mind going over my life time experiences with high nickel alloys.

Being an almost good student some 50 years back, I dredged up that there are three principal groups of SS, Austenitic, Martinsitic and Ferritic. It is the former that most of my experiences revolve around, probably the most common in the processing/chemical industry, 304, 316 and 321. Non of which matters to you I would guess.

What potentially does matter, is that SS, work hardens very quickly, as does other alloys such as Hasterloy, Inconel and materials such as Stellite. So why does it matter? It is both useful and a problem, If bending tubing, it is the work hardening, that prevents the wall continuing to thin locally until the material fails. What happens is other areas to yield as they work harden, and so forth, thereby ensuring a uniform wall thickness. On the other hand this can be a problem. An example if machining a bar, where you have machined, will typically have a hard skin. So what you say, it matters if you just want to take of say 0.001", the tool tends to rub, a little more feed in and suddenly you are taking of say 0.005", oh dear. The same happens when trying to file these materials, initially, it cuts, then the file tends to skate over the surface. The same is true when sawing. The answer is to attempt to file relatively slowly, with high pressure, to get below the surface, so as not to knacker the file or saw blade by overheating the very tips of the cutting surfaces.

There is another phenomena when welding Hasterloy, SS etc, they tend to shrink a lot more than other fabrication materials such as a carbon steel. Not such an issue with light tube though.

I was also thinking about what kills SS, which is Chlorides and a reducing atmosphere (the oxide film does not form or degenerates). I also seem to remember that Hydrochloric acid it not good either, causing crevice corrosion.But then again you will not be cleaning using HCI, will you.

Now of to bed, hopefully to sleep.

 

Edited By Erfolg on 08/09/2014 01:09:09

[Brent]

Well my friend Paul who is the engineer will understand all the properties, he has been an engineer for over 50 years. He specializes in welding Stainless and aluminum parts as well as makes them. he has computerized lathes and can weld just about anything. He spent the best part of 20 years working in Barnsley college as head of engineering. I am more then confident in his experience and knowledge and did ask what I was going to do with the airframe after it had been built to protect the frame from the elements. One option is to have it powder coated which I know of a place that did my motor bike frame after Paul welded that after my crash, or I could get in touch with my other friend Wayne who is brilliant and works with car dealers and goes round repairing paintwork and spraying new panels up for them on new cars, its actually surprising how many new cars have to have repairs to the paintwork before they even clock up any miles or are handed over to there new owner. So I could have him spray the whole frame with a cub yellow or another colour. however either way I will be protecting the frame with a form of paint just sure which method will be the lightest but most durable. As for the expansion of the metals Paul will be testing to find out how much expansion / shrinkage there will be before we start to build the actual fuselage and we will also be carrying out pressure tests on the welded samples and he insists on trying a few different flux's and strengths of solder till he finds one that will be perfect. The last thing I want is they model having a structural failure and causing injury or damage to anyone or anything. Safety will be highly outlined during this project.

I am pretty much sure Paul will do a fantastic job as always with this project and I will be taking notes and photos and possibly some videos of how he prepares the metals and how he welds and I will include this in the build thread so other people can see how it is done, a bit like an instructional video which will be educational to other people thinking of doing the same thing as I am about to endeavor on. I also know we will be building jigs for the sides top and bottom and for the undercarriage to help keep all parts firmly in place,

For safety I will be looking into speaking to my local model shop in Leeds crossgates in regards to a very good quality fail safe for the model and I will be only fitting good quality servos and so forth to ensure the model is always under control and has reliable parts. Safety in this build is first and I will be taking no chances at all. I appreciate your input erfolg and will show Paul your comments because he will understand it better then myself so rest assured your info will be taking in and Paul will no doubt use the info you have provided. so thank you for your input.

[Levanter]

Brent. If you are considering powder coating why go for stainless tubing. Why not use mild steel in which case you have a variety of options to join the tubing and avoid the peculiarities of stainless of which there are many. Stainless steel is notoriously difficult to paint.

Another problem S/S will give you is that all the weld or join areas should be polished afterwards ideally to a mirror finish. Satin would do if you are no where near sea water. If not you will get surface corrosion. This can be done by mechanical polishing or electro-polishing but adds another complication.

[Brent]

One thing I am short of for this build is some info and pictures on the full size cubs control cable system as I do believe the full size uses a pull pull system for both rudder and elevator and this is something I too would like to incorporate into the model for weight saving and also due to it being a scale working part. I also seem to be struggling to find any info on navigation lights of the full size because this is yet another feature I would like to include in the build. I don't want to go over board but would like to have these features included in the model but only if it does not push the weight up by a great amount. I will also have to set a weight limit target but until we buy all the materials and do some math's that will have to wait. There is still allot of info to find and the drawings I have need to be changed and adapted to be of a more scale-ish appearance. It isn't like il be entering a scale competition but I guess this will be a great test and practice for if I do decide one day to build a model for scale competition because it would be a nice achievement to take part in. Its also a shame that most of the scale detail will be under the covering but photos of the build and a build thread will show people what they cannot see on the finished model. I am also hoping that I can hide the servos under the wooden floor like the full size cub has but will have to take some measurements to see if this is possible. if not I could hide them in the luggage compartment behind the rear seat. One thing I want to do is do a full scale cockpit with no wires or servos insight and I am also going to see if Paul can replicate the full size fuel tank to fit behind the cockpits control panel. I will be using a petrol ( unsure what type but may go glow if i can find a small twin cylinder engine for a good price) and Paul can build aluminum tanks for motorbikes so this should be easy for him to do, id also like to have a scale filling cap like the full size which is located just in front of the cockpit window glass on the upper forward turtledeck, Depending on the size I could even have him make a wing tank like the full size which can feed the main tank to extend the duration of flight time which maybe good for a glow twin engine because as of yet I don't have the dimensions of the full size tank to scale down to work out how much fuel it will actually hold. all I do know is the secondary wing tank will have to be perfectly located over the CG point to avoid any balance issues.

This all may seem extreme but I want to create a challenge for myself and be able to enjoy all the research and have the model look just like the full size and have features very much the same as the full size. But more importantly I want it to stand out from other cubs and these so call artf's because in the club I am a member of we have allot of skilled modelers that design there own models and don't entertain artf's so I was sort of taught the older ways by them and I find the hobby so much more fun and enjoyable building a model for months to later see it fly for years to come. I just think its time I took my hobby to the next level and start and push the boundaries a little to make the hobby more interesting.

[kc]

A scale drawing could produce a fair estimate of the length of tubing required and easily estimate the weight. Then it's possible to see if the wing loading is practical for a flying model at that scale or whether it's only going to be a static model.

[Martin Harris - Moderator]

Are you modelling a J3 Cub or a Supercub?

The J3 is a very simple aeroplane and weren't fitted with any lighting although there have been some converted over the years by adding a wind driven generator or uprating the engine to one with a generator.

The fuel tank is actually in front of the cockpit and sits above the passenger's legs - it's flown solo from the back seat although it can be controlled from either of course.

The Supercub was a little more advanced with an electrical system and lighting as standard. The fuel tanks were moved to the wings and this meant that the weight of the fuel was close to the C of G, resulting in the P1 position moving to the front seat as it improved visibility without an unacceptably forward C of G.

Most J3 restoration pictures I've come across show a grey finish to the (hidden) tubing - Supercubs seem to have a zinc chromate (greenish yellow) finish. Almost everything visible in a J3 will be Cub Yellow!

Edit:  I've just remembered that I downloaded this manual which gives a wealth of details of the full size J3 - including the fact that the original tubing could be "metallized" - not sure what this was but the advice to clean it off with Sodium Hydroxide for repairs might suggest an aluminium base...and details of the control systems

Edited By Martin Harris on 08/09/2014 11:10:40