Has anyone considered?

[Golden Eagle]

In the interest of saving quite a bit of weight, has anyone ever considered building the fuselage frame out of carbon fiber tubing?

I have been doing some research and it appears as though the entire fuselage could come in at well under half of the chromoly’s weight.  From my calculations and reading, just the welded steel tubes alone, including the landing gear structure and the wing struts will come in somewhere around 50+ lbs, give or take a pound or two.  But close.

Carbon fiber tubes, if designed properly and given a safety margin to insure plenty of dependability calcs out to be maybe somewhere around 17 lbs.  Let’s just call it 25 lbs, and use a thicker wall tubing.  That has the possibility of saving 25 lbs off of a steel fuselage.

BTW, yes I realize that losing most of the weight would be behind the wing and would require a slight proportion change in the design due to a nose-heavy condition.  Anyway, I was just wondering what your thought were.

Mike

[stevejahr]

I have pondered exactly that same question... it would be expensive compared to CroM and require some special layup skills to create the lugs for joining the CF tubes.

Part of the question once you make any change is where to you stop making changes... a CF fuse frame might suggest composite-based tail surfaces... then you start looking at ribs and spars and wing structure and... 

[Golden Eagle]

Steve,

  Yeah, I know pitfalls of “changing one thing leads to 10 more”.  But I think if a guy were to end up with a Eagle fuselage that is at least as strong as chromoly, AND it had the added benefit of being less than half the weight, I would think for some guys it could be worth it.  For me, I would have to prove to myself whatever CF tube I chose it would have to be zero doubt that it is equal to or stronger than CM.  Yes, I would assume the tail feathers would be CF tubing too.

  For joint the tubing in “T”’s (intersections), I anticipate first coping the tubes exactly like CM gets for a nice tight joint.  Then in a neat fashion, build a coupling out of whetted CF strips.  Layer such strips such that they create essentially a CF version of a plumbing coupling.  (It may sound a bit hokey, but in the final version I would expect it to be quite appropriate and well built.)

  I’m just kicking ideas around.  But imagine how nice it would be to come up with a 25 lb fuselage with tail feathers attached. 

[stevejahr]

Folks have been doing this sort of thing on bicycles for quite some time.  Calfee and Landshark are a couple examples who build CF tandem bikes successfully. They do seem to fit the tubes and add joint reinforcement and the joints angles are whatever they are on any delta frame bicycle (meaning not simple T).

There are some other factors to consider here besides just "strength".  In addition to simple tensile strength there are questions of modulus of elasticity, compression strength, and fatigue characteristics to consider.

The cycling world is extremely weight conscious, bunch of weight weenies, and where you really gain is in tailoring the material and amount of reinforcement to the actual loads. Tubes are generally thinner in the middle than they are at joints for example. Aircraft tend to take a simpler approach with straight non-butted tubing.

Once you make the jump to CF and composites you really have to think about if the whole tubing truss is the best approach or if you need as many tubes, etc. For example how about a foam fuse with strategic corner reinforcement in CF composite? Forget about truss tubes and go shear panels possibly in cheaper FG maybe.

Lots of possibilities here only limited by your time and $$$.

[Golden Eagle]

Hi Steve,

 I’m aware of the standard traits to keep in mind like moduli of elasticities, compressive strength, and fatigue characteristics, etc, and I have also studied the various ways that carbon fiber tubes can be constructed.  The fiber’s orientations has everything to do with the tube’s resistance to “give”.  For compression and tensile strength one would want layers along the axial direction.  For resistance in torsional distortion one would want diagonal fibers wound in the 45 and -45 directions.  Lastly, to resist crushing or bending the fibers would be have to be wound in the 90 degrees direction (in circles)

  A typical layer of CF is approximately .010” - .012” thick.  If you were wanting to make a universally strong tube that has ALL the usual strengths listed then you might use all combinations of the above listed orientations.  E.G., first layer 2x2 twill, then one layer of 0 degrees, then a layer at 90 degrees to resist crushing, then one layer wound at 45 degrees, then one layer in the opposite direction at -45 degrees, then a layer at 90 degrees again, one more layer at 0 degrees for tension and compression and lastly a layer of 2x2 twill.  You can count 9 layers listed.  9 times .010” will produce a wall thickness of approximately .11”.  That’s a carbon fiber tube with a wall thickness of just under an eighth of an inch.  If I remember correctly, this tube would be just under one half the weight of a 4130 CM tube.  This carbon fiber tube so constructed would have the technical description of being stronger than a sumbitch.  If I were to hand you a 4 foot piece of it and I said bend it or twist it, good luck and I’ll be back in a month.  You’d need a sledge hammer.

  With regard to the use of carbon fiber as a replacement in a Legal Eagle frame, empennage, struts, etc., I doubt all of the tubing needs to be built as described above.  Some tubing can be sufficiently strong (for their intended tasks) with only 4 or 5 layers in the proper directions, which is now approaching 1/4th the weight of CM tubing.

  I am in contact with a ‘carbon fiber products producing’ company and am trying to obtain a coupe of specimens to test for sufficient strength for what I have in mind.  I have worked with fiberglass before and am fully capable of making my own CF tubes, which I intend to do.  I will make several samples of varying strand orientations and thicknesses and test them to destruction to find their respective failure limits.

  Referring to the cycling world, and specifically to the manufacturers, yes they do use CF tubing extensively.  The stresses in a bicycle frame would be somewhat similar to those of a typical CM tubed airplane; ‘mostly” tension and compression forces.  There would not be a lot of torsional stresses or bending stresses.  Some, of course, but not a lot, mostly like I said tension and compression.  Of course as a manufacturer they will have fancy reinforcement “T”’s for joining pieces of tubing.  A guy building only one project can make his own T’s like I described in my previous post.  I can guarantee the fitting won’t be a weak point.

  I am in a bit of limbo for a few months at the moment and all this research is just academic at this point.  I am in the process of selling my house and will soon be moving to Boise area, so I am not able to begin actual L.E. XL construction quite yet.  I’m simply doing research and asking a few questions.  I have a fair amount of airplane building experience and a ton of fabrication experience.  I build stuff.  LOTS of stuff.   I don’t approach the idea of a carbon fiber fuselage as an inexperienced novice.  The frame would have to be strong as hell, very light weight and tested enough to prove it will not fail under ALL flight induced stresses.

  I’m not interested in reinventing the Legal Eagle wheel.  If I were to use CF in its construction, then I would simply replace the “as designed” CM tubing.  A fuselage with CF or fiberglass panels is a different airplane altogether.  Could I do it?  Yes.  Do I want to?  No.  If I were younger, maybe I would.  But at this point in my life I simply want to build a lightweight legal (or VERY close to it) ultralight.

  I’m retired, and money isn’t much of a concern.  Maybe a little to my wife, but I won’t tell her if you don’t.  Time I have lots of.  I think I will round up some carbon fiber fabric and toray strands and make a couple of tubes.  Then I will beat the crap out of them (just kidding), I meant test them and see how strong they are.  Who knows?  I may eventually end up back at chromoly tubing anyway, but at least I will have learned a lot about carbon fiber tubing in the process.

[Vince Carucci]

I love where you're going with this engineering exercise. Some time ago I explored the idea of folding wings. I came up with a design that worked (on a scale model) and only added 88 g of aluminum per side. I even build the upper and lower hinges. It was a fun exercise. But Scott in Iowa was right, 1 change results in 10 more. I would have to make a bucket load of small changes to accommodate the hinged wing. And who knows how much more weight I might have added.  I'm gonna stick to the original wings.
I found a design for a tail wheel in Kitplanes magazine that I did build. It's quite a bit lighter but it took a long time to hand machine the parts, and it cost considerably more.
I am in the process making a wood/fiberglass laminate tail spring. The form is assembled and I have all the parts. I will glue them up very soon. The wood is a 7 layer maple kit purchased from Woodcraft.com and the Epoxy resin kit is from Aircraft Spruce. I got the fiberglass at work so that was free but this tail spring is approaching $100 and I don't even know that it will work yet. I love to tinker but the cost of these changes is giving me pause.
One of the benefits of Leonard's design is the simplicity and repeatably. It can be built by any man almost anywhere with any starting skill set.

In the end, as much as possible, I am going to stick to Leonard's design. The only challenge I will take on is gram shaving everywhere I believe it's possible. I want this thing to meet Part 103 requirements. That said, everyone here brings something unique and this is the place to share.

Design on!

[Tom XL-7]

Hey Vince, Just wanting to share something. Let me put myself in the way back machine; and I am standing on the grounds of Oshkosh.
I am talking to one of the Heintz brothers about their 750 stol. I asked why the folding wings did not carry over from the 701 model. He enlightened me with a response of it's not worth all the effort. That he could in fact have the wings off the 750 before the 701 could be folded.
I had always been one of those folding wing fans and that changed it all. 
 Golden Eagle keep us posted, sounds interesting. 
Tom XL-7

[Golden Eagle]

Everyone,

  I am aware of the “one change starts a snowball of changes”.  I can handle it.  If I end up going that route, and I’m not saying I am, I will expect to solve each new difference one at a time.  I also like the idea of a fuselage assembly weighing in at 25 lbs less than 4130 CM steel.

  Incidentally, I thought I’d share how I intend to make the tubing.  Initially I will make a couple of them for testing purposes.  If all goes well, then I will make more.  Here’s the plan:

To make carbon fiber tubing you need something to wrap it around.  This is called a mandrel.  Because the size is about right, I will use 1/2” steel gas pipe.  This is available at Home Depot.  I’ll get a 10’ piece.  I will make a horizontal fixture that it sets in, and it will be able to rotate.  I will sand and polish it fairly smooth.  Then I will do my best to get an even coating of candle wax on it.  The thicker the better.  Next, as it is being rotated I will spirally wrap 2” wide packing tape down its entire length.  The inside layer of the packing tape won’t stick to the wax very well, and that is intended.  It will, however, stick to the back of the previous layer and seal off the wax from the carbon fiber and resin.  Now is the time to apply the various orientations of whetted out carbon fiber, either spirally by adding on the diagonal strands (with toray strands or cut unidirectional fabric) or lengthwise by applying the axial strands of carbon fiber (with unidirectional fabric).  Finish all of the layers off with one layer of 2x2 CF twill, mostly for cosmetic reasons.

That would complete the structural components of the tube.  Then I would wrap the whetted out layers with peel-ply, and an absorbing layer and lastly with a layer of the stuff that lets air escape, called breather cloth.  Next, I’ll slide a previously-made tube of plastic over the entire arrangement, tape it off well on both ends to seal it, and then hook up my vacuum pump and compress the layers as much as possible.  It’ll sit for a day or two while the resin hardens.

To remove the CF tube from the mandrel, I will begin by pulling off and disposing of all of the discarded layers ([plastic vacuum bag, absorbing mat, peep-ply, etc, etc).  Next I will screw on 90 degree pipe fittings at each end of the mandrel, facing up.  One fitting will get a funnel to which I will pour a continuous  stream of boiling water, and the other end will have a small tub to capture the water as it spill out at the end.  Each cycle of water may be left in place for a minute of two to allow the steel pipe to heat up quite a bit.  I should be able to feel quite a bit of the heat transfer by feeling the outside of the CF tube.  Once I am convinced that all of the wax coating around the steel mandrel is liquified, I will unscrew one of the 90’s, pour out the hot water, and slide the CF tube off the mandrel.  The packing tape most likely will stay inside the CF tube, but by simply pulling on the end of end it ought to unspirally release itself and come away clean.  Anyway, that’s the plan.

If I find this plan successful, and it isn’t too difficult to remove the CF tube from the mandrel, then I will be very tempted to buy another piece of 10’ pipe and weld it to the first one, grind the weld super smooth, and make approx. 18’-19’ long pieces of CF tubing.  (The longer the tube, the less waste during the build). Now, we all realize that a 20’ piece of steel gas pipe will have quite of bit of droop to it if we only support it at each end.  Therefore I will make a couple of support rollers to carry the weight of the middle of the mandrel.

BTW, my wings will fold.  The mechanisms will weigh ounces.

Cheers to all,
Mike

[leshoman]

looking firward to seeing the 4 lb fuselage...

[Golden Eagle]

I didn’t say I WAS going to build one, I said I wanted to investigate the idea and test carbon fiber tubing.  But I guess carbon fiber is a dumb idea to use in the construction of an airplane.

I thought this forum was open to discussions about experimental and ultralight aircraft, which would include the materials used to construct them.  I guess if you don’t follow L.E. plans to the letter, this is the wrong place to be.  Thanks for letting me know, Les.  I’ll be sure not to ask any unapproved questions from now on.

[riorex2002]

Les was not dumb ,just excited at this concept There is no such thing as a dumb question on this site.I myself was contemplating making a mould from cf for the carb manifold and this article was full of idea's,Keep the idea's coming

[leshoman]

the standard LEXL fuselage, all welded, ready to paint weights 34 lbs 8 oz.  any weight savings would be good

[Tom XL-7]

Golden Eagle; What you are investigating is good. Even if you choose not to use it for the fuselage there are still four struts. What you learn success or failure will be good for you. Learning and challenging yourself is a part of scratch building an aircraft. Follow your dreams.
 Two things come to my mind. 

welding two sticks of black iron end to end and having a straight not kinked joint seems a serious fabrication test. 
Could you consider a 20' stick of tubing, steel or copper? Plumbing houses usually carry 20' lengths. I would know. I have cut many down in the parking lot.

 Having seen his posts over a considerable period of time, I don't think Les meant anything negative by his comment. He has built two of these and should be considered a  valuable resource. He has been very free with his ideas. There are no trolls here and we are actually monitored
 Tom XL-7

[Golden Eagle]

Tom,

  Thank you for your thoughtful reply.  I need to mention that I never suggested anyone is dumb, especially Les.  I recognize he is one of the leading spokesmen for the Legal Eagle brand, and he speaks from a position of significant experience and authority.  Well earned authority I might add.  I know he is well thought of by all, and that includes me.  I have seen MANY of his videos and dream for the day that I, too, can be boring holes in the sky like he does.

  As I mentioned before, I am in limbo somewhat.  I have sold my house and will be moving to near Boise soon.  That means I can’t really begin my actual LEXL build for quite a few months. So in the meantime, I am trying to figure out ways to make the plane lighter.  I am not a novice builder.  I have built a Kolb MkIII Xtra from scratch, and a Glastar from a kit, plus a Quicksilver and a Wizard J3B.  I also bought and worked on for quite a bit an RV7A, and a KR2S, but I didn’t finish them, I just sold them.  Intermixed with all this airplane building I have owned two Cessna 172’s, and I have accumulated around 700 hrs flying them.  I have also taken 15 hrs of dual ultralight training years ago in a 2-place Quick.
  I picked up an extensive amount of composite experience with fiberglass working on the GlaStar.  I picked up a ton of experience on aluminum building the Kolb.  For over 40 years I have built stuff.  LOTS of stuff.  That is what I do, I fabricate stuff.

  Although I have a fair amount of fiberglass experience, what I don’t have is any experience with is carbon fiber.  I hear that its strength is legendary, almost.  Stronger than steel, lighter than aluminum, etc, etc.  Since I have a few months on my hands and can’t really get a project going, I want to investigate just exactly how strong this carbon fiber tubing really is, and if there is a serious consideration of using it instead of CM steel.  I just want to check it out, that’s all. 

  As far as costs goes, that isn’t much of a concern to me if I thought it would really work.  Money and time I have.  Initially, I would just buy enough material to make test subjects.  Besides, if you buy all of the materials and make your own tubes, it isn’t all that expensive.  If I were to make my own CF tubes, they would be indistinguishable from factory made, both in appearance and quality.  Not to mention I would test the tubes to destruction and find out exactly how well they fared against CM.  I have some extra CM tubing on hand from a fuselage frame I built a few years ago.

  I don’t think anyone has really considered CF tubing.  Mainly for costs, I suppose.  But I don’t think that means that it can’t be done.  I just want to find out if it is really feasible, that’s all.  I certainly don’t want to step on anyone’s toes in the process, but I would also like the same courtesy of not being thought of as some rookie who has never built anything before.

  Like Les said, “any weight savings would be good”.   I greatly agree.  If it doesn’t work out, I’ll have a couple of CF tubes.  Cheers!!

[stevejahr]

I have always figured on using of the shelf CF tubing... you are taking it a lot further than I would have tried to go. The mandrel release has been one of the key obstacles there which you may have solved.

Your thought of using a wax layer to get separation from layup and mandrel is interesting. I also wonder about using aluminum tube with heat during cure and cold for release? This could have some benefits in the CF resin cure with the heat.

Of course for the longer version... a CF mandrel could be stiffer with less droop.

For consideration, instead of vacuum bagging you may get the same benefit from using a spiral wrap of shrink wrap with the advantage of reducing molding lines. I have read of bike builders using Ace bandages in this way. Really anything which provides uniform compression over the surface area of the part will suffice.

The two bike builders I mentioned are really very small production custom bike builders. Landshark in particular are entirely hand built and create "lugs" which are invisible. I have seen a few examples and they are literally mechanical art work. The Calfee are crude by comparison with obvious lugs showing. Some pictures of the process Landshark uses to build a sub-3 lb bike frame here: Landshark bikes "how"

Lot of work with all the tube joints in an airframe... but it would sure gather a crowd anywhere it went.