Graphlite™ pultruded carbon fiber rod...

Discussion in 'Composites' started by Senna, Jan 19, 2003.

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  1. Jan 19, 2003 #1

    Senna

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    OK... calling this cutting edge may be a slight stretch... but I believe this product will benefit designers and builders alike.

    I have been building an ultralight design called the Invader MK IIIB. Well... I have been storing it much longer than I have been building it so it would be more truthful to state I HAD been building this airplane. So what's an Invader MK IIIB you ask??? Well... it's an ultralight design out of the early 80's. Quite advanced for it's time. A fully cantillevered 31' wing when most everything that was flying in the ultralight catagory back then were the lawn chair fabric types. It's a pod and boom pusher with tricycle gear, a V-tail, and fighter-style side contol stick for the aileron/ruddervators. The designer was a Floridian by the name of Nick Leichty. I bought plan set #340 from his now defunct company Ultra Efficient Products, Inc.

    I have the fuse pod, boom, and landing gear completed. I had built and completed the center wing section (the outer 12' panels detach), and both main, rear, & aileron spars. I also cut rib sets for both wings.

    The stated G-load rating was +3.5 at gross 365lbs. Although I had never heard of any problems with this design... I was always in the back of my mind questioning the integrity of the main spar. Maybe this is what kept me from moving forward and completing and flying this craft.

    I retired early and have plenty of free time to indulge myself with hobbies... the computer being one of them. So... I surf the Internet...ALOT! I started reading about the Carbon Dragon (an ultralight sailplane) and that led me to the MARSKE Flying Wing site. I viewed pictures of the construction of Wes Whitehurst's CARBON MONARCH flying wing. The techniques and materials used to build this craft spoke of tremendous strength AND lightness. The use of a product called Graphlite™ carbon rod in the Carbon Monarch's spar caps was of great interest to me.

    I found on Jim Marske's site a link to a paper he wrote detailing the virtues of this particular product. Wow! Amazing stuff! I've since incorporated Graphlite™ carbon rods into the upper and lower spar caps of my ultralight Invader and by doing so I've increased (so far only on paper) the G-load to +6. This certainly has given me greater confidence as to the structural strength of the wings. A static load test will be the proof of the pudding.

    I guess what makes this cutting edge is the cost of this material. It's certainly within my budget! I not only get the carbon rod, I get a little peace of mind... all at $.57 a foot!!! :D

    If you haven't heard of this material... you can find the information here...

    http://www.continuo.com/marske/carbon/carbon.htm

    Build light... build safe...

    Senna
    Crooked River Ranch, OR
     
    Last edited: Jan 19, 2003
  2. Jan 23, 2003 #2

    Jman

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    Great info Steve. I went to the link you provided and I was amazed at how deep the author goes into the subject. The fact that builders are so willing to share info and experiences is one of the best things about this sport. It's why I started this site in the first place. Are you planning on doing a stress test of the entire wing structure or the spars alone? That would make for some very very interesting reading if you let us know how you go about it and how it turns out.

    This was my favorite part of the article:
    Thanks for the post!

    Jake
     
    Last edited: Jan 23, 2003
  3. Jan 24, 2003 #3

    Senna

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    Spar modification...

    This is how I incorporated the Graphlite™ pultruded carbon rods into the spar design. I epoxied three rods on edge into the dado cut into the upper spar cap and two into the lower. (see drawing below)

    The reasoning behind this is in most materials we build airplanes with the compressive yield strength is LESS than the tension yield strength. So when the TENSION LIMIT is reached in a material... the COMPRESSION STRESS is already past it's yield value. With the bending moments we experience in normal flight.... the spar typically sees simultaneous compression on the upper cap and tension on the lower cap. So... with the compression yield strength being less than the tension yield strength.. the compressed member requires the additional strengthing... thus three Graphlite rods vs two in the tensioned member.

    Senna
     

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  4. Jan 27, 2003 #4

    Jman

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    I am not an engineer so please bear with this question designed to stimulate some discussion :D. I was wondering how the different bending qualities of the two materials will affect the structure. Again, I am not an engineer but it seems to me that because the graphite rod is less ridged but stronger than the wood (if I understand it right) than it would not be a problem. But if the graphite rods were stiffer than the wood then, when loads are placed on the spar, there might be a tendency for the rod to want to pop out of the channel. Just wondering what your thoughts are.

    Anyone else have any thoughts? Do dissimilar bending qualities even make a difference?

    Jake

    p.s. Great drawings by the way.
     
  5. Jan 27, 2003 #5

    John Slade

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    I'm not an engineer either, but I've read a lot written by those who are ...

    Changing material in something like a spar cap may cause the stresses to move to somewhere else in the structure. In a composite airplane this could be a VERY BAD thing because all the joints are pre calculated for the amount of stress they can tolerate.

    If you're designing a new airplane, which is really what you're doing when you use unspecified materials in an existing design, then you should be calculating (and testing) stresses throughout the structure.

    Then, of course, the Wright Bros did it all by experimentation.....
     
  6. Jan 27, 2003 #6

    Jman

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    Senna wrote:
    Sounds like he's going down the right track though. Analysis and Testing. I'm looking forward to seeing how it comes out.

    Jake
     
    Last edited: Jan 27, 2003
  7. Jan 27, 2003 #7

    Senna

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    Future static testing...

    I will most definitely be static testing the spar structure! I still need to design and build the test rig. I'm researching the the testing procedure and ruminating on the method to insure the spar remains in the vertical plane under the test loads. Twist would be a very bad thing! I also need to figure out what part of my shop would be the best structural location to bolt the test rig.

    The actual test might be somewhat difficult to carry off. I live in a rural area... in fact just relocated and haven't made a sufficient number of contacts to be on hand to help with the process. This isn't something you want to do alone... although I may just HAVE to do so. Maybe a hydraulic floor jack that I could drop slowly until the spar takes up the load... count off the seconds... take a picture... and jack the load back up when needed. I'll also need to drink a lot of milk between now and then... the jugs are easier to use as weight than sand bags.

    The concern of the modulus of elasticity of dissimilar materials is a valid one. The recomendation of total encapsulation of the carbon rod addresses this concern. The epoxy matrix between the wood and rod will be called upon to carry the shear load imposed while the spar undergoes tension and compression. Since the carbon rods have the greater elasticity... the 'tendency for the rod to want to pop out of the channel' would signify the failure of the wood fibers themselves. If the wood fibers are at a point of failing due to the elasticity differiential between the two... the spruce alone has since reached it's yield limit.

    Also the point made about moving the stress loads to another location in the structure is quite true. Lets say I have indeed doubled the g-load rating of the spar on this ultralight from +3.5 to +7. In no way am I intending to fly the craft any differently than as originally designed. Having the peace of mind knowing the strength is in the spars won't lull me into becoming a test pilot for the remaining structure. No aerobatics planned or desired! :p

    Senna
     
    Last edited: Jan 27, 2003
  8. Jan 27, 2003 #8

    Jman

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    I have some kinfolk out in the madras area just up the road from you. I've been needing to visit them so maybe, when the time comes, I could give you a hand. Sort of trade some unskilled labor for the experience of seeing a static loading test in person :D. I actually used to live in madras and it's where my Dad is buried as well. Let me know if you need a hand. I don't live all that far away from you.

    Thanks for the in depth reply to my question by the way.

    Jake
     
  9. Jan 28, 2003 #9

    Senna

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    Load testing...

    Jake,

    I just may take you up on your generous offer of assistance with the static load testing. It's a bit far off into the future... but I'll keep it in mind! Thanks!

    I did some more thinking about it today and came up with a preliminary plan. I have some hard numbers and an idea for the test rig. I just need to determine if my idea for nylon web straps will point load the spar instead of spreading the load. I may have to come up with something else. Anyway... the following drawing is what I have in mind...

    Senna
     

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  10. Jan 28, 2003 #10

    Jman

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    Senna,

    I think it looks great myself. I'm not sure how you would get around the point loading. Here are some pictures I found of some test structures. They all have multiple points across the span like yours. The only picture showing a spar by itself is somewhat unclear and it's hard to tell how it is setup. I suppose when you are testing a completed wing structure, the wing skins and ribs distribute the load over the spar. Your setup looks great for what you are trying to do, in my unenlightened opinion.

    Jake

    p.s. You are quite gifted with the computer illustrations I must say.
     
  11. Jan 29, 2003 #11

    lou mc carrell

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    point loading

    if i may add my uneducated 2 cents worth on this, it seems to me that if you were testing a completed wing, the load would be carried from the skin thru the ribs to the spars, in effect "point loading" at the spar. that is the same condition, you would be creating with the straps, at least close enough for the girls we date. maybe no. of and spacing of the straps to duplicate the ribs would be a good idea
    lou
     
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  12. Jan 29, 2003 #12

    lou mc carrell

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    load testing

    something else that i'm wondering about, senna. your graphics indicate a stepped loading from the root. assuming, and it may not be the case, a "hershey bar" wing shouldn't the weight be evenly dispersed across the wing?
    lou
     
  13. Jan 29, 2003 #13

    Senna

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    Lift distribution...

    Lou,

    If we built wings of infinite span and constant chord the distribution of lift along that span would be uniform. The air flowing over and under the wing would be the same everywhere. Since we don't build infinite span wings... we experience a condition where the elevated pressure below the wing attempts to equalize the underpressure above the wing and this logically takes place at the wing tips.

    (Recall the Bernoulli Effect... For horizontal fluid flow, an increase in the velocity of flow will result in a decrease in the static pressure. The equation describing this effect is known as Bernoulli's law. The most practical example of this is in the action of an airfoil. The shape of an airplane wing is such that air flowing over the top of the wing must travel faster than the air flowing under the wing, and so there is less pressure on the top than on the bottom, resulting in lift.)

    The equalization airflow created around the wing tips is more commonly called vortices. Caused by the entire top of the wing having an inward equalization flow and the bottom of the wing having an outward equalizing flow. The result is that the lift at the wing tip diminishes drastically. Actually this diminishing lift occurs along the entire span... anywhere there is this equalizing flow of air. Lift is the strongest at the wing root and falls off toward the tips where it disappears entirely. This is called spanwise lift distribution. Hence the static load is distributed to approximate the spanwise load.

    Senna
     
  14. Jan 29, 2003 #14

    Senna

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    Static test ideas...

    I've been thinking about the point load problem with regard to the use of nylon web straps to loop over the top of the spar and suspend the required weight. I've come up with two possible solutions but further research is needed to determine if the use of these 'load distributors' would cause an anomaly and produce false results.

    Here's what I have in mind...

    Senna
     

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  15. Jan 29, 2003 #15

    Jman

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    I would think that, of your two ideas, the second seems to me to be the best. On the first option, if the blocks are rigid, the attach points will become more point loaded as the spar flexes. In other words, the weight will be spread until weight is applied and the spar begins to bend away from the outer edges of the block. The second option seems like it would continue to spread the load even with flex in the spar.

    Lou wrote:
    I see what you’re saying Lou, that kind of makes sense as well. Boy I need more schooling :D!

    Jake
     
  16. Jan 29, 2003 #16

    Senna

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    Moving this thread...

    In attempting to engineer a static test I've concluded the carbon rod modified spruce Invader spar will not take full advantage of the incredible tensile and compressive stregnth of the Graphlite™ pultruded rods. The comparative weakness of the wood fibers (9,400psi vs 330,000psi)... something like 34 times weaker than the carbon... severly handicaps the tinsile and compressive contribution the carbon rod modification would make. Certainly the spruce spar would benefit... just not to the same degree of a redesigned spar where the pultruded carbon rods can be used to their full potential. The spruce has become undesirable as a carrier material for the rods since it will fail at such an comparatively early point.


    So... since we've strayed from the 'CUTTING EDGE' aspect of the original Graphlite™ discussion... I'm moving this thread to the COMPOSITES section as I've made the decision to go entirely carbon with the spar design and build... and not just modifying the 'original' spruce spar.

    Senna
     
    Last edited: Mar 3, 2003
  17. Mar 3, 2003 #17

    orion

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    First of all, nice pictures. One picture is worth a thousand words, and we see the benefit here.

    Just a few comments though. Your are right in your statement regarding the pultruded rods. These products are wonderful and certainly will add a lot of ability for most aircraft designers and builders. The properties are well beyond what you can get through classical laminations and so the end result will be a stronger structre and a lighter one at that.

    A word of caution though. The strength properties you mention are only as tested under ideal laboratory conditions. The actual properties are a function of the application, the quality of mounting and the configuration of the load. Your test will be very interesting and when you do so, I'm sure most here will be interested in seeing the results.

    Two earlier posts had interesting and valid points. First, using dissimilar materials can have negative side effects. The wood has a very low modulus and low strength while the graphite has the properties of steel (although a much lower in-plane shear strength). The differences can potentially cause failure in an unexpected area. Personally, I think I would prefer to see the rods all the way across the cap, sandwiched between the ply webs.

    Also, as the other post mentioned, modifying the structre may cause the stress distribution behavior to be different. I don't think the effect will be all that critical here and it may actually improve the bending characteristics since the deflection will be much lower.

    Remember, a structure in bending is like a spring. the load goes only through the stiffest part of the structure. In this case, you can figure that the graphite material will see all the load, the wood caps will see virtually none. Thinking of this in mathematical terms, the amount of load carried is a function of "E" times "I", where "E" is the Modulus of Elasticity and "I" is the Cross-sectional Moment of Intertia.

    The graphite is probably ten of fifteen times stiffer ("E") than the wood. Since the outer point is at the same height as that of the original spar, we can for this discussion ignore the effect of a different"I".

    The graphite is therefore many times as stiff as the wood and will see all the load. The wood will see virtually none.

    The best example of this was in the old Quickee (the main gear was at the tips of the canard wing). For a time the owners had problems with the nose hitting the ground and breaking the prop. It turned out that the fibeglass wing was too flexible, allowing a prop strike on a hard landing.

    The factory, in their infinite wisdom, had the builders add several layers of graphite. This did stiffen the wing and the amount of impacts decreased. The problem though was that the added graphite was not enough. It was substantially stiffer than the glass underneath so the graphite carried all the load. The amount of material however was not designed to carry an actual hard landing. What generally happened then is that on a hard landing the insufficient graphite broke, forcing the glass wing to again carry all the load.

    The wing was not stiff enough again so the prop hit the ground anyway. However, the problem was compounded. Once the graphite broke, the fracture created a stress concetration, allowing in a few instances a crack to propegate into the primary wing structure. This weakened the wing even more, totally failing the structure. OOPS!!

    In short, be careful in this type of modification. It is usually better policy to design the structure with the material from the start, rather than trying to modify it later.
     
  18. Mar 5, 2003 #18

    dannicoson

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    I've done a load test on two of my spars and have a few recommendations.

    I had built two spars for my verticle tail with a "C" spar cross section that was then filled with foam and a single face ply of fiberglass across the open side of the C-channel. My spars were about 7 feet long, 6" high tapering to about 2.5 inches and about 1.5" thick. My spars are made of glass cloth for the shear web and I have "poor-man's" pultrusion for spar cap material. I took uni-glass and put a good tension on it in a mold as the resin cured. I figured this would get the best strength I could out of the uni-glass.

    My suggestion, you will never keep your spar from twisting without building the full airfoil around it. The wing skins will serve to stabilize any twisting that the spar wants to do. Even with your all carbon construction being more stiff than glass it also holds WAY more load than my glass spars can. Plus your spars are much longer (?) which agrivates this problem.

    My spars were designed to hold almost 2,000 lbs distributed along the length. I had built a steel frame to support the spar at the root end, simulating the attach point on the airframe, the spar hung horizontally and I used large disks of steel (meat grinding plates and exercise weights) suspended below the spar with rope (to keep from damaging the spar).

    I had built a spread sheet that told me which weight to hang where to acheive 4 different load levels (similar to 1 G, 3 G, 5 G and 6G). Somewhere around 860 pounds total I think I ran out of room to hang weight and the spar had some really significant twists starting to form. I stopped so as not to dammage anything.

    A very important point regardless of how you end up loading the part is to ensure that when the weights are removed, the part goes back to the same position without any permanent set. If it doesn't go back to the starting point you have probably damaged the spar (or your mounting technique was not sound).

    I measured deflection by mounting a laser pointer (looks like a writing pen) on the end of the spar. 30 feet away I had a board with paper taped in place. I marked the starting point with a pen and then marked the laser spot at each load point. then I checked afterwards with no load that the laser spot came back to the starting point.

    Good luck with your project. I will be using the same carbon pultrusion on my main spars.

    Dan Nicoson
     
  19. May 29, 2003 #19

    AVI

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    Pultruded Carbon Rod

    Hey, anything going on with the Graphlite rod? Have not seen a posting for some time.

    This stuff's definitely got my interest.

    The rods are being used not only on the sailplanes on Marske's website,
    but there's also a Vision builder in Florida using Graphlite rods in the spar caps.

    Anybody out there with experience who can offer advice on sizing a spar using these rods? And how about Graphlite fuselage longerons?

    Iceman
     
  20. Jun 30, 2010 #20

    planebuilder

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    Hello guys, I'm new to this site. My name is Mark Calder. I am an ultralight aircraft designer. My curerent project the ROBIN, is using a pultruded carbon rod spar. I have built a full size test article and I have fabricated two wing panels (L/H and R/H) Let me relate my experiences. First of all, you must design for deflection, not ultimate strength. My test spar was designed for ultimate load. The deflection of the spar was so great that the wing tip touched the ground before I was able to load the spar to ultimate. Consequently I redesigned to limit deflection. The basic criteria I set for deflection was no D Cell skin buckeling at 2G. This meant that the amount of graphite I had to add to the basic spar cap lay up schedule doubled. The wing is hell for stout, but total deflection at 6G is only 18" at the tip. The spar and leading edge skin for this wing were produced by Vacuum resin infusion, the final laminate was post cured at 180 degrees. The rods were secondarily bonded. I will post some pictures of the test set up and the final wing design. It is also of interest to note that my landing gear is made from pultruded carbon flats bonded to a spruce wood core. This is extremely satisfactory. The total gear weight is 7.2 lbs whic includes all fittings, wheels, tires and axles. I am also posting pictures of this.
     

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