Could kevlar be better for shear webs than carbon or glass . . . ?

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raymondbird

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Hello All,

Been studying Dr Mark Drela's spar design method. Anyone have any thoughts on it? Scaled up to full size of course and with maybe vertical grain pine between caps of pultruded carbon rods instead of balsa.

He is saying the compression side of the +- 45 shear web buckles very early but the tension (much stronger) fibers can pull to the end against the immensely strong vertical grain wood, which makes for a very strong spar.

Makes sense to me and then wouldn't Kevlar be superior. More resistant to cutting where it is layed up over the sharp edge of the spar caps and is unbeatable in tension is it not . . . ?

Cheers,
Ray
 

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mcrae0104

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I suspect he developed this method because it is easy to build at small scale. At full scale, a wood core would be a waste of mass close to the neutral axis--i.e. it would be heavier than it needs to be, and the wood will contribute little stiffness with carbon or kevlar shear webs. However, I think the Gazaile uses wood as a spacer between carbon rod caps--similar concept--and it works.
 

BoKu

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Use Kevlar where you need toughness and don't care much about stiffness. Carbon fiber is a much better deal in terms of dollars per unit stiffness.
 

raymondbird

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According to Dr Drela, you size your spar caps for bending resistance (stiffness). In his math he even shows a grossly overbuilt shear web only reduces bending by ~2% and can be ignored. Again, he sizes his shear web by tension failure alone, hence my question that Kevlar might be a better choice for shear web being superior in tension. It is a rather a sharp edge too around the pultruded carbon rods.
 

raymondbird

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Yes, I think so. My pultruded carbon spar caps are only just over 1" wide (1" deep) at the root. A 2,500 lb gross weight airplane pulling 10g (safety factor) generates a lot of shear compression on that small surface area. Can't afford the carbon to resist that. Vertical grain wood is very efficient at that.
 

Vigilant1

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Makes sense to me and then wouldn't Kevlar be superior. More resistant to cutting where it is layed up over the sharp edge of the spar caps and is unbeatable in tension is it not . . . ?
[/QUOTE
I apologize for not having time to check/back this up with references, but IIRC, Kevlar absorbs and holds moisture to a larger degree than most other fibers used in composites, and this can cause problems with freeze/thaw cycles in some applications, and I suppose it might not be good to be in contact with wood. CF is cheaper and less trouble to work with.
 

wsimpso1

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Kevlar? Works great in tension, like for pressure vessels and tires, which is what it was schemed out for...

Problem with Kevlar is it is closer to carbon for cost, but in compression and shear it is almost E-glass for strength. Since most of our structures (other than oxygen bottles and tires) see both tension and compression and in nearly equal amounts, Kevlar would drive you to nearly E-glass level part thicknesses. Smarter is either glass at much lower cost (and easier construction) or carbon at much lower weight (and easier construction).

My bird has Kevlar in one place so far. I have a pass tube through my wet wings that runs near the fuel filler. The pass tube has two plies of Kevlar on that tube where a careless lineperson could hit the tube with a fuel nozzle. Not saying I might not use a little more bullet resistant fabric someplace else, but so far a lot of airplane is built and no other Kevlar.

I will look up this design method you are talking about, and maybe comment further.

Billski
 

wsimpso1

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Hello All,

Been studying Dr Mark Drela's spar design method. Anyone have any thoughts on it? Scaled up to full size of course and with maybe vertical grain pine between caps of pultruded carbon rods instead of balsa.

Ray
Hmmm. For RC, I see the advantage of balsa. For full size airplanes, there are probably better ways. Certainly lighter ways. Let's start by remembering that the core has two purposes: It is a mold that stays in the part, and; It provides buckling support to the composite layers. Once it does those things, it has almost no other strength requirements.

Looking in detail, for RC airplanes, the spars are small, the cores are tiny and fragile, and if you worked with foam, you would not have a chance at getting it all right and straight. Also, the resin uptake of the surfaces of the common foams would be substantial. To build the small and delicate looking spars for an RC, yeah, balsa has its advantages. Go to a full sized airplane, foam will work great, and balsa may well be expensive, difficult to work, and heavy.

In a full sized airplane, you will have to size the caps to carry the bending and the web to carry the shear and deformed shape of the bending allowed by the caps. Once you have designed the composite elements, the cores, regardless of material and shapes, will be carrying little load. So, if you can accurately fabricate the cores, they should be as light as you can make them. There are places where substantial cores are used, for instance in hard points where connections are made between wing segments or to the fuselage, but those are locally strong, not extending but a few inches along the length.

The other thing that bothered me is his use of "prepreg". From the description, he appears to be using pre-cured unidirectional carbon composite of constant thickness and width from tip to tip. In a real airplane, this approach can be heavy. Bending moments are large near the fuselage, and drop off rapidly as you go outboard. At the tips, you can frequently dispense with the spar entirely. So you would want to tailor the amount of spar cap material in the spar based upon the bending moment vs spanwise position in order to save both weight and cost. Like wise, the shear web carries less and less load as you go from root to tip, and should be similarly tailored. In the caps, this argues for Graphlite rod per Jim Marske, and is tailored by using a few rods that go full span, but many shorter rods. See Marske's site.

He is wise to use precured carbon. Attempts at doing wet layup of unidirectional carbon frequently end with much lower strengths than expected. Straightness and resin fractions are very important to carbon fiber, and we homebuilders have trouble approaching the required levels of straightness and resin fractions in spar type laminates.

I have one other thought at the moment. His lapping the shear web material on the top or bottom of the spar is wasted. If the lap join is made on the sides, those layers in the join are part of the shear web and the spar caps are further out than with each layer of web removing one ply's worth of thickness from the caps.

In total, it looks like a great RC spar method, but can be substantially lighter and better tailored for full sized airplanes.

Billski
 

raymondbird

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IMG_9749.jpg
Core strength and weight is almost irrelevant except where a hard point will carry bolt loads. Use plastic foam for almost all of a full size spar...

Billski
Okay, you're too intimidating to argue with (I've read all your posts many times and think you must be a genius) but if I can find it again I'm going to submit an article I was reading about the failure mode with pultruded rods being compression failure of the foam composite shear web. The rods being very dense and small cross section for their strength of course and therefore harder to restrain than a uni tape cap.


In the meantime, thanks very much all you smart guys for your input! Should be paying you a hefty consultation fee. I'm starting to understand that models are not the same. Very intriguing actually how it doesn't scale linearly. They say Dr Drela would crawl over a mile of broken glass on his hands and knees to save an ounce so it's hard to understand unless it is the scaling factor.


BTW, It was Jim Marske who designed my spar and who I bought the rods from. It does drop out rapidly in rods from root to tip and shear web lams. He also told me I didn't need a vertical grain wood core. That is my paranoid (it seems) idea.


Salute!
 
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BoKu

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That looks like a classic shear failure, all right. We saw a very similar diagonal unzip on the test coupon for the carbon wing spar for our tapered RV wings--except that it occurred at about 2.7 times limit load.

[video=youtube;BBeTSSfe_3Q]https://www.youtube.com/watch?v=BBeTSSfe_3Q[/video]
 
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wsimpso1

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It seemed to me from the initial description what was being described was a Wagner beam. These do need vertical support struts or some other structure to keep the spar caps apart?

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081248.pdf
ALL beams need to hold shape under load to work. The caps have to be held at spacing and plane surfaces have to be kept plane under load. That is both a given for beam design and pretty much inherently covered in most aircraft type spars.

I tried to find Wagner beam, and what I found was tension field theory (invented by Wagner) applied to sheet metal beams. The gist of it is that if you load the shear web up only to shear strength of the metal, it can limit you. The initial failure is wrinkling of the metal panels as you approach shear strength. If you adequately constrain the edges of each panel and keep them small enough (more little panels) you can let them go into the gentle wrinkling and carry the loads with thinner, lighter shear webs. This is tension field design, and it does require more complexity in design and construction and more parts, but done well, it can be a weight reduction. In sheet metal.

Just because you have a sheet metal shear web does not mean you have a Wagner beam. Many airplane designs have sheet metal shear webs that are not designed to Tension Field methods. In many production airplanes, sheet metal gages available drive the decisions. One thickness is too thin even for Tension field, the next satisfies conventional design. So why use tension field with all of its additional parts and work?

Now, in composites, please quit thinking about sheet metal. Metal thinking is BAAAD in composites... The balsa cored, carbon fiber cap, glass wrapped shear web spar described by Dr Drela uses a balsa core that the graphite fiber caps are bonded to, then the glass cloth shear web is wrapped entirely around the core and the caps and is vacuum bagged to the core. Both the caps and the shear web are firmly bonded to the cores and fully supported against local buckling that could be called crippling or result in tension field based distortion of the webs. This works great in composites. Thousands of Long-Ez's and their derivative built with hollow foam box cores for the center section spars. Works great.

The other way to do composite spars is to build channels instead of hollow rectangles. The caps are generally thick enough to take care of themselves until you get out near the tips where the loads approach zero anyway. The Webs are generally built with 2 to 4 plies of shear web wrapped around the outside of the caps, and the rest inside the caps. These two sets of web are generally separated by a foam core which nicely supports even the thinnest of webs against any localized buckling, crippling etc. Where you have need for hardpoints, the foam core is replaced with a denser foam, wood, or other materials that can stand the bearing load and bolt forces of the particular design selected.

So, no, Wagner beam type construction is not needed or even desirable in properly designed composite beams. In fact, by the time bending of the beam is superimposed upon the shear loading in the webs, you must beef up both the caps and the web to make strength, and you have even more resistance to local buckling...

Billski
 

wsimpso1

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Nice photo! Failed parts tell us so much. Trouble is getting them takes so much money and time and effort. It looks like the shear web gave up, yes? I hope that happened at or above the planned ultimate load case. Details would be educational for all of us.

Okay, you're too intimidating to argue with (I've read all your posts many times and think you must be a genius) but if I can find it again I'm going to submit an article I was reading about the failure mode with pultruded rods being compression failure of the foam composite shear web. The rods being very dense and small cross section for their strength of course and therefore harder to restrain than a uni tape cap.
I did say "almost irrelevant". I do not mean to be intimidating, just trying to share the knowledge on how this stuff works.

I would love to see the article you mention. Failure modes exist. The big question is whether the choice of foam will make the difference between an adequate structure and an inadequate one. Almost all foams have little strength - good design avoids putting load into foam cores. I can imagine scenarios where the rods bearing on the foam would show issues in a failed spar. The biggest question then is "Did the foam issues precipitate the failure or did the foam damage follow as the structure collapsed?"

If the foam is in the load path, and the rods deform out of form, the foam can not be expected to restrain them. None of them are strong enough and stiff enough to prevent that. Good composite design practice has the glass or carbon shear web carrying the Graphlite rods, per Jim Marske and others. To get the load from the rods into the foam through the shear web means either not enough web there or the failure seen in the foam happened after the structural collapse was underway. Either way, choice of which foam or balsa core material to use hardly ever matters.

Now, about this photo. If you feel up to sharing, the info on design and load at failure and exactly which things are broken would be very educational to us all.

Billski
 

wsimpso1

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I love that video Bob! Shows us that something has to break, and confirms the analytical work that says the web is usually what goes.

Billski
 

Norman

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Core strength and weight is almost irrelevant except where a hard point will carry bolt loads. Use plastic foam for almost all of a full size spar...

Billski
In this spar design the compression loads and shear loads are handled separately. The core is called the compression insert in Dr Drela's papers and must be strong enough to take the full compression load so that the fibers of the kevlar or glass wrap only see tension. The compression insert is only wood at the wing the root, and dihedral breaks in the case of polyhedral, the rest of the span is foam. I saw a guy bragging about being able to do chin-ups on his spar in a dynamic soaring forum. This type of spar construction is common in dynamic soaring where a 30G pull up is not unheard-of. Phil Barnes describes it better than I could so I'll shut up and let those interested in this spar design read his comments:

Socked spar design
Explanation of vertical shear webs
 
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