dynel cloth as core

G'day,
I was thinking of using some dynel cloth to achieve a thin core in a sandwich.
Carbon twill 2x200g/ dynel 105g/ carbon twill 200g. The resin weight/ fibre weight is about 1/1 for the carbon going into the bag. Epoxy resin.

Some questions if anyone has experience with this:
Mechanical properties for dynel/epoxy laminate? Stiffness and strength values?
Does anyone know how much dynel expands with epoxy saturation? Is it easy to control the thickness by controling the resin ratio during wetout? We can bag with minimal bleed if we want.

The glider control surfaces that this is for are relatively small but we would like to improve panel stiffness a little. I heard dynel cloth had been used befrore for this but I don't have data.

Cheers
Gregg.

 

G'day again,
My email told me I had a reply to my post but I could not see it on the forum, so I'll paste it bellow.

It's from someone called PAR.
***************
Revaluate you abilities to engineer cored structures please. I'm not trying to be rude or coy, but the logic behind the question suggests quite a lack of understanding with sandwich structures.
***************

Unfortunately PAR you are quite knowingly being rude and blunt. I challenge you to actually express your idea. It would help if I knew more clearly why you think this.

Sailplanes commonly have small chord high AR ailerons. Our aileron tip chord for example is 70mm, but some gliders have smaller. The commonly available 3mm PVC core materials are not usefull. Getting a special cut of maybe 1.5mm PVC is trickey, expensive. Expensive of time and difficult to handle.

So when I noticed that some glider manufacturers had used polyester cloth, I assumed dynel or similar as a core material I was interested. But to examine this I need information about the material.

We have 105g dynel available so this is our minimum layer, but we may use more layers. I need to get some idea of what determines the layer thickness. Do we treat it like blotter paper to achieve out thickness, can we use more more layers with less resin and come out lighter. Questions like that came to mind.

It's relatively easy to look at the performance of the dynel in the FEA of our wing, but I have no properties for this material yet.

I'll learn what I can of the history of use of this material as core in the (glider) industry. The application is identical to ours. I think that marine composites designers often don't understand the choices that sailplane composites designers have made. The information about it is not easily available.

So if anyone can help with ideas about the use, properties, processing of dynel please get in touch with me or use this forum. You can see our project at:
www.foamworks.co.nz/sg/

The contact from this website is my email address.

Thinking of PAR again. I challenge you (PAR) to actually express what you think you are actually responding to. I think forums like this can be great if people can ask, answer and debate without being too guarded or trying to look smart. So mine was a simple question, but the way PAR responds undermines the best values of the forum and the process.

Cheers
Gregg

 

Gregg
The best way is to make a test layup. I doubt many here would be considering such thin panels because the requirement is usually driven by rigidity between bulkheads or frames. Seems a waste of good CF to only space them by a fraction of a mm.

Some have mentioned Soric fabric as a separator that allows infusion:
http://www.lantor.nl/index.php/id_structuur/10599/soric.html
It gives the epoxy take-up for the various cloths.

From a strength perspective I expect an epoxy soaked cloth will give higher percentage of the CF strength than a foam core.

However it takes very little effort to make a test piece and load it to destruction. Typically a good foam layup will fail on the compression side around half the tensile strength of the CF - due to buckling. If you get to this then I consider you have done well. In fact you might do better because you have a near solid core. You then have to determine if this gives adequate strength for the project you have.

Rick W

 

viz are you posting someone's private mesage to you on the forum?
Now, wouldn't that be rather knowingly blunt and rude?
and unfortunate.
for you.
It's apples and oranges, buddy.
Yes composites that are real thin behave differently.
So do different epoxies.
This is a boat forum.
In boats we use carbon differently.
Dynel is usually just for abrasion resistance in boats, not a core.
From the original question it seems like you have limited experience with dynel, yet you seem to propose using it in a way that would require intimate knowledge of its properties.
Best of luck with that.
Why not just get a piece of foam and feather it out as needed? Or balsa, that's cheap enough

Finally, the obvious answer:
using cloth will give you something too heavy, unless it is too dry, in which case it will be weak.
Probably best to use something else. Just an opinion.
But I am no airplane expert, maybe it is the wrong forum after all.

 

Rick,
Thanks for the ideas. Yea, I agree some testing will be a good idea. For us this will probably be mostly about processing or handling of dynel. Our torsional stiffness requirement is sizing our aileron shell laminate. The result is just stiff enough in bending under limit aero loads to maintain adequate clearances at the hinge line near the root, even though the hinges are reasonably closely spaced. Anyway, the relative torsional or bending strength or stiffness with this or that core is not a big concern. We were just looking for a little more panel stiffness. The aileron we sofar built has no core and went through the wing bending limit load test without any waves forming. But I would like the insurance of more panel stiffness for buckling. Maybe for the ailerons but certainly for our rudder, which has larger chord. The other issue with the panel stiffness is the profile deformation in normal flight. Quite frankly, it will be easier to explore the use of dynel as a core than to do a good sensitivity study on the profile deformation.

Thanks again,
Gregg

 

My initial post, made this morning was only up for a few minutes, before being deleted. I expected to return to the subject later in the day after I had finished up my thoughts on the questions.

My observations were and still are accurate, thought were not intended to remain posted in that format, in spite of my initial thoughts. Since I can not keep these thoughts to myself now, I'll simple add (again) you need a better grasp of material properties and sandwich composites, at which point the obvious will be as striking as it was to me this morning.

Other then what has been stated above by BWD, the radius of gyration for your proposed sandwich should be closely examined. Viability will become apparent.

 

This is firstly for BWD,
but also for anyone interested in the technical issues.

Firstly the issue of the mysterious email.
I recieved an email from boatdesign.net advising that I had recieved a reply and the content of that reply was included in that notification. When I went to the forum the reply was not there. So no I was not posting a private message on the forum. If for some reason you don't get or believe this I can forward the message to you.

Polyester cloth has been used before in sailplanes as a core, in laminates that would often otherwise be solid, or for control surfaces that might otherwise have PVC. I think I saw some in a fuselage layup schedule for an LS8 and I am sure I have seen some listed for an aileron laminate. So can we just stop the nonsense about this being a strange or inappropriate thing to do. We neded some ideas about the material properties and processing characteristics of this material and boat builders use it a lot so why not ask them? Nothing strange about that at all.

I helped my dad laminate his boat deck with dynel years ago but consider myself to know almost nothing about it. But there is no problem whatsoever with examining dynel cloth as a relatively novel core material for us. If I had some info on the material properties and processes as used on the German prototypes I would be completely happy.

Having pointed my finger at some of the nonsense, there were some genuine technical ideas. Thanks for those.
For or rudder the chord length will allow realistic use of 3mm PVC with a lot of wide feathering. Our aileron gas root c=120mm and tip c=70mm. This poses a problem with available PVc for two reasons. First, the amount of effort to acurately shape the entire PVC area is too much. Even if we could get a say 1.5mm PVC machine cut it will still need feathering and it's fragile at this thickness. Even so this is a possibility. The other issue is that as the core thickness increases even to say 1mm we have a reduction in the section properties, stiffness.

Having a less than accurately ground core thickness (or shape) introduces random variations in the stiffness distribution of the shape. Ditto the mass distribution. With some effort we did an aeroelastic study of the glider to look for flutter and divergence. We can't randomly change the mass or stiffness distribution without affecting the aeroelastic properties. If we use dynel as core we are of course also adding mass, but it's evenly distributed. Still has to be mass balanced though.

Balsa as a core in flying surfaces was done early on. End grain yes. Not sure if sheet was used. But hey the aero modelers have done it a lot. We could possibly apply it to what we are doing. I would need to think about it.

I'm still interested in any ideas about material properties and process of dynel cloth. There is a bit of smoke, noise and nonsense in the atmosphere which makes it harder for good ideas to come out.

Cheers
Gregg.

 

PAR,

It would have helped if you had simply made rational, technical, comment. I've challenged you on what you said:

***************
Revaluate you abilities to engineer cored structures please. I'm not trying to be rude or coy, but the logic behind the question suggests quite a lack of understanding with sandwich structures.
***************

I don't see a realistic cause for this nonsense.

Do you think the core is too thin? It's not even initially specified, except to give that as our minimum core thickness. We would like a little more panel stiffness, but the chord lengths are small (meaning the section thickness is) and we cant affort to reduce the torsional stiffness or bending stiffness much.

Do you not like unsymetric sandwich laminates? They are used very commonly on cliders, so maybe all the sailplane engineers need a lesson from you on sanwich design. Could I suggest instead that you study what they have done and try to understand the choices they have made.

You can take the above questions as rhetorical. No need to reply.


Gregg.

 

Don't get cross

Gregg

You asked a bunch of sailors and boatbuilders about using Dynel as a core and got told it was silly. Dynel is old hat in boat circles. It soaks up heaps of resin and possesses a very low E. It is good for abrasion resistance and for draping.

So when a bunch of boaties tell you its no good they are telling you that because they have used it long ago and have not used it for decades. There is no point getting cross with them. I think it is a silly idea - the core will weigh a heap and Dynel is really hard to sand/ and raises its weave easily. If you do testing as suggested my guess would be that you would do better with single skins. Eric Sponberg says much the same with the laminates for his carbon masts.

If you want to ask about micro laminates I would be asking super kayak makers, fairing builders and sailplane builders. Laminates fit for boats is what this forum is good at

Phil Thompson

 

Take a look at spheretex or simmilar.

 

Dynel is good over flexible wood structures. As a core is simply bad. Cores act as the compression post in a beam. They should be as light as possible.

 

As Ragnar did, I would recommend Sphere Tex, Sphere AX (biax 200gsm) or Lantor Coremat XI (60gsm), the latter needs only 500g resin per m² and is approved by Lloyd's, Registro Italiano Navale, Det Norske Veritas and American Bureau of Shipping.

Richard

 

... all for boating, not for flying...

Still the Coremat XI is an attractive choice.

Did you ever consider filling the complete foil with epoxy foam? I believe www.sicomin.com have a movie on their website...... Found it! http://www.youtube.com/watch?v=1Pm3EC0YpM4

 

dynel as core

I'm going to assume that anyone commenting has read the thread, although it's not always clear that they have.
I get it that maybe lots of the boatbuilders don't like the idea of dynel as core. But I don't hear anyone looking objectively at the material properties or processing in the proposed application.

At leat 2 recent, fully certified german gliders that I have data on have used polyester cloth in an otherwise solid thin laminate. Figuiring out why they chose this is interesting and educative for anyone growing their design/engineering/process knowledge.

If anyone has a set of material properties for dynel/epoxy or knowledge of the processing/handling that may be usefull for me they can contact me direct if they want.

I wondered how many people were on this forum and found hundreds of pages listing the members. In that large number there will be several who will understand exactly the kind of logical thinking proces I am entertaining. It's the same for marine applications, just a different context, set of requirements and constraints. But instead this large forum is dominated by a small highly vocal subset who feel they own it. I would vote for greater democratisation of the process.

Someone said that this was a group of boatbuilders and sailors. In that huge membership I saw there will be some naval architechts, engineers, PE's. composites specialists, FEA guys. But you need to make room for them to be heard. Otherwise maybe a subforum for them.

Gregg.

 

This is as democratic as it gets. Everyone gets their soapbox. It seems that your problem is that we are telling you that your pet idea sucks.