dynel cloth as core

[viz]

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.

[viz]

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

[Rick Willoughby]

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

[BWD]

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.

[viz]

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

[PAR]

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.

[viz]

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.

[viz]

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.

[catsketcher]

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

[Raggi_Thor]

Take a look at spheretex or simmilar.

[gonzo]

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.

[apex1]

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

[Herman]

... 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

[viz]

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.

[gonzo]

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.

[viz]

Gonzo, and anyone else that feels the same.

This idea of dynel as core is not my pet idea. It's something that has been done legitimately before on gliders, so it was a condender. I couldn't myself be so stupid or arrogant as to just dismiss it without thinking about it or learning something from it.

If you are one of the incumbent blockages on this forum that helps retard the flow of interesting ideas and you are telling me that this is as democratic as it gets... then I guess that makes sense.

Unfortunately not everyone gets or should need a soapbox. You really must get wise to the idea that a lot of intelligence will be sitting unutilised in the vast membership. It would be a good thing to enable the participation of this intelligence. It's too easy for a dominant few to take up all the available space.

This probably concludes what has been for me a somewhat unilateral conversation.

Gregg

[SamSam]

Here, take a look at this, it might help a little...
http://sweetcomposites.com/Polyester.html

I was wondering about this in post #7 "The other issue is that as the core thickness increases even to say 1mm we have a reduction in the section properties, stiffness." It seems contrary to what I understand, in that stiffness increases exponentially with thickness.

It might help to give an idea about what this is in relation to, like in a model glider or a full sized glider? And if it is a full sized glider, why so much intensive striving to save a few ounces of weight? Is this a competition type thing or more of an interest in the technicalities of the construction? If this is a full sized project, it seems like you're pushing the limits to a hairs breadth in an endeavor where failure would really suck.

[Rick Willoughby]

Gregg
As a matter of interest, there is a considerable volume of more technical exchange occurring in the background. I am sure some people ARE daunted by the cruel treatment given to some and actually prefer to exchange ideas in the background.

There are some threads that are purely political with very little to do with boating.

There are some participants who take delight in polluting threads with nonsense.

It is the nature of the forum. Unless you want to make this a political thread it is best to stick to the matter of your original question and weigh whatever input as it comes. Ignore stuff that is completely negative or find the tiny pearl of wisdom that it might contain and follow that.

When you make sweeping statements about the adverse contributions you are inadvertently condemning the well meaning. Also it is not unusual for an off-the-cuff remark, an attempt at humour, to be taken the wrong way.

It is a very active forum and often the best to answer the question will take a while to find it. Some actually make a contribution to a thread so they get the automatic update to see how it unfolds. Sometimes threads need to be kept alive until they get the issue resolved. Some threads just go around in circles.

I am certainly interested in any test results you get as I have an interest in low weight high rigidity layups.

Rick W

[viz]

SamSam,

Thanks for the link, I had a look.
It's a full sized glider, but lighter than normal and initially 13.9m span instead of the common 15m span. I included a URL for our website on a previous post:
http://www.foamworks.co.nz/sg/

The ideas about the core thickness relative to the section properties, stiffness. I'm refering to the torsional stiffness and bending stiffness of the aileron. They are long, of small section, but need to be very torsionally stiff, and the bending deflection between the hinge points has to be small. With just 3 layers 200g CFTwill at 45 degrees we are very torsionally stiff and the bending deflection is fine. There is no spar or web inside the aileron. Maybe I already described this. As the core thickness increases we reduce Ixx hence stiffness quite quickly. Our aileron section thickness (height overal) is 34mm at the root and 17mm at the tip so you can see how it happens. A common 15m span sailplane have even less in the latest designs.

We are really pushing our weight limit to fit our intended category (ultralight glider 220kg including pilot and gear). But I didn't remember talking about the weight isue, except maybe to say that the control surafces are commonly mass balanced to avoid flutter, and being careless with the laminate will mean more mass balance at the front of the control surface. We are really short of space for that.

We do have to work hard to keep weight down. Ultralight glider is a relatively new category and I hope we can take some of the world records, as long as we can fit the weight limit. At the same time the dynel as core question, and the whole project, are partly due to interest in exploration of the technologies.

The risks can be higher with things that fly, but the common design and build practices are very conservative. It's common for non certified projects to follow the standards used for certified aircraft. Our project follows CS22 the EU sailplane satndards. Actually it was the use of polyester cloth as core in thin laminates in certified gliders that made me think of it. But we only just started to consider this material. It's just a contender.

I still haven't had time to find some material properties for dynel to begin objective examination of the idea. One idea that occured to me is that the toughness of the material may be a factor, helping damaged parts to hold together after mid air collisions or flutter, or reducing pilot injury from fractured carbon fuselage parts in a crash. Kevlar and kevlar/carbon hybrids are often used on gliders for that reason.

Cheers
Gregg

[viz]

Rick W,

Look I'm quite genuinely sorry if I offended any of the well meaning people. I had a strong reaction to the careless remarks from PAR and it put me in a mood to defend myself.

Thanks for the ideas on how the forum works.

Re the ideas on cores for control surfaces and the use of polyester cloth. I'll probably do most of the following:
1) Review all I can find on such cores as used on the German certified gliders.
2) Find the material properties I need. As used on a prototype or one of the Akafleig student projects. I may already have these.
3) Find some info on their process/handling.
4) Make a calculation that makes it easy to intuitively optimise the core thickness.
5) Apply the core materials within our existing FEA.
6) Test layup(s) to get a feel for the process and handling.

It most likely that we would not do any formal destructive or stiffness test for this. Proviso we can follow design and process practice as per a certified aircraft.

Cheers
Gregg

[pamarine]

Viz,

The best thing I could tell you at this point is make up a sample and work with the local FSDO (well, the NZ equivalent of a FSDO) for a field certification.

I can only give input as an Aircraft Mechanic and inspector, as outside of the marine uses of Dynel I have limited experience with it in composite laminates.

Frank

Rick W,

Look I'm quite genuinely sorry if I offended any of the well meaning people. I had a strong reaction to the careless remarks from PAR and it put me in a mood to defend myself.

Thanks for the ideas on how the forum works.

Re the ideas on cores for control surfaces and the use of polyester cloth. I'll probably do most of the following:
1) Review all I can find on such cores as used on the German certified gliders.
2) Find the material properties I need. As used on a prototype or one of the Akafleig student projects. I may already have these.
3) Find some info on their process/handling.
4) Make a calculation that makes it easy to intuitively optimise the core thickness.
5) Apply the core materials within our existing FEA.
6) Test layup(s) to get a feel for the process and handling.

It most likely that we would not do any formal destructive or stiffness test for this. Proviso we can follow design and process practice as per a certified aircraft.

Cheers
Gregg

[viz]

SamSam,

It won't help if I make mistakes or leave things out in the message.

Re the sectional stiffness of the aileron varying with core thickness. I mentioned area moment of inertia Ixx but forgot area polar moment of inertia J, the most important one for me. All I meant was that as the core thickness increases the inner laminate is displaced so the I and J reduce, the sectional stiffness for bending and torsion is reduced. But some increase in the panel stiffness may come quickly.

Cheers
Gregg.

[Rick Willoughby]

R......
It most likely that we would not do any formal destructive or stiffness test for this. Proviso we can follow design and process practice as per a certified aircraft.

Cheers
Gregg

Gregg
In terms of the destructive testing I was referring to making a sample layup and stressing it in the way it will be stressed in the application. For boat layups the key requirements are associated with good core adhesion and simple bending strength.

If your concern is torsion then maybe a small section that can be torqued. I expect the dynel is acting as a binder for the off-axis stresses. Maybe also the more controlled failure. My original comments assumed it was a separator for bending strength but from what you describe it has little to do with bending.

I have purchased factory made CF tube that is well constructed. It rings when hit. I imagine it is spun with a pre-impregnated strand. It has very high torsional rigidity. A pity you cannot get something like this in the right shape.

Rick W

[viz]

Rick,

We did a torsional stiffness test on the ailerons we made with no core. So we can peg that to the calculations or FEA if we want. Meaning we can tweak the calculation/FEA. From there I would be reasonably happy to accept a calculated value for the torsional stiffness with core.

For us either stiffness or strength can dominate as a criteria for a given object. We don't know untill we do calculations. For example, we have a target stiffness for the wing bending. Wing spars with caps of pultruded carbon rod would have been too floppy if sized for strength. With caps of CFUD fabric sized for strength the spar is just a little too stiff but probably ok.

The polyester core in the certified gliders is as you expect co-cured so the bonding issue I thought would be OK.

I think the torsional stiffness we have now is ok. Our panel stiffness near the root is a bit low. I think we may get a deformation under aero loads a little sooner than I want. We could have almost just added another layer of carbon to the shell near the root.

Cheers
Gregg

[Rick Willoughby]

I think the use of dynel as a filler is heading in the right direction. Probably better filler than using CF cloth. That would be wasteful. It has slightly lower density than CF.

So the carbon is already doing the torsional job. A bit of thickness that adds toughness and stiffness, by virtue of separating the CF, with little extra weight sounds good.

It is amazing how much extra stiffness you get from a little extra thickness. You can do the calculation.

Rick

[apex1]

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.

You would be impressed to see how professional most of the contributors here are, and that they in fact are "the loudest" around. There´s no need for more room.
Moreover you might be surprised that the knowledge about composite building you can find around here is by no means less than you could find on a similar aircraft forum. Imho there is even more around here, because we process a magnitude of composite materials compared to aircraft built.
Maybe one or the other did not like or understand your setup (me included), that does not make it necessary to get rude.
And for a sandwich construction when understood as a I beam structure, Dynel is NOT our choice.

Regards
Richard

[viz]

Richard,

I apologise again if I have offended any of the well intentioned people, yourself included.

My expectations about the level of knowledge and expertise in the forum was/is quite high, basically the same as your self-assesment. We have quite an energetic marine composites industry here in NZ. I sort of just imagined it was similar in other parts of the world. Globally, development and construction of small composite aircraft, including gliders and sport aircraft is like you say or infer, a tiny industry compared to the marine industry (development and construction of composite boats).

So there is a disadvantage of scale affecting glider development and the development of new ideas for materials application. When the Americas cup was big here in NZ I used to marvel at how much money they had for development compared to what (I imagined) was spent on the development of a new competitive glider.

Materials applications on certified gliders and small aircraft tends to be very conservative, and like I said previously, most experimental or otherwise non certified projects are following the certification standards even if not always perfectly conforming. This is mostly about safety, but also about economy. If we wanted to use a novel material or application then we would feel obliged to test it. For a small or self funded project this can be too much.

I haven't yet seen a big forum for light composite aircraft or gliders such as you have here for marine.

Cheers
Gregg