dynel cloth as core

dynel as core

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

 

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.

 

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

 

dynel as core

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

 

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,

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

 

correction and adding to my reply to SamSam

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.

 

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

 

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

 

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

 

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

 

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