Core materials for resin infusion with best strength to weight ratio for use with compound curves

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MotoFairing

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Jul 9, 2020
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I am looking to build an extremely aerodynamic light weight velomobile fairing with compound curves.

I can't use Nomex Honeycomb since the holes will fill up with resin.

Searching has not revealed the best possible appropriate resin infusion core material so I have to ask.

Ideally a material that can conform to compound curves and is about 9mm thick for high stiffness and most importantly work with resin infusion.

Layup will be:
200g Carbon Twill
Core material about 9mm
200g Carbon Twill
60g Aramid (carbon splinter protection)

This has been bothering me so thank you if you can help me find the best appropriate core material.
 

Scheny

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I used this one for my boat and I am more than pleased about it:
and the distributor:

Not the best choice for an aircraft, but for for almost any other solution. It is foam with integrated flow patterns. These will later form a honeycomb built from resin. There are hexagonal, rhombus and triangle patterns and they will consume 70/90/120 grams of resin per m² and per mm of thickness, but they will add shear strength improvements relatively to their weigth. The hexagonal 45kg XPS is as a rule of thumb twice the shear strength than for conventional honeycomb, the rhombus 3 times, etc. For more strength there is also a 120kg PET version, but this is only for hardcore cases (like hitting rocks).

Don't be shocked by the "dead weight". I calculated it against other materials and it is by far superior for any application except for ultimate weight (where you end up with honeycomb).

Your layout (with heavily rounded numbers):
200g carbon: 440g​
64g glass: 150g​
extra saturation: 100g​
honeycomb: 270g​
extra saturation: 100g​
64g glass: 150g​
200g carbon: 440g
Total: 1650g/m²

3Dcore 45kg XPS 10mm (with heavily rounded numbers):
200g carbon: 440g​
extra saturation: 200g​
3D core: 180g​
resin inside core: 700g​
extra saturation: 200g​
200g carbon: 440g
Total: 2200g/m²

But as this material is twice as strong as honeycomb, I would rather go for the 5mm or even 3mm (except you need ultimate stiffness) and then you will end up with:
1760g/m², so almost the same weight as the honeycomb!!! (or 1540g for the 3mm)

BR, Andreas
 

wsimpso1

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I am looking to build an extremely aerodynamic light weight velomobile fairing with compound curves.

I can't use Nomex Honeycomb since the holes will fill up with resin.

Searching has not revealed the best possible appropriate resin infusion core material so I have to ask.

Ideally a material that can conform to compound curves and is about 9mm thick for high stiffness and most importantly work with resin infusion.

Layup will be:

200g Carbon Twill
Core material about 9mm
200g Carbon Twill
60g Aramid (carbon splinter protection)

This has been bothering me so thank you if you can help me find the best appropriate core material.
Best? Strength to weight ratio is your criteria? First, you will have to redefine your criteria. You know, "Define Best". Are we talking axial strength, bending strength, bending stiffness, etc. Second if you have conducted a search, within the search you have already a "local best" choice, you just have not accepted it as such yet... Maybe better still exists outside of your initial search.

Strength to weight ratio is attractive, but what strength do you need and and how do you get to lightness? Bending strength? Impact strength? Torsional strength? Then there is how much must you have and what is the trade for weight in your application? As a long lived and successful product engineer, I know that tradeoffs exist toward the best product. You seem to have already fixed the laminate with the exception of core density, that sets most of the weight of the thing. The guy who invents a decent core that is lighter than air will have something. You will do your project many favors by reconsidering your fiber weight and core thicknesses - you will achieve more this way than by seeking out the absolute lowest density core and accepting whatever other negatives come with it.

I am thinking that adhesion to your resin, low assembled mass, adequate bending and shear stiffness of the assembly, and some level of assembled strength must all be there, but these are all scales with a variety of results. And you get to pick which one you want to end up with. I suggest some sample part fabrication and then assessing their sturdiness to see if they will survive in your build and use environment.

Much of the outside of many sailplanes is the fiber reinforcement you are suggesting. And they already have cores that work well to make a sturdy long lived structural sandwich. Usually around 6mm. For faired racing bicycle, I suspect that this might be quite good, long lived, sturdy in use, etc. And heavier than you will like too. If this is to be a more targeted machine, for racing or record setting, I will suggest that lower mass can be achieved with lighter facings as well as thinner and lower density foam products in exchange for it requiring more care in build and handling. In fact, you might even build a sturdier set of fairings for general use, training etc, then a lighter one and spares for competition work... Again though, it comes back to "Define Best". Then you get to fabricate and test and decide...

Almost all of the foams out there will go compound curve by a cycle of gentle warming and pressing over forms. They each have their own temperatures to do this...

9mm core is pretty thick. My airplane uses 6 pcf PVC foam in about that thickness for much of the structural shell of my airplane, with a design dive speed of 268 knots. Please explain - not for me, but for you - why your velo needs as much or more skin stiffness for its non-structural fairing than the structural skin of an airplane designed to go over 300 mph? It seems to me your defined "needs" are over specified, and can undergo significant adjustments. For instance, you 9 mm core will weigh as much as or more than your laminated facings. Then there are lighter foams, some down to 1.5 pcf.

And you get to choose where on the scale of lightness vs sturdiness you want to be...

Billski
 
Last edited:

stanislavz

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Ideally a material that can conform to compound curves and is about 9mm thick for high stiffness and most importantly work with resin infusion.
Because it is not big part by aircraft standard - i would propose this solution. Out of the box. Poor man prepregs..

1. Infuse your carbon plies on flat table. By infusion if you want. Or using roller between some pe foil.
2. Put you outer layer on mold.
3. Put your nomex honeycomb.
4. Put you inner layer.
5. Add some vaccum to hold it.
6. Turn on the heater :)

Doable with some resins with slow hardener. Or doable with normal resin if you have winter or possibility to build small cold room. May not be best show car.

Or - go from 200 gsm / core / 200 gsm to ~ 300 gsm (done with infusion) + pe foam strips grid (100x100 mm grid as starting point, pe strips 10-15 diameter) to build local reinforcement web ( some manual labor required).
 

MotoFairing

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And you get to choose where on the scale of lightness vs sturdiness you want to be...

Billski
Strength criteria for the fairing for a velomobile with total weight less than 20kg.

1. Resist deformation and vibration that compromises laminar flow from wind at speeds up to 120kmph/75mph
2. Resist damage from careful manual handling
3. Resist abrasion and impacts with the road due to low ground clearance and roll over
4. Protect the rider by deflecting impacts and resisting hard impacts. Example: Velomobile Crash

Lightness:

1) Going up hills is no fun so a light weight is desirable.

I got the idea of 200g carbon twill and 9mm Nomex Honeycomb from an Ecomarathon paper also I have been re-watching the Mike Arnold's videos on Youtube. I won't be able to do CAD or finite element analysis as I don't have these skills so time I spend trying to self learn is usually unproductive.

A test panel may be the most accessible method of evaluating a suitable layup. Experience is invaluable resource.

Thanks for everyone's replies so far I am learning a great deal.
 
Last edited:

kubark42

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Oct 19, 2020
Messages
90
I used this one for my boat and I am more than pleased about it:
and the distributor:

Not the best choice for an aircraft, but for for almost any other solution. It is foam with integrated flow patterns. These will later form a honeycomb built from resin. There are hexagonal, rhombus and triangle patterns and they will consume 70/90/120 grams of resin per m² and per mm of thickness, but they will add shear strength improvements relatively to their weight. The hexagonal 45kg XPS is as a rule of thumb twice the shear strength than for conventional honeycomb, the rhombus 3 times, etc. For more strength there is also a 120kg PET version, but this is only for hardcore cases (like hitting rocks).
This is really neat stuff, and I can see some applications for composite aircraft parts. Unfortunately, their website doesn't go into any detail at all about their product. If you hadn't said that you had experience with compound curves I never would have guessed that's how the product can be used. Do you have any technical reference material on it which you could share?
 

Lendo

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Remembering cuts means openings for Resin to fill, which in turn means extra weight, better to use heat to make the core material more pliable for those weight sensitive jobs - like in Aviation.
George
 

Scheny

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Do you have any technical reference material on it which you could share?
In my opinion the best material for infusion and also suited for compound curves. I only did one typo: the patterns are consuming 75/126/171 g/mm/m² for the different patterns hexagonal/rhombus/triangle.

Here are the datasheet and the user manual (unfortunately in german only):
 

MotoFairing

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Joined
Jul 9, 2020
Messages
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Because it is not big part by aircraft standard - i would propose this solution. Out of the box. Poor man prepregs..

1. Infuse your carbon plies on flat table. By infusion if you want. Or using roller between some pe foil.
2. Put you outer layer on mold.
3. Put your nomex honeycomb.
4. Put you inner layer.
5. Add some vaccum to hold it.
6. Turn on the heater :)

Doable with some resins with slow hardener. Or doable with normal resin if you have winter or possibility to build small cold room. May not be best show car.

Or - go from 200 gsm / core / 200 gsm to ~ 300 gsm (done with infusion) + pe foam strips grid (100x100 mm grid as starting point, pe strips 10-15 diameter) to build local reinforcement web ( some manual labor required).
Having worked with epoxy so far I know I want to avoid wet layups if possible. A skilled person may be able to do what you propose but I can imagine it turning into a sloppy mess in my hands haha.

I have some materials go on now, I will order materials when I get to that stage of the project.
 

stanislavz

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Having worked with epoxy so far I know I want to avoid wet layups if possible. A skilled person may be able to do what you propose but I can imagine it turning into a sloppy mess in my hands haha.
I am not talking about wet layout. Take two pe foil, put your 200-300 gsm cf between them with required blob of epoxy in the middle, close with other foil and work it with roller. I was able to get lower than 40% of epoxy content consistently. Then you remove one foil and put it onto your mold.
 

Richard Schubert

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Carbon fiber cowls have been made with 3ish layers of cloth and no core, they withstand 200mph plus airspeeds. If you have large flat areas that cause drumming or vibration issues, you can add stiffeners.
I am a little dubious of measurable gains from laminar flow at such low speeds.
Hitting a tree stump is a design goal? :) I would work on stability first.
 

Lendo

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Richard, There are gains other than measurable gains, although John Roncz suggests Laminar is better in any condition, for myself I'm thinking softer stall behavior.
George
 

Red Jensen

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Couple things to note. I know you said you don't want a wet layup, but if you'd consider it, the core becomes easy. You have to do it like mentioned above. Wetting the carbon out between plastic is an axcellent teqnique, and makes the process very manageable.

The second option is a modified infusion. You infuse the layers on the face of the mold first and let cure. Then glue your core in under vacuum and let cure. Finally add the outer (inner?) layers as above, wet layup in plastic under vaccuum. Yes its 3 processes, but it keeps the core from filling up.

I'm building some parts using this technique now. I don't mind wet layups but I prefer infusion not only for weight control, but most of all convenience. With just myself I can mamage rather large layups by getting everything in position dry and under vaccuum.

Rohacell is a great option, I can't imagine you'd need more than a 1/8" core for your needs.

Red
 

BoKu

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In your position, I'd do an uncored vacuum bagged shell, then use foam backing rod as core for discrete carbon stiffeners like I did for the CarbonMax aft fuselage. It has turned out to be a reasonable compromise between weight, expense, and build time.

 

stanislavz

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The second option is a modified infusion. You infuse the layers on the face of the mold first and let cure. Then glue your core in under vacuum and let cure. Finally add the outer (inner?) layers as above, wet layup in plastic under vaccuum. Yes its 3 processes, but it keeps the core from filling up.
I do try infusion too, but on thin skins - you are wasting twice amount of epoxy on infusion mesh and peel ply. Not for my "mother nature call" taste. Twin foil + some buddies or temperature control - and you are able to do some really tricky parts..
 

foolonthehill

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I build all the Arion Lightning aircraft composite structural components for Arion Aircraft. We use 4mm Gurit Corecell, which is double scored. Light, strong, and great for infusion.
 
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