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PostPosted: July 21, 2020, 7:16 pm 
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Sam_68 wrote:
KinFung wrote:
...because I have no tools & skill to bend pre cut panel, I can only make 90 degree joint with pre cut panel

If you design the structure right, you can make it pretty much self-jigging at whatever bend angles you want. The angle depends on the width of the 'slot' you rout in the inner face of the panel - which can be calculated quite easily - and you can then fold the panel around profiled bulkheads. You can even do smooth curves by routing a series of parallel slots.

You need little more than a router and a jigsaw as the basic tools, and you're going to need those (plus a hot wire cutter) for foam core work in any case.

In terms of core strength, remember that it's only there to stabilise the outer skins. Certainly you don't want it to delaminate from those skins, but apart from that the main consideration is shear strength, 'cos you're trying to stop the skins moving relative to each other in torsion.


Sam, I somewhat agree with you. The part that I don't agree with is the ability of a foam core to do just what you are asking it to do. With two thin skins made of whatever with essentially a marshmellow in between has no rigidity in torsion. In such a case, the structure relies mostly on the structural performance of the two single sheets alone. Probably less structural performance of a single panel made the thickness of the 2 thinner sheets of the sandwich without the weak "stabilizing" layer.

Now if the core had some features different then marshmellow (a.k.a.foam) which is the OP's question, then it depends on the performance of that layer and the ability of it to stay bonded to the outer skins.

Of course I am not structural engineering expert. It just seems to be a consideration based on my semi educated guess.

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PostPosted: July 22, 2020, 5:20 am 
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rx7locost wrote:
With two thin skins made of whatever with essentially a marshmellow in between has no rigidity in torsion.

That would be the case if there was no connectivity whatsoever between the two skins: for example if you had one perfect cylinder within another, and nothing but core material connecting the two.

In practice, that never happens: you're using 'panels' of sandwich material, with corners and bulkheads that limit the relative movement of the skins.


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PostPosted: July 22, 2020, 10:04 am 
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Chuck, you need to remember that panels tend to distribute loads across the 'membrane', and the same thing goes for the core. If the shear strength of the adhesive is adequate then cores with much lower compressive and shear strength than the surface skins will carry considerable load. On another forum I follow (i550class.org) a builder wanted to substitute a foam-cored glass for a 6mm BS1088 plywood deck panel. He made up samples of 12mm Divinycel or Corecel foam with 8 and 10 oz single skins of glass each side. The panels were stiffer and more puncture-resistant than the ply, and lighter too. But they were more expensive. He did have to plan for where his fittings were going though, because those spots got a plywood core for local compressive strength. As Sam 68 pointed out, his deck panels had edge constraint too, but that tends to be the norm for load-carrying structure. I don't know that I could pull it off: I change my mind about where stuff will go much too often, :-)

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PostPosted: July 22, 2020, 2:40 pm 
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Warren Nethercote wrote:
He did have to plan for where his fittings were going though, because those spots got a plywood core for local compressive strength... I don't know that I could pull it off: I change my mind about where stuff will go much too often, :-)

The kinda solution for that is to use bobbins instead of moulded-in hard points. These can be bonded in place afterwards, and use the flange of the bobbins (usually multiple, for any significant point load) to feed the forces into a sufficient area of the skins. Refer to the pics of my Westfield FW400 on the 'carbon fibre' topic linked above, or to the 'Motorcycle-Engined Racing Cars' book to see how this works.

You do still need to think about how to feed in the really big loads (drivetrain; suspension) right from the drawing-board stage, though, because these usually need to be contrived to be fed into the structure in two planes so that they can be resolved into 'horizontal' and 'vertical' components on panels each side of a corner, in order that each component load is fed into the skins in pure shear.


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PostPosted: July 23, 2020, 7:58 pm 
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Sam_68 wrote:
In terms of the original question, foam core is actually rather heavy, as is wet lay-up fibreglass, but it can certainly be made to do the job.


if I use vacuum bag?


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PostPosted: July 25, 2020, 3:27 am 
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Sam_68 wrote:
RichardSIA wrote:
Frank Costin did the Marcos chassis after successfully designing plywood BOMBERS!

Frank Costin worked for DeHavilland (who had produced the Mosquito plywood bomber during WWII), but he certainly didn't design it. The Mosquito was well before his time and his role for DH was as an aerodynamicist. By the time he was with them, they were producing aircraft like the Sea Vixen and Comet (the first jet airliner), which were conventional stressed metal construction. The last aircraft they did using plywood construction were the Vampire and Venom fighters, which again were before Costin's time.

He did design his own glider out of plywood, though.

In terms of the original question, foam core is actually rather heavy, as is wet lay-up fibreglass, but it can certainly be made to do the job.



...which was designed by ex-Lotus F1 Chief Designer Martin Ogilvie and fabricated from pre-manufactured flat panels of honeycomb core skinned with glassfibre in epoxy.

Monocoques in general, and sandwich panel construction in particular was discussed at length on THIS THREAD.

hot to remove the core like this?


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PostPosted: July 25, 2020, 7:31 am 
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Sam_68 wrote:
Warren Nethercote wrote:
He did have to plan for where his fittings were going though, because those spots got a plywood core for local compressive strength... I don't know that I could pull it off: I change my mind about where stuff will go much too often, :-)

The kinda solution for that is to use bobbins instead of moulded-in hard points. These can be bonded in place afterwards, and use the flange of the bobbins (usually multiple, for any significant point load) to feed the forces into a sufficient area of the skins. Refer to the pics of my Westfield FW400 on the 'carbon fibre' topic linked above, or to the 'Motorcycle-Engined Racing Cars' book to see how this works.

You do still need to think about how to feed in the really big loads (drivetrain; suspension) right from the drawing-board stage, though, because these usually need to be contrived to be fed into the structure in two planes so that they can be resolved into 'horizontal' and 'vertical' components on panels each side of a corner, in order that each component load is fed into the skins in pure shear.

did 1001 core panel have enough strength?


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PostPosted: August 5, 2020, 5:18 am 
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KinFung wrote:
if I use vacuum bag?

A vaccuum bag would be an excellent way of squeezing the excess resin into the core material, where it will permanently add weight.

Commercially manufactured sandwich panels are made by laminating and at least partially curing the skins first, then bonding them to the core material using film adhesive in a heated press.

KinFung wrote:
hot to remove the core like this?

I'm pretty sure that if I could read Chinese (I'm guessing that with your user name you can?), the text to that diagram would tell me that it was a repair patch.
Yes, you can potentially add hardpoints like that, but in practice it's easier to do it with bobbins.

KinFung wrote:
did 1001 core panel have enough strength?


Too dense; too heavy.


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PostPosted: August 5, 2020, 8:34 am 
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FYI, For translation, if you have an android try "Translate" in the Google Play store. Point the camera and read. It really is is that simple. Technology is amazing. It still comes thru as a straight translation and you must interpret what the English really means. Just trying to help a little.

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PostPosted: August 5, 2020, 11:14 pm 
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Sam_68 wrote:
KinFung wrote:
hot to remove the core like this?

I'm pretty sure that if I could read Chinese (I'm guessing that with your user name you can?), the text to that diagram would tell me that it was a repair patch.
Yes, you can potentially add hardpoints like that, but in practice it's easier to do it with bobbins.



it is just Chinese version of "How to Build Motorcycle-engined Racing Cars"

it talk about how to bounded core panel, but the book didn't said how to cut the panel joint


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PostPosted: August 6, 2020, 6:29 am 
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KinFung wrote:
Sam_68 wrote:
it is just Chinese version of "How to Build Motorcycle-engined Racing Cars"

it talk about how to bounded core panel, but the book didn't said how to cut the panel joint

Ah, in that case, IIRC, it's showing how to 'butt joint' two panels together (but the principle is the same as letting in a repair patch).
You rout away one skin and almost all of the honeycomb core material at the edge of each panel, then bond them together with adhesive.

Easy to do with a flat panel on a workbench. Sometimes less easy to do with a panel in-situ on a finished monocoque (whereas to bond in a bobbin with external flanges, you simply need to drill a hole).


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