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Offline Motorman

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« on: February 24, 2018, 09:37:57 PM »
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« Last Edit: December 04, 2021, 09:45:20 AM by Motorman »
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Offline Dan McEntee

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Re: End Grain Balsa
« Reply #1 on: February 24, 2018, 10:14:13 PM »
    Before they got really fire conscious, the floor of some airliners was composite balsa and some kind of planking on the end grain. There was quite a bit of balsa composite with plywood in the DeHavilland Mosquito. Many other commercial and industrial uses like that also. Nothing really new. I have never seen a need for such a construction technique in any of the model airplane disciplines I have participated in; free flight rubber, glider and gas, R/C sailplanes, R/C helicopters, and control line stunt and scale. To keep end grain from soaking up excess glue, you pre-glue by rubbing in some glue and letting it dry, then assemble the part. That's model airplane 101. It's spelled out like that in just about every rubber power model kit I have ever built and every beginners model airplane book or construction article I have ever read.
   Type at you later,
   Dan McEntee
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Offline Brett Buck

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Re: End Grain Balsa
« Reply #2 on: February 24, 2018, 11:36:34 PM »
Quote from: Motorman on February 24, 2018, 09:37:57 PM
Anybody ever do anything with end grain balsa in composite structures? Just wondering how you keep the end grain from soaking up allot of glue.

    My bellcrank has end grain balsa. I let it soak up a lot of glue.

     Brett
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Offline Massimo Rimoldi

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Re: End Grain Balsa
« Reply #3 on: February 25, 2018, 11:33:26 AM »
For the frames of the nose of my new model I made a sandwich composed of: Fiberglass 25 gr/mq - linden 1mm - balsa 2mm - linden 1mm - Fiberglass 25 gr/mq.
All this was combined with HEXION L285 and produced a lighter plywood than poplar plywood, but far more resistant.
The time needed for the job is a few minutes but it reduces the weight considerably and can be worth it when you have to make larges frames.
To avoid excessive resin absorption in the balsa, I previously applied a light layer of very very thin dope.

For the record: I did not invent anything, I limited myself to copy the method from the aero modelers of Eastern Europe.

Massimo
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Offline Tim Wescott

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Re: End Grain Balsa
« Reply #4 on: February 25, 2018, 12:20:50 PM »
Quote from: Massimo Rimoldi on February 25, 2018, 11:33:26 AM
To avoid excessive resin absorption in the balsa, I previously applied a light layer of very very thin dope.

I haven't tried it, but Bob Hunt uses hair spray for this when he's putting wing skins on foam cores.
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Offline Lauri Malila

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Re: End Grain Balsa
« Reply #5 on: February 25, 2018, 01:19:27 PM »
We do, in wing spars. Together with HM carbon pultrusion spars and tight Kevlar wrapping, it's still about the best in weight/stiffness ratio. Spars made wet-on-wet with carbon rowing, rohacell core and carbon hose outer surface may offer a better service life but at cost of non ideal fibre orientation and some extra weight.
You can seal the end grain with a paint mized from white magnesium (oxide?) powder and acetone.
Dope, hair spray etc. is not ideal in this case as you will need a lot of it. The purpose of wood cells is to act as capillary channels, I guess you understand my point.

Lauri
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Offline Massimo Rimoldi

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Re: End Grain Balsa
« Reply #6 on: February 26, 2018, 12:05:02 AM »
Quote from: Tim Wescott on February 25, 2018, 12:20:50 PM
I haven't tried it, but Bob Hunt uses hair spray for this when he's putting wing skins on foam cores.

Yes, the aim is to partially seal the grain of the balsa to prevent excessive resin absorption.
If pieces highly stressed and important, I think it is better to follow Brett and Lauri.

Massimo
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Offline Brett Buck

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Re: End Grain Balsa
« Reply #7 on: February 26, 2018, 12:59:01 AM »
Quote from: Massimo Rimoldi on February 26, 2018, 12:05:02 AM
Yes, the aim is to partially seal the grain of the balsa to prevent excessive resin absorption.
If pieces highly stressed and important, I think it is better to follow Brett and Lauri.

  Agreed, don't make the bellcrank that way, all it will do is change your adhesive from epoxy to dope (since that's where it will peel apart). It's perfectly fine on wing skins because you've got a giant surface area and you are gluing to foam.

    Brett
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Offline Massimo Rimoldi

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Re: End Grain Balsa
« Reply #8 on: February 26, 2018, 02:29:02 AM »
Quote from: Lauri Malila on February 25, 2018, 01:19:27 PM

You can seal the end grain with a paint mized from white magnesium (oxide?) powder and acetone.



Hi Lauri,
it could be an oxide or better a carbonate or a magnesium silicate (talc).
The former is much heavier while the other two have comparable specific weights, the talc is slightly lighter.

Massimo
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Offline Howard Rush

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Re: End Grain Balsa
« Reply #9 on: February 26, 2018, 02:53:39 AM »
I used end grain balsa between maple or carbon engine mounts. I’d put the end grain in a puddle of epoxy and push the epoxy into the balsa until it was saturated. It could maybe have been lighter, but it passed the wishbone test.

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Offline TDM

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Re: End Grain Balsa
« Reply #10 on: February 26, 2018, 06:30:00 AM »
Easy: wet the fiber not the balsa, Blot out the fiber with paper towels then apply to the balsa. Make sure there is no dust in the balsa or you are going to have poor adhesion. A dry roller will work well to wet the balsa.

I define a dry roller one that when soaked with epoxy and rolled to a piece of blue paper towel it will not soak through (towel becomes dark blue color instantly) you see very thin layer of epoxy on it.
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Offline TDM

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Re: End Grain Balsa
« Reply #11 on: February 28, 2018, 06:26:09 AM »
And yes Massimo MGS 285 is the best laminating resin there is to my knowledge. It is my go to resin, It is thin penetrates easy UV protected, good stuff. Actually is the only thing I use. Rarely I use 5min but that is a very rare exception.
And one more thing here Motorman as the resin content goes down make sure the balsa is dust free or the dust will soak all the resin and you get delamination.
The method I explained above is true for any foams or porous material that like to soak resin by the bucket load, best to avoid punting any resin on such materials and instead place the resin on the material that goes on top of them. But if you must put resin on those materials a dry roller is best.
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Offline Brett Buck

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Re: End Grain Balsa
« Reply #12 on: February 28, 2018, 09:58:39 AM »
Quote from: TDM on February 26, 2018, 06:30:00 AM
Easy: wet the fiber not the balsa, Blot out the fiber with paper towels then apply to the balsa. Make sure there is no dust in the balsa or you are going to have poor adhesion. A dry roller will work well to wet the balsa.

I define a dry roller one that when soaked with epoxy and rolled to a piece of blue paper towel it will not soak through (towel becomes dark blue color instantly) you see very thin layer of epoxy on it.

    In any high-strength application, I am concerned that if you do this, or really, anything, to reduce the soak-up, you are also going to have a dry bond line when you are done. The glue at the bond line will get wicked away, into the wood fibers.

       The bottom line - wood works good in tension and not so great in compression, so end-grain wood is only useful in some limited applications, and if the glue required to hold it together is a limiting factor, figure out some other way to deal with it. So far, I use it for my bellcrank construction where I don't care about the extra 5-grams, and engine mount crutches where the peel strength of the bond is not terribly critical, and the reduction of out-of-plan motion is important. The only other place I have used classic end grain balsa was when filling up some gaps in an ARF I helped someone build.

   Otherwise, figure out something else or accept the fact it takes a lot of glue.
   
    Brett
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Offline TDM

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Re: End Grain Balsa
« Reply #13 on: February 28, 2018, 10:13:03 AM »
Bret I have done it many times like this to know that it works. In my application I used it with either vacuum baggin or compression molding. They both had good adhesion and the parts did not delaminate.
Yes the end grain is great in compound curved areas but on the srtaight-ish places going in grain direction will yield a better part but I am sure Motorman has a good reason to go end grain in his application.
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Offline Brett Buck

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Re: End Grain Balsa
« Reply #14 on: February 28, 2018, 10:22:53 AM »
Quote from: TDM on February 28, 2018, 10:13:03 AM
Bret I have done it many times like this to know that it works. In my application I used it with either vacuum baggin or compression molding. They both had good adhesion and the parts did not delaminate.

     In a vacuum, it's a lot different story.

    Brett
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Offline Fredvon4

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Re: End Grain Balsa
« Reply #15 on: February 28, 2018, 02:04:41 PM »
I have followed this an wonder if MM can give hint on "composite structure" he is considering

There are indeed Balsa cores inside a lot of giant arsed wind turbine blades as well as Brett bucks very dinky sandwiched Bell Cranks

I can think of applications were glue weigh load is a Plus and again where it is definitely a negative






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