tschmidt

My Feedback: (1)

I'm thinking of making a glass/balsa/glass laminate fuselage for my pattern design. I've used a glass, kevlar, and foam laminate in the past. I'll be using 1/16 balsa as the core and will be vacuum bagging in a female mold. I guess my question is: How do you prepare the balsa core? Do I need to preshape it? The only thing I can think of is spray the balsa with ammonia water then bag the balsa in the mold before doing the layup. Is it necessary to preshape the balsa core?


Thanks,

Todd Schmidt

dolanosa

Todd,

If you find out how that's done, would you mind sharing that info with me? I'm planning the same thing too but with a 25% scale plane. I'm still working on the plug but hopefully that will be done soon. Then again, I'm finding some imperfections with the plug that's really killing me in that the plug is not as symmetrical as I want. Right now, my project is to make a CNC router to make my parts as well as some 3D parts like the cowl and possibly the fuse as well.

Good luck.

daven

My Feedback: (1)

I've done it on tails and wings, but never a fuse, especially something with compound curves. I would think you could piece it together with preshaped balsa having the grain follow the curve. I would almost try to bag the alcohol soaked balsa in the mold and get it to hold a shape. Then take it out and start your layup. At least then you won't get half way down a road and have issues.

dolanosa

I've seen some euro designs that laminate glass-balsa-glass...absolutely fantastic work that is light and very stiff. I just can't imagine how much work goes into shaping the wood first then laying it up. Sounds like *too* much work. But then again, it makes for very nice planes. OTOH, you can make a template with paper first, right? Put paper in the mold, cut at the parting line, and transfer the part onto card stock or something to trace the balsa. In this case, you'll have less of a mess.

Is that what you do, daven? I'm just wondering how you preshape balsa so it fits correctly already.


Butch

tschmidt

My Feedback: (1)

Hi Butch,

Your approach is exactly what I have done. So far I've made templates, glued the balsa together to make the core, and have vacuum formed the balsa into the molds. It worked okay for the first time, but there are a few places the balsa came apart at the glue seams. I used Sig Bond for glue and the ammonia water caused it to give in a few places. Next time I'll try CA. Below is a pic of the first plane built.

Todd

saramos

I came across a DVD by Bob Hunt and Bill Werwage titled Straight Form & Compound Curve Balsa Molding Techinques. The basics is to make a plug, soak the balsa, form the balsa over the plug, make relief cuts for compound curves, wrap with ace bandages and let dry. I suppose once done, you could then add glass and vaccume bag.
Here's a link to where you can get the DVD

http://www.airbornemedia.com/store/rv-index.htm

Scott

daven

My Feedback: (1)

My parts are relatively flat and not an issue. I suck the balsa down to the mold with about 22" of mercury and have no issues. The compound curves of a fuse are another story, looks like todd's work is the way to go. Nice work!

dolanosa

Todd,

Very cool, man. Thanks for sharing the pics. So basically on the cowl/firewall forward, do you use a carbon to keep the shape of the compound curve? Anyway, do you still use that technique or something else now? I had to put my pattern design on hold so I can make a CNC router. I may just call up Jimmy Hubbard here near Chicago to see if I can test that technique on his old mold. In any case, that's a very nice plane. I remember seeing apicture of that at the NCRCA site for Kansas. Do you sell a kit of that plane?

saramos,

Wow, that sounds like another long, complicated process just to make the balsa skins. Also, I would be very wearly about using my original plug and wrap it in ammonia-soaked balsa. I can make more molds but I really don't want to make another plug, be it a pattern plane or a speed 400 pylon plane. Once my router project is done and finished teaching my buddies the basics of CAD/CAM, I'll go back into my previous project to maka new plug of my DH-Models ZLIN 526 AFS.

Yeesh, I gotta get a decent plane flying next year. No more popcicle-stand 40-pattern planes. This time, "real" pattern planes and scratch built (my plans) 27% aerobatic IMAC planes. At least the molds done and a test plane for each.

Butch

XE521

Hi Daven,

just as a matter of curiosity, how will the 'wet' balsa dry if it's bagged? [sm=confused.gif]

Rgds, Karl.

daven

My Feedback: (1)

Thats a good point, not sure if the moisture would be sucked through the pump or not.

XE521

Hi, if the item is not under continuous suction it will not dry out. We use a similar system to 'outgas' potted items for space flight but they are placed in a vacuum camber that is constantly evacuated. I use the wet principle for balsa but, instead of bagging, use broad rubber bands, hundreds of them.

Rgds. Karl.

tschmidt

My Feedback: (1)

Hey Butch,

From the nose ring back to about 2" into the balsa, I plane to use glass/carbon/glass laminate which is what I've seen on other fuselages made this way.

Karl and Daven,
I put the balse under vacuum for a couple hours, then released the vacuum but left it in the mold which was still in the bag over night. It came out dry and held its shape pretty good. The first time, I pulled it out of the bag still a little damp and deformation took place.

One thing that I haven't decided on is what weight of cloth to use in the laminate. I was thinking 2.4oz on the outter skin and 1.0oz for the inner. Anyone have an idea if this is to much or not enough?

Todd

daven

My Feedback: (1)

For hollow composite wings I am using 1.4oz on the outside and .75 oz on the inside. I am using 3mm (maybe 2mm) Rohacell as my sandwhich stucture, I know many guys using 1/16" or even 1/20" balsa. This results in a very strong, yet light structure.

You may want to start out a little heavier and work your way down.

dolanosa

I have learned more on this thread. This is totally awesome. Thank you. Once the mold is done, I'll definitely try some of the hints here...probably except the one using the plu as a male mold for the balsa. I'll keep my plug and parting sheet in storage.

Butch

Magne

Using the plug as a male mold for the balsa works very well.
I have done this a couple of times, using only water to soften the wood. No need for ammonia.
The first time I tried, I soaked the wood thoroughly with the shower, and tied it on the plug. Because of ecxessive use of water, the wood stretched quite a lot, and cracked a couple of places when it dried out, as it became too tight on the plug.
This was however easily glued together with superglue after taking the balsa off the plug when dry.
The second time, I attached the wood to the plug first, then sprayed water onto it. This time the wood curled up a bit on the "flat" parts of the plug as it swelled.
Next time I would therefore apply water first, but use less than the first time, and thereafter attach it to the plug.
When dry, the "shells" will fit very neatly into the mold. I use a marker pen to draw a line along the mold flange, and cut/trim the wood a few millimeters (1/8 - 1/4" inside the line. Thereafter I chamfer the edges all around, to allow the inner fiberglass skin to attach to the outer skin along the edge.
My plugs are coated with automotive 2-part primer and/or paint, and there has been absolutely no damege due to this "treatment".

Regards,
Magne

tschmidt

My Feedback: (1)

Magne,

That's a very good idea and looks like it works well. When you lay up your fuselage, what weight of cloth did you use on the outter and inner skins? Thanks for the input on chamfering the edges, I hadn't thought of that.

Nice looking plane by the way!

Todd

T_om

There are a couple of great looking planes posted here but... what is the point of using balsa in the layup? Especially in a way that makes no particular use of balsa's positive building properties.

These composite planes could have been built easier, lighter, and stronger, using regular glass layup schedules with perhaps a bit of carbon reinforcement. 'Tortured wood' (which is the actual term used in boatbuilding with similar techniques) does not lend itself to curves found in most fuselage construction. Of course, given a big enough hammer (vacuum bagging for example), almost anything can be forced to fit, be it improper material or not.

This is not a slam on anyone here, just an observation. If I am missing something, I wish someone would point it out.

Tom, Puzzled In Florida

davidfee

Tom,
It is called a sandwich panel. It is many, many times stiffer for a given weight than a solid laminate panel. Thus, for a given design stiffness, the product can be made much lighter. Using balsa is nice because it is moldable (by softening the fibers with water) and because it has a grain structure which can be used to increase panel strength and stiffness in the primary loading direction.

Other common core materials are rigid foams like Rohacell and of course various honeycombs. Balsa is about the easiest to deal with for most people.

-David

Magne

Todd:
I used 2 layers of 49 gram/m2 (approx. 1.45 oz/sq yd.) on the outside, and one layer on the inside.
This gives a surface which is quite dent-proof, and is more than is actually required for flying loads.
I know that people successfully have used one layer of 49 gram/m2 (1,45 oz) on the outside, and one layer of 25 gram/m2 (3/4 oz) on the inside.
I opted for the extra strength, because my model is "light enough" for my requirements anyway.

If I make an other model, I will experiment with thin slices/sheets of white polystyrene foam, approx. 1.5-2mm thick. This is what a lot of manufacturers use (I think e.g. ZN and former PL used this material.) I have made some test pieces, and it is possible to lay this down in the mold after the outer skin, and make it stick to the surface, without the use of vacuum. It should thus be possible to join the two fuselage halves wet, all in one process.
But I would like to carry out a few more tests, before I commit myself to a full fuselage.

Tom:
The layers of fiberglass mentioned above, make up a sandwich panel consisting of 0.108mm (2 x 0,054mm) thick fiberglass - 1/16" balsa - 0.054mm fiberglass. The total fiberglass thickness used is therefore only 0.016mm thick. (0.0006" ) Each layer of fiberglass weighs approx. 90 -100 gram/m2 when laminated, and the balsa weighs approx. 150 gram/m2. Total weight is therefore 420 - 450 gram/m2.
If you made a panel of fiberglass only, which weighed 420 - 450 gram/m2, it would be MUCH softer / more flexible than the sandwich panel.
However, the pure tensile/compressive strength ALONG THE FIBRES would probably be higher, as glass is stronger than balsa. But our models don't fail in tensile/compression along the fibres, but more probably in compression perpendicular to the laminate, or in buckling.

Regards,
Magne

T_om

Hi guys,

I should have been more precise. I am already REALLY familiar with wood (and other) core FRP laminates. It is a primary means of custom boat construction (the area I am most familiar with). So I know the advantages of the construction method.

I was just curious as to why it was used in fuselage construction when other layups would (IMHO) be better suited, given the compound curves involved (which themselves lend strength and stiffness). I would think a glass layup, with simple carbon tow properly applied, should be plenty stiff and lightweight. But like all things in modeling, ya go with what works for ya, so as I said earlier, no slams intended.

Tom

Magne

Hello Tom.
I think that one of the real advantages of using wood for us home builders, is that is possible, as described above, to pre-shape the core material before the lay-up, and that it therefore actually stays reasonably well in place when applying the inner skin.
I have tried other materials, such as Airex/Herex (Cross linked rigid expanded PVC foam) in 1.2 and 2.0 mm thicknesses.
Even in a wing mold, with only single curvature, it was difficult (impossible) to make the core stay in place whan applying the inner skin. I wouldn't even try a fuselage.
I have also made a fuselage with Nomex honeycomb core, 2mm thick. Lots of work, as the outer skin had to be cured first, then the core material was glued in under vacuum, and finally the inner skin was applied under vacuum. Came out slightly lighter than the basla cored version. (less than 2 oz lighter.) Gluing the Nomex in was not easy, as it kept "springing" out of the mold, before I was able to apply the vacuum. Once the vacuum is on, then everything stays in place, of course.
I think that it is possible to make thin sheets (1.5-2mm) of ordinary polystyrene foam stick in the mold (on top of the wet outer skin) without any kind of pre-forming, apart from in the sharpest curvatures. This is what a number of manufacturers do.
However, the foam is much softer than balsa, and you need to use significantly thicker glassfibre on the outside to give enough resistance to indentations, so the whole thing may end up heavier than the balsa-cored version.
Regards,
Magne

JetNoise

Why does the core material have to be in one piece?

If strips were laid down the length of the fuselage, you would still have the same rigidity. Even if they were to have spaces between them. Something like planking a build up fuselage.

I have personally tried thin strips of polystyren, with chamfered edges, laying down the length of the fuselage at 3, 6, 9 & 12 O-Clock positions. Major increase in stiffness and buckeling strength. These were stuck into an existing fuselage.

How this compares to a full balsa or other material sndwich, I dont know. But it might be worth looking into if weight is critical.

Dale

slkret

I have used this method on a .40 size model with great success and it was not terribly difficult and it did not come out heavy. the key to the weight is to be miserly with the epoxy, do not use excessively heavy weight cloth and use contest grade balsa for the cores in the shell. The whole process is outlined in the following link:
[link]http://www.rcuniverse.com/forum/m_4332168/anchors_4341072/mpage_1/key_/anchor/tm.htm#4341072[/link]

Do not be afraid to try this yourself.

Steve

Darryl Usher

I was trying to ask about P-39.
Darryl Usher [email protected]