Molded Balsa - Solid vs. Laminated (?)

LAST EDITED ON Apr-18-02 AT 08:58 PM (CST)

Here are the results of the great Pilsbury Nobler Turtledeck Mold-off of 2002. I need help interpeting them.

Deck One: One solid piece of 3/32 balsa bought from the hobby shop in a bulk lot of blemished balsa, molded into shape and trimmed.
Weight: .175 oz

Deck Two: Three layers of Sig 1/32 Contest Light balsa molded on top of one another. Layers separated. Aerogloss clear applied to inside of outer layer and outside of middle layer and allowed to dry. Bottom and middle layers installed on molding plug. Clear dope applied and .5 oz carbon fiber matte laid over that. Clear dope applied to outer layer of carbon fiber matte and inside of outer layer of balsa. Wrapped and dried. Unwrapped and inner layer of 1/32 balsa removed. Result 1/16 shell of two layers of 1/32 balsa with carbon fiber in the middle.
Weight: .305 oz

Deck two feels a little stiffer when springing between the fingers, but not significantly so.

Deck two resists down pressure on the spine of the turtle deck when placed on on flat table and pressed slightly more before edges slide apart.

The question:

Given either will receive a layer of carbon fiber outside and then the finish and will get the same amount of internal former support, do I really gain anything by going to the laminated version over the single, solid fat version? The single layer version is .130 lighter.

[photo not recovered: 3cb001a952bd6358.jpg]
Photo of Deck One and Mold

Bear in mind this is only a turtle deck on the Nobler, from the cockpit back to the tailpost.

Now let's look at some other results:

In his post showing how to mold balsa at

http://www.clstunt.com/htdocs/dcforum/DCForumID1/1639.html#

Warren claims to have a complete fuselage top for a Skylark that is 1/8th thick and weighing .9 oz

I just pulled the bottom shell off of my bottom mold, a curved and tapering bottom fuselage cover that is 27 1/2 inches in length and is of the same composition as Deck Two, that is 1/16, made up of two layers of 1/32 with a layer of carbon between them.

The weight of my fuselage bottom is .305 oz, which compares very well with the 1/8th Skylark weight. Oddly enough, the weight of the laminated belly skin at 27 1/2 inchs is the same as the laminated turtledeck at 16 inches. Both were laid up from the same weight balsa stock.

Do I go for less weight, or greater strength?

Should I mold a 3/32 belly pan to see if it is lighter than the laminate I have or should I make another laminated turtledeck to see if somehow or another that one got a lot heaver than it should have?

When is weight saving no longer worth time/effort involved?

And, finally, a 'poly-u' glue was referred to in the other post on molding as being the optimum adhesive for laminating wet balsa. What is that and where do I get it?

(No wonder this friggin' airplane is not in the air, what with all this fooling around trying to save .1 oz here and there....)

Sounds to me any gain will be possibly if you plow the bird in. But then will a intact turtledeck br worth all of the time and effort. I think such elaborate effort is best left for high stress areas. But thats just me.

"The enemy of good is better." I believe that if the lighter piece seemed adequate in strength, I would use it. Especially if you are going to CF coat it anyway. Glue is always heavier than balsa. The only qualification to my opinion might be if the lighter one needed bulkheads, then the laminated piece might turn out to have the overall strength-weight ratio advantage.

>"The enemy of good is better."
>The only qualification to
>my opinion might be if the
>lighter one needed bulkheads,
>then the laminated piece might
>turn out to have the overall
>strength-weight ratio
>advantage.

I detect a noticable but not large increase in stiffness in the thinner, heavier, laminated version. As an individual part, it is stiffer.

I am guessing that one former/bulkhead in the front of the turtledeck (back of cockpit) and one near the front of the vertical fin junction will be all I would use in either case. Assuming the fuselage sides are braced and not flexing, then the turtle deck becomes a D tube and that stressed skin is not moving around.

Which argues to just use the single layer, lighter turtledeck.

So much for high zoot.

I would use a turtledeck laminated using 2 pieces of 1/32 sheets. Glue them together with thinned down aliphatic white glue. Do this when you first put the pieces on the mold. I did this on my full fuse top that was 42 inches long. This piece weighted 16 grams and that was with the formers installed. I then glued it on, and finished the plane in 2/10 CF and Sig dope. I put the grey base coat on tonight. I beleive if you make a turtledeck this way it will be plenty strong and light. I also used this method on my Jamison Special and the T'deck weighed 11 grams.

Here is the 42 inch long top block...16 grams
[photo not recovered: 3cbf893b03a207dc.jpg]

Here is the JS T'deck...11grams.
[photo not recovered: 3cbf880a0280aabc.jpg]

Here is the JS in CF.
[photo not recovered: 3cbf883702bea97f.jpg]

Crist,
The turtledeck of a Jamison has pronounced compound curvature. Your moldings look so nice, I wonder what techniques you use to conform to the curves. Do you slice out gores?
Derek Moran

>Crist,
>I wonder what techniques you
>use to conform to the curves.
>Derek Moran

Let me speak for Crist, Al Rabe, Windy, et. al.

What amazes you when you first try the molded balsa technique is the degree to which you can smash the thin balsa sheet (turned halfway to rubber by soaking long enough in steaming hot water) onto a compound curvature.

In extreme cases, like the nose of a long fuselage piece, you may have to slice the balsa and lay the edges together.

But I have found that even with the complex curvature of the Nobler turtledeck, you can relieve the stress and accumulated bunching by pre-trimming your balsa sheet to ultimately wrap and cover the mold where it needs to be covered. Which is kinda magic.

That said, the guy who should answer this is Al Rabe.

Al?

The addition of high tech materials in the hopes of adding strength to a design that has proven itself over the years to be a solid one using low-tech materials in its construction only just adds weight. The single piece shell is the one to use.

As for the carbon adding strength to the model when used on the outside, to double as a finishing aid, should maybe be the topic of another thread. Being so easy to sand through, it provides an eggshell on the model of uneven thickness. This effect is random on the model and varies from clean balsa grain to solid black carbon patches. I do find the material easy to use (I have it on the one I am currently painting), but I think we should be cautious in the faith we put in its strength when used in place of silkspan. It would not surprise me if the silkspan were actually stronger.

Curt

Wow, I can't believe I said that.

I made that mistake, don't do it. Use anything else. Unless I bought the wrong kind or now it is better... It foams & expands when it cures, it is impossible to sand, and it takes overnight to dry. Having your morning coffee is when you discover all the hard foam.

Polyu glue is a pleasure to use when you do it right. I find that it sands almost as easy as balsa. I seldom get any glue to sand though.

The most important thing to remember about polyu glue is that water in any form, indluding the humidity in the air of NC, causes it to foam up. This is a good thing because it will penetrate woodgrain, Styrofoam, etc. Since you know it foams up then the key to success with polyu glue is darn little is needed. I barely change the color of the balsa when I squeegee off most of the glue.

I get my polyu glue at any hardware store or even Walmart. I use Probond by Elmer's Glue. There are other brands.

Ed

Well, I guess I did it wrong. I remembered hearing about its properties and decided to use it to glue the wing in my 2000 Tempest. When it foamed out it was not a pleasurable experience the next morning. Give me your address and I will mail you a full bottle.

I kind of agree. The carbon veil I use (.2 ounce) cannot add any strength and is easier to sand through than silkspan. I like to think it magically adds strength because that is what I want to think. But a single layer with only Nitrate or Butyrate as a binder and no epoxy, I doubt it. It is great for a killer, no-grain finish however. But again, so is silkspan. Oh well. I have 10 feet of it, so I'll use it. It is lighter than silkspan, but on the wings and tails, I end up covering it all with silkspan again anyway. So any weight I save on the fuselage is probably canceled by the wing and tail feathers.

>Here are the results of the
>great Pilsbury Nobler Turtledeck Mold-off
>of 2002. I need
>help interpeting them.
>
>
>Deck One: One solid piece
>of 3/32 balsa bought from
>the hobby shop in a
>bulk lot of blemished balsa,
>molded into shape and trimmed.
>
>Weight: .175 oz
>
>Deck Two: Three layers of
>Sig 1/32 Contest Light balsa
>molded on top of one
>another. Layers separated.
>Aerogloss clear applied to inside
>of outer layer and outside
>of middle layer and allowed
>to dry. Bottom and
>middle layers installed on molding
>plug. Clear dope applied
>and .5 oz carbon fiber
>matte laid over that.
>Clear dope applied to outer
>layer of carbon fiber matte
>and inside of outer layer
>of balsa. Wrapped and
>dried. Unwrapped and inner
>layer of 1/32 balsa removed.
> Result 1/16 shell of
>two layers of 1/32 balsa
>with carbon fiber in the
>middle.
>Weight: .305 oz
>

You have gained a lot of weight with all the dope and carbon fiber between the layers. Carbon mat will not add a lot of stiffness, and putting it between the layers is the wrong place anyway. Neither the dope nor the carbon is doing any good where it is at.

What I would suggest is deck three. Two layers of 1/32 balsa (or three layers if you really want something thick and strong). Let dry. Then remove from the form and put a light coat of Aliphatic or white glue between the layers and replace on the form until dry once again. Use thin layers of glue so you do not build up weight. The additional stiffness from using multiple layers should become readily apparent when comparing with a single layer of equal thickness stock. But there should be very little weight increase, and that only from the glue. And in doing it this way you should need no formers except where it may be visible or is purposely used to seal off an area. We aren't trying to make these stiff enough so you can grab and squeeze. We want them strong enough to hold their shape and hold their structure in the air. Anything more is too much.

I used to think you needed formers, too. You don't.

>What I would suggest is deck
>three. Two layers of
>1/32 balsa and put a
>light coat of Aliphatic or
>white glue between the layers
>and replace on the form
>until dry once again.

OK.

In the interest of science (and the value of the SSW Forum) I'll go off and fabricate Deck Three, which (to fit my mold easily and provide the proper outer dimension) will be 1/32 inner layer, and 1/16 outer layer, SIG contest light balsa with only the thinned white glue between. We'll gain the value of lamination, lose the weight of the carbon fiber and dope and see what we get.

Film at Eleven.....

For what it's worth.

I've been molding these things since 1975 so have had a long time to witness them as they age.

First of all, do not assume you don't need bulkhead supports no matter how rigid the piece might seem. Almost all of the paints we use will shrink over the years and, although loss of strength isn't an issue, appearance is. When the paint shrinks it takes the wood right along with it and the wood will eventually dip between underlying supports. If the turtle deck is installed with no support structure between the bulkheads it will sag noticeably between the canopy and the rudder in not too long a time.

The only way I think this might be avoided is by using a lot of plasticizer in the underlying coats and then painting with epoxy (if you can find any nowadays) which will minimize shrinkage. My original Trivial Pursuit was painted with Hobby Poxy and, after about 12 years doesn't show much sagging.

On the other hand, the Citation which won the first Concours back in 1977 and was painted entirely with dope has a top deck that looks like the wing of a Banshee covered in monokote.

I'd opt for some understructure to support against this potential. The weight involved is little to nothing and will prove beneficial.

I also disagree a bit about the strength of the CF veil. although it can be sanded through when freshly applied if one is not careful, if one is cautious at the early doping stages and leaves a uniform appearance until three or four coats of dope have been applied the resulting surface has great surface strength.

Keith Trostle put his first Chizler (veil applied with hot CA) straight in to concrete and although the nose was "smushed" up a bit the basic fuse remained intact as the other surfaces collapsed around it. Paper comes apart under those conditions...I'm also something of an expert in that area!

OH, just a quick p.s. FWIW

I've tried a variety of both one piece and laminated shells and, with the intention of covering with veil and using adequate understructure, have found no particular advantage in use of the laminated variety. I've pretty much decided that if I can get a piece of good A grain 3/32 balsa that will accept the degree of curve desired it simplifies the whole process without any significant degree of compromise to the final product.

As for sanding through, I am taking the lead from the Bear. Doing it your way Ted, one must very very carefull about when to begin sanding. If you wait too long, it becomes very difficult to sand and you run a higher tendency to get pin holes. Yes you are right, don't sand it right after the initial application or you will just make a mess, but don't wait too long either.

>As for sanding through, I am
>taking the lead from the
>Bear. Doing it your
>way Ted, one must very
>very carefull about when to
>begin sanding. If you
>wait too long, it becomes
>very difficult to sand and
>you run a higher tendency
>to get pin holes.
>Yes you are right, don't
>sand it right after the
>initial application or you will
>just make a mess, but
>don't wait too long either.
>
>
>Curt

No argument with that, Curt. There is a bit of a learning curve. Say hello to the stunt Goddess.

Ted and Shareen

I suppose I took an entirely different track in my last plane. I used 1/32 inch balsa (2) on the mold. Once dry, I poped it apart and used heavily thinned laminating epoxy (West Systems) painted on the inside of the top piece and outside of the bottom piece. The pieces were wiped dry and put back into the mold to dry for 24 hours. This was a turtle deck much like the Nobler deck and weighed about 11 grams when complete and was quite strong by its self; particularly the torsional flexing. Just another idea.