Molding Aft Deck, Material Choices

Gentlemen:

I want to use a molded aft deck on a new 60-size plane. Generally, I hollow blocks to 1/8 thk or a little less, so my original plan was to use two plys of 1/16".

Now I wonder if I can improve on that combination to add rigidity and save some weight.

My other thoughts are:

1. Is 1/16 single ply enough? How about with carbon veil on the inside/outside, with PC?

2. How about a single layer of 3/32?

3. Bulkheads?

Any other notions?

Thanks

Use the 1/16th ply, and make it a box section for strength (the bottom is filled in with sheet balsa, even though you don't see it). You will only need a couple of bulkheads. Actually, using this construction, you don't need ply. Maybe 3/32" balsa for all three pieces would work.

Cap,

My new plane has molded nose block, turtledack and bottom block...all done with 1/16" balsa only. The fuse box was re-inforced pretty well with lateral bracing. The nose had carbon veil wraped around it along with some veil around the stab area, but the rest is just silkspan. No problems so far. The last plane I did like this was crashed in one of those power on, "oh man this is gonna be ugly" sort of ways. It just exploded on impact. Don't think I've ever seen one just turn to splinters like that. But it had no problems all the time I was flying it. And I wan't really designing it to withstand a power on impact with the ground

Curt,
On my newest plane, a 630 sq in SSW 53 powered plane(almost a 60 powered plane), I used 2 layers of 1/32 8# balsa glued together with Pica Gluit during the molding process. I did double up on the number of bulkheads to help eliminate the scalloping sorta like using false ribs. The T'deck turned out to be very stiff and worked great. It is 27 inches long and weighed 18 grams with the bulkheads.

Hi Curt,

I just finished molding a turtle deck for a Katana. 2 1/16" pieces of light balsa with CF epoxyed together. Nice & stiff. I used 3 bulk heads total.
I made the mold from "Dow Score Board" 1.5" blue foam from Home Depot.

Curt,
I'm using a single sheet of 3/32" light (6 lb) A-grain balsa, with 1/8"-thick balsa bulkheads spaced about 6" apart. Molded over a plywood-and-blue-foam mold, as shown on Pat Rowan's picture.
I believe that molding two sheets and laminating them subsequently results in a stronger and heavier shell than necessary (except for the nose sections, where it's justified).
Oops.. Quickly putting my flak jacket on....

--Mike

Guys,

I've done some laminated stuff and they certainly can be strong. I just haven't seen the need. Having built some real porkers the last couple of plane (with the exception of the Bill Simons Shoestring I did last year) I was pretty paranoid about weight with this last plane. I was determined to build a light, straight plane. So the molded top and bottom blocks were one area that seemed a good candidate for insuring lightness. The molded 1/16" pieces were sturdy enough with formers added and were certainly light.

Guys,
How about listing the weights of your T'decks? IMHO a few grams here that would add a lot of rigidity to the aft portion of our stunters would be well spent weight. A twisty tail because we want to save a few grams wouldn't be worth it. If your looking to save a grams, I'd look elsewhere. Like putting on the finish. Not too hard to save a few grams here, just do a bit more sanding and be careful with the spray gun. How about wiping all the excess glue from the joints, a few grams would add up quickly here too. You get my point.

Crist,

Well, I built the fuse box pretty strongly. Diagonal bracing on the fuse sides, top and bottom. The plane would fly fine with little risk of twisting without top or bottom blocks. They are purely cosmetic in this case. They add a bit to the fuse rigidity, I suppose, but not much and any little bit they do add is just icing on the cake.

>Guys,
>How about listing the weights of your T'decks?

Molded Turtledeck Experiment Results

Pilsbury Nobler Turtledeck Mold-off

Warren's original results

Godzilla's results

And no post on molding balsa would be complete without:

Master Molder Al Rabe's tutorial

The Mr. Hyde has 3/32" balsa top and bottom blocks.

No formers on the top, as there is a compound curve sloping down from the canopy.

One former on the bottom at F2.

I do not see need for formers except where the block is cut (like at the back of the cowl). They just cause more trouble than they are worth. Once covered with PC and silkspan they are very tough.

Hi CaptCurt,
Let the flames begin.......!!
Can we really "save weight" by molding? If a good block is available, a piece can be made as light as a molded one. As to rigidity, the molded ones will be stronger. (??) I guess it is open to a lot of variables. If the fuselage box is sufficiently rigid, then the extra rigidity of a molded piece might not be necessary.
I believe the molded parts are really an advantage if the mold can be used on more than one ship, or it is a case of a fuselage like Al, Ron, and Billy build.
We have a molded turtle deck that is a great piece (two 1/16th pieces glued) but the weight is not "light", and "good" wood was used to form it. It will receive the Dremel treatment. (good old "anal" attitude about weight present here)
Example: My complete top block on my old USA-1 (1") was as light as the turtle deck is before any surgery now.
Of course, Aaron's Geo XL has a molded turtle deck, and the mold will be used again.
?????
Bill <><

Hi Guys:

Thanks for all the information. While the "spec" was save weight and add rigidity, a big part of my interest in molding these is to conserve a dwindling resource. I can "recover" alot of 1/16" wing skins by not cutting up a 1x3x36" contest block. Approximately 20% utilization.

Just me.

Hi Curt,
I agree on that point. There is a LOT of waste that I hate to see myself when I carve a block.
Wasn't argueing, just wanting discussion about the "merits" of molding.
My projects DO get "prioritized" as to the wood used on them. Some I build because I want that model. Some to be my "main" ship, etc., so I do sort my wood accordingly. And, of course, Aaron steals some of my BEST wood.
Bill <><

>
>Wasn't argueing, just wanting discussion about the "merits"
>of molding.
>
I understand..didn't even think that.

Some to be
>my "main" ship, etc., so I do sort my wood accordingly.

I do exactly that.

>And, of course, Aaron steals some of my BEST wood.

Man I hate it when that happens.

>Hi Curt,
>I agree on that point. There is a LOT of waste that I hate
>to see myself when I carve a block.
>Wasn't argueing, just wanting discussion about the "merits"
>of molding.
>My projects DO get "prioritized" as to the wood used on
>them. Some I build because I want that model. Some to be
>my "main" ship, etc., so I do sort my wood accordingly.
>And, of course, Aaron steals some of my BEST wood.
>Bill <><

Well, Matt stole all my good wood, but that is another story.

Is it as easy to get a light and solid block by carving? I think it is going to depend on your ability to carve. It is going to be very difficult to carve the outside perfect, then hollow the inside to an even 1/16 or 3/32 thickness all the way across. Remember the old addage, "If you don't go through in a couple of places, you haven't hollowed enough." With molding, we get an even thickness without going through.

Now, that said, Matt makes his out of a couple of sheets of 1/32. You can glue a liner in between, silkspan or carbon fiber, if you want. The glue and liner will add a little weight, but they will add a lot of strength. Remember, we are now at 1/16 of an inch if we are going to two layers. And you can do this without any formers. And you can do this without any hollowing. You can make the mold out of blue styrofoam--cheap and easy to carve. If you want, you can slice it and add ply formers at the various stations that set your limits and establish your shapes. Then carve the mold and use it on one or a hundred airplanes. You have no carving to do at all on the inside of the finished product, so it saves time. You have quick and easy sanding of the mold and virtually no sanding to do on the outside of the finished product, so it saves time. Two layers (or 3) glued together will not have the grain going exactly the same, so there is less tendency to split, and it is much solider to grab ahold of when it is on the airplane. And it is cheaper.

So, it should be faster, it should be cheaper, it should be stronger, I can't think of a downside. Why didn't we think of this 50 years ago? Why did George carve the top block on the Nobler? And why did we all keep following suit and doing the same?

Leonard, Curt,
Our latest molding is for a Rabe Bearcat. Top and Bottom blocks. We used the "blue foam", layered to get the right thickness, with "bulkheads" cut for the mold and correspondingly thicker for use inside the part as formers. It is working fine. I think the 1/32 layered is the way to go.
I do get a lot of enjoyment out of carving and hollowing a block, but I am thinking of the waste more and more.
Bill <><

Ron B.
F4Fguy

Bill:
If you mean two plys of 1/32" laid one over the other,one layer of 1/16" will be just as strong.If you mean cross ply,then you're right.My last Macchi fuse was 39"lg x 9"dp x5"wd.It consisted of two plys of 1/32 6# laid 30deg.to each other and weighed 6oz.with the bulkheads(3plys crossed 1/32" ~4"apart)there was no other internal structure from the wing leading edge back.

On the other hand This was a royal pain to do!It was also structural overkill in most areas.Since then my airplanes have been done with molded 1/20" sheets formed on a blue foam mold and assembled in a jig,laminated in high stress areas to two or even three layers.This gives the best use of material on a strength to weight basis.As an example,the WildcatIIwas three ply to the wing H.P.,two on the sides only to just behind the flap,onelayer from there to the stab,and two under the stab platform.With one wing supported at the fuse side,you could hold the plane level by the stab with no percievable twist.

While on the subject,I've done some primitive testing on various layups(enough to convince myself)with unsurprising results.Carbon,glass,silkspan,etc.reinforcements are dependent on the ahesive,that is,their tensile strength and effectiveness goes up as the adhesive strength(dope<urethane<polyester<epoxy).Stiffness goes up with distance from the neutral axis,as well as with adhesive.None of the materials were effective as a center ply in stiffness.That is to say two plys glued together had the same deflection as the ones with carbon veil etc.in the center.Two plys sans carbon etc.showed some improvement over one ply of the same thickness,probably because grains are always slightly crossed even when samples are from the same sheet.

Putting the veil on both surfaces of the outside is very much a different story.Here deflection was very low for the same load and in proportion to the material and the adhesive.Unfortunately weight also goes up unless adhesive is rigidly controlled.The only effective way I've found is bagging with a peel ply and an absorbent breather ply(not really very user friendly).

Veil on one surface fell,as expected,somewhere in between,with a heavy bias to the bare two ply with the same adhesive.Obviously load direction made a big difference here,carbon/epoxy was almost non-effective on the side opposite the load(this would be the inside of a fuse).

All of the above is relative only to local loads.In a fuselage structure placement of the veil is relatively unimportant as the skins are stressed as part of the section modulus of the whole fuselage.This is one good argument for FAT airplanes.

In sum,I'd say that,as presently used by most,veil is good to stop skin deflections such as"skeletoning"but,it doesn't do much in terms of overall strength.Even then I believe molded balsa compound curves are more effective.

If you eliminate the balsa altogether........that's another story for another time,but it can be interesting.

Ron,
very, very impressive! Both your craftsmanship and the structure analysis you described in your post.
While on the subject of using layups for fuselage construction, did you read an article I translated from Russian (Stunt News, perhaps a year or two ago)? I think the article was titled "The '80s Stunter", and was largely dedicated to the process of producing a sandwich composite fuselage (fiberglass - 3/32" styrofoam - fiberglass) as a vacuum-bagged layup. They claimed the result was 2 oz lighter than the balsa version of the same size and strength.
I also spoke to Konstantine Bajaikine - member of Canadian F2B team; he said his fuselage is made of 100% molded balsa shells (all compound curves, very beautiful) with thin fiberglass on the inside and outside.
I am currently looking into alternative fuselage building methods myself - partly to get rid of the ugly flat fuse sides, partly to reduce building time.
I already have ideas for the quick wing design, but that's another topic.

Ron B.
F4Fguy

Mike:
Yes,I did read the article,great stuff.I've done a lot of work along these lines,and the article confirmed to me that I was on the right track.The big problem I've run into is availability of thin plastic foam sheet material.I've tried,successfully,to hot wire slice 1/16 sheets from blue foam and spider foam but,I haven't been able to form it to my satisfaction(it's very brittle).Zepron may offer some formability,but I haven't tried it yet,and I don't know if it's available in large enough sheets to be used in a fuse or wing for a stunter.
My last Macchi 202 stunter was done in much the same way as the russian stunter.the fuse was two cross plys of 1/32 balsa with 1/2 oz carbon on the outside and 5/8oz glass inside.The wing was carbon/1/20"balsa/carbon,with laminating epoxybonding,and vacuum bagged.internal wing structure was a trussed spar and four ribs(primarily to carry the LG loads),allof carbon/balsa/carbon.Unfortunately,somewhere along the yellow brick road,control of epoxy content got away from me.It ended up at well over 70oz,flyable but not comfortable on 735sq.in. with a piped 61.

THanks guys, for all the input.

Ron: Its good to know that there are others out there that understand the basic structural mechanics of a composite, cored panel.

Mike: I need to dig up your reference on the foam composite stuff--very interesting!

I'm ready to roll on this and right now I guess I'm leaning toward the simple use of a 3/32 single layer with three bulkheads and silkspan/PC over the outside.

I've got a blue foam mold nearly done so maybe by the end of the week.

Thanks again to all.