Composites vs aluminum

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Little Scrapper

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Billski, I'm confused, why the core? Are you saying it's not strong and stiff enough without one?
 

mcrae0104

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If the turtledeck has straight tapers (like the Cassutt), the carving can be done with a hotwire saw and a pair of templates for the outside, and another pair indexed to the outside for removing the inside.
This is exactly how the AR-5 tail cone was done, except sans vacuum (see "How it's Made" video above).

Billski, I'm confused, why the core? Are you saying it's not strong and stiff enough without one?
The added weight of the core is negligible but the stiffness goes way up. Next time you're around some homebuilts poke on the cowls and see how flexible they can be without a core. "Stiff enough" is probably in the eye of the beholder for a non-structural part like your turtledeck.
 

wsimpso1

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Billski, I'm confused, why the core? Are you saying it's not strong and stiff enough without one?
Let's put it in perspective. There are several braces and or ribs in the various plans I have seen for turtledecks, and they all weigh somthing. If you put in the braces, you can go very thin on a coreless carbon and maybe even on a coreless fiberglass turtledeck. But, if you make it with the cores that BoKu and I have been describing, skipping the cores at the edges of the parts, you can skip ALL of the bracing and ribs above the fuselage truss, and then maybe you will save some weight. The thin stuff with ribs and braces and building like metal is called black aluminum thinking. Doing it with cores makes it internally braced and takes advantages of the composites...

Does that help?

Another way of looking at airplane design is that we should be doing things that result in a lighter airplane, not just in making individual pieces lighter. The distinction is important - if all you want to do is make the skin of the turtledeck lighter, you are passing up the opportunity to make the turtle deck and the supporting structure lighter by even bigger margins...

Billski
 
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proppastie

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A .02 aluminum skin on some .02/.016 formers attached to the tubes will be very light, and probably a lot less trouble. Assuming it is not structural and only a "fairing".
 

BoKu

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Billski, I'm confused, why the core? Are you saying it's not strong and stiff enough without one?
To expand on our previous replies: It's hard to appreciate how stiff yet light composite sandwich construction can be until you handle pieces made with it. If there is a glider repair shop near you where you can pick up a piece of composite sandwich sailplane wing skin, that might really help you understand better than all these mass and area numbers we've been throwing out.

--Bob K.
 

Little Scrapper

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I haven't responded to this thread yet because I'm not sure what to do. While learning was my first priority and goal I think composite may not be ideal for the turtledeck.
I figured make a thin composite skin and install it, not a thick sandwich. I certainly don't want to eliminate the vertical uprights in the Cassutt design.

With the information I have here now it's making me think.
 

wsimpso1

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LS,

If you leave in all of the braces inside the turtledeck, the plans design is probably the lightest, and a composite shell will add weight to the bird.

If you want to save some weight, a cored turtedeck will likely allow deleton of the braces, and may save weight.

Gotta run the numbers to know.

Billski
 

wwalton

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I am wondering if keeping the vertical pieces that form the turtleneck is worth it...to me. The Mariah version of the Cassutt used a fairly short after body behind the canopy and it was very fast and I assume light.IMG_0117.JPG
However I can see that the entire fuselage has been reworked so hard to know.
 

TFF

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It goes back to what are you building, an original Cassutt or are you building a modern Cassutt. My personal opinion is it should be original. Improvements are going to be a rabbit hole. I will say the horizontal stab is so small, its crazy; i think I would have to make that bigger; Ha Ha!
 

Little Scrapper

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Older Cassutt but shows turtle deck from a nice angle. I think I'll stick with aluminum for the TD and composite for the tips, pants and cowl.
20170808_115241.jpg

From what I've learned on this thread a basic aluminum sheet of .020" can't be replaced by composites without being heavier. I'm excited to learn composites, hence this thread, but I'm not willing to completely alter the TD to do it.
 

TFF

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I think everyone here is thinking longeron to longeron not a little cap. The whole deck in aluminum or composite.
 

autoreply

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Airframe wise, I think that with a CFRP infused structure you can easily cut the structural weight by 50 to 65%. By structural I mean anything that's riveted on say an RV8.
My experience actually doing that exact thing suggests that that assessment is considerably optimistic. The big bugaboo is inescapably concentrated loads that require both reinforcement and adequate load transfer--things like wing and control system attachments, control surface hinges, walkways, and assembly bondlines. Based on the tapered carbon fiber RV-6-7-8 wings we're building now, I'd say that the weight savings is more along the lines of about 25-40% depending on various factors.

--Bob K.
That's not even remotely what you've been doing ;)

Infusion is the key there. Let's highlight the differences, roughly in order of how much difference in final weight they make:
  1. To avoid pinholes you can use a gelcoat. Or commit to much pinhole filling. A gelcoat adds a considerable amount of weight over a thin PU coat. That alone is a significant part of the weight difference we're talking about.
  2. Hard points. With vacuum bagging, there is no way to accurately locate mount points for controls etc while you're bagging it. Glueing in later will add weight in the glue joint and in both parts (or one). Neodymium might actually make that possible with vacuum bagging too :)
  3. Core bond joint weight. Vacuum bagging will result in heavier glue joints to the cores than when infusing.
  4. Aluminium spars. Obviously adds weight over a CFRP one.
  5. Ribs built into the wings. I've seen some neat work done over the years directly infusing ribs in a wing skin. That's a significant amount of glue joint area removed. Also easier to tolerate tighter to lighten up the other glue joint to the other wing skin.
  6. Fuel tank sealing. Infuse it and if done well there won't be nearly as much sealant required because there's way less void space.
  7. Optimal cores and laminate. With vacuum bagging there is a practical limit to how much you can optimize it. Positioning is unavoidably inaccurate. With infusion you can perfectly position and optimize the laminate.

The seemingly small details quickly add up which is why I usually end in that range; my earlier post mentions some of the numbers I got from infusing parts.
 

wsimpso1

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Older Cassutt but shows turtle deck from a nice angle. I think I'll stick with aluminum for the TD and composite for the tips, pants and cowl.
View attachment 64488

From what I've learned on this thread a basic aluminum sheet of .020" can't be replaced by composites without being heavier. I'm excited to learn composites, hence this thread, but I'm not willing to completely alter the TD to do it.
That turtledeck has a substantial bulkhead (rolloverr protection) at the rear of the cockpit, tube braces at five places along its length, and is mostly fabric covered. Pretty light to begin with, simple, and straightforward to build. That makes it tough to beat. As I understand it, the entire turtle deck is non-structural, that is, it floats and the fuselage truss below carries the loads.

To replace it in composite, a foam cored sandwich that attaches to the same spots as the current braces would be schemed up and built that replaces the rollover structure, the braces, the aluminum fairing at the top, and all of the fabric/dope/silver. To save weight may be a task...

Let's talk about some of the decisions you make in designing airframe exterior parts.

Every part has to be strong enough, but strong enough for what? In the case of the turtledeck, I think that it has to stand flight loads, landing loads, and a one time only rollover (which explains the added sheet metal at the cockpit).

In fabric covered airplanes, we know that tubing and maybe some wooden fairing pieces are used. Then certain standard weight cloth, properly doped and painted will carry the day. Everyone seems to understand that fabric is flexible and potentially fragile (even when it is not) and these fabrics give good service...

Sheet metal airplanes seem to need some standard of thickness to feel substantial and resist dents in use. Again, certain thicknesses have become standard and give good service. If we could get people to treat sheet metal as gently as fabric, we could make the sheet metal a little thinner, but it only comes in certain fixed thicknesses, so we are stuck... .

Then there are composites. If you just use a couple plies of glass fabric, the part will feel flimsy, and maybe you can keep the air-show morons from trying to break your fuselage skin, you know by saying "please don't touch and try to flex my airplane..." Yeah, that and posting an armed guard or two might work. This same thin sheet will get punctured by any idiot with a pencil, so keep an eye out for authors and the like. So add a ply of glass or go to graphite. Still kind of soft feeling... Build it with a foam core, which stiffens it up immensely for a tiny increase in weight, and adds strength too. Trouble is your turtle deck is still as likely to be punctured with a pencil, but now folks get the impression it is strong as granite, and you are more likely to have a moron try to use it as a writing surface or set a squalling infant on it. At some point you see the wisdom of about 21 oz/yd of glass cloth or 12 oz/yd graphite cloth that are common for wings and fuselages and tails and control surfaces on kit airplanes. For flight loads and landing loads, the thing is overbuilt, but it will survive in the real world. Now if it is still lighter than the base tube and sheet metal and fabric/dope setup, you are good to go in composite. If it is not lighter, stick to the plans.

Yeah, the turtledeck is probably best done per plans. But the cowling, wing tips, root fairings, and other compound surfaces may still be best translated into composites because they are compound curved surfaces.

Billski
 
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