# Modern version of TPG - Or the search for "Black Wood"

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by BBerson, Nov 27, 2019.

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1. Dec 2, 2019

### BBerson

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A grid will almost certainly " print through" over time. Not much can be done about that, as far as I know, other than refinish periodically.

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2. Dec 2, 2019

### Vigilant1

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Yep, this has been useful to me, thanks for starting the thread. I think you are within striking distance of matching plywood weight with a flat carbon+ribs-or-waffle of some sort, especially the thicker plywood. >Thin< PVC foam would also likely work, if you can get/make it. I'm assuming the plywood also has some sort of coating on it, maybe on both sides--that will add some weight. The practicality of the synthetic ply would likely depend on just how thick the outer layup has to be to give you satisfactory
damage resistance. The stiffness criteria can be met by varying the rib depth and frequency, or the foam thickness.

3. Dec 2, 2019

### Hot Wings

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Most ply gets varnished inside and out. That does add weight. Lots of ply skins also get covered with 2 oz glass as well. Both give us some of margin when comparing weights.

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4. Dec 2, 2019

### wsimpso1

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It will work, but unless the fiber layer is pretty thin, it will be heavier than the plywoods that are the baselines. I keep trying to point out that the advantage of the composites is that they give your more bending stiffness that allows you to save weight by omitting most of the ribs, stiffeners, etc. Oh, and the plywood can be steamed and bent, the let to cool and dry out in its new shape. To do a curved wing skin in composites with a core, you need to laminate the thing in a mold or do the laminating over the ribs or similar.

Billski

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5. Dec 2, 2019

### wsimpso1

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Syntactic foams are about 10 times denser than 2 pcf foams. Lots of weight in that stuff.

6. Dec 2, 2019

### BBerson

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Syntactic foam is half or less than paper weight.
Also, I might only be talking about a layer of syntactic foam less thick than the syntactic micro slurry normally required on both sides of the foam.
I thought this thread was about a curvable synthetic plywood rather than conventional foam sandwich made in a mold.

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7. Dec 2, 2019

### wsimpso1

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Wow, citation needed, I want to go look at the whole article. Looks like Sport Aviation, but what year and month and who is the author? I want to get to the assumptions.

Some things do not look at all right in Concepts III and IV. Concepts III and IV both indicate that the skins on each face of the foam are 0.036" at the root and 0.018" at the tip. That sounds way high to me at the root and a little thin at the tip. Standard skin for Vari-EZE, Long EZE, and Defiant is three plies of 7 oz UNI cloth, about 0.022 - 0.025 thick, with one ply running the long way on the wings and the other two at +/- 45 degrees. These are used everywhere on these airplanes.

The author has skins over some portion of the span that are 60% thicker than the well established Rutan sandwich skin. We are talking pounds heavier than they should be on these wing panels.

The real weight issue with ribs inside molded skins is two fold - you need glass-epoxy on both sides of the foam core and there is frequently more fiber/resin and dry micro and adhesive holding the rib to the skins and spars than the rib itself weighs.

Between these obvious issues, I suspect that the stated wing weights are way off of what current practice would produce...

Billski

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8. Dec 2, 2019

### stanislavz

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Tailwind was designed around plywood sheet. But even today it is still on the tops. Only few other design can overrun it in bare performance. But taking on performance/price - its still the winner.

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9. Dec 2, 2019

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As usual, we have multiple conflicting requirements.
I am envisaging the "waffle rib" approach as a way of getting minimum thickness carbon panels with hopefully better impact resistance than foam core, because you have the skins bonded together for double thickness.

I would expect it to be somewhat flexible, similar to (dry) 1-3 mm ply?

Obviously it won't do for sharp curves like steamed ply, but that's not the role I had in mind.

I can't see any easy way to have bend-in-place composites without a mould or in-place layup.

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10. Dec 2, 2019

### Hot Wings

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Fold-a-plane The folded part becomes the inner mold.

Just looking for something a lot thinner.

The string grid material seems to be turning out to be one of the better compromises. Even partially molded by draping over a semi-mold while in the green state would get you close enough to pull the fully cured piece into place like dry ply.

11. Dec 2, 2019

### Vigilant1

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It would be most convenient (esp for centralized production of stock sheets) if the simulated ply could be bent around a typical curve in the cured state. Any observations on the approx radius that cured 4oz or 6oz CF biax can make? The practical question, I guess: What's the maximum CF sheet thickness that can be bent around a typical leading edge?

One challenge of the pure Strojnik method is knowing when to grab the sheet and bend it over the nose ribs. If it is too wet/floppy it will sag between the ribs along the leading edge. This is not a problem if there's solid foam forming the LE.

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12. Dec 2, 2019

### Hot Wings

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One trick I learned years ago for a quick ''draped sheet' mold is to use the shrink window film over the form ends. You can also get a sudo-3D shape. I forget the name for this kind of surface, but it is basically anything you can hot-wire you can form with the shrink film and only perimeter guides.

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13. Dec 2, 2019

### Vigilant1

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Here's preliminary info on bending radius of cured CF sheet. Info on bending ability and price of pre-made sheets came from this manufacturer of pre-made CF sheets: ProTech Composites. The CF content (oz/sqyd) came from matching the thickness of the fabrics at Soller Composites to the sheets being sold by ProTech. Approx prices of material in DIY is CF only, we'd need to add epoxy cost and expendables for an even comparison. 0.25mm was the thinnest sheet available from ProTech.

Sheet Thickness......oz/sqyd CF...................Bending Ability........................Price for pre-made sheet........CF price for DIY
0.25mm..(.010")....6 oz (200 GSM)....1/2" radius ("will wrap around a 1" pipe").....$19 psf ($306 for 48" x 48)..........$2.25 psf 0.50mm..(.020")....12 oz (400 GSM)... 2" radius ("will wrap around a 4" pipe").......$20 psf ($320 for 48" x 48).........$3.12 psf

Shipping of a cured sheet from Vancouver, WA (ProTech) to Dayton, OH (Birthplace of Aviation): Approx $50 For comparison: 1mm (3/64") Finnish Birch plywood from AS&S is$2.87 sft (\$46 for 48" x 48") plus shipping

The .25mm (6 Oz/sqyd) CF sheet looks like it would bend around the nose of most typical airfoils. And, with ribs or foam+another ply it could be as stiff as plywood. Weight: Heavier than 1mm plywood (.082 lbs per square foot. 1mm plywood weighs .075 lbs with no varnish), but would be lighter than thicker plywood.

Now, the question would (wood?) be: For sturdiness/damage resistance, how does this .25mm CF sheet compare to 1mm plywood?

Last edited: Dec 3, 2019
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14. Dec 2, 2019

### stanislavz

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I have shown this already :

http://www.reaa.ru/cgi-bin/yabb/YaBB.pl?num=1504196398/4

But this is less structural. Just in place of fabric coverage. Done using 0.5mm fiberglass sheets.

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15. Dec 2, 2019

### BBerson

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That .5mm fiberglass is likely the same weight as .016" aluminum. I would be more interested in .25mm fiberglass.
Not sure carbon fiber would have enough difference to justify the cost.

16. Dec 2, 2019

### Topaz

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Ah, that's the old Sport Aviation article comparing various kinds of metal and composite structural concepts. Mid- to late-80's, I believe.

They basically built five examples of a partial-span wing panel, to the same structural strength, and then tested them for weight.

To the surprise of many, the optimized skin-stringer riveted metal structure is the lightest. the moldless composite structure was the heaviest.

The report neglects aerodynamic characteristics of the different methods - that's left as an exercise for the reader. When you take this into account, the composite sandwich structure is the clear winner, as it's nearly as light as the optimized skin-stringer metal structure, and far more aerodynamic.

The report highlights the tradeoffs a designer has to make in the course of choosing materials and structural concept to be used in building the design at hand. For example, top-performance aircraft clearly justify either composite sandwhich or optimized skin-stringer metal construction. But does your design need that level of sophistication? Moldless composite or traditional riveted metal construction might be heavier, but they're also quicker and easier to build, and those factors might be of higher importance for a particular design. The RV-x series uses more traditional riveted metal construction to good effect, for example, and Rutan and Scaled composites frequently used moldless composite construction for prototypes and their homebuilt designs, because the effort to build was lower and aerodynamic characteristics very good.

And yes, unless you're doing sophisticated sandwich structures like, say, Boku with the HP-24 sailplane, a full-depth foam core wing can be lighter than a low-sophistication "homemade" sandwich composite wing. Not by a lot, mind you, and there are many factors to play with to enable it, but the article's conclusions are not "cast in stone."

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17. Dec 3, 2019

### Aerowerx

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I visited Jim Marske once (he is only about 2miles from me). He showed me a section of D-tube he made as a test sample. Just some epoxy/glass. No foam core.

i don't know how he did it, but he said he wanted to see if it would sag. No it did not. Looked perfect to me. Something for you to look into, maybe?

18. Dec 3, 2019

### Vigilant1

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To look at that series of articles is to realize how far Sport Aviation magazine has fallen. We can snipe about the specifics in that 40+ year old article, but it is meaty and interesting. And every issue had pieces like that-- solid technical articles on landing gear design, aerodynamics, etc. I still page through the current ones when they arrive, but it's largely fluff, human interest stories, ads, pseudo-ads, or stories about megabuck restorations. It's almost indistinguishable from the AOPA magazine. Kitplanes now has more technical content than Sport Aviation does.
Off topic-- sorry.

Last edited: Dec 3, 2019
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19. Dec 3, 2019

### Voidhawk9

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I switched my subscription years ago for precisely this reason. If I wanted to read articles about warbirds and vintage aeroplanes, there are plenty of other magazines dedicated to that.

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20. Dec 3, 2019

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