# Moldless composite (aka "hotwired") --conventional (non-canard) aircraft

Discussion in 'Composites' started by Vigilant1, May 24, 2018.

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1. May 28, 2018

### proppastie

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How about a seat? on a "no suspension" aircraft? Two issues, flexible for everyday use, and spinal crash/hard-landing protection.

2. May 28, 2018

### wsimpso1

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The science of crash protection has made some neat inroads. On seats, what has been found to work well is to avoid rebound as much as you can in the structure and then use either memory foam (3M Confor and others) or a molded foam and bead cushion - these move one way with big energy disappation, but do not rebound. Either way, the seat structure is STOUT. Really stiff to keep rebound tiny, and really strong because 26 g on a 200 pounder is 5200 pounds...

Graphite fiber has little energy absorption in elastic behaviour. It does a great job when building stout. Not good for compliance ...

Billski

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3. May 28, 2018

### BBerson

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I was talking about veneer face sheet on foam core.

4. May 28, 2018

### Vision_2012

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The Personal Cruiser has solid core wings and control surfaces.
http://pro-composites.com/Cruiser info.html
It uses fold-a-plane and is offered as a kit to facilitate "fast build".

I built my Vision EX (plans only) with fold-a-plane and converted the panel build of composite wings to solid core blue foam wings.
The wing fuel tanks were built with urethane and last-a-foam to separate them from wing cores--just like the EZ's.

I laugh when I hear that composites require special skills as I list the skills as reading a tape rule, drawing a line with a straightedge, cutting with a scissors, carpenter's saw and knife, guiding a hot wire to a line, painting epoxy with a paintbrush and squeegee and reading a level. I expended 1700 hours--I estimate my sanding time to be about 50--60 hours.

Rutan adapted moldless composite construction to building prototypes for Scaled because it is a FAST build method. Modern epoxies are less odorous and less toxic. Filling and sanding can be minimized with up-to-date foam cutting and placement. I don't hear people complaining about sanding between coats when painting over fabric. You don't have to finish a plane to Lindy Award standards to fly it.

5. May 28, 2018

### wsimpso1

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I posted several things:

A layer of 6 pcf foam and one ply graphite was heavier than just using 2 pcf and two ply, which is somewhat stiffer and stronger than out standard "sturdy enough" lamination;

Craft paper and veneer are heavy cores compared to any of out plastic foams;

If you really want to know what any lamination has for strudiness, you have to build and do some tests.

Rearranging to make a common sense whole out of it:

We know that 3 UNI glass over blue foam is sturdy enough for 200 mph airplanes. We also know that 2 Plies of 6 oz graphite twill cloth is slightly stiffer than 3 UNI, so should be about as sturdy to abuse over blue foam. We also know that 1 ply of 6 oz graphite over 6 pcf PVC foam is sturdy in sailplanes. And we know that 2 plies graphite over blue foam is significantly lighter than 1 ply graphite over 1/4" PVC over blue foam, so 2 plies graphite over blue foam becomes the new default. Then we started talking about veneer and craft paper as cores, and I told everyone that they are HEAVY and so undesireable.

I also suggested that if anyone wants to know how well any of these actually stand abuse, they would have to make samples and test them, with me even suggesting an impact method using height and gravity for repeatability. I do not know how much wood or craft paper cores over blue foam will influence sturdiness to the slings and arrows of airports and fly-ins and bird strikes, but I suspect they are the heavy way to sturdiness. Testing will tell.

Billski

Last edited: May 29, 2018
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6. May 29, 2018

### BBerson

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I thought post 30 was a question about paper as "intermediate" face sheet on foam core.
His mention of TPG (Taylor Paper Glass) was confusing because Taylor didn't use foam as core.

7. May 29, 2018

### proppastie

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What is the weight of that UNI?....and in the UNI direction is it stiffer?

8. May 29, 2018

### poormansairforce

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Yes, that's exactly what I was saying. Having read up on the TPG and the discussion about a PVC foam shell triggered it. Replacing 2 layers of glass with linerboard over the foam. It should give a much harder shell than just foam and maybe allow a lighter single layer of glass. Just an effort to save money. It was just a thought so no big deal.

9. May 29, 2018

### wsimpso1

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I too thought that craft paper over blue foam was the suggestion. My point is that craft paper is a HEAVY core. A 1/8" core of 6 pcf PVC foam is lighter and known to be sturdy enough, at least for racing sailplanes, which are assembled and disassembled every flying day, and have pretty significant Vne's.

Since we have a common acceptable solution that is light, and we know that craft paper or veneer will add weight, why go there? If someone has comparative impact data on the the various sandwich skins and cores or if someone wants to build samples and do tests, I will gladly read their report. I am not spending my effort on chasing an answer in need of a question.

Billski

10. May 29, 2018

### wsimpso1

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7 oz UNI is customarily laid up on the outside of wings with one ply at zero degrees (spanwise) and one ply at each of +45 and -45. The laminate is somewhat stiffer in the spanwise direction than in the chordwise direction, and I used the spanwise Ex in my comparison. I assumed +/- 45 degrees on the graphite fiber cloth and had calculated spanwise Ex as well.

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11. May 29, 2018

### wsimpso1

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It does so at a weight penalty over the other solutions that appear to be fine at handling the world... Why bother with more weight?

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12. May 29, 2018

### proppastie

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If one was to design a fabric covered aircraft out of CF using the "black aluminum" concept....Should the spar caps be uni or twill. I would guess the spar web would be twill or bid....not sure I understand the difference there.

13. May 29, 2018

### Vigilant1

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Spar caps are usually made of UNI, or made from bundled rods of extruded CF ("Graphlite")

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14. May 29, 2018

### Voidhawk9

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Twill is a type of weave. Bid is bi-directional, describing the direction of fibres.

15. May 29, 2018

### Vigilant1

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Alan,
I hadn't realized you'd gone with solid core wings, that's very neat. What layup schedule did you use for the wing skins, and do you know how their weight came out compared to Steve Rahm's Vision SP wings or those of another Vision XP built per plans? I'm sure it is possible to build great wings per the plans, but hot wired solid cores looks to be a lot simpler and less risk (esp the blind fastening of the skins to the ribs/flanges).

If you've got a link to how you did the build, our wouldn't mind giving a quick synopsis here, that would be very interesting. (lay up the spar shear webs on the blue foam? etc)

Last edited: May 29, 2018
16. May 29, 2018

### pictsidhe

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Welcome to my world of Coroplast!

Well, aircraft spruce sells 1/4" H100 for a mere $6 per square foot. No 1/8. That adds up over a 150 sqft budget ultralight... H45 1/8 is 'only'$2 per sqft, but much softer.

17. May 29, 2018

### wsimpso1

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Serious non-sequitur there proppastie. We were talking skins, not spars, but I will bite:

Spars with caps and webs nicely distribute loads.

Caps end up seeing little shear and lots of either tension or compression. In a composite spar, you use UNI tapes or prepregs tapes or Graphlite rods all at zero degrees.

Webs set the cap spacing and hold them there, keep plane surfaces plain when the spar is loaded and thus deformed. So webs end up seeing almost all of the shear load seen in the shear and bending moment diagram. Close to the caps, they end us seeing the same extension or compression as the caps, so their loading is a little more complicated. Webs are most efficient with the fibers near +/- 45 degrees, and cloth on the bias is conveniently right there... To connect the web between to the caps, the webs either wrap on the caps from below or around the outside or some do both. So webs tend to be BIAX cloth or BID cloth on the bias or prepreg uni tape put on in alternating 45's.

Billski

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18. May 29, 2018

### Vision_2012

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As I was the first to build three piece wings built from Scott's spars, as far as I know, there is no other Vision EX (or Vision SP) 3 piece wings to compare weight to. The plans build a one piece wing. I suppose my wing would be heavier than normal Vision wings. But not by a lot. I applied two BID Style 7725 and one UNI Style 7715 plies--no difference in plans here, but fewer internal ribs. The decks for entry (center section) where four plies BID over locally placed higher density foam and a supporting rib. There are beefy ribs to secure the main gear struts and box in the rear spar. I lined the fuel bays with two plies BID with a lot of extra epoxy. They now have fuel in them and don't leak. To raise the fuel vents above the wing tips, I made winglets--also not in the plans. Wing tips and wing/fuselage fillets are integral with the wing, not separate per plans. The main advantage was ease of building. I don't think I saved any money, probably spent more. But it was fun to compute airfoil templates for hotwiring and see the wing take shape in hours and build in a smaller space. My empty weight with the Corvair came out at the top of the Vision range 850-950#. I also like that my wings are removable separately.

If anyone read the plans, building the spars and then the wing in place as it laid out in the plans would easily find this intimidating. Check out Scott's Freedom: It's the only detail photo album on wing fabrication that I've found. Scott is more patient than I am and does great work.

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19. May 29, 2018

### proppastie

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Sorry, but you confused me when you said 2 Twill stronger than 3 uni. you answered that. The link in post 58 shows the wrap around of the web....I was thinking you glued the cap to the web like you rivet the caps.....boy that would have been a mistake. That "black aluminum" concept is not what I though it was.

20. May 29, 2018

### wsimpso1

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Did I say 2 Twill? My apologies if I did... I believe what we had been talking about was 1 ply and 2 plies of 6 oz graphite cloth as a skin on foam. This is a common graphite fiber cloth which has equal parts of fiber in 0 and 90 directions and is frequently used either in 0-90 or in +/- 45 orientation. When we talk about the skin standing up to the real world, we are mostly talking about how easily contact with things can do one of several things:

Elastically dent the facing far enough to delaminate it from the foam underneath;
Elastically dent the facing far enough to tear the foam underneath;
Elastically dent the facing far enough to break the fibers in the laminated cloth;
Dent the facing far enough to break the resin in the laminated cloth
Dent the facing far enough to break the between laminated cloth and foam;

There are several things that contribute to the ability to resist these dents. Stronger cloth, stronger foam, more cloth, stronger resins. If we are talking two different types of cloth on the same foam, the distinguishing feature is EI of fiber/resin system. EI is the bending stiffness of laminate or structure, and determines how easily the dent is formed. E is Young's Modulus of the cured cloth-resin layup, and I is t^3/12 where t is the laminate thickness. So, if we know that 3 UNI on blue foam is adequate for absorbing impact without damage, then another laminate with as high or higher EI would also be adequate on that foam. 2 Plies of 6 oz graphite bidirectional cloth in epoxy is (it says here) has EI 12% higher than the traditional 3 UNI (7715) in the 0 degree direction. 2 plies of 6 oz are even stiffer than 3 UNI in other directions, but not hugely so. By this analysis, I would expect 2 plies of 6 oz to be fine on blue foam.

With a stouter foam, less laminate thickness might be OK. BoKu tells us that 6 oz graphite on 6 pcf PVC foam also holds up well in sailplanes. That is pretty darned light, about 0.007" facing, but since it is well backed up by the foam and it is graphite fiber, well, they have found it to work well. You could then find other skins with similar EI to the one ply of 6 oz graphite fiber cloth and infer that they too would be adequate on 6 pcf PVC foam.

Now does it make sense? What confuses me is the use of the word twill. Twill is a specific type of weave used in forming a cloth. Has it become shorthand for a specific graphite fiber cloth common in composites? If so, which one?

Billski