Single-seat ultralight puddlejumper: the "Carbonmax"

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BoKu

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Great job!
2 questions:
- 8 oz/yd² is ~270g/m² for us european, right?
Yes, I believe that's true.

...how do you join the molded pieces ? That's something that still is a bit of mystery to me, how just tape around a butt join is enough structurally ?
Good question. There's a bunch of ways of doing that, and the one way I'm pretty sure I wouldn't do is with wet tapes on the outside.

What I'd prefer to do is mold a joggle into one edge of each skin using tooling wax or perhaps just several layers of tape. Then you'd bond the parts together at the joggle using a bonding paste made of epoxy plus flox and cabocil. That's how we join our sailplane wing skins and fuselage halves and similar assemblies.

Another possibility is to mold a carbon fiber channel about 2" wide with 1" tall flanges, and use the 2" face to bond the skins together on the inside surface. The channel would serve as both a bonding element and as a stiffening longeron. It would add a bit of parts count and weight, but you could compensate by eliminating the longitudinal stiffeners from the skins. Doing it that way simplifies the layup of the skin, since you don't need to include the joggle and longitudinal stiffeners during the layup.
 
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Deuelly

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The LT-1 was a side project for Andy. He usually stays busy trying to squeeze every inch of speed out of Lancairs and NXTs.

Brandon
 

BoKu

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With your process it wouldn't be hard to make something similar to the Aerochia LT-1 for a lot less cost.
I haven't studied the LT-1, but it looks like the airplanes would be only somewhat similar. The CarbonMax would be optimized to reduce dependencies on complex parts and processes, and maximize the things you can do using common tools and processes under relatively primitive conditions. The only thing you'd have to buy from me would be the molded forward fuselage and cowling. And if you wanted you could rent the molds for a weekend and come out here and make them yourself. Ideally, it would use an industrial V-twin engine with a PSRU. One big difference is that the lower gross weight and greater wing area would give CarbonMax about half the wing loading as the LT-1, yielding a much lower stall speed.
 

Vigilant1

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About the wings aft of the spar: fabric certainly cuts the weight, but going with a thin carbon skin would allow builders to avoid learning a new set of skills (same reasoning you are avoiding an AL fuselage aft).
A very interesting project!
 

BoKu

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...going with a thin carbon skin would allow builders to avoid learning a new set of skills (same reasoning you are avoiding an AL fuselage aft)...
Good point. However, I think that the raw numbers make the fabric pretty compelling: A minumum carbon skin would be 2 ply of 6oz, which comes to about 24oz/yd^2 with epoxy and finish, but doesn't include stiffeners and such. Generic polyester comes in at about 2oz/yd^2, and even if a light finish triples the weight you still come out way ahead. Over the 10yd^2 or so of the mid-chord area covered by fabric, it looks like it's about a 10 lb weight savings.
 

Will Aldridge

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Good point. However, I think that the raw numbers make the fabric pretty compelling: A minumum carbon skin would be 2 ply of 6oz, which comes to about 24oz/yd^2 with epoxy and finish, but doesn't include stiffeners and such. Generic polyester comes in at about 2oz/yd^2, and even if a light finish triples the weight you still come out way ahead. Over the 10yd^2 or so of the mid-chord area covered by fabric, it looks like it's about a 10 lb weight savings.
Ive read and re read your last few posts so excuse me if I missed it but I haven't seen anything about cores. Is this tail cone stiff enough without cores? All the bonded in stiffeners you've mentioned take their place? I do remember seeing that it's a minimalist structure so it would be more susceptible to hangar rash and air show morons?
 

BoKu

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Ive read and re read your last few posts so excuse me if I missed it but I haven't seen anything about cores. Is this tail cone stiff enough without cores? All the bonded in stiffeners you've mentioned take their place? I do remember seeing that it's a minimalist structure so it would be more susceptible to hangar rash and air show morons?
Correct, the aft fuselage has no core, just a carbon fiber shell and a few molded-in and bonded-on stiffeners. Is that enough? I think so, and testing will show for sure one way or the other. And, yes, this is a very lightly built airplane, and would indeed be somewhat more vulnerable to mishandling. That's one of the tradeoffs inherent in designing for minimal power and minimal expense.
 

sming

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A minumum carbon skin would be 2 ply of 6oz, which comes to about 24oz/yd^2 with epoxy and finish, but doesn't include stiffeners and such.
I've linked that product in another thread but what about that: http://www.bcomp.ch/en/products/powerribs
Apparently triple the stiffness for the cost one 6 oz ply... it feel likes a good deal! Well perhaps not for an ultralight but I remembered Sonja motorglider with non-cored skins and wondered if it could help that case...
Drifting off-topic, with this "sustainable" future world going on, do you think a natural fiber composite aircraft would be possible/not too compromised? In my world (surfboards), I've seen experiments and they fare very well.
 

cluttonfred

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You can make anything out of almost anything if you allow for the materials in the design. During WWII, the Blohm & Voss BV 246 Hagelkorn ("Hailstone") glide bomb had high aspect ratio wings made of cement over spot-welded steel sheet. That sounds terrible as a choice for anything that flies (and the bomb could glide for over 200 km when dropped from altitude) but it carried a payload twice it's own empty weight. I think that sustainable fibers and non-toxic resins would be welcome in composite aircraft construction if they could be show to be as easy to use and predictable as synthetic materials.

Drifting off-topic, with this "sustainable" future world going on, do you think a natural fiber composite aircraft would be possible/not too compromised? In my world (surfboards), I've seen experiments and they fare very well.
 
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mcrae0104

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Drifting off-topic, with this "sustainable" future world going on, do you think a natural fiber composite aircraft would be possible/not too compromised?
Yes, picea sitchensis is a popular natural fiber composite used for aircraft. You might call it carbon-sequestering fiber. When the aircraft has reached the end of its life cycle, the fiber's embodied energy can be extracted in an exothermic process that produces a naturally-occurring compound that is one of the main catalysts to produce more of the fiber composite (cradle-to-cradle I think is the term that's popular these days). Also it is 100% organically produced using solar power and rainwater harvesting. In terms of sustainability I'd say it's very promising.
 
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cluttonfred

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Hehe, unfortunately I think the days off reasonably-affordable, aircraft-grade spruce are numbered. Finnish birch aircraft plywood seems to be very sustainable in terms of how it is managed and produced, so I think that a basically plywood aircraft reinforced at the wing spars, longerons, etc. with natural fiber-based cloth (jute, hemp, bamboo, etc.) and organic epoxy is likely one path to take in the near future.
 

wsimpso1

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Is there a way to make the stress analysis simpler than matrices? Marske used the calculated strength from Mil-hnbk 17 and then treated the sheet of material as though it was aluminum. Could this be done?
You can, but you do need to make some assumptions about what the mix of stressors are the structures and then about the mix of strengths in your materials. The beauty in using advanced materials is they allow lighter structures than the other materials do, but that does mean you are also working at least as near to limits as was the previous design. If you skip working near limits, the ability to be lighter just got ruined. This approach is likely to result in an overspecified and thus overweight structure, makes you dependant upon a thorough load test program, with test to failure, and may still have some surprise failures which drives redesign of the product after test. As I see this, these schemes are more guessing and less knowing what is going on with your structure.

Many parts of an airplane like the one Bob is proposing will be at min gage for build and ground handling, with excess strength for flight. But in the places where we can build to strength instead min gage, weight can be saved with good design, and overbuilding it puts weight back into the airframe.

Billski
 

wsimpso1

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People that own airplanes need to know how to buck rivets.
They do? How so? Lots of folks own and/or fly airplanes that really know nothing about riveted joints. And a bunch of folks now are building airplanes with pulled rivets instead of bucked rivets... I gotta disagree with the premise, although I agree that broader knowledge is not a bad thing...

Billski
 

wsimpso1

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But...what are the issues with carrying a cantilever wing with a skinny fuse?
What BoKu seems to be proposing is the two main spars over lap each other through the fuselage like a sailplane, and are then pinned in two places to each other and the fuselage. When you do the skinny fuselage, the pin joints may have to be pretty close to each other, meaning bigger loads in the pins and joint than with a wider space, but it is all amenable to good design work. Look at a sailplane with a less than two foot wide fuselage and a 50 foot span - the joint spacing looks pretty small there too...
 

wsimpso1

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I ran across this a few years ago, but have never had the time to go back and really study what they were doing:

http://www.jeccomposites.com/knowledge/international-composites-news/composide-and-prof-stephen-tsai-collaborate-demonstrate-new
"As a result, only a few simple tests are required to characterise composites materials. This approach simplifies composites design to the extent that it is similar to designing with aluminium."
I did go through it. One of the authors is also one of the authors of one of two widely used textbooks on the topic Introduction to Composite Materials. I have a copy here within my reach. While they feel it will decrease the work to design in composites, decrease is a relative term. Composites are still more involved in design and estimation of strength, and as Tsai mentioned in the video, it is conservative, that is, it errs on the side of safety, adding weight in the process.

Matrix algebra is not that hard and makes all of the composite plate analysis straightforward.

Billski
 

BoKu

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Update: I just did a quickie re-draw of the inboard profile for a bubble canopy version with a blown forward canopy and a flat-wrap aft section.
Carbon Max notchback.png
I like the looks of this one a lot better, and it also makes the three aft fuselage skins more similar in area. I'm also fiddling with fuselage cross-sections that will be friendly to the ruled-sheet molding process.
 

sming

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Wow, love it, well done!!

Still thinking (shipping fuselage part on the other side of the pond, not practical) about the ar-5 and its flat sides over the wing roots, if that would not be a solution: mold everything with your proposed mold technique, and just for the 1 or 2 foot of compound curves joining the tail cone to the seat and forward fuselage, glue them to some xps and let the builder shape it manually, and laminate in place, quickie style?
 

BoKu

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...Still thinking (shipping fuselage part on the other side of the pond, not practical) about the ar-5 and its flat sides over the wing roots, if that would not be a solution: mold everything with your proposed mold technique, and just for the 1 or 2 foot of compound curves joining the tail cone to the seat and forward fuselage, glue them to some xps and let the builder shape it manually, and laminate in place, quickie style?
I really want to keep that down to a dull roar. As Joa pointed out earlier in this thread, the properties of vacuum bagged carbon are much more consistent, and more consistently good, than those of ambient wet layup carbon. I don't mind adding ambient fairings and stiffeners, but I want the main skins to be vacuum molded.

It looks to me like the three forward fuselage skins (right side, left side, top/glareshield) would nest into a cardboard carton 1600mm x 700mm x 300mm. Is that an unreasonable size for international shipment?
 

cluttonfred

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Standard UPS size limit is "up to 165 inches in length and girth combined" and "up to 108 inches in length" (4.19 m and 2.74 m respectively) and 70 lb (31.75 kg) without excess weight charges.
 

BoKu

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Standard UPS size limit is "up to 165 inches in length and girth combined" and "up to 108 inches in length" (4.19 m and 2.74 m respectively) and 70 lb (31.75 kg) without excess weight charges.
If I'm doing the math right, my box would clock in at 142" using the UPS standard "once down and once around."
 
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