Single-seat ultralight puddlejumper: the "Carbonmax"

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by BoKu, Feb 14, 2020.

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  1. Feb 14, 2020 #1

    BoKu

    BoKu

    BoKu

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    In response to a couple recent threads, here's an idea for a fun little toy airplane: Take the dimensions of the Minimax and develop an airplane that uses the same engine and general configuration but with a simplified carbon fiber airframe.

    Before continuing, let me describe the general benefits of going with carbon:

    * In terms of dollars per unit strength and stiffness, it is about the same cost as usable wood, cheaper than the equivalent 4130 steel, and much cheaper than either spruce aircraft lumber or aircraft plywood. Now that the A380 is almost done and 787 has peaked, worldwide supply exceeds demand so carbon prices are declining against inflation. Composites salvage firms are once again offering low prices on seconds and overstocks.

    * Contrary to common opinion, carbon fiber is easy to work with. By mass and person-hour, most of my high-performance sailplanes are built by amateurs in their first or second week of carbon fiber experience. I generally have people making small parts on their second day in the shop.

    * Carbon fiber and epoxy are easy to ship, much more so than long sticks of lumber or sheets of plywood for which the shipping often exceeds the cost of the materials.

    * As demonstrated by Dick Schreder's HP-18 and RS-15 kit sailplanes, the judicious combination of pre-molded and field-fabricated parts can result in a simple, lightweight aircraft that is sleek and attractive.

    * The judicious use of pultruded carbon fiber allows for a lightweight cantilever wing that can be attached and removed in the field by a single person with no tools.

    Okay, here's a sketch of what I propose:

    Carbonmax_sketch1.jpg

    The main element is a molded carbon fiber shell for the forward fuselage tub and cowling that also includes the wing side-of-body fillet. The neat thing about making these as molded composite parts is that they can have a compound curves that smoothly transition from the tapers of the aft fuselage and the wing. These molded parts would really jazz up the aesthetics of the airplane with only a modest increase in expense. The tub would be supplied as raw shells that the builder would join along a joggle, and in which they install simple bulkheads, ring stiffeners, and reinforcements. Most of these structures would be foam shapes that the builder cuts from sheet, glues into place, and skins with carbon fiber. Along the aft edge would be a joggle that the builder attaches to the aft fuselage skins.

    The aft fuselage would be a carbon fiber shell that the builder would fabricate using simple molding techniques. The builder takes a 4x8 sheet of 0.032" aluminum, curves it to fit into wooden cradles at each of the ends, and uses it as a mold for each of the three aft fuselage skins (right, left, and top). Each skin has simple longerons and ring stiffeners molded into it. Vacuum bagging of the skins is optional.

    The wings and tail structures would be primarily moldless foam core, with wing spar caps of pultruded carbon fiber. The wing would probably have fabric covered ribs aft of the spar. The ribs would be pan-molded parts made in simple particle board molds. The overall effect would be parts that are plenty strong enough for flight loads, but that marginal for handling loads so you'd have to treat them carefully. But that's no different from a Minimax.

    The canopy could be either a blown acrylic or PETG bubble, or a three-section Galland hood pieced together out of flat stock. Or just a windshield for open-cockpit flying.

    As I see it, the big downside against the Minimax is that wood is fun to work with in a way that carbon fiber isn't. Wood smells nice, sands easy, and is relatively hypoallergenic. So if what you want to do is spend pleasant hours fiddling in the garage, the Minimax is what you want. By contrast, you need gloves to work with epoxy, carbon sands to an itchy black dust, and the whole affair smells like Dick Cheney's wet dream. But you make progress fast, and in the end you have an airframe that is much more impervious to age and the elements than a wood one, and for which you don't need external bracing wires or struts.

    Of course, just like the Minimax, this airplane would have no great utility. It would be light and slow, and any wind or turbulence would really beat you up. It would be an airplane for early morning and calm weather flying. But it would assemble faster than the Minimax, with wings that go on with a pair of pins and ailerons that automatically connect. The wings would be light enough for a single person to attach at the airport.
     
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  2. Feb 14, 2020 #2

    Jay Kempf

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    Nice complete thought. And we will call it Valentine!

    Why mold the tailboom over aluminum sheet when you can use the aluminum sheet? Would be a minimum of effort to rivet a few seams and a few bulkheads. Tail could bolt on an aft bulkhead and incorporate a tail cone of some sort out of composite. It follows on your HP18 heritage...

    I built an 11' layout table 5' wide (room size limited) in the last month that I can vacuum bag against. So now everything I am thinking of molding fits that envelope until I outgrow it and build a new one. Larger things can always be joggled together and sub assemblied into chunks of a finished design. (My vacuum manifolds, piping, pump, accumulator, switches, yadda all hold vacuum over 24 hours, NICE!)

    If what you are talking about is a reduced parts count and task design I think figuring out the build table and fitting every single task to it is a good way to reconcile it... An airplane that can be finished in large sub assemblies that are brought together in the last month of construction can easily be managed. Rigging that needs to be done to align things can be done on a nice day or clean out the garage. Then reduce back to sub assemblies that are easily stored, hung up, stood vertical in the corner, etc...
     
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  3. Feb 14, 2020 #3

    cluttonfred

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    Very cool, I love seeing someone with your experience tackle the low and slow, fun flyer category. Composites are not my thing for the reasons you mention, but I think this type of design could attract a lot of people.

    On the canopy, when you say Galland, I think Me-109, but I suspect you mean canopy of curved flat sheets as seen on the PIK-26?

    I am curious about the choice of a Rotax 447-582 as a powerplant since only the 582 is still in production. What about optimizing for 35-40 hp for a four-stroke industrial V-twin/Verner Scarlett 3V/half VW) a modern two-stroke like the Polini Thor 250?
     
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  4. Feb 14, 2020 #4

    Jay Kempf

    Jay Kempf

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    And one more thought: while you are arranging the whole front end for the kit builder why not just mold in flat wrapped joggles for the majority of windscreen and side window and then provide a nice door on one side that fits in a nice door jamb The door could wrap up to an overhead strip of carbon roll over protection and have a matching window in it (matching the other side).
     
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  5. Feb 14, 2020 #5

    BoKu

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    Sure, the aft fuselage could be aluminum. But my thinking there is that it's easiest to do good work when all the work uses the same materials and techniques. Rather than make builders learn two entirely different skill sets to the degree necessary for aircraft quality, I'd rather make them learn one skill set and expand it slightly to cope with the field-molded parts.
     
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  6. Feb 14, 2020 #6

    BoKu

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    When you look closely at the topology of the Erla Haube "Galland hood" fitted to the Bf109, you see that it is made of three pieces of simple-curved acrylic. The long center piece transitions through two different simple curvatures, the result being that the whole appears to have some compound curvature, when actually it is all simple curves. It's a pretty neat piece of design that uses materials and processes judiciously.
     
    Last edited: Feb 14, 2020
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  7. Feb 14, 2020 #7

    BoKu

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    The idea there is to make the molded parts as simple and inexpensive as possible. And part of that is making the parts small enough to fit into UPS-shippable cardboard cartons. Making the tub extend above the cockpit rail makes that more difficult.
     
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  8. Feb 15, 2020 #8

    plncraze

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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? I went to Soller after you shared the link for the$20.00 a roll carbon. That's a great deal! Would the insulation sheets from the box stores work as core?
     
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  9. Feb 15, 2020 #9

    Jay Kempf

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    People that own airplanes need to know how to buck rivets. This follows the logic that people that own computers need to be able to use a sliderule. I might be biased.
     
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  10. Feb 15, 2020 #10

    Jay Kempf

    Jay Kempf

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    Yup with joggles you can fit anything in a UPS package.
     
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  11. Feb 15, 2020 #11

    BoKu

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    Here's what I'd do for a typical wing section. The wing would be modestly tapered, because that has relatively little cost in carbon fiber. Although, the full-span aileron or flaperon would be untapered, and the drag spar channel likewise the same height root to tip.

    Carbonmax wing section.jpg

    The pan ribs would be made in particle board molds made of repositionable sections. You adjust the mold to the specs in the table in the manual, make a rib, then readjust it for the next rib. You'd probably have two of these molds so you can make the right and left concurrently.

    The D-tube would be blue styrofoam core. You'd hotwire the three or four sections, then skin the aft face and the notch where the pultrusions go. When that cures, you bond in the pultrusions. Once that cures you sand the upper and lower faces of the spar flush and skin the D-tube with carbon. The airfoil is a fat 23000-ish, probably 16% to 18% thick, so you can easily wrap the carbon around the leading edge without vacuum assist.
     
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  12. Feb 15, 2020 #12

    BoKu

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    I would probably just design to gauge thickness or TLAR and test the proto for compliance.
     
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  13. Feb 15, 2020 #13

    BoKu

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    Yeah, OK, I can understand you might feel that way. But I've got 50 lbs of pop rivets that disagree.
     
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  14. Feb 15, 2020 #14

    Dillpickle

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    You know, I like it! But food for thought. For two whole days at Copperstate, I watched guys crawl on and off my buddies airbike—the same airbike he flew 900 miles one way...in some pretty fierce winds. Old crippled guys like me, big tall guys (6’4”), fat guys, little guys, and people just loved it. It is a whole lot easier to get ON than IN. I recently made a post about finding video on getting in and out of a Legal Eagle, and a kind member provided a link. The Airbike is a lot easier. Step over, sit down, and put your feet up. Skinny airplanes are cute. Erikk has something there. But...what are the issues with carrying a cantilever wing with a skinny fuse?
     
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  15. Feb 15, 2020 #15

    plncraze

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    In your drawing of the wing what does "MFC" mean on the D-tube? Also thank you for contributing this idea. You could be the Ken Rand of the new Millennium.
     
  16. Feb 15, 2020 #16

    BoKu

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    Thanks!

    Apologies, as a tech writer I ought to know to define my TLAs (three-letter acronyms). MFC is moldless foam core composite.
     
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  17. Feb 15, 2020 #17

    pwood66889

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    Well thought through, BoKu. Especially like:
    * making the parts small enough to fit into UPS-shippable cardboard cartons.
    * I would probably just design to gauge thickness or TLAR and test... Though I have no problems with matrix algebra.
    * using only the one material.
    Don't live far enough west, but will cheer you on.
    Keep this idea going!
     
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  18. Feb 15, 2020 #18

    BoKu

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    One more thing about this scheme: One justification for the molded forward fuselage tub is something I learned from the first HP-24 glider, Brad Hill's Tetra: The first thing to do on a kit airplane is make a thing the builder can sit in and make airplane noises. Once you can sit in it, it's much easier to conceive of it as something that will carry you through the sky, and it's easier to stay motivated.
     
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  19. Feb 15, 2020 #19

    pwood66889

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    "Once you can sit in it, it's much easier to conceive of it as something that will carry you through the sky"
    Another excellent thought.
     
  20. Feb 15, 2020 #20

    Hot Wings

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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/knowle...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."


     

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