Duncan's got a Flea

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Aug 3, 2017.

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  1. Aug 3, 2017 #1

    rtfm

    rtfm

    rtfm

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    Hi,
    Well, having had to set aside my Aeromax build for lack of funds to buy the next sub-kit, (which isn't cheap), I got re-inspired by all the talk about Fritz' Piojo. I've always liked the Pou's, so I've been reading as widely as possible, collecting design rules-of-thumb, trying to intersect these with academic reports, plugging numbers into spreadsheets - and playing around in Sketchup.

    I have arrived at a little design which I think will be great. I worked hard at Fritz' concept of a sit-on-box - but couldn't get the bloody thing to work the way I wanted. So I reverted to a sit-in fuse. Tri-gear. Basic wing layout taken directly from the HM-293 - but I changed the wing plan a bit to look more "modern".

    I decided that if I were to build the little plane, I would need to gather some hands-on building experience, so I grabbed some 3mm MDF sheets I had in the workshop, and together with some 20mm x 20mm pine, began building a full-size "proof of concept" or practice fuselage. Just as well I was using cheap materials, because I wasted quite a lot of material making mistakes. But I was sufficiently happy with the results to take some of my meagre funds and I ordered a few sheets of 1.5mm AA grade plywood, and some lengths of 19mm x 19mm Hoop Pine.

    I'm waiting for the wood to arrive (2 week delay). So, this morning I resurrected my copy of DevMold, and spent a pleasant hour or two developing a mold from my Sketchup files. And this is what I have so far:

    DevFuse 1.gif DevFuse2.gif DevFuse3.jpg DevFuse5.gif

    I strongly doubt that a composite fuselage will be as light as a wooden one, but I was intrigued by the capability of the software. These are STL files. However, I have some terribly basic questions regarding STL files, and what one does with them. It strikes me that I may be able to use just the turtledeck with the wing cut-out as well as the cowl area and fix these onto the wooden fuselage. Dunno.

    Question: OK, so now I have pretty looking STL files. What do I do with them? Take them so someone with a - what? - milling machine? CNC router? How does that work? Do I have to slice the thing into pieces and get each CNC'd separately? I have absolutely no idea what one does next.

    Help?

    Duncan
     
  2. Aug 3, 2017 #2

    Dana

    Dana

    Dana

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    What happened to the Razorback project?

    STL (STereoLithography) files aren't terribly useful. All they are is a collection of polygons representing an approximate surface, intended to go into a stereolithography machine or 3D printer, they're also used by ray tracing and rendering programs. CNC machines generally require a true surface file containing analytic or spine surfaces.

    Dana
     
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  3. Aug 3, 2017 #3

    rtfm

    rtfm

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    Hi,
    Thanks for that. DevFus has a function to "Create Molds from a STL file" - so perhaps that produces true surfaces? Maybe...

    The Razorback project came to a sudden and calamatous end in a huge hail storm some time back. The almost finished plug was outside at the time of the sudden storm, and with hail stones bigger than eggs, I couldn't venture out to retrieve the plug for fear of getting brained! It was all over in seven or eight minutes, but the damage had been done. The plug was ruined, and I just didn't have the heart of begin again.

    I then went into a long period of just spinning my wheels, trying to muster the enthusiasm to start again - and eventually bought the first of the sub-kits for the Aeromax. I've built the wings, but with no work since Sep last year, I don't have the funds to buy the next sub-kit.

    So it's all a matter of messing around on the laptop, and fiddling in the workshop with spare bits and pieces.

    Duncan
     
  4. Aug 3, 2017 #4

    FritzW

    FritzW

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    Looks good Duncan.

    STL files are probably the most common files used for 3D CNC work, which is what you'd be doing (as opposed to 2.5D like you'd use for cutting out ply).

    You'll have to slice your model into separate parts that will fit under the Z axis of the CNC machine and shapes that a 3 axis machine tool can get to ...unless you know someone with a 4 axis machine. If you can find a machine with enough Z axis travel it looks like you could get away with just slicing your model in half and cutting the left and right sides separately.

    You, or your CNC guy, will just bring up the STL file in their CAM software and lay a MOP (Machine Operation) on it and save it as an .NC file that will run the machine.
     
  5. Aug 3, 2017 #5

    rtfm

    rtfm

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    So, if I can get the software to produce true surfaces, what then? What sort of shop will be able to cut this for me? Or do I slice it and cut the pieces on my CNC hotwire? This is new territory for me, and I don't even have the vocabulary to ask the right questions.

    Duncan
     
  6. Aug 3, 2017 #6

    rtfm

    rtfm

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    RTFM-Flea profile2.gif
    Anyway, here's what I'm currently putting together out of MDF. Just the boat, mind you. And just to check construction methods.

    Duncan
     
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  7. Aug 3, 2017 #7

    FritzW

    FritzW

    FritzW

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    Your STL files are fine, STL's are what you want for what your doing.

    Even a sleepy little hamlet like Brisbane is bound to have a few CNC shops (I saw about 50 on a quick google search). I'll ask around on the CNC forums for a hobbyist in your area, it might save you a few bucks.

    1) find a guy to do the work
    2) find out what size material he can handle (hight, width and length)
    3) slice up your STL model the way YOU want it sliced so it'll will fit on his machine
    4) send him the STL files and drop off the foam
    5) come back a few days later and pick up your parts
     
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  8. Aug 3, 2017 #8

    rtfm

    rtfm

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    Hi Fritz,
    That easy? Wow. That's great news - but for the moment, I'm going with the plywood version. Not as pretty, but cheap as chips. And if someone wants to make one for him/her self, doing so out of wood will be the only way to go. But the composite fuse sure is tempting... Definitely something to consider once I get some funds.

    Thanks for your help guys,
    Duncan
     
  9. Aug 3, 2017 #9

    Victor Bravo

    Victor Bravo

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    Absolutely go for it Duncan! Wood, plastic, metal, steel tube... whatever suits youur needs and your skills is the right material.
     
  10. Aug 3, 2017 #10

    Dana

    Dana

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    I didn't know modern CNC machines could read STL files. Everybody I work with wants SAT or STEP files, which are true solid models.

    The thing to watch with STL is the polygon size. As it's an approximation of the surface using thousands of flat polygons, the polygon size (usually a program setting) is important... too large and you get a rough visibly faceted surface, too small and the file size becomes unmanageable.

    Dana
     
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  11. Aug 3, 2017 #11

    wsimpso1

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    I am way more accustomed to .stp for solids, tooling, etc.
     
  12. Aug 4, 2017 #12

    Dana

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    Looks like the only CAD formats Sketchup supports are dwg and dxf, and then only in the $700 "pro" version. I guess Google's not interested in interoperability with the mainstream CAD world. But stl files support the 3D printer market, and that's probably a far larger market than mainstream CAD. But if a CNC machine can import stl files, then I guess that's all you need.

    How you prepare the models depends on how they're going to be used. If I understand correctly, you're using wood stringers over plywood bulkheads, correct? If so you don't need to export surfaces; you would take slices at each bulkhead to generate the 2D flat patterns, and send those to the CNC... but I don't know what format you'd use since stl is a 3D format. If you can't export the 2D slices, then the CNC software would have to be able to make slices from the imported stl file at locations you'd specify. Depending on the software the CNC guy uses, that may or may not be possible.

    Dana
     
  13. Aug 4, 2017 #13

    Topaz

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    Sketchup was spun off from Google a few years back. It's owned by Trimble now. Not one of Google's properties anymore.
     
  14. Aug 4, 2017 #14

    FritzW

    FritzW

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    If you just want to cut out plywood bulkheads, DXF files are the way to go.

    ...just some semantics:

    2D CNC = things like bubble jet printers and pen plotters.

    2.5D CNC = things like cutting out ply bulkheads. The "half" in 2.5D is because you control the Z axis (ironically, 3D printers are only 2.5D CNC machines).

    3D CNC = things like cutting out full 3D stuff like fancy carvings on raised panels, or 3D fuselages. The machine has to "hit" every single X,Y,Z point. ...sort of raster vs. vector

    So a typical CNC machine can do 2.5D and 3D jobs.

    The machines (printers, routers etc.) don't except DXF or STL files. The CAM software does and adds machine control information and exports .NC files that the machines control software uses to run the machine.

    I'm not being a wise guy, just clearing up how CNC typically works.
     
  15. Aug 4, 2017 #15

    rtfm

    rtfm

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    Hi guys,
    It's all basically a foreign language to me, but it's good to know there are people I can call on if/when I get there.

    What I did was to use the DevMold software to model the fuse. It still has some touching up to do, but it is basically finished. HOWEVER, that project is going to have to wait. I did it more as an exercise than anything else.

    What I did this morning was to carefully study the HM-290 plans (in English) and figure out how to use the plans to construct my own fuselage using MDF and pine strips - still in the process of figuring out how to build - not actually building the plane yet. That's still a couple of weeks away when the wood arrives. I've got my four bulkheads sorted, plus the vertical keel done. So now its off to the workshop to cut me some wood. My first attempt proved to be very fiddly, and I wasted a lot of (cheap) wood. I wasn't too happy with the results, so I'm trying it again. The process has to be simple, and easily repeatable for it to work. My first attempt wasn't, on either count.

    Regards,
    Duncan
     
  16. Aug 4, 2017 #16

    deskpilot

    deskpilot

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    There's a download for Trimble SketchUp which enables the export of .stl files for use with a 3D printer. Just go to their file warehouse.
     
  17. Aug 5, 2017 #17

    cheapracer

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    There are numbers of 3rd Party "File Export" addons, and also independent conversion programs out there.

    Sometimes need to be careful between conversions with what scale it comes out the other end.


    That's what I do often right now, draw a 2D, or a slice, in Sketchup, export it to .dwg*, and then clean it up in Librecad 2D (Sketchup does not do true circles unfortunately). The local CNC Lasers all accept .dwg/.dxf no problems.

    If you're fond of Sketchup then Libercad 2D performs the closest in selection techniques, could have been made by the same people type usage feeling.


    * to export a .dwg out of Sketchup, you must lay the shape flat (perpendicular to the vertical blue Z axis).

    Select Camera drop down, Standard Views, Top.

    Tick Parallel Projection.

    Select File drop down, Export, 2D Graphic, select .dwg or .dxf.
     
    Last edited: Aug 5, 2017
  18. Aug 6, 2017 #18

    Thunderchook

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    Wouldn't another title for this thread be "Duncan takes a pou!"
     
  19. Aug 6, 2017 #19

    Thunderchook

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    I too occasionally have my Pou du Ciel obsession.

    I even met up with Patrick Martin who has an HM290/3 (an HM290 fuselage with a 293 wing) but, sadly, I found it a little snug.
    The seat made me feel like I was balancing on a three-legged stool, with my knees around my ears.
    It might be fun for maybe half an hour until the wobbly seating arrangement started to annoy cr@p outa me.

    Also, there's the whole 2 axis thing which I can't get past (Yeah, yeah, I know - Cossandy flaps.)

    I'd be interested in a Pou designed with larger pilots in mind with a better seating arrangement, and a full three axis control.
    I know that this get brought up often but never goes anywhere.

    Duncan, are you planning either of these with this design?

    Thanks,

    Thunderchook
     
  20. Aug 6, 2017 #20

    rtfm

    rtfm

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    Hey Thunderchook,
    Hi. My design will be 22 inches wide at the shoulders (when I can figure out how to print full size templates)... Actually, that's 22 inches around the chect, since it is an open top plane. So quite roomy.

    RTFM-Rangipou profile.jpg

    I'm still unsure about ailerons/Cosandy flaps. One of the attractive things about the Pou's is their simplicity - both to build and to fly. That, and their inability to get crossed up and go into a spin. Adding ailerons/Cossandy flaps to the rear wing makes spins possible. But is that such a terrible thing? Millions of planes have this setup. What niggles me most is the fact that I've never flown a Flea, and don't actually know what a Flea feels like in a turn. If it skids and mushes around corners, I don't think I'll be a fan. Anyone care to enlighten me?

    As far as seating is concerned, I'll be fitting an ergonomic seat with "memory foam". I like to be comfortable when I fly.

    Also - I've decided to call my Pou a "Rangi-po" (Rangi is Maori for "Sky"). Also, the Black Gate of Mordor scenes were shot in the Rangipo Desert of New Zealand, and "Rangipo" was the name of a champion Australian race horse trained in New Zealand. So, on a number of levels (both Aussie and Kiwi), "Rangi-po" is a great name for a Aussie/New Zealand bred Pou... And if you add in the ANZAC/French crossover, "Rangi-po" is a "Sky-Flea".

    The 19mm x 19mm Hoop Pine for the longerons arrived on Friday. Now just waiting for the 1.5mm plywood. In the meantime, I have 3mm MDF and 18mm x 18mm el-cheapo pine from Bunnings in the shop, together with some 9mm MDF for the bulkheads. As soon as I can figure out how to print full-size templates from Sketchup, I'll be cutting and trial-fitting the parts.

    Regards,
    Duncan
     
    Last edited: Aug 6, 2017

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