3D Cad (Lesson 101)

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by superjacent, Jun 19, 2009.

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  1. Jun 19, 2009 #1

    superjacent

    superjacent

    superjacent

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    I'm new to 3d (and 2d) CAD. I'm interested in learning and how I'll learn is by inputting my Cozy plans (when I get them) into a CAD file. This will be for my own benefit (learning).

    I was following this thread and many CAD programs were mentioned and some technical jargon used to describe certain features which at this present time is foreign to me. So in simple terms the following is what I would want from a 3d CAD program.

    I'm assuming that sub-parts are created ie bulkheads and accordingly dimensioned (inches/cm). Other parts created, dimensioned. Then along the way connect all the sub-parts to eventually end up with a complete airframe. I'm also assuming that it's possible to work backwards in that a fuselage is created, then point and click to a spot for an automatically dimensioned bulkhead to be inserted only for some fine-tuning maybe (thickness comes to mind).

    From a completed file, I'm also assuming that if a change is required, say widen the cabin by 5" that all the other parts are accordingly re-sized to suit.

    Specify material type.

    Print out accurate 2d drawings of any given sub-part.

    Lastly, making the airframe into a solid, skin it, colour it etc.

    I'm thinking (hoping) that the above is standard 3d CAD stuff. As I mentioned I'm new to this and so I'm not sure whether my assumptions equate to basic, intermediate or top end CAD programs.
     
  2. Jun 19, 2009 #2

    Tom Kay

    Tom Kay

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    Hi Steve;

    I'm literally a few weeks ahead of you in the 3D learning campaign, so I can't answer all of your questions, nor can I really give a fair comparison between various software systems. What I can do is tell you that in the short few sessions I've had with Rhino Version 4 (demo) I've learned a ton about its usefulness and how to put together the skeleton of an airframe on my screen. I have not played with skinning the frame yet, and I'm sure that will be a challenge.

    But I've approached the airframe exactly the way you just mentioned, albiet using lengths of tubes, not fuselage frames (bulkheads). However, the bulkheads, assuming they're flanged metal, would be created the same way a metal wing rib is made, so I see no major challenge in doing them. Can you resize the fuselage by 5", especially in the middle? Not sure, but I suspect there will be some individual re-sizing of each frame to get the size you want, then adjustment of the longerons and stringers. It would be great if I'm wrong about that.

    Another good thing about Rhino is the support available on the web, including a news group that has given me almost immediate answers to questions that were really important for me to continue. Lots of tutorials, and stuff to keep you occupied with all kinds of drawings, not just airframes.

    I can also say that I wish I'd started 3D years ago, and I find it very exciting to watch a complicated structure with hundreds of parts rotate in front of my eyes, to be viewed as if I were standing there observing it.

    The journey seems very worth it so far for me. Probably for any aircraft enthusiast/designer.

    Good luck. Tom.
     
  3. Jun 19, 2009 #3

    Mac790

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    Steve,

    Yes it's possible and relatively easy with programms like Solidworks etc, sorry but I don't have experience with cheaper substitutes. For example in Solidworks and in many other engineering softwares to achieve it, you have to create your parts in an assembly module rather than in a single part module. You can use different methods, for example you can extrude part up to surface (longeron) pic1, in result you will receive a perfect connection between parts pic2, every time you change your outside shape inner parts resize to suit it. Of course there are many other methods. The assembly module will let you save each part separately, so no problem with creating 2D drawings for each part separately.

    Seb
     

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  4. Jun 19, 2009 #4

    Dana

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    Exactly how you approach the modeling depends on the software you're using, but in general, you don't create and dimension parts and then put them together into an assembly. Rather, you model all of the individual components in the context of the assembly, modifying and tweaking each part as you go along based on its relationship with other parts.

    In some programs, like KeyCreator, you create individual components on separate "layers", which can be turned on or off for clarity as you're working with different areas. In others, like Solidworks, each part is a separate file, with an assembly file which displays all of them together.

    Modifications also differ depending on the program. I the parametric modelers like Solidworks, parts are tied together so modifying one can automatically adjust the adjoining part; in other programs you must make changes to each part individually (though you can "window select" groups of parts. However, often the time required to set up the inter-part relationships in the parametric modelers can take longer than manually making the changes later... it depends on what you're doing and your style of designing.

    Either way, dimensioning the drawings is the last thing you do, after you're satisfied with the 3D model... not that you won't revise them later after you start building the "real" parts.

    It's a little different if you're copying an existing design and inputting the parts exactly as somebody else designed them. There, you may well create each part separately and assembly the model afterwards... but generally it's still better to create the model as an assembly, so you can adjust things that don't quite fit (if the original plans were hand drawn, this is certain to be the case).

    -Dana

    The family that shoots together..... shouldn't be messed with!
     
  5. Jun 19, 2009 #5

    Tom Kay

    Tom Kay

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    Hi Steve;

    You got me thinking about whether a quick change could be made in Rhino (and I'm sure, other modelling programs) to widen the fuselage by 5" or so. So I did a quick sketch.

    It's basically a canoe, or at least the framework. I didn't jot down the actual dimensions, but it's comparative, so here goes.

    The 2 pictures are a wireframe sketch and rendered picture of canoe ribs and obviously the left one is a slim canoe, and the right one is wider. To at least partly answer your question, all I did to get from the slim canoe to the wider one is highlight all parts, then expand the x axis by a factor, say 1.5, which you can do easily in Rhino.

    If I had drawn longerons, stringers, and other framework, all of those parts, if highlighted, would have "gone along for the ride."
    They would have been widened with everything else. Perhaps a downside to that, is increasing the thickness of a skin or stringer, from a nominal size to something that isn't even available from the aluminum store. That would happen, given the method I used for this quickie sketch. But hopefully, as I get further into this, I'll find remedies for that sort of problem.

    Does this demo help? Widen your cabin with all associated parts, I mean?

    Cheers, Tom.
     

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  6. Jun 19, 2009 #6

    orion

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    One other area where designers get into trouble is in tolerancing. Simply said, just because it's in the computer doesn't mean that that's the way it'll go together in the real world. Even machined metal parts will have tolerance issues, especially when considering the stack-up of a number of toleranced parts, assembled into the whole.

    And all this really gets really tricky when you start working with composites. Here you not only have looser tolerance characteristics of the parts but now you also have issues with tooling, bond line thicknesses, tolerances as a function of material choice and of course, the quality of the build.

    It is nice to have a stable and well referenced computerized database but it's not unusual to find yourself going back to the original models and making changes as you do the build, as each time you realize that the part you designed will not fit within the constraints of the actual surrounding hardware. True, if you're doing a one-off it's not as critical but if you're serious about doing this so that you learn all that's involved within the design process, you'll probably find that it's a lot more work than you think.

    And one additional note regarding what you posted above: It's virtually impossible to go and simply scale a fuselage to make it a few inches wider. If you scale it as a whole then you also change areas that you maybe wanted to stay the same. Also, if you scale it wider and you have set cross sections (like an interface to a prop spinner), then obviously that fixed section will now be an distorted from the original. In order to incorporate a change like this you'll have to go back in and redo your body surface loft based on new cross sections and control curves. It is therefore important that you have the outer loft of the airplane done and set in stone prior to defining the internal structure and component layouts.

    It will also be important for you to understand at least the basis of some of the math behind the geometry. In that way you'll better understand what the software can and cannot create and why sometimes you'll get shapes that make absolutely no sense. Remember, in CAD, the package will always do what you tell it to do as a function of its own rules, not what you want it to do. Understanding the difference will make your work that much easier.

    In brief terms, the CAD process i use is as follows:

    1) Design what you want the airplane to look like. The initial aspect of airplane design is mostly artistic - it has to meet your personal aesthetic goals. This can be done in 2D on the computer but I've found that I work better with pencil and paper. The drawing can then get scanned into the CAD system and further tweaked.

    2) From the basic lines you create the longitudinal control curves, along with several key cross sections. These are then lofted into an overall surface. This make take numerous iterations as you learn how the software thinks. Most lofting is fairly straight forward but there are difficult areas that usually take some time to get right. Some of the most difficult parts to loft include the engine cowl and even simpler things like wing tips - the latter especially so if you're after creating a very specific shape.

    3) You then to the same for the flight surfaces, after which you'll start in on your control surfaces, doors, windows, cowl part lines, and other components that involve cutting the outer shell.

    4) Next you start in on the internal components. This is often also combined with the structural analysis phase where you'll need to define gauges of materials before you can start in on the details of the internal parts.

    5) Only when the loft and the structure are defined can you start in on the other details. At this point much of your design work must also consider the manufacturing of the parts since that will involve things like offset flanges, alignment geometries, door frames, and on and on. And as you design your structure, also don't forget your control runs, access panels and room to get your hands in on internal parts that may need maintenance, inspection or replacement. Nothing more embarrassing than having to cut a hole in your wing just because you forgot that you have to get back at your fuel sensor or main gear bolts. these were easy to install while the parts were being built but now that the whole structure is closed off, getting back in may be a major undertaking.

    6) And so, most importantly, learn to think at least two or three steps ahead - that way you wont have to backtrack as much.
     
  7. Jun 19, 2009 #7

    superjacent

    superjacent

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    Thanks everyone for the advice, comprehensive comments indeed.

    I suppose I was looking at the process of creation in a backwards sense since I intend to copy existing plans, as-is, so to speak. Even so, as now pointed out, I suppose even in a copying process starting from the shell and drilling down is the way to go.

    As regards the 'make the cabin 5" wider' aspect and all sub-parts changing is doable but obviously requires thought behind the process. Though I didn't mention it in my initial post I was thinking more along the lines of selecting an area (group of controls ie. cabin area) scale up a certain percentage or actual measurement so only that area affected. After the change then fix up the components forward and aft.

    No doubt when I decide upon a particular software package this will (should) become clearer.
     
  8. Jun 21, 2009 #8

    lr27

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    I agree with what Dana said and I suspect I'd agree with what Orion said if I had the relevant experience.

    I've used 3D CAD a lot. Most recently Solidworks. The more you use the bells and whistles the more chance you have to activate the bugs and also bog down your computer. They may be worth it, but it takes a lot of time to learn that stuff. Probably creating the Cozy in CAD will be a good exercise, but start with the simple shapes before you get into lofting, surfacing, etc.

    If you start scaling things, you'll probably uncover lots of things that will need to be fiddled with to make them work, but then that's probably easier doing with just electrons rather than with epoxy and glass.

    Way back when I started to use 3D, I think it was quite a while before I was more productive with 3D than with 2D. However, if you don't have a lot of 2D experience, this may not be a relevant factor.

    I used to use CoCreate and I wonder if it's less likely to blow up your computer than Solidworks is, and if it's less of a drain on resources. If it's cheap, it might be worth a try. I suspect the free version will be too limited for an entire Cozy. Of course I have no idea how long PTC will support it, now that they've acquired it. The other thing is that hardly anyone uses it any more. I may be biased about it's reliability as I mostly used it on Unix workstations, so it may be the operating system that kept it from crashing so much.
     
  9. Dec 17, 2009 #9

    Woodenwings

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    Some word of warning for SolidWorks - get a SolidWorks approved video card with no less than 512K of ram. If the video card is adequate the computer won't bog down at all. I have an entire airframe and components modeled with occupants and I’m fine on my laptop.

    I love SolidWorks! It is very easy to use and hasn't any bugs compared to inventor or solid edge. I also like the fact that the rules are the same whether you are in part, assembly, or drawing mode (or simulation mode!). You can also run a virtual wind tunnel which is pretty useful. Cosmol is another great simulation tool. I save a lot of time with SW because it is easy to use and powerful.

    Weight and balance is also very easy! CLICK! I have verified actual assemblies that weigh thousands of pounds to within a single cubic inch of where the computer predicted. I have great confidence in placing lifting lugs EXACTLY where they need to be and it is great for checking tool clearance and accessibility. I also model the paint as a separate part/s and can see how painting the structure will affect the weight and balance. WOW 135 lbs of paint! Darn!

    It is a journey well worth the effort!
    Good luck!
     
  10. Dec 17, 2009 #10

    GarandOwner

    GarandOwner

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    Each CAM (Computer Aided Modeling) program is different. While I am not familiar with the program you are working with I am fluent in CATIA. If some of the same features are available in the software you are using, you can use knowlegeware to link some of the values so that if you change one all associated values will change as well.

    I am sure you will have to create each part individually, then use some kind of assembly design to constrain the parts together. Depending on the program, it may make the drawings for you, or it make create the solid model from your drawings.

    The easiest way for you to start would most likely either try and create a single solid that mimics the final model. That way you can dimension the finished product right off the bat. Once you get more familiar with the software you can go back and create each individual part. Be aware that it is a LONG process. Don’t get discouraged, the more practice the better you get. GOOD LUCK!!
     
  11. Dec 17, 2009 #11

    Woodenwings

    Woodenwings

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    Hi yeah I’ve been using the 3D cadd stuff for about ten years. I’m pretty ok with it all.

    I model the entire airframe approximately based on predetermined standard components. Then I run some simulations to see if there is any naughty flow, check for cooling distribution and sizing of control surfaces. If it looks ok I then get down to creating an assembly of parts linked to the original single model. I will break the links when I’m happy and fully define components/sub assemblies to each other or without links.

    The latest project can be seen here. This is an older version of the current design.

    Propulsive Wing Projects: Albatross Airplane
     
  12. Dec 17, 2009 #12

    flywulf

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    Hi all,

    I have been trying to teach myself to do 3D modeling using autocad 2K. Just using the tutorials in the program and a book I bought that is written specifically about this version of the program. To make a long story short, I have been working with it for over a year, periodically in my spare time and can not produce anything like the canoe picture in this thread. I can produce 2D drawings no problem. Does anyone here use this program?

    :lick:

    thanks,

    Ed
     
  13. Dec 17, 2009 #13

    Mac790

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    I'm afraid that you have to try another software, autocad is great for 2-D drawings but not for 3-D, of course it's possible, but it's really not the software for the 3-D job.

    Seb
     
  14. Dec 17, 2009 #14

    Woodenwings

    Woodenwings

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    [FONT=&quot]I learned 3d in AutoCAD first. It was dreadful! Don’t feel bad that you have been struggling. As a 2d program it is still good - possibly the best still produced. There have been better ones but I can't see them in use anymore.

    Get a free 150 day trial for Solidworks and enjoy the learning process. I teach Solidworks to kids after school and they are eating it up. They don't know any engineering stuff and still have fun. Find out who distributes SW in your area and give them a call. They will probably be very helpful.

    Feel free to ask me directly any questions you may have re: Acad (if I still remember how to do it.).

    Good luck![/FONT]
     
  15. Dec 17, 2009 #15

    AVI

    AVI

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    Quite the understatement, eh?
     
  16. Dec 17, 2009 #16

    Woodenwings

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    Engine cowls are hard for lots of reasons! not to mention how t odirect airflow to actually cool the engine properly! "No.6 cylinder overheating???"

    Feel free to post any autocad drawings and i can try to loft them 4 U. if it helps.

    WW
     
  17. Dec 18, 2009 #17

    AVI

    AVI

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    Thanks so much. TO is not that far away so perhaps I will call upon you. Thanks for the offer. I will PM.
    Meanwhile, the approach that seems to be working best for me is the same one that Orion uses - pencil and paper!
     
  18. Dec 18, 2009 #18

    Dana

    Dana

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    Autocrap, shudder! Although it can do 3d... sort of... it's an exercise in masochism.

    As much as I like KeyCreator if I was looking at starting from scratch with a 3D design package, I'd probably get a copy of PTC's CoCreate. They were offering a free downloadable version for personal use, though I don't know if it's still available.

    -Dana

    First Law of Debate:
    Never argue with a fool-- people might not know the difference.
     
  19. Dec 18, 2009 #19

    KeithO

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  20. Dec 18, 2009 #20

    GarandOwner

    GarandOwner

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    While I do love AutoCAD above anything else for drafting, it is mostly designed to be just that, a CAD program. If you want something to do 3-D you are better off moving to a program that was designed with modeling in mind (a CAM program) AutoCAD, although used by some mechanical engineering firms, is more tailored for the architecture industry.
     

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