Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Topaz, Sep 10, 2014.

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  1. Sep 10, 2014 #1

    Topaz

    Topaz

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    A couple of days ago, over in the Cheap Air Racing Class thread, I basically volunteered nerobro to share his design process "out in the open" on the boards. He accepted the challenge and he's moving forward with his design in the Designing the Cheap Air Racer thread. Definitely take a moment and check that out.

    In fairness, I've decided to put my time and money where my mouth is, and do one myself. Matt G. put it best:

    I'm going to start and maintain two threads. The main thread, in the "Build Log" section, will show the work on the project. Build Log threads are locked to everyone but the OP, so I'll be the only one posting there. This thread, in the Aircraft Design sub-forum, will exist so that I can talk with you folks about the project, ask for your advice, and answer your questions. I hope you'll learn some things, and I hope you'll teach me some things.

    Thanks for your interest!
     
  2. Sep 10, 2014 #2

    Jay Kempf

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    Excellent. Needs to be done.

    Can't way to see how it develops. I know how I would proceed. The first is the definition of inexpensive and how good of a glider it will try to be.
     
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  3. Sep 11, 2014 #3

    Matt G.

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    I've been thinking the same thing. I suppose I should follow your lead and try to collect enough of my thoughts to start a thread.

    Hopefully when the soaring season is over I will have time. :)
     
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  4. Sep 11, 2014 #4

    BBerson

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    I would like to see the material list for the "$500 glider". Must be covered with cheap plastic or something.

    Also, a link between these threads should be on each post, or we will get lost.
     
  5. Sep 11, 2014 #5

    proppastie

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    whats it made of, how much will it weigh, how much a pound for the material, add in instrument cost, and engine. whats the problem
     
  6. Sep 11, 2014 #6

    Brian Clayton

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    There was that russian? fellow that popped on here last year with a basic glider he built himself. It was bare bones, couldnt have been that expensive or difficult to build.
     
  7. Sep 11, 2014 #7

    Topaz

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    That's exactly what this is all about. There's a well-defined process for getting from "dream" to that point.
     
  8. Sep 11, 2014 #8

    Topaz

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    That's a good idea. I'll try and remember to do that.
     
  9. Sep 11, 2014 #9

    proppastie

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    so you are talking about more than expense, Would the title be better as Conceptual Design......as I see it the expense is a direct function of the weight and materials, and I do not see any mystery there.
     
  10. Sep 11, 2014 #10

    Topaz

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    You can't do a weight * cost of materials calculation until you know the weight of the materials - which means that you need to know the exact size of all the parts in the airplane. Which depends on the requirements, reflected through the process of design.

    How about letting me actually get into the process before tearing it down as needless?
     
    cheapracer, Apollo, nerobro and 3 others like this.
  11. Sep 11, 2014 #11

    Victor Bravo

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    I'd be very thankful if someone (who has the background and engineering ability) was going to go through an exercise like this out in public. The opportunity for a cheapskate balsa basher like myself to learn more about real aero engineering, just for tuning in to this forum, is just fantastic. As a soaring pilot, the concept of an inexpensive soaring machine of any type is worthwhile.

    As a guy with a powerplane half torn apart up on blocks for almost two years, looking out at the August cumulus clouds floating over the mountains only 3 or 4 miles away from my home 'drome... laughing at me... it's maddening not having a sailplane :)

    If your selection/design/materials/cost process happens to yield a cleaner, lower parts count, faster build, and higher performance version of the GOAT - I'll buy plan set #1 cash on the barrelhead.
     
  12. Sep 11, 2014 #12

    Pops

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    And the greatest tool for setting your wing is a good digital level. Don't leave home without it. Dan
     
  13. Sep 11, 2014 #13

    Jay Kempf

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    What is the real world cost and time to build a goat? The electric assist version of these sorts of gliders is pretty impressive. A slightly rearranged version for more performance (L/D) would be a pretty good target. The glider on a stick method of construction ALA Strojnick S2A with an attention to simplification could be another benchmark. Those are real world examples of existing airframes that probably bracket the ends of the cost range and maybe the performance range as well. Of course I am partial to twin booms to facilitate a pusher folder.
     
  14. Sep 11, 2014 #14

    bmcj

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    To that end, you might browse the threads in the design/aerodynamics section for educational discussions: Aircraft Design / Aerodynamics / New Technology

    Pay particular attention to the "sticky" threads at the top of each forum... they have been deemed to have extra value to all members.
     
  15. Sep 11, 2014 #15

    Hot Wings

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    If you are going electric trade the twin booms for twin pusher motors. If you're worried about asymmetrical thrust just cant the motors in so the thrust line makes a "motor out" a non-event.
     
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  16. Sep 11, 2014 #16

    proppastie

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    Sorry its your show, I need to engage brain before typing, I will be looking forward to your stress so you can size the parts to get the weight. that is were I am stuck.
     
  17. Sep 11, 2014 #17

    Topaz

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    No worries at all.

    Unfortunately, it's not my plan to be getting into loads and stress analysis for this study. If I decide that I just can't live without building this airplane, obviously I'll have to do that, and structures aren't something I'm strong at (yet), so I'll be learning along the way just like you. IIRC, you seemed more stuck about how to get the loads analysis done, so that you knew what loads were going into the piece of structure you're wanting to analyze. It might be profitable to pick up a copy of FAR 23 from the Government Printing Office, or review the Structure section here. Nice workups on developing loads for a given aircraft.

    Just for everyone's reference, Raymer puts forward a good example of the difference between the Conceptual, Preliminary, and Detailed design phases, using the wing spar as an example, in his Figure 2.3:

    overview.jpg

    For Conceptual design, we're concerned with coming up with a configuration and a viable overall concept that meets the mission requirements. Preliminary design "can be said to begin when the major changes are over. The big questions such as whether to use a canard or an aft tail have been resolved. The configuration arrangement can be expected to remain about as shown on current drawings, although minor revisions may occur. ... During preliminary design the specialists in areas such as structures, landing gear, and control systems will design and analyze their portion of the aircraft." "Assuming a favorable decision for entering full-scale development, the Detail design phase begins in which the actual pieces to be fabricated are designed. ... For example, during conceptual and preliminary design the wing box will be designed and analyzed as a whole. During detail design, that whole will be broken down into individual ribs, spars, and skins, each of which will be separately designed and analyzed." - Raymer, Aircraft Design: A Conceptual Approach, Third Edition, pp. 4-7

    That's geared towards commercial design and production of larger aircraft, but it gets the idea of the kind of work in each phase across rather well. I'm going to be doing a conceptual design study only. I'll put up the first new post over lunch today.
     
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  18. Sep 11, 2014 #18

    Topaz

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    Rather than copy and paste every time I post, I put the links in my sig line. How's that work?
     
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  19. Sep 11, 2014 #19

    Hot Wings

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    Is this really appropriate for a one of a kind homebuilt? In the commercial setting this may be the most efficient from a corporate point of view but what happens if at the preliminary design stage if you discover that one of the assumptions isn't going to be as cheap or straightforward as expected. The big boys can throw money and time at the problem to force the out of round design into the round hole. We don't have that luxury and if the budget is set up for common materials and methods but it turns out that only a Titanium forging is going to work for a major item then we have to go back and start over.

    Experience with the process does let you develop some "feel" for what will work but if you already have that "feel" then you probably have a pretty good handle on your process?

    I'm not intending to second guessing your plan, just posing a question.
     
  20. Sep 11, 2014 #20

    Topaz

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    Then the conceptual phase wasn't completed properly, in a nutshell. That's a flippant answer, but it's still strictly true. Part of the conceptual phase is researching materials and other questions about the requirements and design sufficiently that you are confident in your assumptions. That's probably going to become more evident as I start developing the requirements set over the next few days. I'm sure some people will find it intolerably slow, but I think it's essential to getting a good machine out the other end of the process.

    The reason I take this approach over something else is that bad assumptions are a danger with any approach - and I consider them an even larger danger with some of the other design methods, where you don't know for sure that the aircraft will meet the requirements. In Raymer's approach (which is similar to what Orion used, nearly identical to Roskam's, etc.) when you're finished with the conceptual phase, you have a specific geometry of aircraft that, if your research during the process was sufficient, you know meets the mission requirements and is optimized for them. Even John Roncz's method from "Designing Your Homebuilt" skips the sizing and optimization steps completely, and whether or not the aircraft can actually meet the requirements is unknown until you start flight testing.

    Another advantage, I think, is that Raymer's method supplies the information you need for each phase simply by looking back at the previous one. Conceptual feeds geometry and aero information that you need to do the loads analysis in Preliminary. Preliminary provides the more-detailed geometry that you need to start breaking assemblies apart into individual pieces for final design and drawings. It's a more rational and organized method, IMHO.

    It's a completely appropriate question. As I said in my preamble, this is a method that I choose to use personally. I make no presumption that it's the best method. I'm sure someone else could do the job in a different manner. The story I've heard (possibly apocryphal) is that the original Bower's Flybaby started out as a chalk outline on the garage floor. But then, Al Bowers was already a heck of an aerospace engineer by that time...
     
    Last edited: Sep 11, 2014

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