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

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

    Quote Originally Posted by Topaz View Post
    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?
    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.

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

    Quote Originally Posted by proppastie View Post
    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.
    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:

    Click image for larger version. 

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    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.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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

    Quote Originally Posted by BBerson View Post
    ...Also, a link between these threads should be on each post, or we will get lost.
    Rather than copy and paste every time I post, I put the links in my sig line. How's that work?
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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

    Quote Originally Posted by Topaz View Post

    That's geared towards commercial design and production of larger aircraft,
    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.
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

    "--and pompous fools drive me up the wall. Ordinary fools are all right; you can talk to them, and try to help them out."
    Richard P. Feynman

    “Without deviation from the norm, progress is not possible.”
    Frank Zappa

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

    Quote Originally Posted by Hot Wings View Post
    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...
    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.

    Quote Originally Posted by Hot Wings View Post
    I'm not intending to second guessing your plan, just posing a question.
    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 by Topaz; September 11th, 2014 at 01:26 PM. Reason: Cleaned up some typos.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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

    I moved this reply from the other topic, because it's way more appropriate in this one.

    Quote Originally Posted by Topaz View Post
    I obviously didn't speak clearly. Sorry. It's been a busy week.

    I wouldn't suggest anyone post the entire process here - napkin sketch to detailed assembly drawings. It would be great but, as you say, a mammoth undertaking that would be more than is reasonable to ask of anyone.

    No, what I meant was just the first part of the process: the conceptual phase stuff that Dan Raymer covers in his books. Getting from napkin sketch to a cleanly articulated design concept that is known to meet the mission requirements. One that is ready to later be analyzed for loads, then structures, then for detail design of individual parts. In other words, everything that's lacking in the "What do you think of my new design?" posts we see here so frequently. Right now, people come with a dream and a pretty picture. I'd like to see them be able to translate that dream into an actual design concept that they know will meet their needs, with numbers to back that up. If we gave them nothing more than an insight into how that first-order process works, I think it would be tremendously worthwhile for both HBA and this nascent racing class project.

    In my third edition of Aircraft Design: A Conceptual Approach, Raymer's first example study (a single-seat aerobatic ship) occupies 42 hand-written pages. Simply repeating that process with another set of requirements would be sufficient, I think, to show how an airplane gets from dream to rational design concept. The next steps - loads analysis, structural design and analysis, detail design, etc. - can either be "left to the reader" or handled by someone else. Just knowing that their design concept actually will meet their requirements with a reasonable level of confidence might motivate more of these new designers to undertake the study needed for those next steps, and maybe we'd see more of these airplanes actually built.
    The problem is that – unless you start off with an existing, proven concept – the “conceptual approach” doesn’t actually work that well, because it’s all too often the small details that are show-stoppers, literally. Virtually every design, even of experienced designers had such an “oopsie”.

    Once one is willing to accept an “underperforming” plane wrt cost or other performance parameters, it works perfectly fine, but not necessarily with pleasing results. After all, the average of a statistical analysis is... average.

    And obviously, this only works for fairly conventional designs. With any pusher for example, center of gravity (range) is going to be a huge issue and unlike in tractors, it is highly non-linear too.

    Weight, build time and cost are other factors where it’s the details that are a go/no-go. A good example is Raymer’s (and Roskam’s) approach to weight (or cost) estimate. Those are based on the existing planes. Using that approach I reach twice the wing weight of what some of the better existing designs have. Other existing designs are 3 times that. In other words, if you assume up-front you’re going to have this weight or that cost, it’s probably indeed going to be twice as heavy and 3 times as expansive.
    As for build time, for all the focus on the main structure (fuselage, wing, spars, tail surfaces), this is often only a fraction of the actual build time spent.
    Quote Originally Posted by Topaz View Post
    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.
    Spot on.

    With Raymer/Roskam, you'll likely end up with a feasible design.

    The other approach, that can yield considerably better aircraft, is to keep the design conceptual to the end and optimize everything. My wing has about 40% of the weight that Raymer would apply for it. But you can only do that if you know (from experience, other tested flight structures) that what you're designing is actually feasible. Same thing for production or a cost break-down. Without being deeply involved in either, any designed reduction in build time or cost is nothing more than a "design hope".

    About a month ago I gave a presentation about my design to a few Aerospace engineers. Took 3 hours and despite an in-depth presentation and 2 hours of Q&A's I went away thinking I barely scratched the surface with regards to all the considerations that go into a design.

    Probably one of the best examples of that process is the Boomerang. Here a short one:
    Design Explanation

    http://books.google.nl/books?id=4MOI...ionale&f=false

    // Ending rant for now. Sorry Topaz
    Kennis vermenigvuldig je door het te delen.
    (You multiply knowledge by dividing it)

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    Registered User Hot Wings's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by Topaz View Post
    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,
    I may have not been clear when used the word assumption, or I'm still misunderstanding the distinction between concept and preliminary stages.

    Lets presume that your concept involved using an off the shelf aluminum tube for the tail boom. Lets further presume that this is based on the fact that similar planes have done so successfully making this a reasonable "assumption". Unless you stop at this point and actually calculate if the tube is sufficient, based on the concept sizing and loads, you won't know until you get to the next step if there is in fact an off the shelf tube that can take both the torsion and bending loads. At this point you then have the choice to have a custom tube extruded that will take the loads (busting the budget) or go back to the conceptual stage and either reduce the tail loads or plan on using a different method to attach the tail.
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

    "--and pompous fools drive me up the wall. Ordinary fools are all right; you can talk to them, and try to help them out."
    Richard P. Feynman

    “Without deviation from the norm, progress is not possible.”
    Frank Zappa

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

    Quote Originally Posted by autoreply
    Sorry Topaz
    No worries. As I said, people approach the process differently. This is one way.

    BTW - you're going to see that I don't slavishly use Raymer's curve-fit equations for sizing either. The aircraft from which the curves were derived are sufficiently different from mine that the results, as you say, are inappropriate. I have my own way around that, but I'm getting ahead of myself.
    Last edited by Topaz; September 11th, 2014 at 03:38 PM.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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

    Quote Originally Posted by Hot Wings View Post
    I may have not been clear when used the word assumption, or I'm still misunderstanding the distinction between concept and preliminary stages.

    Lets presume that your concept involved using an off the shelf aluminum tube for the tail boom. Lets further presume that this is based on the fact that similar planes have done so successfully making this a reasonable "assumption". Unless you stop at this point and actually calculate if the tube is sufficient, based on the concept sizing and loads, you won't know until you get to the next step if there is in fact an off the shelf tube that can take both the torsion and bending loads. At this point you then have the choice to have a custom tube extruded that will take the loads (busting the budget) or go back to the conceptual stage and either reduce the tail loads or plan on using a different method to attach the tail.
    How can you stop and "actually calculate if the tube is sufficient, based on the concept sizing and loads", when you don't know what those are yet? The process I'm doing is what gets you to the point you can do that. You're talking about questions that come up in preliminary design. To get there, to get to the point where you can do that, you have to develop a concept that can give you the information you need to answer that question with a reasonable amount of accuracy.

    If the assumption to use a tube proves invalid, you have to go back and re-address that portion of the design process. There's no guaranteed method that rules out the possibility of error. Airplane design is an iterative process. If you make invalid assumptions, it becomes more iterative still.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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

    Quote Originally Posted by Topaz View Post
    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.
    Well I hope I am past that and someone might consider Basic Glider Criteria FAA. I describe it as Part 23 for Gliders.

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    Registered User Hot Wings's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by Topaz View Post
    How can you stop and "actually calculate if the tube is sufficient, based on the concept sizing and loads", when you don't know what those are yet? The process I'm doing is what gets you to the point you can do that. You're talking about questions that come up in preliminary design. To get there, to get to the point where you can do that, you have to develop a concept that can give you the information you need to answer that question with a reasonable amount of accuracy.
    I think I got it now. It's my interpretation of the concept phase - that is out of phase.
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

    "--and pompous fools drive me up the wall. Ordinary fools are all right; you can talk to them, and try to help them out."
    Richard P. Feynman

    “Without deviation from the norm, progress is not possible.”
    Frank Zappa

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

    Quote Originally Posted by Topaz View Post
    How can you stop and "actually calculate if the tube is sufficient, based on the concept sizing and loads", when you don't know what those are yet? The process I'm doing is what gets you to the point you can do that. You're talking about questions that come up in preliminary design. To get there, to get to the point where you can do that, you have to develop a concept that can give you the information you need to answer that question with a reasonable amount of accuracy.

    If the assumption to use a tube proves invalid, you have to go back and re-address that portion of the design process. There's no guaranteed method that rules out the possibility of error. Airplane design is an iterative process. If you make invalid assumptions, it becomes more iterative still.
    Topaz, you're on the right path. Having been through this on the Apollo and many other projects, the conceptual/preliminary/detail design process is valid even for small aircraft. On large aerospace programs, these phases are delineated by design reviews and customer (or management) approval prior to advancing to the next phase. On a homebuilt design, the phases are more fluidic and resembles more of a continuum of design progress, and you can jump back and forth as required.

    Of course, not everyone works in an organized way. But the odds for success are higher when following a logical and proven design process. It benefits us more when someone demonstrates proper and disciplined design methodologies rather than providing another poor example (which already exist at HBA). Your comment about aircraft design being an iterative process is a fundamental truth.

    I do have one question: After determining the mission requirements, will you be performing trade studies for different aircraft configurations? Or should we all shut-up and let you get on with it? .

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

    The cross dependencies in airplanes are a real pain in the rear. How do you know how much a tail section will weigh, if you don't know the forces it needs to support. You don't know the forces it needs to support until you design the tail empennage. The size of the tail feathers varies based on the weight of the rest of the plane. But the weight of the plane is affected by the material you use to make that tail, and tailplane.

    *tears hair out* :-)

    And that's why you make a SWAG. Then you try to make that math work. Then you revise your guess. Then you do that math. Wash, rinse, repeat, until you're close. once you're close, you start designing pretty good parts. Then you run the numbers again. Then you design real parts. And wonder where all your spare weight went. :-)

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

    Starting with a single seater is a very good plan.
    It will test design skills, building skills, test pilot skills, and most important, ability to finish it.

    Worked for Mr. Vangrunsven. (Actually, he started with a partial re-design first)

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

    Quote Originally Posted by Apollo View Post
    ...I do have one question: After determining the mission requirements, will you be performing trade studies for different aircraft configurations?
    Part of the point of this project is to keep it as simple and quick as possible, not just for the possible later build, but also for the design process. I just posted the first "ten-thousand foot overview" of the goals for the design, and one of them is simplicity - this isn't a "moonshot", and I'm not trying to stretch state-of-the-art in small motorgliders with this aircraft. I'm therefore limiting the trade studies in terms of overall configuration to two. They're significantly different from each other, and they represent a "low-risk" and "high-risk" pair in terms of design effort and confidence in matching data such as We/W0 supplied by comparable designs. Naturally I'm in love with the "high risk" concept. Figures.

    Quote Originally Posted by Apollo View Post
    Or should we all shut-up and let you get on with it? .
    Heh. No worries. I know the pace of this is (and is going to be) seemingly glacial compared to some "design studies" we see here. While I don't want to foster frustration in anyone, I'm just going to take my time and do it in the manner I think is right. At the moment, I'm just copying across and clarifying work that I already have in my notebook for this project. I started work on this study back in mid-July, so I'm farther ahead than you see here on HBA. Eventually I'll catch up with that and transition to this thread being my "design notebook", which might actually slow things down even more. Sorry about that.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

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