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

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    Post Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    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:

    It would be a good way for me to learn more about things I don't know as much about, and for others to learn about stuff I do know a thing or two about.
    I'm going to start and maintain two threads. This one, in the "Build Log" section, will be the main thread showing the work on the project. Build Log threads are locked to everyone but the OP, so I'll be the only one posting here. The other 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.

    One thing I can guarantee is that this won't be quick. I own and operate a buisness, and have plenty of "life happening" right now on top of that. I'll work and post as I can. I'm actually several weeks into this design study already, before I decided to start this thread, so I'll be able to post a lot in the beginning fairly quickly. After that, I'll try to post at the same rate I can work on it - a "lunchbreak" worth of work a day, on those days when work allows me to get away from my desk. I don't know if this is going to work - nerobro and I both started these threads today - but I think it's worth a try.

    My Design Process
    The process I use is very close to the one shown in Dan Raymer's Aircraft Design: A Conceptual Approach, with some stuff from John Roncz's "Designing Your Homebuilt" series in Sport Aviation thrown in here and there, and some other additions from other sources. If you want to follow along with the same resources I'm using, I have the third edition of Raymer's book (other editions should be the same, for the portions I'm using, but equations might be on different pages, etc.), and you can download a PDF of Roncz's article series from here. I'll post other references as they come up. For a preview of what to expect in terms of process and end-result, check out Raymer's first example study in the title above, which is pages 683-725 in my Third Edition.

    I don't presume to say that this is the "right" way to design an airplane, or that I have some special knowledge or experience that means it ought to be done this way. It's one way to design an airplane. It works for me, to the extent that it gets me from a starting point to where I want to go. If there's any legitimacy to it, that's because it's mostly Dan Raymer's way of designing an airplane, and he's a recognized authority on the subject. None of my design studies have ever been built or flown, so take this whole thing for what it's worth - a description of a partially self-taught design process by an inexperienced amateur. Caveat Emptor!

    Thanks for your interest, and let's get started...
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    Many of you know of my main project, a two-seat motorglider. While that project is moving along, the various "cheap aircraft" threads keep catching my eye rather severely. It sure would be nice to have a very small single-seater to use as an experience builder, a testbed to try out some of my ideas before committing to them on the larger project and, when it's done, a simple fun-flier while the two-seater is in the build process. The low-cost aspect especially interests me, much as it does many of you.

    We have much disagreement on that score, with some people opining that it's impossible to design and build an airplane for less than "X" dollars, with X ranging from $15,000 to $50,000 and higher.

    Well, that's absurd, folks! This little glider was built for $500 dollars a few years ago, according to its designer/builder. And it most certainly is "an airplane".



    So the real question isn't "Can you build an airplane for $X?".

    The real question is, "How much will it cost to build Y airplane with Z capabilities?"

    And that's what I want to do here.

    A conceptual design study, according to Dan Raymer, answers the following questions:
    .
    What requirements drive the design?
    What should it look like?
    How much should it weigh?
    How much should it cost?
    What technologies should be used?
    Do these requirements produce a viable airplane that can meet the requirements in the real world?
    .
    The purpose of this study is to see if I can come up with a viable conceptual design for a cost-conscious single-seat motorglider, and then to have a rough estimate of what it might actually cost to build. Those answers are the results. I won't have plans. I won't have structural analysis or even loads analysis. But I'll know enough about the airplane to make an informed decision as to whether it's worth moving forward with those efforts, pointed towards actually building one.

    Next post: Developing a viable set of requirements.
    Last edited by Topaz; September 10th, 2014 at 06:11 PM. Reason: Swapped out the static photo of the $500 glider for a video
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    The first step in any aircraft design process starts with deciding what you want to do with the airplane. You're setting the requirements for the design. The requirements drive every decision you make later. They literally design the airplane.

    WHAT DO I WANT?
    At the most general level, what am I looking to accomplish with this project? What do I want?

    My two-seat motorglider project is going to be awesome - a distillation of everything I think I want in an airplane. That makes it a bit of a daunting task for someone who's never completely designed and built an airplane before. No pressure, right?

    I keep seeing the various "cheap airplane" threads and find myself drawn to the idea of a very small, very "inexpensive" (whatever that actually means) airplane that I could use to gain design and construction experience for the more-advanced two-seater. NASA didn't go to the Moon straight-off; they did Gemini first to test ideas and methods, and to find out what they didn't know. I personally think that's a good approach.

    I also happen to think a little single-seater would be a heck of a lot of fun, all on its own. Something to putter over to the next airport for breakfast or a burger (we have some decent airport cafes in the area), or to attend an airshow (we have several decent shows around here, too). Maybe down the coast a bit on a pretty day, or to visit and do the Dawn Patrol bit with my friend from Boeing (who is also working on his own design). I'm also a soaring nut, and being able to head out to Elsinore any day I have open (whether a tow plane is available or not) and trade thermals with the hawks would be perfect. A motorglider would be great as a soaring cross-country trainer, too, so I can build my skills without having to make The Call Of Shame for a pick-up and trailer home.

    So no major powered cross-country. No flight levels. No night. No weather. Probably not very fast. I don't have to make a meeting using this airplane and, if the weather isn't nice, I just don't fly. Simple. Fun. Uncomplicated.

    This sort of airplane generally falls into the category called a "touring motorglider", as distinct from a self-launching sailplane or something like a Hummelbird or Sonex.

    Another consideration is that I'd like the airplane to be a learning tool - a place to experiment with ideas I have and techniques with which I really don't have any real-world experience. My mixed throttle and air-brake "quadrant" is the sort of thing I'd like to try out - and rip out if it doesn't work. As such, this airplane doesn't have to be "fancy". In fact, it shouldn't be. Simple is the order of the day, both in the design process and in terms of the build. "Good enough" to do the job, but "quick and dirty" enough that I can get the process done relatively quickly.

    This enters into cost as well. This isn't my "ultimate dream airplane". It's my "good enough training airplane." The design and construction equivalent of buying a used C-150 to build hours. So spending a bunch of money on it would be counter-productive. I don't know exactly what the number is yet, but there's a dollar value above which it just won't be worth it to me to spend on this kind of airplane.

    And lastly, there's the question of "everyone else". This is an "X-plane" for me. A testbed. I guarantee there will be things I'll learn that I would want to change before selling plans or kits. No, this one is just for me. A one-off. If it turns out to be awesome and I get enough requests for plans or a kit, I can develop a "production" version of the airplane that incorporates all the lessons-learned. But that's down the road. This one bears the weight of no such considerations.

    That's the 10,000 foot overview. The start. The gist.

    Now I need to get back to work. Clients are waiting!

    Next Post: Starting to break down the "general wish list" down into a detailed requirements list.
    Last edited by Topaz; September 11th, 2014 at 04:44 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    I'm waiting on a call-back from a client, so I have some time to move on to the next post.

    So, I have a "big picture" view of what I want out of an airplane like this. That guides the design process, but it's not enough to actually do any design. To get there, I need to start breaking that "wish list" into some real numbers. This isn't part of Raymer's book, but the process I've developed for my own work accomplishes this task by asking four discrete questions about the airplane, and then answering them in as much detail as possible.

    The four questions are:

    Who? - Who will be flying the airplane? Who will be flying in the airplane (meaning you include passengers, too). How big are they? What do they weigh? Who's the biggest and smallest person the design will accommodate?

    What? - What kind of flying will our intrepid pilot be doing? Aerobatics? Soaring? Powered Cross-Country? Soaring Cross-Country? $100 Burgers? You have to pick one, maybe two at most. We all want the perfect airplane that does everything, but it just doesn't work that way: "Jack of all Trades and Master of None" is what you'll get. Have some discipline when looking at this one. For what kind of flying do you want the airplane to be especially suited?

    Where? - What kind of airports (specifically, or actual specific airports, if you know) will the airplane frequent? Are there any "edge cases" that drive the design? What are the runways like? How long? Every airplane uses a "runway," even if your machine is highly-STOL and that runway is a 50' long sand bar on some river somewhere. Also included in "Where?" is the airspace and altitudes to be frequented. Are you going to be cruising up in the flight levels or smashing bugs down in the weeds?

    When? - "When" is kind of a catch-all: Not just time of day (Day flying only? Night too?), but also the weather conditions. I know, that seems arbitrary (it could fit in "Where," too), but I think of weather as a time-based phenomenon that changes constantly, which probably gives me a bias for putting it into the "when" column. That might be a sailplane pilot thing, since we're watching the weather and its changes so intently, all the time. So, "when" will you be flying the airplane? VFR only? IFR? Fire-bombing a raging forest fire? What are the "worst" transient flying conditions in which you want to be flying this thing?

    I know this is a somewhat controversial list, and I'm sure every designer has their own way of accomplishing the same thing. The goal here is to break down the "ten-thousand foot view" in my last post into hard "conditions" and data. Things to which we can attach actual, hard numbers that the design must accommodate or for which the airplane must be suited. This is actually a lot harder than it first appears, and the "four questions" are my attempt at organizing that process and making a rational division of information. Your mileage will very probably vary on this one.

    No call from the client yet (argh!), so let's get into the first question:

    "Who" is going to be in this airplane?

    Given that this is a one-off, truly experimental airplane, the obvious first answer is me.

    Topaz
    Height: 5' 10"
    Weight: 175#

    Who else? Just for the sake of appreciating the long, strange, airplane-design road we've been through since we met in college, I'll probably want to trade cockpits at least once with my friend at Boeing - we'll call him "Joe" - who's also designing his own airplane.

    Joe
    6' 0"
    200#

    Anyone else? I know a few people who'd probably want to fly the plane - and whom I'd let do so - but I'm pretty sure they're all within an inch of Joe's height and between the two of us in weight.

    So let's call the "biggest pilot" for this airplane to be as follows:

    Maximum Pilot
    Height: 6' 1"
    Weight: 200#

    "Minimum Pilot" is actually quite a bit harder in my particular case. I'm either the shortest or lightest pilot I can foresee flying this airplane, off the top of my head. My girlfriend is not a pilot, and I don't foresee her getting her cert in the future. I guess I can sandbag anyone smaller who comes along, and design the airplane to handle someone a bit shorter than myself.

    Minimum Pilot
    Height: 5' 6"
    Weight: 175#

    As a single-seater, there will be no passengers unless something at the Elsinore dropzone has gone terribly, terribly wrong.

    Baggage? Yeah, I'll probably want to bring something, especially to airshows and such. I'm not fond of paying $25 for a hot dog and coke at an airshow, so maybe I'll bring some lunch of my own, and a big bottle of water. If you were doing a powered cross-country "traveling" airplane, what I'd do is actually pack for the kind of "biggest" trip you expect to take in the airplane. I mean really put the clothes and toiletries in your bags, and then add a little weight for bringing home knick-knacks and "lousy T-shirts" for family members. Then measure and weigh the bags. You might be surprised.

    For this airplane, it's a bit easier. I'll assume the baggage requirement is as follows:

    Baggage
    Size: One "day-pack" style bag. Measuring my favorite day-pack, it's 20" tall, 15" wide, and (packed) 10" deep.
    Weight: In total, including bag: 20#

    You'll note that I made this kind of a heavy day-pack for one containing just lunch and water. There's method to my madness, and here's where having done this process a few times comes in handy in terms of "experience". I know that virtually any actual "trip" made in this airplane will be done by me, not Joe or anyone else. I also happen to be the lightest of the list of likely pilots for the airplane. Since any of the heavier guys aren't going to need to bring baggage, I can count the baggage allotment against their weight. It may cause weight and balance troubles (and I'll be checking for that later), but this is an opportunity to keep the weight of the aircraft down. For every pound of useful load, there's "X" pound(s) of airplane needed to carry it. Since that structure isn't weightless itself, you actually need "X + Y" pounds of airplane structure to carry that extra pound of useful load. It really starts to add up! I can't assign numbers to those variables yet for my airplane, but it's really important to carry no more useful load than you really expect to use over the life of the aircraft. Designers who guess this amount properly have the benefit of a lighter, better performing, and less expensive aircraft resulting from their work. Designers who don't either end up with an airplane with insufficient useful load, or one that's heavier, worse performing, and more expensive. Take some time to really think this through. I spent a lot more time looking at potential pilots and baggage allotments than you see here. Even so, you'll guess an answer, as I have above, and then you'll go down and answer the "Where" question and probably have to revise your "Who" answer for baggage and people based on that. It's probably better to answer the "Where" question first, but I've gotten into the habit of answering "Who" first and then updating my answer based on "Where", probably because a particular non-pilot "Who" has a big influence on choosing the "Where"'s we'll visit. Funny where women end up wanting to go...

    So, for the moment, I'm going to revise my "Maximum Pilot" down in weight, by the weight of the baggage he won't be carrying:

    Maximum Pilot
    Height: 6' 1"
    Weight: 180#

    The beauty of this is that I can now pretty much dispense with this "Maximum Pilot", "Minimum Pilot" nonsense. They're only five pounds apart. Now I have a "Design Pilot":

    Design Pilot
    Height: 5' 6" to 6' 1"
    Weight: 180# (Up to 200# without baggage)

    That worked out nicely for me here, and it's a trick you'll see even in Cessnas and other Part 23 aircraft - the designer trades baggage, fuel, and other weight for passenger weight. It's why you hear an obviously four-seat aircraft so often referred to as "not really a four-seater". You can carry four adults, or you can carry two adults and full fuel and baggage. But you can't do both without going over maximum takeoff weight (MTOW), which pilots call "gross" weight.

    If you're going to do this for a two- or more-seater, you'll end up with a definite "maximum-minimum" range of weights, from smallest-possible-solo-pilot-without-baggage on one end, to "load everyone and everything up and let's go to Bermuda for a month" on the other.

    I'm not going to worry about fuel weight yet. That comes quite a bit later.

    Next Post: Continuing to answer the 'four questions'.
    Last edited by Topaz; September 12th, 2014 at 09:52 AM.
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    Moving on to the next question of the four: What?

    Here is what I said about the kind of flying I want to do with this airplane:

    I also happen to think a little single-seater would be a heck of a lot of fun, all on its own. Something to putter over to the next airport for breakfast or a burger (we have some decent airport cafes in the area), or to attend an airshow (we have several decent shows around here, too). Maybe down the coast a bit on a pretty day, or to visit and do the Dawn Patrol bit with my friend from Boeing (who is also working on his own design). I'm also a soaring nut, and being able to head out to Elsinore any day I have open (whether a tow plane is available or not) and trade thermals with the hawks would be perfect. A motorglider would be great as a soaring cross-country trainer, too, so I can build my skills without having to make The Call Of Shame for a pick-up and trailer home.
    So this airplane will be doing:

    1. Casual low-performance powered cross-country flying.
    2. Casual around-the-airport self-launch soaring.
    3. Cross-country soaring training.

    Breaking this up into powered and unpowered flight modes, I can draw some conclusions from these "general" missions. I'll get much more into the specifics for the powered mission in answering the "where" question, but there's information we can pull from that even now.

    Powered Flight
    1. I'm not asking the airplane to do any aerobatics (and with the long wings of a motorglider, it'd be a pretty sluggish aerobat anyway), so a Normal Category g-rating is sufficient.

    2. $57 burgers, airshow runs, and flying down to Oceanside don't require a lot of speed, especially since it's all going to be fairly short-range stuff. I'd naturally like it to be faster than a car, but part of the appeal is to stay fairly low and watch the countryside roll by. Last time I drove down for an airplane design work session with Joe (we trade off locations, with him driving up here every other month), I checked the time and distance, and came up with an average speed of 56 mph. Checking on Google Maps today (reflecting another day's traffic conditions) gives almost exactly the same expected average speed. Doubling that speed would be nice, so I'll set the goal cruise speed at 110 mph for now. It's not really fair to compare door-to-door speeds for a car to cruise speeds for an airplane (I'm not including the drive to the airport, for example), but you have to start somewhere. I'll revisit the goal cruise speed when I look at an actual list of destinations in the "Where?" question. Maybe something will pop up there to change it but, for now, goal cruise speed is going to be 110 mph [96 knots].

    3. Cruising altitude. Totally going to be a local thing. Flying around the LA/OC basin, I've always found 4500-6500' MSL to be a good VFR cruising altitude here. You're up out of the worst of the surface turbulence, but you're not up into the approach traffic for LAX, SNA, or ONT. Now, I haven't done any SEL flying in the Basin in a while. Anyone who lives in the area and is reading this, would you mind chiming in on the discussion thread about your thoughts here? We're talking casual Day-VFR flight in this area on, say, a $57 burger mission. What altitude suits you for cruising around here? Pending that discussion, I'm going to set the powered-flight design cruising altitude at 5,500' MSL. This is another area that I'll revisit when I get to creating a real list of destination airports.

    Time to go to work. I'll come back to this at lunchtime.

    Next Post: Continuing answering "What?" for soaring flight.
    __________________________________________________ __

    CURRENT SPECIFICATIONS LIST
    as of this post.

    PAYLOAD

    Design Pilot
    Height: 5' 6" to 6' 1"
    Weight: 180# (Up to 200# without baggage)

    Baggage
    Size: One "day-pack" style bag. 20" tall, 15" wide, and 10" deep.
    Weight: In total, including bag: 20#


    PERFORMANCE

    Powered Flight
    Design powered-flight cruise speed, Vc = 110 mph [96 knots] (161 fps)
    Design cruising altitude = 5,500' MSL (standard day)


    RESTRICTIONS AND CONSTRAINTS

    Limit load rating: -1.5g to +3.8g (Normal Category)
    Last edited by Topaz; September 12th, 2014 at 11:23 AM. Reason: Adjusted the price of the "burger mission", for Hot Wings. We're going for "inexpensive" here!!! :-)
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    INTERLUDE

    We work before we play. :-/

    A client pushed up a deadline by an entire week - they do that sometimes - and so yesterday turned into a hectic day that has spilled over into today. So no airplane work for me yesterday, and today isn't looking good either. Sorry for the delay.

    I do want to speak a moment about how much effort I'm putting into developing the requirements and specifications for the project. I'm sure more than one of you are saying, "Hey, when are you going to start actually designing something?". It's a fair question.

    The answer, for me, is that setting up rational and realistic requirements is probably the most important part of the entire design process. These conditions and numbers will drive the entire rest of the project, and the airplane will be optimized to work best under these conditions. Get it wrong - set up requirements that don't actually reflect how you'll really use the thing - and you're going to be designing and building the wrong airplane. What comes out will be something you don't really enjoy, however you actually are using it, and it won't perform as well in that other set of conditions.

    Setting requirements and specifications isn't something to be done quickly or lightly. Take your time. Get it right.
    Last edited by Topaz; September 13th, 2014 at 02:11 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    Repeating my previous call for aid. I haven't flown SEL in the Los Angeles/OC Basin in quite a while. For those of you who have flown the Basin in the last ten years, what have you found to be convenient and useful VFR cruising altitudes? I'm particularly interested in altitudes for local flights where both ends are within the Basin.

    Please reply in the discussion thread, linked below in my signature line. Thanks!
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    That set of catalogs with the rushed deadline is about ready to go to press, on the new schedule, so I have some time to start getting back to the "What?" question for my project, now for the soaring missions:

    Soaring Flight
    1. Sailplanes pilots seek out what power pilots call "turbulence", because it usually reflects convective lift that a sailplane can use to gain altitude. However, it can still be a bumpy ride sometimes. Because of this, sailplanes generally are designed to a higher limit load value than the FAR 23 Normal category. The factor of safety is the same: 1.5. Since my airplane will be soaring, I'm going to increase the limit load factor value to reflect the JAR 22 Utility category values for sailplanes, instead of the FAR 23 numbers I used earlier. JAR 22 has only Utility and Aerobatic categories, and doesn't have a "Normal" category. JAR 22 actually sets four limit load values, with positive and negative numbers for both VA and VD, the latter of which we call VNE here in the States. JAR 22.337 sets limit load values of +5.3g and -2.65g at VA and +4.0g and -1.5g at VNE for Utility category gliders. For the sake of simplicity at this point, I’m just going to use the greater-magnitude VA values

    2. Quantifying the minimum sink rate needed will depend on the local conditions where I'll be soaring most of the time, so I'll get to the details of that in the "Where?" section for soaring. For the moment, I'll use the minimum required rate of descent listed in JAR 22.71(a): -1.0 m/s, or roughly -196 fpm. That’s about the same as an SGS 2-33, and I know I want to do better than that. How much better will be determined by the local conditions in which I expect to fly - another "Where?" answer. But this will do for now.

    3. For casual near-the-airport soaring and ridge soaring, L/D isn't as important as a low minimum sink rate. L/D is always important for intra-thermal glide and looking for thermals, but being able to use weak thermals or orographic lift to stay up will count for more days that don't turn into sled rides. For the moment, and based purely my experience of flying sailplanes with the LESC, I'm going to set an initial soaring L/D goal of 30:1. This matches the SGS 1-34.

    Next Post: Getting into the nitty-gritty of performance requirements: "Where?"
    __________________________________________________ __

    CURRENT SPECIFICATIONS LIST
    as of this post. Values in red have have been changed or added by this post.

    PAYLOAD

    Design Pilot
    Height: 5' 6" to 6' 1"
    Weight: 180# (Up to 200# without baggage)

    Baggage
    Size: One "day-pack" style bag. 20" tall, 15" wide, and 10" deep.
    Weight: In total, including bag: 20#


    PERFORMANCE

    Powered Flight
    Design powered-flight cruise speed, Vc = 110 mph [96 knots] (161 fps)
    Design cruising altitude = 5,500' MSL (standard day)

    Soaring Flight
    Design minimum sink rate: -1.0 m/s [-196 fpm]
    Design maximum L/D ratio: 30:1


    RESTRICTIONS AND CONSTRAINTS

    Limit load rating: -2.65g to +5.30g (JAR 22 Utility Category)
    Last edited by Topaz; September 16th, 2014 at 11:47 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    Wow. It's great to have all the business in the shop, but it's not leaving a lot of time for airplanes!

    Last time I posted, I'd finished up the "What?" question - what kind of flying will I be doing with this airplane? That filled in the blanks in my requirements list for limit load ratings and gave me preliminary values for cruising speed and altitude for powered flight, and for minimum sink and L/Dmax for soaring flight.

    Now it's time to get those latter "preliminary" values nailed down, using the characteristics of specific locations to drive the requirements for my airplane. Since this airplane is just for me, this is largely a data-gathering exercise. Where will I be flying, and what are the characteristics of those places? From this information, I'll get:
    .
    • Takeoff and landing distances
    • Runway surfaces to accommodate
    • Actual powered-flight range required
    • Cruise speed for powered cruising flight
    • Climb rate at sea level, for an adequate climb rate hot-and-high at real airports
    • Minimum sink required for my most-frequent soaring spots
    • L/Dmax needed for the kind of soaring I'll really be doing


    First thing is to gather together a list of places I'm going to be flying from and to with this aircraft. Since it's not intended to be a long-range cross-country aircraft, the list will be relatively short, and mostly local airports. Out of that list, I've pulled out the ones that are close to my home base airports and with really long, smooth runways. Those aren't going to drive any requirement, so I'll save some time by not having to collect data about them.

    Further, I'm splitting the airports into three categories: Home Bases, Frequent Destinations, and Occasional Destinations. When I'm pulling requirements from these airports, it makes sense to keep an eye on them to see if any one place is really pushing a characteristic of the airplane much farther than any of the others. If that's a home base or some place to which I'll be flying a lot, it makes sense to really consider accommodating that need. But if it's someplace I'll only fly occasionally, and it's really driving a big increase in wing, or engine, or some other part of the airplane, I may want to just throw that out as a destination for this airplane. Perhaps I can rent a plane to fly there, or find some other way to get rid of that particular extreme requirement.

    For each airport, I need four pieces of information:
    .
    • Elevation - Combined with likely "high" temperatures for that airport, it gives me a worst-case density-altitude for takeoff and climbout.
    • Runway Dimensions - The shortest runway is going to drive my actual takeoff length requirement. Runway width can have an effect on certain landing gear types. In each case, I want to look at the shortest, narrowest runway at each location. The bigger runway may be closed or operations may be restricted to certain kinds of aircraft. While I should be able to learn about any runway closures by calling the airport in advance, there's always the chance of something unexpected happening, and I'd hate to enter the pattern at a destination airport, only to find out that the runway I need isn't available.
    • Runway Surface - Mostly drives the design of the landing gear (tire sizing, etc.), but dirt runways also present a FOD hazard for pusher propeller installations, so this can affect the final overall configuration.
    • Obstructions - Are there any large obstructions off the ends of the runway? This could drive an "Angle of Climb" requirement.


    I got this information for each airport from AirNav.com, a nice pilot resource that happens to be handy here, too. Rather than re-type my list of airports and the data about them, I'm simply going to attach a PDF of the pages out of my study document. You should see that PDF below. Clicking it will download it. If you don't have a reader for PDF files, you can get one for free from Adobe here.

    Next Post: Pulling takeoff and landing requirements from this list.
    Attached Files Attached Files
    Last edited by Topaz; September 20th, 2014 at 04:17 PM. Reason: Grammar. Sheesh.
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    Takeoff and Landing
    Okay, let’s take a look at the list. For the takeoff run, the things that matter are:
    .
    • Runway length
    • Potential for high density altitude
    • Runway surface


    Looking at my list, the shortest runways are:
    .
    1. Hemet-Ryan (HMT) - 2,045'
    2. Oceano (L52) - 2,325'
    3. Crystal (46CN) - 2,700'
    4. Oceanside (OKB) - 2,712'
    5. Santa Paula (SZP - 2,713'
    6. Skylark (CA89) - 2,800'


    The airports with the potential for high density altitude (high geographic altitude and frequent high heat) are:
    .
    1. Mountain Valley (L94) - 4,220' and summer highs in the 90°F+ range: DA=7,396', density ratio=80.09%*
    2. Crystal (46CN) - 3,420' and summer highs in the 90°F+ range: DA=6,420', density ratio=82.52%
    3. Banning (BNG) - 2,222' and summer highs in the 90°F+ range: DA=4,957', density ratio=86.28%
    4. Twentynine Palms (TNP) - 1,888' and summer highs in the 90°F+ range: DA=4,549', density ratio=87.35%
    5. Hemet-Ryan (HMT) - 1,512' and summer highs in the 90°F+ range: DA=4,089', density ratio=88.57%**
    6. Skylark (CA89) - 1,253' and summer highs in the 90°F+ range: DA=3,772', density ratio=89.42%


    * Calculated on www.pilotfriend.com. These were all calculated for 90°F, 29.92 mmHg, dew point 65°F.
    ** Catalina (AVX) is higher in geographic altitude than the last two airports on this list, but even summertime temperatures are kept below 80F by the surrounding ocean (Catalina is an island 22 miles off the California coast). Calculated density altitude for this airport at 75°F is 3,245, well below the others on the list.

    And lastly, the worst runway surfaces are:
    .
    1. Mountain Valley (L94) - “Asphalt/Dirt, in fair condition. Partially paved portions of the runway 20 feet wide.”
    2. Skylark (CA89) - “Packed sand, clay.”
    3. Catalina (AVX) - “Asphalt, in fair condition. (Potholes and loose pavement fragments on runway.)”


    Well, that data tells a tale! Three of these airports see density altitudes during the day of nearly 5,000 feet and even much more. The air density at Mountain Valley (L94) on a 90°F day is only 80% of that on a “sea level, standard day”. At least that airport has a long runway!

    I’m not sure that trying to accommodate all of these runways mid-day is a smart way to go. Takeoff performance is largely driven by power-to-weight ratio and wing loading, and getting better performance requires more engine or more wing, or both. I’m trying to keep this airplane small and inexpensive, so I want less of both.

    Without running actual numbers, I don’t know which of these airports is going to end up really driving the takeoff performance the most. It's going to be a combination of runway length and density altitude. My guess is that it’s going to be either Hemet-Ryan (HMT) because of the short runway or Crystal (46CN) because of the high density altitude. Mountain Valley (L94) has a much longer runway, and the reason I’d be going there is to attend the Experimental Soaring Association fly-in, so I’d likely be spending the night and be able to fly out in the morning when it’s cool. In the case of Hemet-Ryan (HMT), there is a much larger runway that normally I could use, so if the short runway I’ve chosen there drives the design too far, I can perhaps qualify that particular runway as “optional”.

    Fortunately, none of these airports are on my “frequent destinations” list, so I can consider them more carefully when I go to determine the wing loading needed for takeoff. So this will be my first “trade study” to do - finding out if any of these airports significantly drives the requirements of the aircraft, and whether I can plan to fly there at a cooler time of day or time of the year. I’m starting a list of trade studies that I’ll want to do at the appropriate time in the design process.

    In terms of runway surfaces, nothing really jumps out here. I have extensive experience with Skylark (CA89) and, while it’s dirt and clay, it’s fairly hard-packed and shouldn't impact takeoff performance much. I've called out Mountain Valley (L94) and Catalina (AVX) for the possibility of potholes, broken pavement, and debris on the runway. I haven’t flown at either of those, so I don’t know how bad they are, really, but given that both have operations on a regular basis, they can’t be much worse than Skylark. I think all of these will be more relevant to FOD damage potential than for impacting the takeoff run.

    Next Post: Defining my takeoff performance requirement.
    Last edited by Topaz; September 20th, 2014 at 10:07 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    It's frustrating, but there doesn't seem to be any guidance anywhere about choosing a takeoff distance for a single-engine airplane, given available runway lengths. Multi-engine airplanes get all sorts of stuff for balanced field length, etc., but it seems to be the assumption that a single-engine airplane will use up all the available runway and, if there's a power loss on takeoff, you just end up in the weeds. Anyone have any input on this? Post it to the discussion thread, please. You can find the link for that in my signature line below.

    None of the runways I'm listing are particularly short for a small, non-STOL, single-seat aircraft. Unless it adversely affects the choice of wing loadings, I can afford to "save" a little runway to put the airplane back down again if there's an engine failure right after lift-off. So, let's take a look at what that might mean.

    The shortest runway on my list is Hemet-Ryan (HMT), at 2,045'. Assuming the airplane were at an altitude of 50' at a point 60% of the way down the runway (1,227' from start), I'd have 818' left to get the airplane down and stopped. Neither of those numbers seem completely unreasonable, and they're even easier on the next-shortest runway, Oceano (L52), at 1,395' and 930' respectively.

    I'll check these numbers carefully when I'm looking at wing loadings for takeoff and landing but, for the moment, I'll specify that my takeoff and landing requirements will be as follows:

    Takeoff distance: 60% of the available runway length for the most-adverse runway (length and density altitude), and the airplane will be at 50' AGL at this point.

    Landing distance: 40% of the available runway length for the most-adverse runway, starting at an altitude of 50' AGL.

    I still don't know which is going to be the "most adverse" runway, so I'll run the takeoff and landing wing loading numbers for the following two cases:

    Airport Runway Length Takeoff Distance Landing Distance Density Altitude Density Ratio
    Hemet-Ryan (HMT) 2,045' 1,227' 818' 4,089' 88.57%
    Crystal
    (46CN)
    2,700' 1,620' 1,080' 6,420' 82.52%


    Whichever comes out as requiring the lowest wing loading will be the "worst case" that will drive the design. You'll recall that I'm "throwing out" Mountain Valley because I'm going to be able to reasonably avoid high density altitude conditions by virtue of the reasons I'll be traveling to that airport, plus it has a much longer runway. I’ve looked at all the other airports, and the “worst case” is going to be one of these two. I’m betting it’s Crystal, myself.

    Next Post: Defining a climb rate requirement.
    Last edited by Topaz; September 21st, 2014 at 06:25 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    I jumped the gun - the next item on my list from post #9 is setting the range requirement, not climb. Soooooo...

    Cruise: Range, Speed, and Altitude
    Using Skylark Field (CA89) as the starting point, here are the four farthest destinations on my list of airports. I'm using the great circle distance to figure out which ones I should check for an actual flight distance. I calculated these using a neat online tool: Great Circle Mapper.
    .
    1. Fresno-Chandler (FCH) - 223 nm (257 statute)
    2. Oceano (L52) - 187 nm (215 statute)
    3. Mountain Valley (L94) - 104 nm (120 statute)
    4. Santa Paula (SZP) - 98 nm (113 statute)


    These are all fairly trivial ranges for a sportplane, even one like this, and especially so since I’m hoping to use a fairly small motor for this airplane. I don’t see any reason to discard any of them, since I don’t think they’ll greatly affect the sizing of the aircraft. The farthest two are both “$57 burger” destinations (I hear the food is good at Fresno-Chandler - ahem - and I know from experience that Oceano is a great place to go for a bite of lunch), so if, somehow, those destinations prove to make the airplane too costly, I can always discard them and make those trips in two stages with a fuel stop.

    Next Post: Calculating actual flight range to the furthest two airports and setting the design cruising speed, range, and altitude requirements from that data.
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    How Far Is It?
    The distances in my last post above are all great-circle routes - the closest thing you can get to a straight-line distance between two points traveling on the surface of a sphere called Earth. Unfortunately, even a great circle route usually isn’t the way we get to fly our airplanes. There are Restricted areas, MOA’s, Class B and C airspace, and natural obstructions like mountains that get in the way and force diversions. For a little bug-smasher like this thing I’m designing, all of those have to be taken into account to find the actual range to a given destination.

    Using an online tool called Skyvector.com I’ve sketched out rough cross-country flights to the two farthest airports in my list above. I didn’t take the time to really dig into them in detail, with all the navigation, but I plotted a likely-looking flight path that avoids the mountains, Class C airspace (where possible), Restricted areas, and so on that lay between Skylark Field (CA89) and each destination. Here’s a screen grab of one of them. The pink line is the route I’ve selected.

    Click image for larger version. 

Name:	Skylark-Fresno.jpg 
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ID:	34916

    As you can see from the bottom of the Flight Plan dialog, the real-world flight path distance between Skylark and Fresno-Chandler isn’t the 223 nm of the great-circle route, it’s 252 nm. Looking at the ground altitudes en route, it looks like the highest terrain under the airplane is about 5,000 feet MSL just north of Quail Lake Sky Park (CL46). So much for my 5,500 foot design cruising altitude! This is why I go to these lengths when writing requirements for an airplane. My original assumption was just wrong. Not way wrong, but it's good to have the right numbers in there, and who knows what the future will bring for destinations?

    Taking this into account, I’ll set the design cruising altitude to 7,000 feet MSL: half-way between the outbound VFR cruising altitude of 6,500 feet MSL and the return altitude of 7,500 feet. Not all of the route requires these altitudes, but it’ll be nicer to get up out of the surface thermals for a cross-country flight. I won’t get bounced around as much.

    I’ll also note two more datapoints: At my current design cruise speed of 110 mph (96 kts), not accounting for climbout, etc., the estimated time en route to Fresno-Chandler is two hours and thirty-eight minutes. The estimated time en route to Oceano is two hours and nineteen minutes. We’ll come back to that in a later post.

    An inland flight to Oceano (L52) proved to be 223 nm, with similar en route altitudes.

    So now I’ve set my design cruising altitude and my required design range. Next comes the matter of extra range, to account for headwinds, unexpected destination airport closures, etc. Bumping the Fresno-Chandler range up to 300 nm gives me nearly 50 nm over the expected still-air range, and that’ll do for the moment. That brings the design cruising mission to 250 nm (no reserves) plus 50 nm (reserve), all at 7,000 feet MSL. Into the requirements table they go!

    Next Post: Revisiting en route cruising speeds.
    Last edited by Topaz; September 22nd, 2014 at 11:49 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    A Better Look at Cruise Speeds

    Now that I have a very good idea of the actual range specification, I'll take a quick look back at my initial cruise speed specification. That was 110 mph, which works out to 96 knots.

    My loose goal was to be "twice as fast as a car" for the same trip. I've got two explicit trips to look at now, so how long would those take by car? Google Maps to the rescue.

    Skylark (CA89) to Fresno-Chandler (KFCH)
    According to Google Maps, it'll take me 4h 32m to drive from Skylark Field to Fresno-Chandler at the moment. Maps says the trip would take 4h 14m if there were no traffic issues.

    Half of that time is 2h 7m, which, over 252 nautical miles for the air route I chose earlier, yields 119 knots, or 137 mph to get there in half the time of driving.

    Skylark (CA89) to Oceano (L52)
    Google Maps says it’ll take me 4h exactly to drive to Oceano County Airport from Skylark, without traffic. Half that is 2h, which yields a 112 knot cruise (128 mph) for the 223 nm flight path.

    Both of those are a little faster than the 96 knots (110 mph) I’d specified earlier. BBerson noted in the Discussion thread that getting 110 mph from a small motorglider would be a challenge without a large (expensive) motor, and he’s quite possibly right. But I don’t know yet, and that’s part of the point of this design process: How much will it cost to meet these specifications I'm developing? The Stemme S-10 cruises at 140 knots (161 mph), so it’s not like a cruise like this is impossible for a motorglider.

    For the time being, let’s call the design cruise speed out as 115 knots (132 mph). This may, indeed, prove unrealistic for an “inexpensively” small engine, so I think it’s prudent to set a lower bound, or “threshold”, on cruise speed. In other words, the lowest cruise speed I’d accept if I got everything else I want out of this design. If the final design falls somewhere between these two numbers, it’ll be “good enough”.

    Getting there at the same speed as a car seems, to me, to be setting expectations too low, so let’s say getting there in three-quarters of the time to drive the trip in a car is “good enough”, instead of half the time. Plugging that into the more-critical Fresno-Chandler case, I get a flight time of 3h 11m, and a cruise speed of 79 knots (91 mph). That seems reasonably attainable for a clean motorglider with a smallish engine, considering the Monnett Moni gets better than that - (96 knots / 110 mph, exactly my initial specification) - on its 30hp KFM. Let’s call the threshold value 80 knots (92 mph) for simplicity, and have done.
    "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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    INTERLUDE - Pause and consider...

    Initially I'd been putting a log of the specifications and requirements I've developed up on this thread, as they were completed. As more and more stacked up, and some of them started needing both "goal" and "threshold" values, I took to using a table in my design notebook to tabulate my specifications. (I'm using Google Docs for the design notebook, BTW.) Unfortunately, when using HBA in "code" mode, writing up tables is a real PITA. So you've stopped seeing a running tabulation of the specifications, and instead are just getting the analysis that leads up to them.

    Just to give everyone a clear view, I've attached the specification sheet for this airplane as looks like right now. Some of the fields still contain "???" because I don't know those values yet. A few fields are filled in that I haven't discussed here; mostly stuff that's called out by JAR 22 and that I want to adopt verbatim for this design. There are some fields that don't show up yet, but will be added later.

    This sheet is the point of all the analysis so far. This document, when completed, will drive the entire design of the airplane.

    DS54-Draft Specifications 09-23-2014.pdf

    Looking at these numbers, you can already start to get a feel for what this airplane will be like, can't you?
    Last edited by Topaz; September 23rd, 2014 at 03:57 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
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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