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

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    Registered User Matt G.'s Avatar
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    Re: Discussion: 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?

    My (probably worthless) opinion as a low time (<200 hr) pilot is that I like to have about twice the runway I need. I am used to flying in the midwest, where there are usually multiple decent-enough off-airport options in the event of an engine failure. I'm guessing there are fewer options like that in your neck of the woods. So, half your smallest runway's length to clear a 50' obstacle?
    Last edited by Matt G.; September 21st, 2014 at 06:49 PM. Reason: Fixed the quote code.

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

    Quote Originally Posted by BJC View Post
    Lots of homebuilts have grown over the years to accomodate / be more comfortable to more "normal-sized" people.

    Examples include the Pitts Specials, Van's RV series, Glasairs / Glastars / Sportsman, Lancairs, etc.

    So if a design is intended for more than just one or two known people, why not make it comfortable for the masses?


    BJC
    Masses?
    The adult world average weight is 137 pounds. Body weight - Wikipedia, the free encyclopedia
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    Moderator Topaz's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by Matt G. View Post
    My (probably worthless) opinion as a low time (<200 hr) pilot is that I like to have about twice the runway I need. I am used to flying in the midwest, where there are usually multiple decent-enough off-airport options in the event of an engine failure. I'm guessing there are fewer options like that in your neck of the woods. So, half your smallest runway's length to clear a 50' obstacle?
    I'm a pretty low-time pilot too - a bit less than you, as a matter of fact - so your guess is as good as mine. It's why I wanted input, so thank you for letting me know what you think. Skylark has a lot of flat ground off the end of the runway, as do most of the other airports on my list. I think most of them are a lot like yours. The urban airports - Cable, Fresno, etc. - really don't. Catalina drops off steeply from near the end of the runway on one end, and that would be... bad. These airports are the point of the exercise, really.

    I chose 60% since I know I can land in a shorter distance than take off. Half the runway also sounds reasonable, too, in that it's a bit more conservative. I suppose I'll need to run the numbers for both cases. I can save some engine power/weight/cost if it's 60%, but the numbers will tell in the end.

    Anyone else have any thoughts on this?
    "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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    Moderator Topaz's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by BBerson View Post
    Masses?
    The adult world average weight is 137 pounds. Body weight - Wikipedia, the free encyclopedia
    Interesting. The average United States male body weight is up to 195 lbs. (same source)

    Ah, gone are the days in the late '80's when I was, at 175 lbs, still the "FAA Standard Person". JAR 22.25(a)(2) specifies a "minimum" pilot weight for MTOW calculations of 110 kg (242 lbs), while FAR 23.25(a)(2)(i) only requires 170 lbs for Normal category and 190 lbs for Utility or Acrobatic categories.

    Okay, when I do the initial sizing, it should be a pretty quick trade study to see what accommodating a larger pilot would do to the airplane. I'll add that to the list of trade studies to be done. I'll use the JAR 22 maximum pilot weight of 242 lbs, plus full baggage. If you weigh more than that... You're gonna have to take it out of baggage weight or start eating more salads.

    We'll see what it does to the size of the airplane and, more importantly, the size (and cost) of the engine required to meet my requirements list.
    "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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    Registered User proppastie's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by Topaz View Post
    .

    I chose 60% since I know I can land in a shorter distance than take off. Half the runway also sounds reasonable, too, in that it's a bit more conservative. I suppose I'll need to run the numbers for both cases. I can save some engine power/weight/cost if it's 60%, but the numbers will tell in the end.

    Anyone else have any thoughts on this?
    The Pilot operating manual of every aircraft I have flown has a performance section where distance over a 50' obstruction is shown. Perhaps this is useful for this exercise.

    This is the only manual I have now, perhaps a C-150 manual is a better fit.
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    Last edited by proppastie; September 22nd, 2014 at 05:36 AM.

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

    Hey Guys,

    I am a bit late to this thread, but a year or so back, I was looking to build the same thing. Upon Topaz's recommendation, I bought raymers book and read about half before things got in the way that made me have to shelve my idea. I too stare at those same CU clouds that Victor Bravo mentioned and I use to be an Hanglider pilot flying form the hill I can view from my local airport. I left HG due to my feeling that weight shift was more of a suggestion than actual controll and every now and then the thermal reminds you by tossing you one direction you don't want to go at the worst time and your along for the ride. Yet I really enjoyed soaring off that location thus my hope was to build an electric aircraft with the sole goal to be able to fly out of KWHP over to the HG site soar a bit then head home. This is one of the few scenarios that makes Electric power feasible. As with others I looked at electrifying a goat but then diverged toward a variation of two designs. Either an electric version of the Marske Monarch, or an electric version of the souricette which has already flown electric. Here is a link: MB 02 SOURICETTE

    On the Souricette, I was trying to simplify the construction and was leaning toward the overall layout of the Souricette with the construction methods of the Baslee airdrome aircraft: Airdrome Aeroplanes ~ Holden, MO
    Or another construction method I liked was the pro composites personal cruiser: Pro-Composites

    Either method would make a quick fuselage and the wings and empenage could be built more conventional.

    Marc

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

    Quote Originally Posted by proppastie View Post
    The Pilot operating manual of every aircraft I have flown has a performance section where distance over a 50' obstruction is shown. Perhaps this is useful for this exercise....
    I have pilot manuals for the C-152 and Piper Tomahawk. Always a great source of information for the aspiring designer! The issue I've run into is that the manuals specify T/O and landing distances for the airplane, from which you can determine if you can get into a given airport or not. But what if you're the designer, and doing it the other way 'round - you have a list of sample airports and how do you decide how much of that runway to use?

    I think either my 60% or the 50% Matt suggested are about right, but I don't know. Power pilots out there... When looking at a particular airport with your POH in hand, how much of the airport's available runway do you allow for takeoff, for matching up to the distances listed or computed from the POH? The whole thing? 60%? 50%? Some other amount?

    It's been many years since I've been PIC of a powered aircraft, and I was a student pilot at the time. Any other advice on this score from anyone?
    "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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    Registered User BBerson's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    For clearing the trees, it is mostly about climb angle.
    I design the aircraft to get the best climb angle for the engine I design around.
    That is about it. I am not concerned with speed.

    What is your main design point?
    My design point is best climb angle and everything revolves around that.

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    Moderator Topaz's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by BBerson View Post
    For clearing the trees, it is mostly about climb angle.
    I design the aircraft to get the best climb angle for the engine I design around.
    That is about it. I am not concerned with speed.
    Well, with the question at hand this moment, I'm concerned with neither speed nor climb. I'm looking at takeoff distance, using the "distance to 50' altitude" specification so common in both takeoff and landing specifications. While that implicitly includes a short climb segment, in all the books from which I'm working it's considered separately from the climb phase proper.

    So say you're on a trip with your G-109. You know the destination airport and you have the POH for the aircraft. You look at the POH and you see that for the conditions you're expecting the day you arrive, the POH says you need "X" distance to take off and get to 50'. Now you look at the length of the destination runway, online or in whatever you use for that. Are you checking that the runway length is greater than "X", greater than "X times two", or ... what? What's the "safety factor" you like when determining whether you want to take off from a given runway or not?

    That's the open question to all powered-aircraft pilots reading this. I don't have any recent experience upon which to fall back, so your input is welcome.

    Quote Originally Posted by BBerson View Post
    What is your main design point?
    My design point is best climb angle and everything revolves around that.
    I'm not focusing on any one particular design metric. Instead, the "design point" is the entire requirements list. While it's certain that one performance parameter or another will eventually decide the final sizing of the aircraft, I don't yet know which one that will be. I can tell you that, while a lot of the work I've shown so far in establishing requirements has been for powered flight, the emphasis on the design is going to be focused on self-launch soaring, since that's what I'll be doing most often. In the end, there is going to be a lot of optimization of the wing to ensure that I can get a reasonable climb rate, cruise speed, and at least a fairly smooth ride (higher wing loading), while absolutely maintaining the soaring performance goals I set.
    Last edited by Topaz; September 22nd, 2014 at 05:58 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

    Just a quick note. This question has been vexing me, so I took another look in JAR 22, the rules set I'm generally using at a guideline for requirements. JAR 22.51(a) specifies that the takeoff run should be less than or equal to 500 meters to an altitude of 15 meters. That's 1,640 feet to an altitude of 49 feet. JAR 22.51(b) specifies particular speed conditions that apply.

    That's shorter than the full runway length for both of my "critcal case" runways, so I'll use that as a threshold "absolute" value for takeoff length, and then try for better in matching the 60% runway lengths I've already listed.
    "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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    Registered User BBerson's Avatar
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    Re: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    I never look at the POH. It is for a new aircraft tuned to perfection and a bit of exaggeration.
    The real aircraft performance is learned from experience and takeoff needs has to be " guessed at" from experience in similar conditions.
    Just too many variables, density altitude, wind, slope, grass, etc. The runway length isn't very important, but I like at least 2000 feet of runway. The Grob needs 700 feet (solo) to lift off. For a passenger I like to have 3000 feet of runway, which gives about 150 feet at the runway end.

    You need to visualize the climb angle ( not rate) through the initial climb and abort early if something seems wrong.
    The real problem is figuring how far to the trees or other obstacle.
    Some sort of sighting inclinometer would be neat.

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

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

    Quote Originally Posted by addicted2climbing View Post
    ...I am a bit late to this thread, but a year or so back, I was looking to build the same thing. ...
    I thought of you when I started this project. It's quite similar to what you and I talked about, with a bit more emphasis on powered cross-country flight. Who knows, maybe if I ever build this thing, you and I can talk about an electric version...
    "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
    Just a quick note. This question has been vexing me, so I took another look in JAR 22, the rules set I'm generally using at a guideline for requirements. JAR 22.51(a) specifies that the takeoff run should be less than or equal to 500 meters to an altitude of 15 meters. That's 1,640 feet to an altitude of 49 feet. JAR 22.51(b) specifies particular speed conditions that apply.

    That's shorter than the full runway length for both of my "critcal case" runways, so I'll use that as a threshold "absolute" value for takeoff length, and then try for better in matching the 60% runway lengths I've already listed.
    This looks like a good specification to use. As for checking length before take off, I will check length of the runway, but it usually is not an issue. I am usually solo, or at a certified and paved runway which has a minimum length to accommodate standard aircraft. I probably can handle 1600' or greater for landing or takeoff in all the aircraft I have flown, but that does not include trees... say with a fully loaded C-150. Pilot skill and judgement is very important in these situations. I am much more comfortable with at least 2000' and 3000' is not an issue. If I can not trust my engine I should not be flying.
    Last edited by proppastie; September 22nd, 2014 at 09:07 PM.

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

    Quote Originally Posted by BBerson View Post
    I never look at the POH. It is for a new aircraft tuned to perfection and a bit of exaggeration.
    The real aircraft performance is learned from experience and takeoff needs has to be " guessed at" from experience in similar conditions.
    Just too many variables, density altitude, wind, slope, grass, etc. The runway length isn't very important, but I like at least 2000 feet of runway. The Grob needs 700 feet (solo) to lift off. For a passenger I like to have 3000 feet of runway, which gives about 150 feet at the runway end.

    You need to visualize the climb angle ( not rate) through the initial climb and abort early if something seems wrong.
    The real problem is figuring how far to the trees or other obstacle.
    Some sort of sighting inclinometer would be neat.
    Very true. Know your aircraft. The POH is OK as a reference but that's all. (Sort of like a woman with a lot of make-up Not the real world) , that is with a test pilot, the best engine, etc. Don't expect the same from your aircraft. Dan
    Pops

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