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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 cheapracer View Post
    Besides blatantly not true verified by hundreds of motorsports crashes every year
    We're talking about airplanes here however. Those structures you mention work just fine. If strong enough however they'll be prohibitively heavy, which is why we don't see them in airplanes. Maintenance on a crop duster is a good teacher there.
    [...]no one, including you, is building aircraft with Formula 1 spec baked carbon fibre monocoques that have had millions spent on development inclusive of crash testing. Their builds include strict regulations as to where thicknesses and weights are applied - the weight and style of build is simply out of the question for an aircraft period.
    You're entitled to your opinion of course.

    Unfortunately for you, most LSA's and microlights use exactly the same materials they use in F1. T700/T1000 are common in both. Same for resins. Same for prepregs and manufacturing methods. Same for test labs even.

    F1 is 700kgs with a 150kg power/drive unit, so that's 550kgs/1200lbs for a small single seat airframe, empty, no engine, etc - not going to happen.
    An F1 monocoque is roughly the same mass as a sailplanes ahead-of-wing fuselage mass or me. Both work fine. Pretty sure that safety cockpit design and test expenses are pretty similar.

    Fortunately for us, much of those test reports are more or less public (many via the Idaflieg) and thus we don't have to do anywhere near as much testing ourselves.

    Sure, there's the myth of incredible complexity in making such "high-tech" parts. Having witnessed several such parts being designed and built, I could build every single one of those within tolerance in my shed, except for the lack of an autoclave. Even that is changing with a major shift to OOA, notably infusion.


    Back to the real topic; I'd simply copy the structural layout of the Lange Antares or the ASW27. As good as it gets and calculating the required lamina thicknesses is pretty straightforward since the loads are too.

    For crushing, alternating layers of foam with thin layers of glass is pretty light and very simple to tailor to your specific loads. It's also very easy to implement in the rest of the design, which is the biggest difference with almost every other approach (alu foam, collapsable seat supports and so on)
    Kennis vermenigvuldig je door het te delen.
    (You multiply knowledge by dividing it)

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

    Gentlemen: Might you please break this discussion off into its own thread? 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: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Well, I have to admit I haven't read this whole thread. I HAVE read the design thread.

    Topaz:
    I know you've already decided on an airfoil, but I think there MAY be better choices, especially if you don't like building flaps. Click image for larger version. 

Name:	polars for Topazs cheap motorglider.jpg 
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    The polars on some of the airfoils above are reduced to correspond with the reduced chords you could use due to their higher maximum lift. To be fair, you'll have to look at the numbers for correspondingly high lift coefficients, and reduce the drag coefficient corresponding to the reduced chords. For instance, if we wanted to compare the Eppler 749 to the Wortman airfoil at a Cl of 1 with flaps, we'd look at the Eppler's figures for a Cl of 1.32. That's about 0.011. However, due to the reduced chord, we'd multiply by 0.76, giving an equivalent Cd of 0,0084, compared to the Wortman's 0.008. The Wortman is using flaps, though, where the 749 is not, so in the real world it might be a wash. Similarly, the Wortman, at 0 flap, has a Cd of about 0.005. The E749, at a Cl of 0.26, has a corrected Cd of .005 too, or close to it. I threw in the Ara D 20 percent because of the obvious structural advantages of a 20 percent airfoil. Plus the magnitude of the pitching moment is much less than the E749, which is worse than the Wortman. You'll find that the Ara D looks good thinned to 15 percent as well, and in both cases seems to work well with flaps, if you are so inclined. But there may be better options with flaps. Another advantage is that upper surface transition is much sooner, meaning less area to make smooth enough to be laminar. Maybe the rear 60 or more percent could be fabric covered to save weight. The Ara's shape is simpler than the others too. Be sure to cut down the trailing edge width. The Ara was, I think, designed for a wind turbine or propeller, so, unfortunately, the thinner versions are meant for higher Reynolds numbers.

    You might also be interested in my hobbyhorse airfoil (I didn't design it, I just like it), the FZX ng-7, which I think is found on the Xfoil Yahoo group. Another one, with the silly name of Yahoo! AF Group Hi Lift GAV 4, is found on, surprise surprise, the Airfoil Yahoo group.

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

    Oops. Somehow this post slipped "below the fold" on the New Posts search without me seeing it. Sorry for the delayed replay.

    Thanks for the suggestions, lr27. I'm a little confused by the mention of "reduced chord" and changing the wing geometry for these new airfoils. My wing loadings, chord, span, and area are set by the performance requirements of the airplane, and an airfoil chosen to suit, rather than the other way 'round. That process is detailed in the design thread.

    In the end, I'm very happy with my choice of the FX79. It meets my design requirements, and is a known and understood quantity. I just don't feel the need to go through the airfoil analysis process again, when I already have something that meets my requirements very well. If you'll recall, exceeding the design performance requirements is not a plus for this project. I want something that performs exactly to the specifications and requirements I've layed out - not worse, and not better.
    "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: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Topaz,

    You can change the wing loading depending on maximum lift. The higher the maximum Cl, the less area you need to get the stall down to a reasonable speed. Let's say you had a 100 square foot wing on your airplane, and you wanted to keep the stall the same while switching from a 4412 to, for some reason, an 009. The max Cl for the 4412, according to Profili, is about 1.7 at a Reynolds number of 1.5 million, but the 009's is only about 1.1. Seems to me that you'd have to increase the wing area by 1.7/1.1 or 55 square feet! Of course, that would increase the Reynolds number, so you might not have to add as much if you did it by enlarging the chord, but it would still be a substantial amount.

    One of the other advantages I think you might be able to get with these other airfoils (not the 4412, but the ones I mentioned in an earlier post) is a broad enough lift range that you didn't need flaps. You wanted simple and cheap, right? The other is less area that has to be critically smooth, since the flow goes turbulent anyway at something like 35 or 40 percent chord on the top.

    It seems to me that one can't really divorce airfoil from wing area.

    What am I missing?
    Last edited by lr27; January 14th, 2017 at 05:55 PM. Reason: eliminate duplicate sentence, clarified what I meant by "other airfoils"

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

    Finally got around to reading the 'build' thread and have dipped into this one. Suggestion, put a bigger fuel tank in, you don't have to fill it. But if you ever get an urge to attend Oshkosh or sun n'fun, you may appreciate it. For such rare use, I'd leave everything else as is and make allowances for longer TO and lower g's if you use it. 30lb baggage is unlikely with your case size.

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

    Quote Originally Posted by lr27 View Post
    Topaz,

    You can change the wing loading depending on maximum lift. The higher the maximum Cl, the less area you need to get the stall down to a reasonable speed. ...
    Well, that's not strictly true. If you'll recall from post #37, the maximum allowable wing loading for this airplane to meet all my specifications is 7.6 pounds per square foot, and there are two mission requirements that are equally driving that number: Stall speed, and takeoff run. I can't arbitrarily decrease the wing size (increase the wing loading) without adversely affecting the takeoff distance - which is already pushing my threshold limit for this airplane. Best L/D glide and minimum sink glide start to become affected at 11.6 and 11.2 psf, respectively, so there are definite limits that I have to respect in terms of wing loading, or else I'll start to sacrifice key performance parameters.

    So while yes, there are higher-lift airfoils out there, they really don't gain me anything, in this particular case.

    I'm really happy with my choices so far. I appreciate discussing this with you, but I'm going to stick with my choices as-is, unless some later calculation invalidates them.

    Quote Originally Posted by pictsidhe View Post
    Finally got around to reading the 'build' thread and have dipped into this one. Suggestion, put a bigger fuel tank in, you don't have to fill it. But if you ever get an urge to attend Oshkosh or sun n'fun, you may appreciate it. For such rare use, I'd leave everything else as is and make allowances for longer TO and lower g's if you use it. 30lb baggage is unlikely with your case size.
    Heh. Right now, I'm more worried that the fuel tank location I have will hold enough fuel - the whole 10 gallons. I haven't done the volume calculation on it yet. It's not hard, I've just been too danged busy.

    I see what you're saying, though. Yeah, within weight and balance limits, I could trade baggage for fuel. If the volume available proves to hold more than 10 gallons, I may just take all of that volume and do exactly what you say. The dude on the other shoulder is saying, however, "Do you really want to sit in a low-wing-loading motorglider for hours on end, being beaten to death (and distracted by) every thermal you cross?" Yikes!
    "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: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by Topaz View Post
    The dude on the other shoulder is saying, however, "Do you really want to sit in a low-wing-loading motorglider for hours on end, being beaten to death (and distracted by) every thermal you cross?" Yikes!
    Focus.....you can always have two aircraft.....sell the Porsche.

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

    Quote Originally Posted by proppastie View Post
    Focus.....you can always have two aircraft.....sell the Porsche.
    "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: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

    Quote Originally Posted by Topaz View Post
    snip I can't arbitrarily decrease the wing size (increase the wing loading) without adversely affecting the takeoff distance - which is already pushing my threshold limit for this airplane. Best L/D glide and minimum sink glide start to become affected at 11.6 and 11.2 psf, respectively, so there are definite limits that I have to respect in terms of wing loading, or else I'll start to sacrifice key performance parameters.

    So while yes, there are higher-lift airfoils out there, they really don't gain me anything, in this particular case.

    I'm really happy with my choices so far. I appreciate discussing this with you, but I'm going to stick with my choices as-is, unless some later calculation invalidates them.
    snip
    The dude on the other shoulder is saying, however, "Do you really want to sit in a low-wing-loading motorglider for hours on end, being beaten to death (and distracted by) every thermal you cross?" Yikes!
    I'm having a bit of trouble figuring out how a high lift airfoil won't help you keep equivalent performance with less area, as long as you keep the span the same. Unless the shortened chord makes the Reynolds number too low, or the wing too thin. And some of the high lift airfoils are quite thick. One other possible problem might be that you'd need a higher angle of attack for max lift.

    An additional benefit of a high lift airfoil combined with a higher wing loading is that you'll bounce somewhat less, as the lift change per degree of change in angle of attack will be less. The lift coefficient change ought to be the same, but since, instead of going from, say, a Cl of 0.5 to a Cl of 0.7 because of a gust, you might go from a CL of 0.75 to 1.125, and the percentage change will be less.

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

    BTW, if you sell the Porsche, if you can find a 1976 Ford Fiesta, they are very economical and very fun to drive. If someone put brail messages on the road, you could read them with the steering wheel. ;-)

    But only if rust isn't much of a problem in your area, and you aren't addicted to low zero to 60 times. Unfortunately, mine kind of broke on the tire jack due to rust. I kept it for a while after that, but the driver's door never closed right again. Before that, there was the time I ran through a puddle at 40 mph, the repair in the floor let go, and a fountain appeared around the cut in the floor carpeting at the stick shift.

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

    I doubt his Porsche will yeald an airplane. My wife always says that about my cars, but they would not bring in enough. Cheaper to keep that fun.

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