Issues and Solutions for kit- and plans-based sailplanes and motorgliders

Discussion in 'Soaring' started by TerryM76, Sep 3, 2019.

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  1. Sep 10, 2019 #201

    BBerson

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    Not interested in thermalling anymore. 99% of the flights will be cross country while powered or just fun glides. Thermals are rare here.
     
  2. Sep 10, 2019 #202

    PiperCruisin

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    Ground looped once...didn't appreciate it. I was mainly pointing it out since nose draggers are fairly rare in gliders (at least ones designed to stay that way and not plop on the tail when the people step out). I also tend to touch down at a high angle of attack. Sounds weird when you are landing an ASK21 an you hear the tail wheel turning, but you have not landed yet. :oops:Here is another:
    upload_2019-9-10_13-42-4.png
     
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  3. Sep 11, 2019 #203

    Topaz

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    Getting a wing to stay fully in the drag bucket at both low and high speeds and have good stalling characteristics, without resorting to full-span variable camber (camber flaps and "drooping"/"reflexing" ailerons) is a bugbear for me. Doing partial-span camber flaps (for example, not including the ailerons) just ups the induced drag at low speeds (distorted lift distribution) and parasite drag at high speeds (unaligned flap/aileron segments, and especially the resulting gap between them). My own project may be too low-performance to need this kind of sophistication, so I may have it easier, but without any kind of camber change, you get into off-design fuselage angle of attack at one end or the other - or both ends if you optimize for the middle. My current airfoil has a drag bucket just big enough to accommodate both thermalling and cruise, without camber flaps, but that's as much a function of the low cruise speed as any sophistication in the airfoil itself.

    Not trying for absolute "peak" performance, as you are, I'm satisfied with a conventional tractor configuration for the simplicity and weight benefits, and will likely have some kind of "cowl flap" arrangement and a self-feathering prop to keep the drag at least "down" when soaring.
     
  4. Sep 11, 2019 #204

    Jay Kempf

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    Camber changing is a must have for me. It has it's complexity but it is what it is. No engine on the nose for me. The composite parts are actually the most straight forward for me. It is just a lot of floor space. Welded parts are minimal and I don't trust myself to make flight parts yet but I do trust myself to make mockups and the parts. Plenty of good welders out there. Some machined parts I can do. Some I wouldn't cause I just don't have the machinery. Canopy is one of those things you just suck it up and buy. I can make the drape mold. The engine package is not a big deal, neither is the folder. Cooling drag is a big design task but not impossible.

    You might consider putting your base airfoil in its flapped cruise trim and putting up with that for thermalling. You have an engine. You won't be able to slow down to sniff out the marginal stuff with a low sink rate a low speeds but you can push the button and fly to non-marginal conditions. That would up your cruise efficiency and if you are careful about drag you can still do well overall. I think you are under estimating the drag of your feathered prop.
     
  5. Sep 11, 2019 #205

    bifft

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    If you're willing to give up cruise and just accept takeoff and climb on the engine, wouldn't that simplify the wing design? The climb and thermal Cl should be the same or similar.
     
  6. Sep 11, 2019 #206

    proppastie

    proppastie

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    Perhaps there should be a distinction between a power launch glider and a motor glider, and maybe between a contest machine an "other glider" ..We also have electric, ICE, turbine, pusher, tractor, ........Of course that further fragments the population and lowers your potential market.....but we all know how to make a small fortune in aviation.
     
  7. Sep 11, 2019 #207

    Jay Kempf

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    There is already a distinction between only launch powered gliders and touring motorgliders. And there is a BIG performance difference between the two in terms of soaring performance. Published data for the DG1001T is 46.5:1 at max gross with the 20M wingtip extensions. Aeromot AMT-200 Super Ximango (Brazil) publishes 31:1 was designed by Fournier and is 17.5 M span. Real world probably less than 30:1.

    Those are probably the two best in class of the two types of gliders with motors. Neither has what I would call stellar climb performance but the Ximango is very conventional except near 70' of span.

    So maybe 50:1 is tough on a two seater but it is all about drag budget management and 40-45:1 is very doable and more than adequate for real soaring in all but the most marginal conditions. Wing loading will drive that. 20 meters (65 feet) is way too much to be practical for a motorglider. But 17M might be fine with AR somewhere north of 22. The DG publishes capacity for 353lb of water ballast. No need for that in a motorglider. So use that budget to get the wing loading of a smaller higher aspect ratio wing in the reasonable range.

    Topaz, the airfoil set I am using is from the ASW 27. Very well tested, well behaved, seriously low drag at both ends of my spectrum. Lots of published data. Why reinvent the wheel. It is designed flapped @85%C but can be used unflapped. In these Re regimes it has across the board benefits. I think I have coordinates for 13 through 14.5% t/C seems to have a real sweet spot at AOA around +5°.
     
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  8. Sep 11, 2019 #208

    Topaz

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    Yes, it would, to an extent. But even "pure" unpowered sailplanes are optimized for flight at two design points: Thermalling near the minimum-sink airspeed and intra-thermal flight at or near the airspeed for best L/D. Modern ultra-high performance competition sailplanes have such good sink-rate performance at all speeds that it's actually becoming easier to design them, so to speak: They can be optimized more for higher-speed flight near or at best L/D and they really don't stop to "circle" in thermals, rather just "porpoising" through them in a roughly straight line. They gain altitude when crossing the thermal, and lose it again when racing at high speed to the next one. There's much less emphasis on low-speed performance.

    As Jay noted, there is already a distinction between "self-launching sailplanes" and "motorgliders," especially that subset of motorgliders known as "touring motorgliders."
    • Self-launching sailplanes are very accurately described - the engine is used only to replace a towplane, winch, or auto-tow for the launch, then shut down for (ideally) the remainder of the flight. In the real world, the engine is also used for "saves" (to avoid an out-landing), and for hunting lift if it proves to be outside of the gliding range area. The latter types of operation have, with internal-combustion engines, been a bit problematic, as the operations to extend, retract, and cool-down an ICE take highly drag-producing time, and the soaring performance of the aircraft is greatly reduced during this period. Electric-powered self-launchers are now the most-popular type of motorized sailplane on the market, because converting to powered flight is usually a "flick of a switch", and there's little or no "cool-down" period required after using the engine, before you can clean the glider up for maximum soaring performance.
    • Motorgliders, and "touring" motorgliders in particular, are expected to spend much more time under power, cruising from one place to another. It's more of the philosophy that, "I'm going to fly to my destination and then soar, rather than trailer the airplane there and launch from that location." They also can be useful for limited cross-country under power without any soaring at all. The compromises to make powered flight more practical tend to reduce their soaring performance somewhat: More emphasis on proper engine cooling, more-robust (heavier) landing gear that can accommodate true taxiing at an airport, etc.
    This is a Ximango that was at the recent ESA Western Workshop. The wings are folded in this shot.

    IMG_20190901_124244.jpg

    Interesting. How the heck did you get a hold of those? ;) We should probably talk via PM or e-mail. You'll recall that I'm doing a touring motorglider that is, in concept at least, very much modeled after the ASK-14. Once I scaled it down from the image last seen in my design thread, and in further developing the design, my two biggest airfoil constraints turned out to be "thinness" to keep the weight of the full-depth foam down, and good performance at low Reynolds numbers, ranging as low as ~390,000 at the tip in hot-and-high conditions at stall speed. I had been planning on the Wortmann FX79 when the airplane was larger, but that well-known airfoil completely fell apart at the kind of Reynold's numbers I'm looking at now. I ended up in the oddest of places for an airfoil, based on a suggestion by John Roncz. The published original version of the Eppler 361 (mod) that I'm currently baselining was originally designed as an advanced helicopter blade airfoil. (LOL!) Turns out that it's got drag characteristics almost as good as the FX79, operates down to ridiculously low Reynolds numbers, is only 12% thick, and has a nearly zero pitching moment coefficient, for reduced trim drag at all speeds. I had to increase the camber slightly to bump up the Clmax just a bit, but that turned out to have almost no effect on the rest of the characteristics.

    It's a thought. I'm still battling in my mind about the final wing design. Span versus buildability in a "standard" 20'-deep garage, choosing my final wing loading, flaps, etc. As I mentioned earlier, I'm at the point where the wing can operate well at both design points un-flapped, but fuselage angles of attack are not exactly optimum at either point. That may push me to flaps in the end. I don't want the complication, but since I need ailerons anyway, that "complication" is rather limited to the mixer necessary to schedule them in concert with the plain flaps inboard. I just don't know yet. Part of the problem is that I'm pushing for the aircraft to qualify as an LSA, so the flaps can't be used for meeting the LSA stall-speed requirement. It forces the wing loading fairly low (which was the FAA's point), which brings best-L/D speed down quite a bit anyway. Ugh.

    No, not really. I know it's "a lot." But recall that I'd be perfectly satisfied with 28:1 best-L/D performance, very happy with 30:1, and ecstatic with 35:1. I'm mostly going to be using this airplane for house thermals at either Skylark (CA89 or Mountain Valley), ridge-soaring at Skylark CA89, and similar flying at destinations in the southern half of California, reachable under powered-cruise. I have no real intentions of using the airplane for unpowered cross-country - as you say, I have "the start button" for that. For this kind of work, a tractor feathered prop is "good enough", especially if I can close off the cooling flow with some sort of cowl-flap arrangement once the engine has cooled down. If electrics had the endurance I need to meet the powered-cruise range specification, I'd probably be looking at those, just to avoid the cooling-drag-while-cooling-down situation altogether. If anyone else ever wanted to build another one of this thing, they could easily fill the "gas tank" volume with batteries and have a mid-performance self-launcher instead of a touring motorglider.
     
    Last edited: Sep 11, 2019
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  9. Sep 11, 2019 #209

    Jay Kempf

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    That is not important and it is on a need to know basis :)
     
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  10. Sep 11, 2019 #210

    Hot Wings

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    All of us participating in this thread have a "need to know". ;)
     
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  11. Sep 11, 2019 #211

    BoKu

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  12. Sep 11, 2019 #212

    Topaz

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  13. Sep 11, 2019 #213

    BoKu

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    I did that exact thing two years ago. Steve Smith analyzed the results. Great if you want to spend a lot, and spend a lot of time going fast.
     
  14. Sep 11, 2019 #214

    proppastie

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    is it one airfoil or does is change out to the tip.....might need more than a day.
     
  15. Sep 11, 2019 #215

    BJC

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  16. Sep 12, 2019 #216

    BoKu

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    We identified three different profiles along the semispan, there may have been more. We also identified an interesting kink in the washin/out distribution. All told we used about two hours of access.

    I do that pretty often with my own stuff. But when it comes to borrowed machinery in the hundred AMU sector, owners get kind of nervous when you break out the bondo. Also, it's difficult to find someone with a hi-res scanner that accepts chunks of plywood.
     
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  17. Sep 12, 2019 #217

    Jay Kempf

    Jay Kempf

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    This is what I have been working on behind the scenes for a few years now. 15 meters. 2x 95th percentile males for reference
    upload_2019-9-11_19-42-33.png
     
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  18. Sep 12, 2019 #218

    Topaz

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    That's beautiful, Jay!
     
  19. Sep 12, 2019 #219

    proppastie

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    On the Carbon Dragon wing, I modeled ruled surfaces between the tip rib profile and root rib profile...are there discrete steps in the wing you measured or is it a smooth transition?
     
  20. Sep 12, 2019 #220

    Jay Kempf

    Jay Kempf

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    Smooth! There are 4 different airfoils for the wing and one for the winglet. I cheated. I was given coordinate files. Didn't have to do any transfer molding. Ground handling configuration. Wide gear. Remove wings and still taxi it around. Folding tips and go right in a hangar. Tips way clear of landing lights
    upload_2019-9-11_20-55-3.png
     

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