Has anyone be thinking of designing a more modern, light weight powered sailplane ?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Pops, Nov 11, 2019.

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  1. Nov 28, 2019 #121

    Victor Bravo

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    I resemble that remark ! I still have one or two of my old soaring hats, and I think they make a fine statement of fashion.

    Regarding another post, a retired United Airlines pilot told me that their DC-8's had a L/D of 20-1 in the clean cruise configuration.
     
  2. Nov 28, 2019 #122

    stanislavz

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    My small contribution about in flight starts "helper" autogyro in far 103 class as a rotor - prerotator do use just an big cordless drill. Small shaft with hex end.. It helps to stay in 254 empty lbs category.

    Same could be sued here if needed..
     
  3. Nov 28, 2019 #123

    Pops

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    The glide ratio is far better than that. At about 45 knots it wants to just hang in and floats. Get about 40 or 42 you can tell the sink rate goes up. What is the glide ratio of a T-Craft? Its about like that at least.

    Did a ROC chart, wished I had done a rate of sink.
     
  4. Nov 29, 2019 #124

    bifft

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    I find 10:1 for a Taylorcraft (https://www.aopa.org/news-and-media/all-news/1996/march/pilot/new-pilot-(2)).
    Putting in Cdi and Cdp for 10:1 @ 45 kts says only 28 hp needed for 80 mph, 399 fpm min sink.

    Expanding to 38 span kicks that up to 12.7:1 @ 40 kts, 82 mph on 28 hp and 283 min sink.

    So, by my crude calcs the glide ratio wouldn't go up much without considerable attention paid to cleaning up profile drag, but just increasing the span really helps with the sink rate.
     
  5. Nov 29, 2019 #125

    BBerson

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    Well, glide ratio and sink vary together. (Double glide ratio will cut sink rate in half)
    But that's only at constant speed.
    Cut the speed (as in post 124) and now the minimum sink may improve more than the glide ratio.
    The problem is real airplanes get heavier with span increase and may not get a slower glide speed.

    Hang gliders have a low sink speed with rather poor glide, with very low wing loading.
    And that DC-8 in post 121 with a L/D- 20:1 has a horrible sink rate.
     
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  6. Nov 29, 2019 #126

    Topaz

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    Sailplane pilots obsess about L/D because that's what wins sailplane races - getting as far as possible between thermals, as fast as possible. Recreational glider pilots (and hang-glider pilots) are much more concerned about the minimum sink rate, since that determines how weak a thermal will keep them aloft. If your minimum sink rate is -200 fpm, you need to find a thermal or ridge lift that's going up at +200 fpm, just to sustain level flight. If you're just hanging around locally, L/D is, within limits, far less important.

    From his description so far, I seriously doubt Pops is going to be doing any "distance" soaring in his new design, and instead will be catching local thermals and ridge lift, staying in the local area. As such, reducing minimum sink is going to be the dominant soaring performance goal. That means reducing induced drag is the most important concern, and that means span. Since the minimum sink airspeed is usually just a little above the stall speed, parasite drag reduction is going to be less important, and whatever effort is made there will primarily be for powered cruise, not soaring. Span will be considerable, compared to a normal tube-and-rag airplane, as will be aspect ratio. For a two-seater, you'll be looking in the 45-50 foot span range. Either struts or a carbon-rod capped wing spar would keep the weight down.

    I would consider -200 fpm the very worst minimum sink rate you could tolerate and still have some fun soaring on reasonable days. The SGS 2-33 training glider has about this minimum sink rate, and if your area has a good ridge that works often, or local "house" thermals that will work that hard, you can do some decent soaring. A better minimum sink rate means you can stay up in weaker lift, and therefore will be able to stay up more often, and for longer. A good sailplane will have a minimum sink in the -140 fpm range or less. You probably won't achieve that in this airplane, but somewhere in-between the two values would make for a decent-performing "local fun" motorglider.
     
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  7. Nov 29, 2019 #127

    Pops

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    I like this. Weathered in at my place for 2 weeks.

    283 min sink sounds good. Build light and clean. On the SSSC I put all the control cables inside including the elevator horn. Used 500 x 5" wheels and tires for several years ( 800 x 6" cost 2 knot of cruise. Flying with open windows and door folded down cost about 3 knots of cruise. The cruise of 80 mph is at 2650 rpm burning about 2.9-3.0 gph.
     

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  8. Nov 29, 2019 #128

    BBerson

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    image.png 2-33 specs above. Note: the L/D is same for both 1 person or two.
    But minimum sink is much less with one person. Weight has no effect on L/D but a huge effect on minimum sink.

    So the motorglider problem is that engines and the extra wheels add much weight. So how can a motorglider compete with a glider on minimum sink?
    The upside of motorgliders is the option to look for better thermals. Or like I do, just glide down in still air for fun.
     
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  9. Nov 29, 2019 #129

    leifarm

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    I have a Pipistrel Sinus like the one that weathered in by you, but with the tailwheel in the proper place. It's got the soaring performance you want, but visibility when circling isn't great. Compared to the higher wing loading TMGs like the Super Dimona I think it`s much better in marginal lift. Build a U2, not a Starfighter!

    Also used to have a Super Koala. Fun little airplane.
     
    Last edited: Nov 29, 2019
  10. Nov 29, 2019 #130

    billyvray

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    Whisper motorglider. Started as a kit. Now evolved to a fast sport plane, Whisper x-360 or something. On Facebook the original builder has started a scratch build and will sell plans for about $500. 2 place, sbs, all foam, rotax 912 power typically.

    WHISPER.jpg
     
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  11. Nov 29, 2019 #131

    Pops

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    I will never forget the 42 mph on the 2-33. Sort of special for me. In my instruction in the 2-33, if I went 41 mph the instructor would lean forward and say with a loud voice, " why are you going to stall , get the speed back to 42" and if I went to 43 mph he would be saying " get it out of the dive". He died about 30 years ago, like to hear him again. Very nice person and a great pilot. Been an instructor in powered, gliders, float planes since 1937. We went to OSH together many years and he would take me to sailplane contest with him where he was flying. Lot of fun.
     

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