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 12, 2019 #41

    Pops

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    Wing platform -- 4' cord, straight wing to 9' ( wing strut attaches at 9') then tapered for the next 10' with a 3' cord at the tip rib with straight LE. Then a one foot long wing tip. With a 24' wide fuselage = 153 sq ' of wing area. 800 lb GW = 5.228 lb sq ' wing loading
    Say a EW of 500 lbs and my 230 lbs and 40 lbs of fuel = wing loading of 5.03 lbs sq'. Total Fuel cap= 10 gal.
    1835 cc, 60 hp , VW engine weight about 140 lbs firewall forward, so that leaves about 360 lbs for the airframe less firewall forward engine weight. Culver 60'x 24"/26" prop. Engine rpm @ 2640 is about 2.9 gph. That is about 55% power. The SSSC cruises at 80 mph with the door up and the windows close at that rpm and fuel burn. So the motorglider should have less drag and maybe cruise at 85 mph or so.

    Comments == Am I far off ?
     
  2. Nov 12, 2019 #42

    bifft

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  3. Nov 12, 2019 #43

    Vigilant1

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    Pops,
    If we assume a no-flap 2d Clmax of 1.4 for the airfoil (that's what TOWS gives us for the NACA 2415, at 14 deg AoA), and reduce that by 10% for real-world 3d lift, then the clean stall speed would be about 35 KTAS.

    I'm sure you've considered this, but, with the wings on (40 feet total span) the plane won't fit in some common hangars. I know the plane is for you, but I'll bet the performance hit for a couple less feet of wing would be minor, and might be worth it from a convenience standpoint (e.g. rain moving in, just want to get the plane out of the elements overnight without having to de-rig the wings, etc).
     
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  4. Nov 12, 2019 #44

    Pops

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    I was thinking of that. My hanger at the county airport for about 20 years was 40' wide. Had to put tape on the concrete floor so I wouldn't hit the wing tips on the end of the opening with my C-172's with a wing span of 36' 6". My hanger door at home is 44' wide , so no problem with a 40" wing span but for putting in most hangers looks like the 36' would work the best and take the small hit.
    I think the 35 KTAS clean stall is about right.
     
  5. Nov 12, 2019 #45

    Vigilant1

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    I just looked at a few Cessnas. You know that some of the heavier planes would probably do better with a bit more span, but it looks like the marketing guys convinced them to stay under 39 feet.
    C-172: 36' 6"
    C-182: 36' 0"
    C-210: 36' 6"
    C-337 (ahem!): 38' 9"
    C-310: 36' 11"
    C-206: 36' 0"

    I thought she'd stall slower, even clean. Oh, well.
     
  6. Nov 12, 2019 #46

    Hot Wings

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    One semi-related data point: AV-36
    Stall = 36 mph (it doesn't really stall and being a wing is probably higher than a 'normal' plane)
    Wing area = 157 ft^2
    Span = 42 ft (folds to about 22)
    2D airfoil max Cl = 1.8 (Again, being a FW it will never be quite as good as a 'normal' plane)
     
  7. Nov 12, 2019 #47

    Pops

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    Just guessing, but the stall of 35 knot with that airfoil sounds about right. Haven't decided on an airfoil. I like what Fisher used on the Koala and Super Koala, but they never said what it was. Flat bottom and a 16% airfoil. Very good airfoil for the airplane. In ground effect the SSSC wants to lift off at about 27 knots in ground effect with the Helo airspeed. No flap wing and a stall buffet starts about 32-33 knots and a very small break at about 30 knots ( non-event). For crosswinds, I wouldn't want any slower stall. Bob Barrows has been trying to talk me into using the Riblett airfoil. He says low speed about the same but a little better cruise speed. I know the Bearhawk Patrol and the LSA are very good on both ends with the Riblett airfoil.
     
  8. Nov 12, 2019 #48

    Jay Kempf

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    36 mph seems suspect and is probably based on the 1.8CL max which is a overly optimistic 2D number I am sure and also suspect.

    I always see these sorts of estimates on non flapped airplanes. On FW's especially the CL of the 3D wing is severely discounted against the 2D numbers. If you could get perfect lift distribution and if you had a near infinite AR you could make claims like that maybe.
     
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  9. Nov 12, 2019 #49

    Hot Wings

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    Numbers based on flight testing for the AV-36. The polar is also from flight tests = 24/1 L/D @~55 mph @ 496 pounds.

    Real world 3D CL = 0.96
     
  10. Nov 12, 2019 #50

    mullacharjak

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    Exactly what was ordered but in wood.
     
  11. Nov 12, 2019 #51

    Topaz

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    Not to veer this off into the AV-36, but one characteristic of "flying plank" aircraft is that, to command and trim a nose-high attitude, the elevator has to be deflected trailing-edge up, essentially making it a "reverse flap." That section of the span which contains the elevator will produce a much lower lift coefficient at the minimum controllable airspeed, as compared to the original airfoil section. With these kind of designs, you have to figure in this effect when you're calculating stall speed. Flying planks, like any pure-tailless design, aren't able to generate as high a lift coefficient on the entire wing as a conventional or even a canard design could do.
     
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  12. Nov 12, 2019 #52

    fly2kads

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    Some of the Fournier manuals are here:
    http://www.fournieruk.com/technical/Manuals/manuals_index.html
    They address how to operate the system, but don't provide any description of how it works. The general procedure is:
    1. Ignition off
    2. Throttle closed
    3. Dive to prescribed airspeed
    4. Pull decompression knob
    5. After prop starts windmilling, release decompression knob and pull out of dive
    6. Ignition on
    From what I can piece together, the "decompression knob" is attached to a spring-loaded lever system that presses down on the rocker arms for the exhaust valves. How many valves? Is one bank sufficient? Both banks? Now I'd like to find more!
     
    Last edited: Nov 12, 2019
  13. Nov 12, 2019 #53

    Aesquire

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    Truth! I've spent a lot of time soaring in a 5-6/1 L/D hang glider. 2 1/2 hours with an Eipper Flexi II in ridge and thermal lift. ( personal best with that glider )

    Going anywhere with that low L/D is very challenging. Double that to 10-11/1 and it makes a huge difference in how high you need to be to cross a gap or make the downwind run to the next ridge. Double that again & you hit "practical" cross country flights. 17/1? That would be fine for a motor glider for recreational use. 30/1 is more than enough....

    But you could have 100/1 L/D and if you sink faster than the, say, 1/3 average thermal in your area ( western NY and Texas aren't the same ) while banked, you aren't staying up except on high wind days in a narrow band close to the ridge, or in a wave. @300-400 fpm sink it's a lead sled.
     
  14. Nov 12, 2019 #54

    BJC

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    To make it “more modern”, I would use an EarthX battery and light weight starter, but no alternator / electrical system.


    BJC
     
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  15. Nov 12, 2019 #55

    Aesquire

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    Anecdotally.... A F-106 that flames out at high altitude has a startling range, 12/1 L/D!!!!!

    But you aren't soaring anywhere but a mountain wave.
     
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  16. Nov 12, 2019 #56

    Pops

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    These are indicated airspeeds. But the indicated at cruise speed is very close to the GPS speeds. Never tested any the lower end of the airspeed. Close enough for me.
     
  17. Nov 13, 2019 #57

    erkki67

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  18. Nov 13, 2019 #58

    erkki67

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  19. Nov 13, 2019 #59

    erkki67

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  20. Nov 13, 2019 #60

    erkki67

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