Flying wing as cheap and simple option for basic fun flying.

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Hot Wings, May 18, 2017.

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  1. Aug 4, 2017 #901

    Autodidact

    Autodidact

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    Topaz is right, and I felt a little convicted by his statements and so I went to do a little more analysis of the "campstool" (it's not even a lawn chair), and I went at it pretty much exactly like you said; I went to the 103-7 appendix and looked up the lift factor for a double surface airfoil with no flaps (since it's a tailless, can't have flaps) which was 1.6, and found the wing loading would need to be no more than ≈ 3.13 lb/ft^2, goofed up and used the wrong weight and got 121.7 ft^2 instead of 105 something ft^2, figured up an alpha of about 1.7° for a 3d Cl/Calpha which corresponded to a Cm of 0.03188 or so for the Marske XM-1D airfoil and figured up a moment (taking into account tip losses) of about 188 lb/ft at 60 mph. If the gross weight is 380 lb, then that puts the CG about 6" forward of the 1/4 chord point which I show in the sketch here, but I didn't quite get the estimated CG - based on the estimated component weights - quite forward enough which is probably OK since the wing really only needs to be 105 ft^2 and there is a cut-out at the TE for the prop so I can avoid putting the pilot way out in front of the wing a la Breezy - which should reduce the moment a little. Anyway, I approached it just as you described and found out what the moment might be at 60 mph and then tried to arrange the weights so that the CG corresponded. A few more iterations and mistake corrections and it should be ready for some basic structural calcs. I think the structural weight of the wing and the sling, minus the engine, should be 85 lb or less, hopefully. It's veerry tempting to try and build this because it's so simple (or simple looking, anyway).

    As far as the faster Mitchell style wing I'd like to design, yes, it needs a lot of thought, and I'm fully prepared to find out that the symmetrical airfoils are not up to it. But I just can't get over how good some of them are; the Eppler 479 symmetric airfoil has almost the same Clmax as the NACA 2415 and a much better drag polar...

    campstool2.jpg campstool1.jpg
     
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  2. Aug 4, 2017 #902

    Hot Wings

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    And this is the reason I keep thinking just tacking on a canard, while not being the 'pure' way, is very practical for a part 103. Imagine a U-2 with a canard, not for pitch control, but for trim. This lets us call the junkers elevons - eleflaperons.
     
  3. Aug 4, 2017 #903

    pictsidhe

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    The appendix stall calcs aren't applicable to flying wings. A swept flying wing can use flaps. It might be possible to use LE flaps on a plank?

    Edit, just checked the appendix and I can't see planks being excluded from the stall calcs.
     
    Last edited: Aug 4, 2017
  4. Aug 4, 2017 #904

    Hot Wings

    Hot Wings

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    But if it has a canard it is no longer a flying wing.;)

    Per the AC we can't use the area of the canard in the calculation, but a 10 foot square canard that can't be considered is a very good trade for being able to use the higher lift factor. It also helps considerably with the CG problem that different pilot weights creates.
     
  5. Aug 5, 2017 #905

    Aesquire

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    Autodidact, like that one! Good visibility in the directions that need it. Cushions for a lighter pilot to move him forward?

    Silly question. What's the minimum non-flapped wing area for a pt. 103? By the formula.
     
  6. Aug 5, 2017 #906

    Aerowerx

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    Assuming a 200 pound pilot, and a max CL of 1.4

    24 kts is 40.5 ft/s. 254 pound empty. 200 pound pilot. 30 pounds fuel.

    The formula is...

    S=2*484/( 40.5^2*0.00238*1.4)

    Which gives 177 square feet.
     
  7. Aug 5, 2017 #907

    pictsidhe

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    I believe that a 170lb pilot is used for stall calculations.
     
  8. Aug 5, 2017 #908

    Aerowerx

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    Ok, 166 square feet then. And no more donuts!
     
  9. Aug 5, 2017 #909

    pictsidhe

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    I've shed a few doughnuts myself recently, but I really don't look good at 170...
     
  10. Aug 5, 2017 #910

    Aesquire

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    Not even Krispy Kreme? Their signature glazed weighs near nothing.

    And magically converts 3 times it's mass to fat when I eat them... Maybe I shouldn't buy the 2 dozen for the price of one?

    Thanks for the formula.
     
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  11. Aug 5, 2017 #911

    Norman

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    You're going to be very hard pressed to get that from an airfoil with a positive Cm and low drag. If you don't care about drag it's not too hard though. Here's an example:
     

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  12. Aug 5, 2017 #912

    Norman

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    So can planks but only one obscure type. Split flaps can have Cm=0 if hinged at about 40% of the wing's chord. Farther forward they even produce Cm>0. In this position they do increase lift. Think of it this way: if you move the hinge all the way to the leading edge it would be a Kruger flap.
    http://www.homebuiltairplanes.com/forums/showthread.php?t=14378&p=157650#post157650
    http://www.homebuiltairplanes.com/forums/showthread.php?t=14011&p=152351#post152351
     
    Last edited: Aug 5, 2017
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  13. Aug 5, 2017 #913

    Aerowerx

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    I just picked that number out of the air**, Norm. 1.2 would be more realistic, I think.

    -------
    **Genuine pun.
     
  14. Aug 5, 2017 #914

    pictsidhe

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    I have a paper somewhere comparing dive brakes. It is one of the very few papers I found on split flaps. Another ARC paper showed power assistance for a split flap. The back was a bellows and the chamber formed was vented forward or aft to open and close it. A droop nose flap will give +Cm, that could be combined with down elevon. Or do a SWIFT, and let it trim the aircraft for lower speed? It seems that a 1.4 Cl would be a valid appendix calculation figure for a plank.
     
  15. Aug 5, 2017 #915

    Autodidact

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    Reality aside, the lift factor that the plank wing would be allowed to use in the the 103-7 wing loading chart is 1.6 which works out to 3.13 lb/sqrft - just divide estimated empty weight plus 200lbs (170pilot + 30fuel) by 3.13 and there's your area. No reason for a simple wing to weigh 254 lb....
     
    Last edited: Aug 5, 2017
  16. Aug 6, 2017 #916

    pictsidhe

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    With 51% dive recovery type flaps, a lift factor of 2.0 would be valid. 3.9 lb/sqft? This is rather interesting.
     
  17. Aug 6, 2017 #917

    Autodidact

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    You're right; it doesn't say that the flaps have to be attached to the trailing edge, it just says how much of the span they have to cover. If the plank weighed 130lb empty, it could have an area of less than 85 sqrft.
     

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