Theoretical question about wing thickness

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by geosnooker2000, Jun 10, 2019.

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  1. Jun 10, 2019 #1

    geosnooker2000

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    Using the Zenair CH 640 as an example, Mr. Chris Heintz claims "I guess fat wings are one of my trademarks. The depth lets me build a strong wing that is very light, and gives excellent stall characteristics as well. Under 200 mph, the thickness doesn't contribute to significant drag, so why not use it for its other advantages?"

    This is a question about the effects of thinning the design of the wing.

    I am assuming that would result in less drag and a faster stall speed? The stall would be more abrupt?

    I know that going with a different airfoil design would have an effect, but for now, let's assume the same camber and generally the same airfoil design, just slimmer.
     
  2. Jun 10, 2019 #2

    pictsidhe

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    Mr Heintz is right, you won't gain anything at low speeds from a skinny wing. Unflapped Clmax/Cdmin is maximum around 15%. There is little loss above that. 18% is almosr as good. Using a wing of less than 15% will actually add drag as well as make rhe plane heavier. Add flaps, the optimum is even thicker than 15%.
     
    Last edited: Jun 10, 2019
  3. Jun 10, 2019 #3

    wsimpso1

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    Sorry to be a grammar nut, but I neither get requests nor questions out of the above. Since I am wired a bit weird, I might be able to guess as the intent...

    At Mach numbers in our range, min drag on turbulent flow wings and on well executed laminar flow wings occurs in the 12-15% thickness range. There is no point in going thinner than 12% even on tail sections, and little penalty in going to 18%. The Boomerang, the best performing light twin ever flown, uses 17% thick wings.

    Thinner foils require more weight to make strength, can be higher drag at cruise, can have lower stall Cl. There are exceptions and the data will show that.

    As to stall behaviour of a real airplane, there are lots of things involved besides the airfoil selected. Good stall behaviour can be achieved lots of ways. Bad behaviour is available lots of ways too. More on that later...

    If you want to explore the effects of foil thickness, camber, and base profiles, there are a couple books your should have on your shelf:

    Theory of Wing Sections by Abbott and von Doenhoff
    GA Airfoils by Harry Riblett

    Neither is expensive and both have LOTS of info on whole families of foils. TOWS is test data by NACA, Riblett is done in software, but have proven out very nicely in a bunch of different airplanes. I have it on good authority that while Harry was a bit of a nutjob, his airfoils are good.

    Billski
     
  4. Jun 10, 2019 #4

    mcrae0104

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    Two wings with the same camber but different thickness will produce the same amount of lift when they fly at the same angle of attack.

    Generally, for two wings with identical camber, the thinner one will have marginally less drag but the thicker one will stall at a lower angle of attack. Therefore the thinner wing will have the lower stall speed. (There are limits to this, such as an extremely thin wing with a leading edge radius so small that it drives the Clmax AoA down.)

    I would add one book to Billski's recommendations, and I think it's the best one to start with: Airfoil Selection: Understanding and Choosing Airfoils For Light Aircraft by Barnaby Wainfan. It's 60 or so pages and will give you a good background when you dive into TOWS or Riblett (both of which are excellent).
     
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  5. Jun 10, 2019 #5

    geosnooker2000

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    I Find this comment very interesting. I will check out the readings. Thank you everyone.
     
  6. Jun 10, 2019 #6

    Riggerrob

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    Leading edge radius often determines stall characteristics, with large radii stalling at steeper angles of attack.
    Note that most of the after-market STOL kits (Robertson, Sportsman, Wren, etc.) primarily increase the leading edge radius on stock Cessna wings. Cessna even adapted one of those STOL kits to later production C-172s.

    In an extreme case, a very thin wing - with the same camber - would have such a sharp leading edge that it trips airflow earlier than a fat (18% thickness) wing. The other disadvantage of too thin a wing is that spar weight increases dramatically as they get thinner. Spar weight is inversely proportional to the square of the spar depth. Under-cambered wings are also a pain to build with their concave surfaces. The majority of homebuilt wings have flat bottom skins to simplify jigging on flat tables.
     
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  7. Jun 11, 2019 #7

    TFF

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    12-14 ish percent thickness is pretty much the spot for all around performance. Unless you are only interested in speed or STOL, the middle is the best overall. I would pick your airfoil and then see if you can make a solid wing design in the 12-14% range. If you want to bias it one way or another, 1% at the most. Anything else, I would be picking a different airfoil at a sold doable spar depth.
     
  8. Jun 11, 2019 #8

    bmcj

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    Remember that airfoil shape has an effect on control forces. Some of the high-lift kits for Cessna tend to make the controls feel a bit heavier.
     
  9. Jun 11, 2019 #9

    mcrae0104

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    I didn't think that was possible. o_O
     
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  10. Jun 16, 2019 #10

    geosnooker2000

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    I was able to get a plans elevation view of the Sling TSi airfoil with dimensions. Turns out it is about 14.4%. I can't find a good elevation (section) view of the CH640 wing to compare the two, short of buying the $475 plans. Any CH640 builders out there who would care to share the figures?
     
  11. Jun 17, 2019 #11

    Heliano

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    Very interesting thread. Let me propose a somehow more pragmatic aproach: how about surveying those designs that worked out right and see the kind of airfoil that was used? One thing I've noticed is: those wings with higher aspect ratio have greater thickness, for a very good reason: thicker wings are lighter under high bending moments, and they are stiffer too, which means less risk of aeroelastic problems. However lower aspect ratio wings do not exibit such great tickness. Examples:
    1 - Heron, an UAV from Israel with AR of 18: root airfoil 21% tip 15%
    2 - Fokker 27, a turboprop that was popular in the 60's wing AR of 12, root 21% tip 21%
    3 - Glasflugel Libelle, a german glider, AR of 23, wortmann airfoil 18% thickness.
    Therefore it seems that high AR wings need greater thickness for structural reasons. Lower thickness wings don't. But it seems to me that when it comes to safety the point is stall abruptness and stall propagation. For example Airfoils such the 5-digit 43015, for example, should be used with great care due to the abrupt stall. 43012 is even worse. In these cases washout and/or slotted ailerons are a must.
     
  12. Jun 17, 2019 #12

    BJC

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    All of the airplanes that I have flown with the Clark Y and USA 35B use washout to improve handling near the stall. Very common. Ditto the 23012 / 2301X on multiple, well known, well performing airplanes. Leo won the world aerobatic championship with a 23012 / 2301X without washout.

    The Ercoupe (with its limited elevator authority) uses a 43013, and I see that there are several non-USA light aircraft using the 43012. Does anyone here have first hand experience with one of them? I know that we have some Ercoupe operators here.


    BJC
     
  13. Jun 17, 2019 #13

    pictsidhe

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    If you are wondering about airfoils for a low aspect delta wing, they are a different bucket of bananas. Deltas have a huge amount of induced drag at high Cl, rendering high Cl foils somewhat pointless unless you have SR71 amounts of thrust.
     
  14. Jun 18, 2019 #14

    Heliano

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    Answering your question, BJC, we had an aircraft here in my country - delivered to the Air Force for primary training in the late 60's - which had exactly the same Ercoupe profile (43013). I've flown some 10 hours on it. This aircraft did not have washout. Did not have slotted ailerons or any other way of improve stall propagation. It killed a test pilot - the aircraft went into a flat spin and could not recover. Another 5 or 6 were also lost due to spin or due to a sudden uncommended snap roll in the traffic pattern. The aircraft had a very malign stall characteristic. The aircraft was named Aerotec T-23 Uirapuru. I do not mean that all aircraft using this type of airfoil will be a killer - but the designer must exercise care. The Corby Starlet, a popular australian design, uses a 430xx airfoil and does not seem to have problems. But if you look at the attached CL vs. alpha graph , you can see the sharp drop in CL at stall. That is an indication that such airfoil is not very forgiving.
     

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  15. Jun 18, 2019 #15

    pictsidhe

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    I just read the wikipedia page on the Uirapuru. The ventral fin stall fix it got is the same that fixed the Hurricane's reluctance to stop spinnning. Nothing about it's stalling behaviour. But I am deeply wary of 5 digit airfoils as they can be very nasty in certain configurations. I'll leave it to better aerodynamacists than me to work round their quirks...

    Edit: It appears to lack wing root fillets. Fillets radically changed the stall behaviour of the P-40 evolution of the P-36 and of early Spitfire predecessors.
     
    Last edited: Jun 18, 2019
  16. Jun 19, 2019 #16

    BJC

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    Thanks for the reply.

    I am not familiar with the T-23. According to WiKi, they added a ventral fin to cure a spin problem, and that is logical, because unrecoverable spins in GA type aircraft usually are the result of too little yaw stability or yaw control force.

    I knew about the sharp stall characteristic of the 2-D airfoil. As you point out, having a 3-D wing that stalls inboard first tames actual whole-wing behavior.


    BJC
     
  17. Jun 19, 2019 #17

    pwood66889

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    Thank you, BJC, for the opportunity... :)
    I believe the 430xx airfoils have hysterisys; that is to say, they do not "unstall" at the same Angle of Attack (AoA) as they stall (I am not be saying that well, and probably misspelling it to boot).
    The Ercoupe up elevator is set to keep the wing from being held beyond critical AoA. Flow over the top of the `foil would resume after elevator authority limit reached. Had to demonstrate this breaking in CFI's that were new to the `coupe during the BFR. This may the reason behind the manditory placard: “This aircraft characteristically incapable of spinning.”
    I pointed out to Mr. Harry Riblett that the `coupe was tractable despite the 430xx series. We "agree to disagree." He avers that series is dangerous, I say I'm flying the whole plane, not just the airfoil! And I have over 300 hours PIC in type.
    In his book "From the Ground Up," Mr. Fred Weick said 13% thickness was the best compromise he could come up with.
    Hope this is useful to Mr. Heliano.
     
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  18. Jun 19, 2019 #18

    BBerson

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    The Schweizer 2-22 I soloed had about the most benign stall with 43012 airfoil. Same for the 2-33.
    Wouldn't spin.
    From the NACA graph in post 14, it has a sharp initial drop but then regains some lift up to 30° AOA.
     
    Last edited: Jun 19, 2019
  19. Jun 19, 2019 #19

    Scheny

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    Gliders normally fly at lower induced angles of attack due to high aspect ratio.

    BR, Andreas
     
  20. Jun 19, 2019 #20

    pictsidhe

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    One of NACA's few test failings is that they did not do test hysteresis on airfoils. A lot of hysteresis is bad for a docile stall.
     

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