Revisting thicker skins for laminar flow

Discussion in 'Sheet Metal' started by LHH, Jan 17, 2020.

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  1. Jan 17, 2020 #1

    LHH

    LHH

    LHH

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    Found an older post on this subject, hence the subject line
    Question:
    Are laminar flow wings/fuselage always impossible with aluminum construction regardless of construction methods?
    Does no construction method exist to solve this issue?
    Prebent/molded thicker wing skins, aluminum over foam or other substructure, spar less, high AR wings with small distance between front and rear spars etc... does nothing solve this problem?
    Understand the solution may not be worth the effort or weight, but still wondering if a solution exists.

    Thanks in advance
     
  2. Jan 17, 2020 #2

    BJC

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    Stretch-formed thick (0.040”) aluminum skins can be made to laminar flow accuracy.

    See the SX 300.

    Thin skins bonded to PVC ribs can achieve reasonable laminar flow.

    See the HP-18.


    BJC
     
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  3. Jan 17, 2020 #3

    LHH

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    Familiar with HP-18, but not the SX-300, will check it out.
    Any other examples or methods?
     
  4. Jan 17, 2020 #4

    BBerson

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    Schweizer 1-35 sailplane. All metal skin with a fabric layer sanded smooth.
     
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  5. Jan 18, 2020 #5

    pictsidhe

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    I don't know if it is laminar or not, but the cri-cri wing construction could be good enough with care.
     
  6. Jan 18, 2020 #6

    lr27

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    Earlier HP's also had this same kind of construction, as I recall.
     
  7. Jan 19, 2020 #7

    pfarber

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    Laminar flow over a wing has nothing to do with the material, but the design of the wing. Typically the wing max thickness is farther back as a % of MAC. ie a perfectly smooth wing does not make it laminar. The local renyolds number, pressure gradient, and thickness do.

    A smooth surface can reduce the depth of the boundary layer, but most laminar flow research is actually using specially designed 'rough' surfaces that control this skin friction drag and allow the air to 'stick' to the wing longer.

    Granted you need a mostly smooth surface for laminar flow.. but there is always a thin (.05mm?) layer of skin friction drag that is always there and can be manipulated to ASSIST laminar flow.
     
  8. Jan 19, 2020 #8

    LHH

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    Is it fair to say:
    A preformed aluminum wing skin greater than .05 thickness with an underlying structure that keeps the wing from deforming under load is the preferred approach to this issue?
     
  9. Jan 19, 2020 #9

    BJC

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    For homebuilt airplanes? No.


    BJC
     
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  10. Jan 23, 2020 #10

    pfarber

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    The wing rigidity is a tiny, tiny part of the issue. Keeping the air attached to the wing (or minimizing the high drag boundary layer) is the real problem. Some wings are designed with slots/holes to either suck the air back on the wing or blow out more high energy air to re-attach the boundary air to the surface.

    Its a little easier with thin wings, but you still get boundary layer separation, but rigidity is not the issue. You should google laminar flow wing theory, you're just shouting out random things.
     
  11. Jan 29, 2020 #11

    LHH

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    Understanding laminar flow is not the issue, getting it is the issue.
    What I was looking for was the best-documented method for getting close to laminar flow if aluminum was used.
    Aside from snide opinions, the best data I could find was from Dick Johnson's sailplane tests. He always noted the airfoil waviness and measured laminar flow. For example, the last aluminum sailplane commercially made was the Let 33 solo( that I know of). According to oil flow studies, the 33 achieved 50% laminar flow over wings.
    Udo Rompf HP-18 achieved 60% to 70% laminar flow all the way to zigzag tape, however this was a custom airfoil formed using micro ballons over aluminum.
    Swearingen SX 300 used a NLF airfoil, but unclear on the actual wing testing. It was suggested in one reference book SX achieved 70%, but could not find actual test data.
    1-35 sailplane did not achieve laminar flow, but handicap is same as early fiberglass DG-100 and LS-1 and slightly better than other glass planes like the Libelles.
    Let 33 is 14 meters, but has the same handicap as Pw-5's and AC-4's (13 meter)
    Only common factor I can find to date is preformed skins for smoother wing with less wrinkles, creases and so on.
    If anyone knows where to find the Swearingen tests would be very helpful.

    It is very likely that club class performance can be achieved at pretty low cost, but after that glass or lots of sanding will be needed.
     
    Last edited: Jan 29, 2020
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  12. Jan 29, 2020 #12

    pictsidhe

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    NACA published something in the 40s on the required surface accuracy and finish to achieve laminar flow. It may be in TOWS.
     
  13. Feb 2, 2020 #13

    dog

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    cant remember the thread,similar topic,where a poster was scoffing(gently and from mass experience) getting laminar flow in any reasonable
    and sustainable way,what they talked about was spending endless hours with very long sanding boards,getting and then keeping the wing surface smooth enough for laminar flow on competition glider wings, and stating that temerature changes
    and differential expansion were more than adequate to disrupt laminar flow, and back to sanding.Everything else I have read,says ya but no.
    Then I think about watching the dew on the hood of my battered old truck,bead up and run in nice curving lines all the way back.
    The competition glider crews are the ones who are
    realy into laminar flow,and anything else that gets a net gain in lift,amazing stuff, blink and they have lept ahead again.
     
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  14. Feb 2, 2020 #14

    pictsidhe

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    If you read the experimental reports and data, you'll have an idea how much surface accuracy and finish affects transition. Different airfoils do have different requirements. That depends mainly on the pressure gradients before transition. Even a flaw (or mosquito) well upstream of transition in a proverse gradient region will affect where transition occurs. The information is out there. The glider guys are constantly battling for that last 0.001% of performance, hence the huge efforts they make. VB has spent some time with giant sanding boards, perhaps he will chip in.
     
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  15. Feb 2, 2020 #15

    lr27

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    Material doesn't have anything to do with laminar flow on a wing fresh out of a perfect factory, without any lift. Otherwise, it does.
     
  16. Feb 3, 2020 #16

    Norman

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    The problem with getting and keeping laminar flow on a metal skin is not necessarily the stiffness of the skin but rather it's the rivets and panel joints. A laminar boundary layer simply can not cross a gap without being tripped and rivet heads never conform to the nominal profile even if they are perfectly flush with the surface around the edges. Rivets also constitute stress risers that will ripple a thin skin in operation. Bonding rather than riveting would definitely be a step in the right direction but gluing aluminum is a bit tricky and of course you can't easily remove a glued down skin panel for maintenance. Even with a bonded skin it still has to deal with wing bending and torsion loads which will once again try to ripple the skin. Thickness is the only way to prevent these wrinkles but the thickness required to stay under the waviness tolerance is probably much thicker than the structural requirements so metal laminar flow wings would be heavy. That's no reason not to use a laminar airfoil cross section though. Even though the performance will degrade with age you probably won't lose all the drag advantage over say an NACA 4 or 5 digit section and its lift characteristics hardly change at all.
     
    Last edited: Feb 3, 2020
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  17. Feb 3, 2020 #17

    BBerson

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    The Questair Venture had thick, one piece stretch formed wing skins. No panel gaps.
    The rivets can be filled with bondo. But I don't know if they bothered with a laminar airfoil. Top speed 300 mph.
     
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  18. Feb 3, 2020 #18

    BJC

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    23000 series, 17% at root, 10% at tip.


    BJC
     
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  19. Feb 3, 2020 #19

    BBerson

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    That's what I suspected. Powered aircraft owners are not interested in the effort required of a competition sailplane pilot. No reason the metal wouldn't work for laminar, if someone wanted to fill and sand.
    Actually, the people at Oshkosh thought the Venture was composite until they knocked on the skin.
     
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  20. Feb 3, 2020 #20

    LHH

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    .004 "waviness" over 2 inches was the gold standard in the flight tests. .006 appears to be a common number for fiberglass planes. A few were in the .004 range, but almost all regardless of construction method needed work to achieve .004.
     
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