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  1. Jan 3, 2012 #1

    Georden

    Georden

    Georden

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    Just wondering what the current theories are for wing tip design for light aircraft? Specifically those with constant wings.
     
  2. Jan 3, 2012 #2

    orion

    orion

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    There are a lot of theories out there regarding tip design, and even some published information, although that's pretty limited. The bottom line however seems to be that for most relatively lightly loaded wings as we have in GA, even the most "optimized" wingtip treatment will provide little benefit, in some cases even immeasurable over something more generic. Improvements in induced drag or other penalties are often most visibly realized on larger aircraft with heavier wing loads (50 psf and up) so for our lightly loaded planforms it's often just as good to design something that meets the aesthetic eye rather than getting too fancy. Interestingly enough though, a poorly designed elaborate tip (like a winglet) may actually harm the performance, and that is measurable.
     
  3. Jan 3, 2012 #3

    topspeed100

    topspeed100

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    Is there any correaltion between a winglet and a boxwing...I recall one boxwing maker claims his plane span has been increased by let's say x 1,3 ( cannot remember the figure right now ) due to the boxwing.
     
  4. Jan 3, 2012 #4

    autoreply

    autoreply

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    Constant chord wings you mean? Taper will cause the biggest decrease in induced drag (for a given wing weight) and if you limit it to around 0.7, you probably can avoid twist and other complexities.

    Winglets cause a large decrease in induced drag, especially on shorter wings. But induced drag is only significant if you fly far slower that most GA aircraft cruise (only useful for higher climb rates). Designing them correctly is very hard though. As Orion noted, one degree of "toe-in" or "toe-out" off and you've just increased drag by possibly as much as 5%... Not to mention the structural implications (noticeably flutter), higher spar loads. Bottom line, stay very far away from winglets, even aftermarket options, unless it's someone who's done a full windtunnel program with them...
     
  5. Jan 3, 2012 #5

    Aircar

    Aircar

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    I used to be pretty anti retro fit winglets but keep hearing a lot of good reports of improved handling (in roll) -- got an open mind on the subject now (having first flown the Vari Eze with winglet rudders which are an essential part of the design probably doesn't count and the case for simple extension vs winglets seems lineball at best except for 15M limited .

    Topspeed is correct in quoting an improved 'e' value for box wings or C tips or non planar wings -- I guess to some extent an upturned and inward lifting wing throws the tip vortex further outboard and also the 'upwash' becomes 'inwash' so affecting a slightly greater streamtube of air --the added area at a large span is considerable but of course the up bend of the wing itself reduces the notional stream tube and the lift vector is no longer all lift so a conceptual understanding is still unclear ( and why an inverted V tail should be better than an upright V tail for normal tail downloads for similar reasons --Wil Schuemann's 'thought experiment' with swept back and polyhedraled wings --and his reworking of an ASW 12 to 15 meters , influenced all the German manufacturers to adopt the concept --from a homebuilder using intuition to later theoretical justification (just like about every other aspect of civilization )
     
  6. Jan 4, 2012 #6

    Georden

    Georden

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    Yes I meant constant Chord wings. Taper doesnt seem to offer any real gain that is worth the added complexity for my very basic design.

    So if I understand Orion correctly I can simply end my wing with a squared off tip and not be losing any measurable amount of performance?

    The design I'm starting on is meant to be simple and quick to build, so a lot of features are not optimized for the sake of simplicity. eg the fuselage is squared off, wings are constant chord etc. The intention is to gain back some of the slight losses with a lower weight that will result from the simple structure.
     
  7. Jan 4, 2012 #7

    ClippedCub

    ClippedCub

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  8. Jan 4, 2012 #8

    orion

    orion

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    If you want another company's understanding of wing tip design, simply look at one of the most efficient production planes out there - the Mooney. Although the wing is tapered, it is very simply squared off at the tip. As such, you can square it off or maybe do something like the aforementioned 45 deg. angle cut, which is then closed off with a flat rib (I think Sonex does this).
     
  9. Jan 4, 2012 #9

    autoreply

    autoreply

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    That depends on a lot of things, but taper is by far the most effective way to decrease induced drag. If you can't be bothered with that since you're mainly interested in cruise, use a straight wing ;)
    A slightly rounded tip, with roughly the radius of half your spar depth at the tip will help prevent separation and thus reduce drag a bit. Simply rotate around the chord line and it's fine. Fairly easy to make with a bit of foam and sanding and it gives you a nice "bumper" when you're hitting something, plus a good mounting location for lights.
    Which type of structure? In metal it makes sense to stay constant chord, in composites not so much.
     
  10. Jan 4, 2012 #10

    Aircar

    Aircar

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    The theory about the square cut wing tip is to force the tip vortex to develop just a little bit further outboard than on a rounded wingtip which encourages the flow to wrap around and appear on the upper surface --the Schreder HP sailplanes had simple straight cut wing and tail tips based on that idea .

    The traditional wing tip (semi circular 'bow') like on Piper Cubs actually had a lot going for it because, probably unwittingly, it behaved like a Zimmerman or ARUP disc wing which does not stall until 30 degrees or more and then gradually --rapidly 'sheared' wing tips --roughly triangular in planform could also act a bit like a delta wing and retain lift also to high AoAs without stalling (the delta wing is often described as "two wing tips stuck together" and functions from the 'tip' vortex roll up also.

    Squirting air out of a wingtip has the effect of blowing the tip vortex further out and adds virtual span so it would actually be a good place to run engine exhaust and might get some free de icing along the way as well as decent noise suppression .
     
  11. Jan 4, 2012 #11

    ClippedCub

    ClippedCub

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  12. Jan 4, 2012 #12

    fly2kads

    fly2kads

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    I have a question about how a wing with raked tips (swept l.e.) would be analyzed. I seem to recall one source that suggested that, for purposes of calculating the effective span and aspect ratio, that one should use the mean span (~halfway out the swept tip) and not the full tip-to-tip span. Comments? Would one also do the same when looking at the wing bending moments? Just curious.
     
  13. Jan 4, 2012 #13

    ClippedCub

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    Last edited: Apr 16, 2012

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