Solidworks Airfoil lofting question

Discussion in 'Workshop Tips and Secrets / Tools' started by Hot Wings, Oct 15, 2018.

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  1. Oct 16, 2018 #21

    Hot Wings

    Hot Wings

    Hot Wings

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    I remember this from the good? old days of hand lofting these things on grid paper. Kind of makes me wonder exactly what shapes are getting tested in the wind tunnels these days? Are they testing a CNC cut from file airfoil with this 'artifact' intact or are the test models actually built to conform to the ordinates the underlying math spits out?

    Makes no difference to me. I'll just build with the contaminated pepper and no one but the fly and me will know...............
     
    Last edited: Oct 16, 2018
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  2. Oct 16, 2018 #22

    Hot Wings

    Hot Wings

    Hot Wings

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    Sometimes just figuring out how SW wants us to do things takes a bit of trial and error. And there so many ways to error :gig:

    Remember what you are doing is for plans, not CNC bent parts. I tend to fall into the same trap as well - and why I started this thread in a way. We try to put in too much detail - because we can. A customer looking at one of our 3D PDF isn't going to notice, or care, if we show one flute and the rest depicted with a dotted line. We could probably even get away with not showing any flutes and just making mention of them in the directions, or manually add them min the 2d prints, or include a picture of a finished rib. These are just visual aids for non draftsmen builders that don't know a first from a third angle projection.

    For things like the flutes you are trying to draw I've at times drawn just one side of the part using patterns/cut and then shelled the part to the sheet metal thickness needed.

    Going to have to try that. I'm self taught and haven't run across this yet. Looks like a tool I need to learn!
     
    Last edited: Oct 16, 2018
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  3. Oct 16, 2018 #23

    Jay Kempf

    Jay Kempf

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    Take this with a grain of salt because the context is a large development program where you are sharing data back and forth between multiple CAD/Analysis packages and multiple agnostic formats for interchange:

    It doesn't matter in the end. When you are designing before you start worrying about how to build the stuff below the aero surfaces you use either new aero curves or surfaces created to get a certain aero optimization value to be what you want. At that point coordinate systems are a bit important because all people involved need to be on the same system or be able to get back and forth between datums with an offset. At that point it matters a bit but the resolution isn't as high as what you are worrying about normally because the airfoil is positioned around for instance the .25C point anyway. So the real thing to keep common is that XY zero and orientation of the chord line, not just a line drawn between the front and back in CAD. I always use the zero angle chord line and the zero, zero point on the airfoil regardless of whether the curve actually goes through it just because that is the coordinate system that your aero/wind tunnel data is based on. The rest is just manufacturing tolerances. Curve fits based on data sets use different approaches. Just an annoying detail to be ignored for the most part.

    Back to programming.
     
  4. Oct 16, 2018 #24

    Norman

    Norman

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    I've been chastised for trusting the computer too much but these splining "errors" are often smaller than hand drafting errors so what do you do? The NACA 231 airfoil sections provide a good example of the splining problem. The NACA slope+radius method introduced an actual problem (early stall due to a spike in the pressure distribution near the LE) so I don't see any reason to try to reproduce it. The leading edge issue doesn't seem be much of a problem for the 4 and 5 digit airfoils but it is for the 6 digit sections. The method described by Harry Riblett addresses the LE issue nicely. Felix Wartmann also addressed that problem. Riblett actually did it the hard way. With software that can do inverse design you just set the AoA high enough that the spike shows up and then cut it off with a tangent spline. The sharp stall issue with the NACA5 digit sections is due to the extreme forward camber and not because of the nose radius so the nose mod wouldn't fix it. Fortunately 3D planform effects are more important to airplane stall than 2D section characteristics.
     
    Last edited: Oct 16, 2018
  5. Oct 16, 2018 #25

    BJC

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    My guess is that there is almost no actual wind tunnel testing being done these days.


    BJC
     
  6. Oct 16, 2018 #26

    addicted2climbing

    addicted2climbing

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    HW,

    I agree I can fall into that trap too. However, these ribs show the flutes since the airfoil shape is formed by fluting a straight piece of bent aluminum. The flutes give the builder an idea of where they go and how to get the shape. If it was purely a visual thing, like flutes on a bulkhead, you can bet I'd not even bother with them. Also as I mentioned the plan originals were missing the rib so I needed to draw it anyhow so may as well add the detail. Originally the plans had the builder cut the plan in half and tape together to get a full size rib. I am supplying a full size template and possibly even a drawing with the build jig base laid out so the builder can just drill a few clearance holes in the jig base and be ready to go. I am also planning to offer pre bent material to build the ribs and may even replace the rib widgets with 3D printed parts in glass reinforced filament. None pressing and will add these in the future.

    Also there will be a few items either waterjet cut or CNC milled as well... I'm not going to draw the entire plane but will draw much of it in my free time as I move toward subkits.
     
  7. Oct 16, 2018 #27

    lr27

    lr27

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    A search on "wind tunnel" on the NTRS gives 104 results published in the last year. So I guess there are some things which are still too complicated for CFD.
     
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  8. Oct 17, 2018 #28

    Jay Kempf

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    Too complicated for CFD isn't the point. The only thing you can do in CFD is derivatives of things that have been verified in real life. So the wind tunnel is used for the innovative stuff to make sure the theory is real. Then that data point can be used for all kinds of things where CFD can tell you all kinds of things.

    Boeing can only design close derivatives of things they have already spent decades verifying. If they get off in the corner of the known universe and want to innovate they are right back in the tunnel.
     
  9. Oct 17, 2018 #29

    BJC

    BJC

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    The discussion is about airfoils, so that was the focus of my comment. Perhaps I should have written “My guess is that there is almost no actual wind tunnel testing of airfoils being done these days.”

    I checked the first 50 recent NASA reports with “wind tunnel” in the descriptor, and found no examples of airfoil testing.


    BJC
     
  10. Oct 17, 2018 #30

    proppastie

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    In 2d autocad I would construct a 3 point ark or circle at the LE and Pline or spine from there, Practically speaking your LE when metal or wood would be an ark at the nose, I guess in plastic you could make it something else.
     

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