Airfoil data

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by LBarron, Dec 6, 2010.

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  1. Dec 8, 2010 #21

    Topaz

    Topaz

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    No, the air chemistry changes along the length of the vehicle in real time, under steady-state flight conditions. As in, one composition at the nose of the vehicle, and another near the tail, at the same moment. And varying in stages along the way, as different species break down at different points. With energy transfers from the chemistry adding to the drag of the vehicle. :speechles
     
    Last edited: Dec 8, 2010
  2. Dec 8, 2010 #22

    addaon

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    And of course, laminar-turbulent transition increases heat transfer, which changes the chemistry and densities downstream, which changes the positioning of secondary shocks... and the transition point can easily oscillate, we well, making things exciting. Just because you're hypersonic and rarified doesn't mean you can ignore turbulent and transitional effects. Plus, theres a lot of space (80 km altitude +) where not only does the density and composition of the atmosphere change drastically day to day, but we don't even really know the composition with any real confidence.
     
  3. Dec 8, 2010 #23

    Topaz

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    Yeah, have fun with that. :gig:
     
  4. Dec 28, 2010 #24

    HumanPoweredDesigner

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    I downloaded Profili. It is VERY user friendly and robust. Well, robust compared to what I plan to use it for. Who knows what you need it for.

    The free download does not let me check the airfoils that I want, but does demos on the first airfoils in the alphabet. I can scroll through the full list of airfoils though, and the list is much more extensive than the 1700 in Worldofkrauss.

    The reason I'm downloading it is, like everyone said, the wind tunnel tests at worldofkrauss were done in different tunnels. No tunnel or program will give near perfect results, but the goal is to compare airfoils head to head using the same program or same wind tunnel.

    For 15 Euros via paypal, I think this program is a deal.
     
  5. Dec 28, 2010 #25

    Mac790

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    Wind tunnel? where is that info coming from?, I think we are looking at two different pages, those "charts" at worldofkrauss were generated by Javafoil
    besides check out those Re numbers, those Re are for RC planes not for real ones, unless someone is building a kite, it's clearly stated
    Seb
     
    Last edited: Dec 28, 2010
  6. Dec 28, 2010 #26

    HumanPoweredDesigner

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    Javafoil... That would explain why they have the clark Y and the clark Y smoothed, and some graphs are smooth and others very jagged. It depends how many coordinates they used. But that means the airfoils are compared head to head as long as they have enough points and smooth graphs. All the naca foils are very smooth.

    Profili got very similar results to Javafoil for the 12% Joukowski at mach 0.05 and 120 iterations. It predicted better results with the smoother coordinates. The only differences where at 4x the Re (400k vs 100k) there was 1/2 the drag, the linear portion was longer, the angle of L/D max was a bit higher, and of course maximum Cl was higher, though I don't trust those values. At the same alpha, Cl was the same if in the linear region.
     
  7. Jan 10, 2011 #27

    Retiree

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    A couple of points:
    Codes like Eppler and XFoil are empirical. Their results are based on airfoil data. They do not try to calculate the physics of the CFD codes. I know from experience that Eppler over predicts the pitching moment of the airfoil.
    CFD codes do not get the answer exactly right because we still cannot represent turbulence with fundamental equations. Turbulence is klugged in.
    The best way to use codes like Eppler has already been mentioned. Take a published airfoil similar to the one you want to analyze. Use your favorite analysis code on the published airfoil. You can then see where the program is "off". When you analyze your airfoil you can make approximate adjustments to get close the "right" answer.
    ;)
     
  8. Jan 13, 2011 #28

    c p skeates

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    my experience in airfoils is with models, homebuilts ,and hangliders and all are diffreent .models are very sensitive to exact thickness to cord ratios about 11% .. homebuilts are trying every thing and putting up with it if its not exactly right and hangliders are flexable so the only airfoil that has worked is having the camber as far forward as possible which ridgid aircraft cant do because of bad stalling characteristics where as a hanglider flexes so stalling is not a problem and they are trimmed at minimum sink and not at best lift-drag ratio position like ridgid aircraft so it all depends on what your trying to acheive and on what .if your willing to learn all the strategic reasons why an airfoil works and its limitations you will realize that the more pacific you try to be in the macro sence the more it dont work because your trying to get something for nothing in the micro sence ...camber position..camber height ..camber curve length ..thickness .. thickness position ..thickness increase and decrease ..surface roughness ..laminar flow,transitional flow,turbulent flow type ,vortecies gates and position ,reflex,leading edge curve ..each thing makes a difference but only to a degree after which you are trying to get something for nothing ..lastly reynolds number is every thing because of the viscosity to inertia ratio for lowest wetted area rule..
     
    Last edited: Jan 13, 2011
  9. Jan 13, 2011 #29

    Mac790

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    Just one note guys, be careful with some .dat files from worldofkrauss, I've seen some errors in them in past, and here is latest example (maybe not the best one, but like said it's current), just compare those charts for two versions of same foil FX-79 K144, one is from David webpage (black one) other from worldofkrauss (navy blue), there is no problem for un-flapped "analysis", but when you try to simulate -3 flaps problem appear, in short when I set flaps at -3 for black one, I can set panels number at 290, but when I try to change panels numbers (for -3 flaps) with that airfoil form worldofkrauss, airfoil changes shape, so I was forced to "analysis" it at lower number of panels (around 100), difference is huge (actually there is no point to test it at so low panels number), but even with good coordinates I can beat it with modified AH-93 with ease :) special with Cm and L/D.

    Seb
     

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  10. Dec 16, 2011 #30

    topspeed100

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    What kind a margin would use use in defining the ball park rightness between xfoil study and real life...1-3 % or more ? Can it ever be detected ? How much would you put emphasis on the butt-feeling just at looking at the foil ?
     
  11. Jan 24, 2017 #31

    Aerowerx

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    This is an old thread, but I would be interested in the answer to this question myself.
     

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