candidate for magic airfoil section?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Starflight, Dec 3, 2010.

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  1. Dec 3, 2010 #1

    Starflight

    Starflight

    Starflight

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    I know the title is 'corny' but after cruising through those hundreds of sections
    graciously provided by "krauss", I couldn't help wondering why the 18.3% thick
    Gottingen 679 showed such a superior L/D ratio (64.8) when that number is
    usually reserved for thinner airfoils. The plotted polars add up (divide that is)
    and I know the Rn is low at 100000. Could any of the owners of aerodynamics
    programs please run the numbers for me at R=500000, 1million and 1.5million?
    The last mentioned number approximates the landing approach speed for typical
    ultralights. Are there any stall problems with that section in your opinions? Thanks.
     
  2. Dec 4, 2010 #2

    HumanPoweredDesigner

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    http://www.worldofkrauss.com/foils/1419

    I'd like to see it at Re = 500,000 too.

    I don't know why the L/D is higher. I do know it gets the extra lift because the top keeps curving down even towards the trailing edge. This does give it a higher pitching moment. Reflux does the opposite on both accounts.

    You can mitigate the pitching moment by putting the spar further back, but who knows what affect that may have at other AoA's. The nice thing about most airfoils with a spar at 75% is they self stabilize: if the AoA increases, the pitching moment helps pitch it back down. I don't know if this would remain true at a different spar location.

    Also note worthy is that the max L/D is achieved at a peak. If you increase the AoA even slightly from there, the drag jumps up dramatically. I think that is a separation bubble. The higher the Re, the later it develops. The reason the L/D max is there is because that is the highest Cl before the huge drag sets in.

    Also due to the peak nature of that L/D max, if you lower the AoA, the Cl drops fast and you no longer have the great L/D.

    I would think that airfoil would be great for fast take offs, since you can get a high Cl at low Cd, just as long as you are aware of how fast L/D change with AoA.

    http://www.worldofkrauss.com/foils/208
    That one has a lower pitching moment, but also a lower L/D max.
     
    Last edited: Dec 4, 2010
  3. Dec 4, 2010 #3

    orion

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    That's actually not accurate - the highest l/d numbers (notice lower case - applies to sections only) tend to be for sections in the 15% range, although there is some variation up and down, depending on particular family. Surprisingly enough, a substantial number of thicker sections tend to exhibit excellent ratios.

    One critical issue however is at which point those l/d ratios are evaluated. And I repeat, this is very critical. Some sections may exhibit the highest l/d values at low lift coefficients (such as needed for cruise) but in comparison to others, may actually have poor l/d performance at higher lift coefficients such as those you might need for take-off and climb. Keep in mind that cruise is not the only design condition, especially for low powered airplanes such as ultralights.

    Not likely, especially at the low wing loadings ultralights will tend to operate with.
     
  4. Dec 4, 2010 #4

    topspeed100

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  5. Dec 4, 2010 #5

    orion

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    That may be but keep in mind that there are hundreds, if not thousands of different airfoils out there. Given the typical geometry and flow requirements a typical wing has, it is unsurprising that many sections tend to look alike, especially the fat low speed ones. There's only a few sections that tend to be relatively distinct: for instance the five digit series (23012) tend to be easily identifiable, even to some casual observers.
     
  6. Dec 4, 2010 #6

    HumanPoweredDesigner

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    For 15% I think clark Y m 15 trumps the USA 35B. The Goe trailing edge curves down more than the 35B. They don't look similar to me.
     
  7. Dec 4, 2010 #7

    HumanPoweredDesigner

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    I have XFLR5 on my system. I can post screen shots and you can tell me where to click. I can't figure it out. I got a NACA 4412 to pop up on the screen from the library, but I could not get any data about it.
     
  8. Dec 4, 2010 #8

    Norman

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    Here's a comparison polar with 3 other sections of similar thickness and camber. The Gottingen 679 doesn't look that good to me. When looking at that old wind tunnel data keep in mind that the turbulence factor was not controlled very well, if at all, and the experimental procedures and equipment weren't identical. If you're comparing results from different tunnels your conclusions probably aren't any better than the 10% or so error from this type of code. If Human can get any results from Xflr5 that look different from what I have it's because his settings are different. I'm using:
    Ncrit=11
    Mach=0.1
    FILT=3
    And the version of Xfoil in Profili only runs in viscous mode. Any of thes can change the results.
     

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  9. Dec 4, 2010 #9

    HumanPoweredDesigner

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    Wow. Each of those at Re = 500,000 don't get nearly the coefficient of lift that the 100,000 airfoils are claimed to get by that database. That is a pretty big difference in testing methods.

    I could not find the NACA airfoil in the data base, but this one appears to be the closest:
    Airfoil Investigation Database - Showing NACA 63(3)-618
    Its minimum drag is much lower than the GOE, at 0.02. Also, the L/D max occurs in a round area, where L/D does not change much with AoA, which is good.

    This suggests to me that I can't really know how much coefficient of lift to expect from an airfoil. 10% accuracy sounds believable. I guess computers are just to get you in the ball park so you can do real testing. Same with scale models. Sad.

    I would still go with the GOE simply because the trailing edge is easier to fabricate.


    My advice: google the stall speed of real airplanes, and look at their airfoil and aspect ratio. That is where you get some real Cl data, though ground effect and tip extensions may mess that up. As for Cd, airplanes have so many sources that you can't know what it is for the wings.


    What I do find reassuring is that the GOE and many turbulent naca foils posted on here at Re = 500,000 do not seem to have separation bubbles in their polars. The polars seem to be smooth curves, unlike what I see in other airfoils and lower Re airfoils. I also think a fabric wing would not get the low drag potential of the laminar section, which probably has to have perfect ribs too.
     
  10. Dec 4, 2010 #10

    topspeed100

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    They don't look similar..for sure..but I really did mean the USA 35 B is a thinner Goe 679..they are not identical.

    The USA 35 B also has very high L/D considering how thin it is. There is a relation anyhow..IMHO. I believe what I see...and strangely enough..the numbers seem to indicate similarities as well.
     
  11. Dec 4, 2010 #11

    Starflight

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    HPD-you hit upon my second choice :O) the Clark YM-15; there are many alternatives as well.
    Norman-thank you very much for the comparison graph; the NM and Eppler look like THEY are the "point"
    airfoils and the -818 has too much camber(nice gradual drag rise). How large a chord would these models
    be at that 500K R#?
    Orion- My continued interest in thick airfoils was based on their primary advantage; light structure(stiffness).
    They usually have very good stall manners. My multi-engined U/L (20hp total) would have a Vc of about 65mph
    since the thrust velocity of the off-the-shelf props is less than 80mph. I would like to get the best climb for
    the available power (800 foot runway) which also means balancing drag and pitch moment. High lift devices
    are not preferable, and I am operating in a sub-PiperCub envelope. This is why I am looking for the most lift
    at a minimum AOA; thus the cruise and climb will not be separated by more than a few degrees. Maybe I can
    make one exception option...full span,narrow chord ailerons with up-travel for cruise(reflex)? :O)
    TS100- that is a nifty avatar you have...highly modified BD-5? What engines are you using?
     
  12. Dec 4, 2010 #12

    HumanPoweredDesigner

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    Depends on the speed, air temp, and altitude. I forgot what the viscosity of air is in the range I used. In answer to your question though, at 30 mph, probably ball park 19 inches.


    You sure are getting excited by this airfoil when two different data sources had strongly conflicting results. Sure, it depends what other data values are assumed. I would pick the airfoil that is easier to build and easier to make strong.
     
  13. Dec 5, 2010 #13

    Starflight

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    HPD-Norman set the velocity at Mach=.1 (75mph? at sea level STD) That would be Vmax for my 4foot+ chord wing with an Rn close to 2.5million.
    My objective with the design is to build and test numerous wing sections and see which ones fly more pleasantly/efficiently/safely than some others.
    Slip in one set and go climb like hell, or another and up the speed by a few knots; sort of a REAL full-scale scientific experiment. Get other pilots' inputs too. :O)
     
  14. Dec 5, 2010 #14

    HumanPoweredDesigner

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    I think that is what you'll have to do, since this software is kind of unreliable. It is good for finding the good candidates. Then you test the top 3, and hopefully the first one you test is good enough.

    I'd actually prefer the Clark YM-18 because of the option to fly upside down.
     
  15. Dec 5, 2010 #15

    HumanPoweredDesigner

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  16. Dec 5, 2010 #16

    karoliina.t.salminen

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    For example HQ 2.5/14 gives quite good results. The l/d curve has quite flat top and gives quite high l/d for wide angle of attack (from cruise to climb aoa) according to simulations. The pitching moment is not as big as on some other widely used airfoils while not as low as some lowest moment airfoils. It of course depends on the use case how much importance the pitching moment and trim drag has, depends on the drag breakdown in the plane. Attached is a pic where the HQ 2.5/14 is compared to one my airfoil. This simulation: Re = 2 000 000, NCrit = 9, trip location = 1.0. The flatter l/d curve on the bottom right is the HQ 2.5/14.

    Simulation produced with XFLR5.
    Download XFLR5 here: http://sourceforge.net/projects/xflr5/files/xflr5 v6.01_Beta/
    XFLR5 is a multiplatform airfoil simulation tool which is available for MacOSX, Linux and Windows. It is also open source, so you can validate (and improve) the algorithms used by yourself should you disagree with the results.
     

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    Last edited: Dec 5, 2010
  17. Dec 5, 2010 #17

    karoliina.t.salminen

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    Added simulation which includes göttingen 679. (Cl vs Cd simulation graph)
    The drag at low lift coefficient is fairly high. It does not look magical at all to me, rather poor instead I would say. I had to smooth the original coordinates which had poor resolution with only 33 points. Loses quite a bit to e.g. HQ 2.5/14 in every aspect. The worst line (on the right) is the goe679.

    On the l/d graph, the goe679 is the turqoise line which has the lowest l/d until very high Cl of the compared airfoils.

    Because the reduced size attachments are hard to read, I uploaded the original files to our server:
    http://www.katix.org/karoliina/airfoils/goe679.jpg
    http://www.katix.org/karoliina/airfoils/goe679ldgraph.jpg
     

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  18. Dec 6, 2010 #18

    Starflight

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    Gee; HPD, what are you using for power? I kinda like what NACA was trying to do with those 4 digit airfoils too.
    Early on, I was enamored of the 5 digit ones for their low Cm, but can not now stomach the sharp stalls; too many
    pilots have turned their last finals with these (hint). At really low R (under 300000)they are softer somewhat but I
    will leave them to the fighter jocks. I have to fly upright with only 20+hp on tap and a wing load around 4#/sq.ft.
    Formula for no-dispute Canadian UL is: (metric) 165Kg max launch weight(empty+fuel) minus 15 divide by 10= 15sq.meters wing area.
    Minimum legal wing area is 10sq. meters. For the min. wing area= 107.64sq.ft. your launch weight= 254# (FAR part 103 legal :O)
     
  19. Dec 6, 2010 #19

    HumanPoweredDesigner

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    I'm definitely afraid to try anything funny looking. The clark Y has been around for ages, so I'd only pick a naca foil or other foil that somewhat resembles it, though 12% is too thin for me. Besides reliability, a smooth airfoil is much easier to make ribs for, and probably also to attack skin to. You can make the most efficient airfoil there is, but if it is so thin that your spar is heavy, then you will need more wing and more drag, and only do as well as the regular airfoil that was easier to build.

    I don't know how much power is needed for aerobatics. I think the main issue with flying upside down at least is cambered airfoils get worse L/D upside down, though flaperons probably help with that. A loop probably takes more power, but not if you dive into it. Could be dangerous if you don't have it all calculated out before hand.
     
  20. Dec 6, 2010 #20

    NorthwestJack

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    I wound up using a gottingen 324 airfoil for my design. If the data is to be believed , the 324 has very high lift and fairly low drag at larger angles of attack, which would seem to be ideal for a slow moving airplane.
    Jacq
     

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