candidate for magic airfoil section?

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

Help Support HomeBuiltAirplanes Forum by donating:

  1. Dec 27, 2010 #41

    topspeed100

    topspeed100

    topspeed100

    Banned

    Joined:
    May 4, 2009
    Messages:
    4,063
    Likes Received:
    63
    Location:
    Oulu/Finland
    Funny FX-73 ( reflexed ) is closest to the foil that I drew intuitively for the small plane I have on avatar.

    or FX 84 http://www.worldofkrauss.com/foils/765

    Thickest point is not 50% but less tha 40%..this could be maneuverable in short plane ?

    -------------

    Could GOE 425 like foil be better in aerobatics ? Better spin recovery ?

    http://www.worldofkrauss.com/foils/245

    -------------

    Any comment on foils like JPM3-07 where the thickness and camber are the same point ( 42.1 % of the chord ). Easy pitch moment ?
     
    Last edited: Dec 27, 2010
  2. Dec 27, 2010 #42

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
    Top keep in mind that that high cl, means usually high cm, which means higher trim drag, it's not easy to use those foils in planes, special with short tailcone.

    Another tool to play with. Some results are really strange, hmm
    Kevin's Online Panel Code - Version 2.0

    Seb
     
  3. Dec 27, 2010 #43

    Topaz

    Topaz

    Topaz

    Super Moderator Staff Member

    Joined:
    Jul 30, 2005
    Messages:
    13,824
    Likes Received:
    5,460
    Location:
    Orange County, California
    Hi Seb,

    Just want to throw in the point that simulating the performance of an airfoil out to the "nth" degree is largely a fruitless exercise. Remember that the construction tolerances on almost any wing we might build as homebuilders is roughly of the same magnitude as the errors in the various codes such as XFoil, which is why those codes weren't developed to a higher degree of accuracy. There simply is little point in determining the exact performance of a given airfoil, when what will be on your airplane will not be that airfoil, to that same degree of "exactness". None of our aircraft are so performance-critical that we need to nail performance numbers exactly.

    At some point you have to say "good enough" and accept that the real article will differ in performance from your calculations to some slight degree. When someone says XFoil is no longer accurate enough, they've probably passed that point, IMHO.
     
  4. Dec 28, 2010 #44

    HumanPoweredDesigner

    HumanPoweredDesigner

    HumanPoweredDesigner

    Well-Known Member

    Joined:
    Sep 6, 2009
    Messages:
    1,030
    Likes Received:
    2
    Location:
    Arizona
    If you want a thick airfoil with good L/D, how about this one:

    Airfoil Investigation Database - Showing SIKORSKY DBLN-526

    It is 26% thick with L/D = 53 at a high Cl. Only problem is I don't know how you'd attach ailerons or flaps. The Cl max is over 2. The lack of a low drag bucket is probably not good for a speedster, though.

    Do you believe that wind tunnel data? Even more interesting is with the same front half and a pointier back, the performance drops dramatically:

    http://www.worldofkrauss.com/foils/show_compare/?id[]=1244&id[]=1950&chord=6.5

    I suspect this airfoil gets laminar flow over most of its surface, which makes up for the blunt trailing edge. Funny looking airfoil though.

    Here is a 22% thick one that has less high speed drag and more aileron potential:
    http://www.worldofkrauss.com/foils/show_compare/?id[]=1244&id[]=1566&chord=6.5


    I use that site extensively for estimating the Cm at non 25% cord locations. But the Coefficient of drag is so unbelievably low, unless that is hydraulically smooth and Re well above a million.
     
    Last edited: Dec 28, 2010
  5. Dec 28, 2010 #45

    orion

    orion

    orion

    Well-Known Member

    Joined:
    Mar 3, 2003
    Messages:
    5,800
    Likes Received:
    135
    Location:
    Western Washington
    A two dimensional l/d of 53 at high cl is downright awful
     
  6. Dec 28, 2010 #46

    HumanPoweredDesigner

    HumanPoweredDesigner

    HumanPoweredDesigner

    Well-Known Member

    Joined:
    Sep 6, 2009
    Messages:
    1,030
    Likes Received:
    2
    Location:
    Arizona
    That is at Re = 100,000. It is probably much better above 500,000. The clark Y smoothed has a 2D Cl of 54 in the naca wind tunnel at Re = 100,000.
     
  7. Dec 28, 2010 #47

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    919
    Location:
    Grand Junction, Colorado
     
  8. Dec 28, 2010 #48

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
     

    Attached Files:

    Last edited: Dec 28, 2010
  9. Dec 28, 2010 #49

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
    Norman, last question:) what do you think about DesignFoil, have you tried it? you can download demo for free, I tend to have more faith in DesignFoils results than in XFLR5, just compare NACA 66-2-215, charts from DesignFoil seems to be closer to Abbott results (cl is almost identical) than XFLR5, unless I messed with some settings.

    Seb
     

    Attached Files:

  10. Jan 10, 2011 #50

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
    I must said that when Jarno wrote in "Lancair 414F" thread about new sailplanes airfoils with Cd around 0.002 I didn't have much faith in it, besides I was expecting very high Cm for those foils which might turn those airfoils into useless for GA application, today I found some time, and decided to try that HQ-35 (which is relatively new airfoil, pic4), the problem was that coordinates which are available, aren't extremely useful, in short you can't generate an airfoil directly with them, because lack of coordinates, plus I have also a feeling that at least few of them are incorrect. But like I said found some time today and decided to give it a try, first I created an airfoil based on those coordinates, next I used Direct Foil Design tool available in XLFR5 and created a new airfoil around it.

    Despite I didn't make it extremely accurately (lack of time, plus that was only a test, you can see it form charts, I bet that the drag curve should be more vertical), still I must say that I'm extremely surprised, I got following specifications for Re 5x10^6.

    For .5 AOA
    Cl 0.280
    Cm -0.33
    Cd 0.003
    L/D 93

    For 2.5 AOA
    Cl 0.516
    Cm -0.35
    Cd 0.03
    L/D 194

    For 5 AOA
    Cl 0.777
    Cm -0.35
    Cd 0.008
    L/D 95

    If you take a look at charts you will notice that the lowest Cd is around 0.0026.

    Because like I said I didn't put much care in it, I can bet that original airfoil has even better parameters, of course I'm aware of difference between 2D and 3D effect, and I know limitations of the XFOIL, but it's still the best airfoil I've ever played with, I would love to see those newest ones.

    btw, when I find time I try to see what I can do about kl002 (pic 1,2), there are no coordinates, but I should be able to get some of them wit CAD software.

    Ok now back to the reality :gig:.

    Seb
     

    Attached Files:

  11. Jan 10, 2011 #51

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    919
    Location:
    Grand Junction, Colorado
    As far as I know The Xfoil code behind profili has not been altered. What you pay for when you send your "donation" is a user friendly shell that also dose some nice things like draw ribs for arbitrary planforms with spar slots and leading edge blocks. It also does templates for hot wire cutting and aparently there's even some CNC output capability. All in all a good deal for modelers
     
  12. Jan 10, 2011 #52

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    919
    Location:
    Grand Junction, Colorado
    I haven't used it, Seb. Probably won't either because I just don't have time, or patience anymore, to learn new software
     
  13. Jan 10, 2011 #53

    topspeed100

    topspeed100

    topspeed100

    Banned

    Joined:
    May 4, 2009
    Messages:
    4,063
    Likes Received:
    63
    Location:
    Oulu/Finland
    How about low reynolds number E201 ( tip ) and E210 ( root ) and E374...I love the looks of those. Good for a 1 metre spanning R/C model ?
     
  14. Jan 10, 2011 #54

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
    Norman I'll take a look at it, but it's a pity that their demo version isn't very useful, I would like to test it first (of course it's cheap so the risk isn't big), I wondering which version of Xfoil was used in Profili, so far I noticed that DesignFoil is good for NACA 6- series but it's terrible for some sailplane airfoils, XFLR5 seems to work fine with sailplane airfoils (some charts were at least similar to those from windtunnel), but on the other hand it doesn't work very well with NACA 6- series, I'm wondering about Profili. I read discussion about Profili a few days ago, (didn't bookmark it so can't give a link for it) anyway someone was claiming that there is a difference in charts between Base and Pro version of Profili, he was talking something about simplified approach which was used for calculations in base version, I don't have personal opinion about it just writing what I've read. But it seems that price may vary because Pro version has more tools in it, not because it's more accurate.

    Seb
     
  15. Jan 10, 2011 #55

    autoreply

    autoreply

    autoreply

    Moderator

    Joined:
    Jul 8, 2009
    Messages:
    10,732
    Likes Received:
    2,542
    Location:
    Rotterdam, Netherlands
    A quick lookup shows only the DG-600 as a user of this airfoil. It's an excellent performer, but has a bad reputation as being very aggressive in the stall. Whether it's because of the very thin airfoils (and a bit thicker might help) or the airfoil itself, I don't know, but more than one remains high in the Alps, shattered in pieces after a stall/spin..
     
  16. Jan 10, 2011 #56

    Norman

    Norman

    Norman

    Well-Known Member

    Joined:
    Nov 29, 2003
    Messages:
    2,880
    Likes Received:
    919
    Location:
    Grand Junction, Colorado
    I think the version of Xfoil behind Profili is not the latest version so if you're comfortable with XFLR5 and don't need the rib design functions there's not much reason to buy Profili. Profili also limits your access to the airfoil design functions and some output types in the basic version (and alpha is limited to 13 degrees). So if you realy want to use all of Xfoil's potential the $15 version ain't gona satisfy you, you'll have to shell out a bit more money for Profili Pro. I don't even know what he's asking for that because at the Re I'm usualy looking at most airfoils stall at prety low AoA
     
  17. Jan 11, 2011 #57

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
    Jarno, you are right this HQ35 is very thin, leading edge it's extremely thin, but on the other hand it doesn't look bad at charts, of course without proper coordinates it's hard to compare, that chart which I did with original foil.

    But I must say that it doesn't matter :), I found something much more interesting, it has slightly higher Cd but overall it's a real killer :), you probably heard about it before it's AH 93-K-132/15, I know you probably say, I've never heard about a glider with that foil... of course but keep in mind that it's a relatively new airfoil, and take into account that all modern sailplanes (besides Diana:)) use Boermans airfoils so who is gonna to try something else (besides one crazy Dutch and one mad Pole:roll:), besides I found something http://www.icas-proceedings.net/ICAS2008/PAPERS/319.PDF

    Of course he recommended it for different purposes, I did comparison between AH 93-K-131/15 (which he recommended) and AH 93-K-132/15, and it seems that 132 is even better.

    Take a look at those charts: Re 5x10^6, Mach 0.28, NCrit 9

    black AH-93K-132/15 (mod)
    violet AH-93K-132/15 (mod)
    light green NACA 671-215
    navy blue NLS 414F
    orange I tried KL-002, but as you can see with little success (without proper coordinates).

    Second chart (pix1) for Cl/Cd at cruise.

    I was going also to include FX 79 in those charts, but it's a piece of %&^% compared to those.


    Norman,

    I think I'll stay with XFLR5, I think I solved all problems, the key is to use at least 290 panels, previously I was using around 100, NCrit 9 instead of 12, and calculate every 0.5 ideally 0.1 AOA, setting like that works fine even with NACA 66-serries foils :).

    Seb
     

    Attached Files:

    Last edited: Jan 11, 2011
  18. Feb 4, 2011 #58

    Navy guy

    Navy guy

    Navy guy

    New Member

    Joined:
    Feb 3, 2011
    Messages:
    2
    Likes Received:
    0
    Location:
    Indianapolis in 46239
    I am still looking for that magic airfoil that has low pitching motion, high lift, a slow stall and easy to build. So far i have tried GEO387 and GEO384 airfoils with a full length Junkers flap of an eleven inch cord. Lots of rear airfoil chamber when the flap is deployed (too much pitching motion I should think) which would likely result in a first flight crash. Not good.
     
  19. Feb 4, 2011 #59

    flat6

    flat6

    flat6

    Well-Known Member

    Joined:
    Jan 9, 2011
    Messages:
    99
    Likes Received:
    0
    Location:
    monaco
    this is very interesting. where can we get data for this airfoil?
     
  20. Feb 4, 2011 #60

    Mac790

    Mac790

    Mac790

    Well-Known Member

    Joined:
    Jun 22, 2008
    Messages:
    1,529
    Likes Received:
    19
    Location:
    Poznan, Poland
    You can find it here Airfoil Investigation Database - Showing AH 93-K-132/15 or here http://www.ae.illinois.edu/m-selig/ads/coord_database.html#A to get decent performance in cruise, you have two choices, first lower camber to around 2.3 (if I remember correctly), second use flaps at around -3 deg.

    You will also find coordinates and graphs in Stuttgarter Profilkatalog II: Niedriggeschwindigkeitsprofile, you can order copy here http://www.iag.uni-stuttgart.de/laminarwindkanal/profcat1.html

    Seb
     
    Last edited: Feb 4, 2011

Share This Page



arrow_white