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Thread: Fixed sum airloads on an airfoil

  1. #1

    Join Date
    Nov 2017
    Posts
    27

    Fixed sum airloads on an airfoil

    Hello,

    Reading books on aerodynamic/design/conception/construction of aircraft gives you
    (by date of pub.) by authors
    Munk
    Abbott (and related NACA 814 reports)
    Peery
    Roskam
    Bruhn
    Raymer (on design)
    Snorri Gudmundsson
    and a lot of others authors (naming here Americans ones). I haven't read
    all, but some (or part of it).

    In France we have also old literature and old engineering regulation (that's
    so different from now for example giving moment coefficient in wind tunnel
    at leading edge when NACA was giving them at 25% (aerodynamic center) chords).

    But old literature about old regulation is giving sometimes good and relevant
    numbers for homebuilt airplanes.
    In immediate post WWII age, in France was a regulation Air2004B that some french
    authors illustrated in a rather easy way.

    A pitfall in conception is the travel of center of pressure
    at different Cm/Cl that made the NACA 23012 a wide accepted airfoil that has
    Cm in 0.01 range meaning that center of pressure is almost near the aerodynamic
    center(no aft 30%). Harry Riblett gives you a bunch of information on this airfoil
    that is prone to icing (on ATR family of aircraft) and exhibits an abrupt stall
    that is the heads of the (no center of pressure traveling) tails.

    Correction : H.Riblett writes about the NACA 43018 airfoil for ATR family
    of aircraft.
    Airfoil 230XX and 430XX are depicted in NACA report N610 related to Forward camber
    airfoils. For Airfoil 43018 Cm is also in the -0.01 range (cf p.19 of report)


    Air2004B regulation gave 4 different fixed sum airloads on an airfoil depending
    on center of pressure location i.e Cm/Cl

    The "weird" thing is CoG of surfaces gives you (algebraically) the position
    of the center of pressure, and sum of airloads on airfoil.


    I tried to find the same calculus in different books and didn't found anything
    that resemble it. If you have seen something close to this in a textbook or have
    any commentary on it, you're welcome to share it.


    This post "Fixed Sum Airloads on an airfoil" is for educational purposes only and may contain errors. Any attempt
    to use the results for actual design purposes are done at the user's own risk.
    Attached Images Attached Images       
    Last edited by JLS; 09-17-2020 at 03:53 AM.

  2. #2

    Join Date
    Nov 2017
    Posts
    27
    My previous post was just plain wrong; formulas were for a
    center of pressure under 1/3 chord, such as ironically
    NACA 23000 series of airfoil.


    For center of pressure over 1/3 chord the fixed sum
    airloads on airfoil is as illustrated below.


    The center of pressure at Cl 0.2 (0.717 chord) is
    far to aft and over the wing chord that's make
    the calculus at this point erroneous.


    So if you have an idea for that (Cl 0.2 limit
    and acceptable center of pressure/wing chord
    ratio) you're welcome to share it.

    This post "Fixed Sum Airloads on an airfoil" is for educational purposes only and may contain errors. Any attempt
    to use the results for actual design purposes are done at the user's own risk.
    Attached Images Attached Images       
    Last edited by JLS; 09-11-2020 at 02:08 PM.

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