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Old 08-01-2009, 03:27 PM   #33 (permalink)
aerohead
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putting wings to rest

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Originally Posted by aerohead View Post
I've been thinking that we should do a separate thread exclusively for wing sections.I've been playing a game of catch up,reading a little deeper into my texts and see the wings with mixed feelings.Here are some random observations1) wings operate in free air which is virtually turbulence free.(2) automobiles operate in air which is all turbulence.(3) Virtually all the drag of a wing (unless it stalls) is skin friction.(4) Only about 7% of an automobiles drag is from skin friction,with 55% from profile drag.(5) Wings reach their lowest Cd(min) at aspect ratios of 4-5,and "practical" aspect ratios for wings are in the range of 3-9,and as high as 20 for sailplanes.(6) a wing used for a automobile body could have only a fractional aspect ratio,its tabular data unusable according to Abbott and von Doenhoff.(7)"Flight" Reynolds numbers of 6-million can be achieved at Re 2,000 due to turbulence in ground-effect,and at 20-mph.(8) What would otherwise be a "laminar" wing in free air,will transition to turbulent boundary layer at normal automotive ground clearance and low road velocity.(9) crosswinds would be the same as angle of attack change to a vertical wing section,Cl would quickly climb,as would drag( for example,a Clark-Y in a 17-degree relative wind would see a 180% drag increase),creating a pitching moment about the aerodynamic center.(10)0.0005-inches roughness or less is permissible for at the leading edge of a laminar wing,so dried bug juice and insect remains would be enough to scuttle the low drag of these high performance sections,not to mention free turbulence.----------------------------------------------------- Wings are great and I love what they do.I'm just having a time of it trying to wrap my brain around using their performance criteria in the context of an automotive body.Jaray's form does resemble a Clark-Y from the side,but with all the rounding off of edges on the side and nose,"pumpkin seed" seams a better fit for a descriptor.With the "mirror-image" my brain sees the body of a "pair-of-grins" falcon in full stoop at 250-mph.
This is an addendum to what I presented above in hopes of tempering enthusiasm for symmetrical wing sections,and providing a context in which to contemplate their use1) The fundamental assumption for all sub-supercritical -velocity wing theory models used to construct section data is that of 2-dimensional,chord-wise-only flow. (2) The fundamental assumption for all road vehicle aerodynamics is that of 3-dimensional flow,hence,the Navier-Stokes Equations of Spherical Coordinate System,employed in CFD modeling. (3) To presume the "applicability of section data" as presented in "THEORY OF WING SECTIONS" for 3-dimensional flow,in ground-effect and "outside turbulence",violates the fundamental premise under which all the data were conceived,and is stated so by the authors on page#28,paragraph#4,sentences-#2,#3,and #4 of their text.
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