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Originally Posted by theycallmeebryan
I would like to hear what aerohead thinks about what i have talked about so far.
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theycallmeebryan,good feedback and good eyes.Perhaps you can help out on Marchaj's table for sections.My thought was that at the Reynolds number listed,the relationship between drag and fineness ratios/thickness, would be be maintained across the Reynolds number spectrum,as all airfoils perform as a function of Reynolds number ( all else being equal )and that skin friction is the primary source of drag for sections up to stall,where profile drag would dominate.From my fluids text,for sections in 2-dimensional flow,thickness ratios below 2:1 are dominated by profile drag.Above 4:1 skin friction is the dominant player.--------------------------------------- With respect to the 67-series wing section,I have only data for the 67,1-215 section.It shows a drag minimum at Re 6-million-------------------- Here's the freaky thing about wings.In my Abbott and von Doenhoff' Theory of Wing Sections,on page 28,Chapter-1 it says that"section data are not applicable to wings of low aspect ratio.In fact,an entirely different theory applies to wings of very low aspect ratio." It cites Reference 61,Jones,Robert T.:Properties of Low-aspect Ratio Pointed Wings at Speeds below and above the Speed of Sound.NACA TN No. 1032,1946,as source of data for this situation.------------------- The other thing is that for anything other than zero crosswind,a wing,standing on it's head is experiencing a change in angle of attack,lift is induced,which creates a pitching-moment,and also,friction drag increases due to velocity increase due to lift,and pretty soon your Cd min ( @ zero "angle-of-attack") is right out the window.----------------- Sylph was designed around a NACA 66-section( P-51 Mustang ) and while it tunneled out in 1/4-scale with Cd 0.109 it's still 11% dirtier than GM's pumpkin-seed Sunraycer.--------------------------- And evidently,some wings are chosen for "flight Reynolds number" which would not be experienced in a car until over 200-mph.---------------------- I know BicycleBob and others have a much deeper grounding in struts and wing sections,and the station positions make it great to produce these forms,and I'm not in opposition to their use,however,until I dig a little deeper,it seems like we have to temper our enthusiasm for them when considering the context of their design and limitations imposed by ground-effect,crosswinds,extremely low aspect ratio,spanwise circulation,etc..