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Old 07-25-2009, 01:31 PM   #18 (permalink)
aerohead
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speed/open-closed

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Originally Posted by orange4boy View Post
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The above is the intro the the templates which were posted separately by aerohead.

Thanks for the charts and template. Good timing for me because I'm going to start my kamm-back soon. I have several neophyte questions that have not been answered so far (that I could find).

1. Does the optimal fineness ratio change with speed? I am hoping that if I make mine lower, that is to make the slope steeper, but I stay below say 55mph, will the flow stay attached? My aim is to reduce the size of the tail, and I could live with the slower speeds to get the wake and tail as small as possible. I don't think the Egg will ever go "super-critical". Sounds like you would need a flux capacitor for that.

2. Does the kamm-back or boattail have to be closed off to get the best result? How much difference is there between a closed off and an open ended tail?
(1) The ratio does not vary with speed,as it is fixed by the critical Reynolds number which for cars,occurs at about 20-mph.At 20-mph and above,you've established the turbulent boundary layer and Cd minimum which remains constant up to the beginning of transonic flow,at about 250-mph.--------- If you lengthen the "pumpkin-seed" you will increase friction drag,although friction drag is only about 7% of total drag for a car so it's not a biggie.The only reason I depicted the 2.5:1 ratio,is that below that you run the risk of early separation and high form drag.--------------------------------------------- (2) Sepp has reported that an open shell did not offer as much drag reduction as a closed shell on his Nissan truck.Bob Parson's open boattail for his Vanagon did not return as high a savings as the closed unit on my Transporter but his "lifestyle" dictated an open plan and he was happy to live with the difference.Open units may allow"circulation" which always has to be paid for at the pump.
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