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Old 01-29-2010, 07:05 PM   #31 (permalink)
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Originally Posted by ChazInMT View Post
Aerohed, the red and green templates are both based exactly on the clock face template you described. I made a circle, found its exact center, then projected a line from the center at 25. I then traced that shape. This yields the templates I made.

I just analyzed these further and found some stuff out.

If I make a teardrop out of your "4 Seconds" diagram, I get a 2.75:1 shape using a semi circle as the front. The circular section on top travels through 25. (Red)

If I make a teardrop that is 2.5:1 using a semi circle front, it yields a top circular section that travels 28 or 4.5 seconds after midnight. (Blue)

More questions & graphics later, I would like to nail this down pat as I'm sure many who are looking in on this would.
Chaz,excellent question and I'll beg your indulgence as I attempt my convoluted logic.
From Fluid Mechanics,the professors will tell you that,below 250-mph,forget about the nose,a hemispherical front is all you need for this incompressible flow.
They'll also tell you that the aft-body is where the action is.If your fore-body prevents separation ( which the hemisphere will provide),then focus your attention on a long tapering tail.
The 2.5:1 L/D ratio body of revolution,as cited by Hoerner( and depicted in the "Aerodynamic Streamlining Template")is the "shortest" body of revolution with a Cd 0.04 that I've run across.
When used with Paul Jaray's "ground-reflection" technique,if the upper portion is used,it creates,from what I can gather,the "minimum" curvature for an aftbody which will maintain attached flow in ground-effect for it's entire length.
So the answer to your question,is that while the clockface will "work",the aft-body of the 2.5:1 teardrop represents,from an engineering point of view,the least amount of structure that will deliver a separation-free flow.
You can chop the tail off wherever you like,just as Koenig/Kamm/Korff/Morelli advocated,and you should be guaranteed clean flow right up to the "chop."
As I mentioned in the preamble to the clock-face thread,this would be something one could use in the face of having no other tool to work with.
My opinion,is that for the time and money,the aft-body architecture of the 2.5:1 teardrop is the most appropriate model we might use to construct with.
And this is the teardrop with the traditional semi-ogival, prolate-ellipsoid nose,something to compare to with the simplified "hemi" nose clockface.

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