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Originally Posted by sendler
Correct. And it is way too long and skinny to help us estimate the difference between the two Vetter tails pictured. The first 20% truncation of the fuselage basically chops off all of the boat tail already.
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Ground effect: Any minimal ground effect that the two bikes have will be quite similar and can be tossed out as a common factor. We are only concerned with the comparative values. Nothing absolute. The 22* ice cream cone is closer to the actual bikes although it is too short. An 11* ice cream cone, or better yet, 4 feet tall hemi front rounded wedge at 11* on each side would give quite a good means of comparison as we chop off length in the front or the back.
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Foil sim shows the x,y code for the given shapes but it can't be edited.
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*If you can extend the lines of the Vetter fairing out to a point and then compare that to where they made the truncation you should be able to get a handle on the percentage of chop.
*The % drag difference per 10% of truncation difference won't really vary with fineness ratio.
*If you know the Cd of the profile your starting with,you should be able to predict the Cd at any given truncation.
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*One thing about the straight taper.It has been shown to have twice the drag compared to the more complex teardrop taper.Eiffel measured this.DVL measured it.Junkers measured it.
*If you run a straight,conical taper as long as the 'Template' as Lay,K-Fachsenfeld,and Kamm did,you can get to very low drag.
*If you go from the hemi nose,directly into the angle,without the Mair contour transition,you'll lose the game right there.Sorry,but it's hard science.
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*The thing about 'angles' is that they are measured at a tangent to the body,and will occur only as a constant after the curved Mair transition zone.
Without it,you'll have separation and a good chance that it will never re-attach.
*You can only cheat so much.
*You want to pull the body in as freebeard does with his redwood bender-board.I've been using 1" Schedule-40 PVC pipe as a 'french-curve' held over bulkhead station points which define major body vectors.The redwood or pipe will perform the calculus for you,plotting a smooth curve profile for fabrication.
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*It's better to have a truncated body of proper contour than a full-tailed body of pseudo-streamline form.This was the entire premise of Kamm's research and university lectures.
*If you can go in the 11-degree territory it will return great dividends compared to the 22-degree which would only yield around Cd 0.23 even with a full tail.
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I don't think you'll learn anything useful from the CFD.