Thread: Kammback size?
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Old 01-18-2013, 06:09 PM   #12 (permalink)
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Originally Posted by ennored View Post
First, let me say, my brain hurts! Lots of thinking going on lately.

I had read the entire thread mentioned above. Saw the images on page #25. I was having a little trouble with the second batch of images quoted above, the piggy bank looking box. If the Dryden van and all the boattails out there were right, surely the the boattails shown there hurting or barely improving the drag couldn't be right. What was wrong? I think it's in the last piggybank. the shaded area I assume is the sides being angled in, duh, addressing on the top wasn't doing too much by itself. Right?

And something else I have been playing with. The shape of the tail, that's where this thread kind of started. I'm still a bit stuck on the Dryden van, but I have also now read threads about Cinderella, and Tiger Woods. I'm an engineer, there must be a spreadsheet, right? There is:

(X and Y scales are in inches, based on the Dryden van that was 80" wide.)

I was doing OK until the Cinderella threads. 4 seconds, 5 seconds, 22, I'm still processing. A simple "4 second" tail is much longer than the template. Easy to make the following observations about end of the clockface tail:

3.66 seconds - 22
4 seconds - 24
5 seconds - 30

I get that a simple clock doesn't a template make, but thought I'd be closer. A "5 second" clock starts to get close to the template, but has 30 of taper in it. It ain't so simple! But you all knew that. I guess my question is, just what clock is worthy of a basic template? One at something like 5.5 seconds, limited to 22 would be close to the template. I may narrow that down, but haven't seen it here, I must be off on an unreasonable path?

And back to my graph. Both the Dryden van, which was tuft tested, and the Trailer Tail, which was optimized for angle, are very close. They both have limited lengths (80" and 48"). Both seem to work very well. Just what is a good model for a practical tail? (Recall I'm thinking about large vehicles, 100" wide, so the template tail is like 178", not practical at all.)
The analog clock face represents a 3-D sphere(dimpled golf ball if you will) moving from right to left with supercritical flow and turbulent boundary layer.
The forward stagnation point occurs at the 9:00 position,and the separation point occurs 115-degrees downstream,at 4-seconds after.
Flow separation always seems to occur at 4-seconds after,so the arc,from 12:00 high,to 4-seconds after,or 6:00 low,to 26-seconds before 6:00 will describe a curvilinear architecture which will support attached flow between these limits.
If you use a Rolex watch for the 'nose,' and Big Ben for the 'tail,' you get,roughly,a streamline body of revolution of 2.5:1 fineness ratio,with the 1/2 convex-hemispherical forebody,and bifurcated hyperboloid aft-body with completely attached flow for its entire length and Cd 0.04 in free flight.
It's not the best 'template,' but when all else failed it could function as such.
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