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Old 01-28-2010, 05:55 PM   #26 (permalink)
aerohead
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my mind

Quote:
Originally Posted by Christ View Post
Phil -

I gotta be honest here; I'm not entirely sure I understand exactly what is going on in your mind with this one.

I'm desperately trying to follow, but from what I can tell, your pattern (it's yours, right?) suggests that for attached flow, we'd need something along the lines of a 30 foot long vehicle with a bull nose, which doesn't seem to follow the logic of a 3:1 relationship between length and frontal area.

Could I trouble you to elaborate a bit more?

EDIT: I think I understand a little bit better, after having re-read the text and taking a more detailed look at the images. I do, however, still have a question - since the profile of the divine shape is a curve, at what point do we measure the angle of descent?

I notice in your image that it states a 22* angle at approximately 1/4 of the distance from the tail tip to the "12 o'clock", or the area at which cross sectional diameter is the largest. Is that the proper measurement there, or do we average the slope of the full form against the flat center line?

Also - does this apply as is to a land-vehicle, when given the ground effect?
Christ,This thing is just a thought experiment.The "dimple-hysteria" got everybody obsessing over golf-balls,spheres,laminar and turbulent boundary layers and drag.
The sphere seemed the perfect tool to introduce what would morph into wing theory,fuselage theory,and finally automotive body theory.
The 'clock-face' is a SURE THING.But that's not to say that I's use it.
For attached flow,you need at least the aft-body portion of the 2.5:1 teardrop.It can be longer but the drag will be higher due to increased skin friction.
Will the skin-friction increase make a big difference? No! Skin friction is only about 7% of the overall aerodynamic drag of an automobile.So even if aggravate it with a lot of wetted-area,the overall net increase is negligible.I'm sure Bicycle Bob wanted to throw knives at me when I first introduced the template with 2.5:1 L/D ratio due to it's "shortness".Stretching a body out l-o-n-g-e-r wouldn't really beat you up at the gas pump.
I just wanted to build a consensus on theory which states that there is a point where profile drag and skin friction,together,present the lowest drag,which to the best of my abilities occurs for a body of revolution of 2.5:1 L/D.
As to the 30-foot-long thing,for my T-100,this IS about the length I'll need to hit Cd 0.12 or lower,due to it's height and width,and is why I chose to go the trailer route.A 7-foot boat tail seemed out of the question.By adding some length,I get a trailer I can camp with,and it looks like the trailer will pay me to pull it.
If you have Hucho's book,or have access to it,when you look at Jaray's ground-reflection for a body of revolution in ground-effect,which was the basis of his 1922 Cd 0.13 design,it looks eerily similar to the clock-face segment in reflection which creates the template.Also Kamm's development model for his K-Cars.
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