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Old 05-18-2011, 07:19 PM   #161 (permalink)
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Thanks for info aerohead. My RV, and especially Randy's RV, I think will benefit from aerodynamic studies on semis, since the 'taller than wider' shapes are similar. My RV is also 8 feet wide, so the ideal length for a boat tail 'template' design would be about 14.5 feet long, according to the width X 1.8 equation. Looking at the curve of the template, I'm surprised it's not longer than that. I think for practicality's sake, though, this one will have to be 6 to 8 feet long.

The info from the link to the NASA/Dryden project that wyatt posted in his email above, notes that leaving off the last four feet or so of the pointed tail in their van project only resulted in a net gain of 0.004 drag, so I'm hoping the shorter length won't be too detrimental.

Doing a full belly pan on my RV is going to be a pain in the neck, but if the truncated boat tail yields sufficient gains I'm game.

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Old 05-20-2011, 04:33 PM   #162 (permalink)
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Originally Posted by orbywan View Post
Thanks for info aerohead. My RV, and especially Randy's RV, I think will benefit from aerodynamic studies on semis, since the 'taller than wider' shapes are similar. My RV is also 8 feet wide, so the ideal length for a boat tail 'template' design would be about 14.5 feet long, according to the width X 1.8 equation. Looking at the curve of the template, I'm surprised it's not longer than that. I think for practicality's sake, though, this one will have to be 6 to 8 feet long.

The info from the link to the NASA/Dryden project that wyatt posted in his email above, notes that leaving off the last four feet or so of the pointed tail in their van project only resulted in a net gain of 0.004 drag, so I'm hoping the shorter length won't be too detrimental.

Doing a full belly pan on my RV is going to be a pain in the neck, but if the truncated boat tail yields sufficient gains I'm game.
orbywan,that's correct.The last bit of boat-tail creates a virtual tail( phantom tail )when chopped off.Air skipping over the small wake as if it were a solid cone.
For the 'Template',when doing a car:
* @ 100% = Cd 0.13 ( with no wheel fairings )
* @ 90% = Cd 0.13
* @ 80% = Cd 0.133
* @ 70% = Cd 0.14
So you can see that,and as others have published,there is little benefit doing the entire tail.
I chose 75 % for the Prius tadpole.
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Old 05-20-2011, 05:15 PM   #163 (permalink)
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Thumbs up

Thanks for the info. A phantom tail, that's a good way to put it. So I guess the curved surfaces at the rear help the air 'make the corner(s)' at the rear and meet up, theoretically, at the point where the four planes would converge if the tail were complete?

If this works I may try the belly pan if I can find the right materials. Do you have any idea how to 'treat' the dually rear wheels? There's got to be some crazy things going on there as the air goes over, around and through the tires.
Should there be some kind of diverter in front of the tire sets to help direct the airflow? How about behind the tire sets?

What kind of improvements did you realize with the Prius 'tadpole' tail?
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Old 05-20-2011, 06:50 PM   #164 (permalink)
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corners/duals/Prius

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Originally Posted by orbywan View Post
Thanks for the info. A phantom tail, that's a good way to put it. So I guess the curved surfaces at the rear help the air 'make the corner(s)' at the rear and meet up, theoretically, at the point where the four planes would converge if the tail were complete?

If this works I may try the belly pan if I can find the right materials. Do you have any idea how to 'treat' the dually rear wheels? There's got to be some crazy things going on there as the air goes over, around and through the tires.
Should there be some kind of diverter in front of the tire sets to help direct the airflow? How about behind the tire sets?

What kind of improvements did you realize with the Prius 'tadpole' tail?
orbywan,with respect to the tail,the air will separate right at the chop,but the turbulent wake will take the form of the continuation of the 'Template' outline.Air will follow over that turbulence and meet.
--------------------------------------------------------------------------
With respect to the duals,the only 'solution' for 18-wheelers with open under-carriage is an invention by the fella at Nosecone Corp.
He created a fairing which forces all the underside air out,along a semicircular path to the edge of the duals,preventing any air to collide with them and attempt to flow through.
The recent skirts you see today on 18-wheelers will do the same.Skirting the duals themselves cut drag about 6% more.
-------------------------------------------------------------------------- As far as the Prius goes,it remains an unknown quantity until tested.
The aerodynamic science surrounding its design is very mature and I have an exceedingly high confidence in its potential.
-------------------------------------------------------------------------- At 1,100 pounds 'overweight' my T-100 returned 5-1/2 mpg better than stock,pulling the un-finished Viking boat-tail trailer in December 2009,in arctic air,and 15-mph crosswinds.
-------------------------------------------------------------------------- My first 2-wheel trailer,with no belly,and no gap-fillers caused only a 2-mpg penalty for my CRX at 1,175 pounds over curb weight on a round trip from Denton to Los Angeles.
-------------------------------------------------------------------------- My first 1-wheel trailer returned 48 mpg ave. behind the CRX,over 4-states,over mountain passes as high as 10,600-feet,and speeds as high as 80 mph,all with 2 vacuum lines completely burned through on the carburetor.A condition I was unaware of until returning home.
--------------------------------------------------------------------------
I feel pretty certain that the Prius is going to pick up 7.5 mpg.
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Old 05-20-2011, 07:53 PM   #165 (permalink)
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More good information, thanks aerohead. I kept looking at the side deflectors on the semi's on the road and wondering exactly what the goal was there. Duh.
The undercarriage of my RV is obviously different that a semi's trailer, but do you think I should try the same approach, long diverter pushing the air to the outside of the each wheel set? That would be assuming no belly pan.

With a belly pan, would the general design be the same do you think?
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Old 05-21-2011, 01:12 PM   #166 (permalink)
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sides/belly

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Originally Posted by orbywan View Post
More good information, thanks aerohead. I kept looking at the side deflectors on the semi's on the road and wondering exactly what the goal was there. Duh.
The undercarriage of my RV is obviously different that a semi's trailer, but do you think I should try the same approach, long diverter pushing the air to the outside of the each wheel set? That would be assuming no belly pan.

With a belly pan, would the general design be the same do you think?
There are two schools with respect to sides:
(1) make the flanks of the vehicle flush with the outside of the wheels and put a smooth wheel cover on.Allow some ventilation of the cover to help brake cooling on mountain downgrades.
(2) inset the wheels a bit,allowing a full skirt to be added.Provide shark gill exits to blend brake cooling air along the sides of the rig.
------------------------------------------------------------------------
The belly would be done as any.Renault demonstrated a 17% drag reduction with a full pan on VIRAGE.
Fachsenfeld got an OMNIBUS inter-city bus into the low Cd 2s with a pan.Under Cd 0.2 with inflated tail.
------------------------------------------------------------------------- On low drag vehicles the wheel drag can be half the drag.It's worth going after.
See what you've got and maybe take the easiest path.
--------------------------------------------------------------------------
PS,I looked up the Omnibus in Hucho's book.This is actually the first K-Tail,the K coming from Koenig in Fachsenfelds name.He beat Kamm to the patent office.Although Carl Breer had done a 'chopped tail' in 1934 with Airflow.
The bus,with inflated boat-tail had Cd 0.158.

Last edited by aerohead; 08-13-2011 at 04:49 PM.. Reason: added data
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Old 08-13-2011, 04:27 PM   #167 (permalink)
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This is probably one of the most useful and best tools on ecomodder, as long as it remains Un Sticky..I'm gonna bump it when it falls off the top page of aero threads.

Any one think this is a Bad idea, Let me know. I'm sure 99% of you think it's a good one, no reply needed.
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Old 08-13-2011, 05:31 PM   #168 (permalink)
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A little knowledge is a dangerous thing, and this template is the least amount of information that can make people feel wise about airflow when they are not. I suspect that this thread does more harm than good.
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Old 08-15-2011, 11:25 AM   #169 (permalink)
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No substitute for during your homework starting with fluid mechanics basics then proceeding to aircraft design and automotive applications. Check local college libraries and book venders such as Amazon. The Society of Automotive Engineers list reference books and articles for a price. "Race Car Aerodynamics: Design for Speed" by Joseph Katz covers basics for DIY mechanics and has many references at the end of each chapter. Just remember modern race cars are more concerned about downforce than drag.
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Old 08-15-2011, 12:25 PM   #170 (permalink)
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Quote:
Originally Posted by Bicycle Bob View Post
A little knowledge is a dangerous thing, and this template is the least amount of information that can make people feel wise about airflow when they are not. I suspect that this thread does more harm than good.
I figure it's a start to git people going in the right direction...and that is a good thing!

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