EcoModder Forum BOAT TAIL Drag Reduction Estimates

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 The Following 3 Users Say Thank You to aerohead For This Useful Post: BamZipPow (04-15-2010), jime57 (03-07-2012), Piwoslaw (04-16-2010)
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Quote:
 Originally Posted by aerohead Note: The aft-body of the example fuselage is responsible for a 567% drag reduction when added to the point of max camber. From zero-to-20% chopped-off,the drag rises 3.95% 30%-chop = 12.5% drag increase 40%-chop = 29.6% drag increase 50%- chop = 59.6% drag increase 60%- chop = 99.85% drag increase 70%- chop = 257% drag increase 80%- chop = 330% drag increase 90%- chop = 426% drag increase
So,if I interpret this correctly, the best one can hope far in about 3.95% worse than the ideal template, with a boattail that is probably impossible long and low. RobertSmall's proposal looks to be about a 30-40% chop, if one degrades it a bit for the upward slop of the bottom, so he is looking at something like 20 % drag increase from the perfect template. The stock Insight looks to be something like a 70-80% drag increase from the template. Am I in the ball park?

 04-15-2010, 10:40 PM #3 (permalink) Master EcoModder     Join Date: Dec 2008 Location: Southern WI Posts: 827 AlienMobile - '00 Honda Insight Team Honda 90 day: 80.05 mpg (US) Thanks: 101 Thanked 558 Times in 191 Posts I'm just hoping for a decrease in drag similar to Metro's. Jim.
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Quote:
 Originally Posted by jimepting RobertSmall's proposal looks to be about a 30-40% chop, if one degrades it a bit for the upward slop of the bottom, so he is looking at something like 20 % drag increase from the perfect template. The stock Insight looks to be something like a 70-80% drag increase from the template.
I can't study this thread tonight, but I had to at least crunch these numbers.

If the stock Insight's drag is 75% greater than that of a full length streamlined car, then the latter has a Cd of 0.14 or so. Apply a 20% penalty to this, and you have 0.17. These numbers use a baseline Cd of 0.25, which is a stock Insight with mirrors, grille openings, and stock underbody coverage.

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Quote:
 Originally Posted by jimepting So,if I interpret this correctly, the best one can hope far in about 3.95% worse than the ideal template, with a boattail that is probably impossible long and low. RobertSmall's proposal looks to be about a 30-40% chop, if one degrades it a bit for the upward slop of the bottom, so he is looking at something like 20 % drag increase from the perfect template. The stock Insight looks to be something like a 70-80% drag increase from the template. Am I in the ball park?
I ran the profile out with some minimum constants for cooling system drag ( 0.011 ),'features drag'( gaps,cut lines,door handles,antenna,wheel openings,etc.) ( 0.007 ),and then subtracted out 0.015 for a complete wheel fairing set using data from the Sunraycer.
It appears that if we do everything 'right',we could get a car down to a conservative Cd 0.126,at 80 % of the template length.
The last bit of drag would be insignificant,residing within the 'phantom' tail.
Running the tail out to 100 % would only get the Cd down to 0.123.

 The Following User Says Thank You to aerohead For This Useful Post: jime57 (04-17-2010)
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Insight

Quote:
 Originally Posted by RobertSmalls I can't study this thread tonight, but I had to at least crunch these numbers. If the stock Insight's drag is 75% greater than that of a full length streamlined car, then the latter has a Cd of 0.14 or so. Apply a 20% penalty to this, and you have 0.17. These numbers use a baseline Cd of 0.25, which is a stock Insight with mirrors, grille openings, and stock underbody coverage.
I'm seeing the stock Insight at 30 % aft-body.
If I use Cd 0.25 and add the Sunraycer's original wheel fairings,I'm seeing Cd 0.241 at stock length.
If I run the tail out to 50 %,and with cooling system mods( ductwork) we might be looking at Cd 0.163.
With a one-wheel range-extender trailer with gap-fillers taking it out to 80 % template,Cd 0.119.

 04-17-2010, 10:05 PM #7 (permalink) Left Lane Ecodriver     Join Date: Aug 2008 Location: Buffalo, NY, USA Posts: 2,257 Prius C - '12 Toyota Prius C Thanks: 79 Thanked 286 Times in 199 Posts What do you make of my grille block, in terms of its contribution to Cd: Insight grille block I will probably also block most of the are to the driver's side of the license plate, leaving <10in˛ of cooling air inlet low in the bumper, and ducted to the front wheel wells. I see in the other thread you translated the above percentages into Cd figures: Code: ```zero------ Cd 0.488 10 %----- Cd 0.375 20 %----- Cd 0.299 30 %----- Cd 0.240 40 %----- Cd 0.194 50 %----- Cd 0.162 60 %----- Cd 0.135 70 %----- Cd 0.125 80 %----- Cd 0.118 90 %----- Cd 0.118 100 %---- Cd 0.115``` It is interesting to compare your table to this one: Specifically the half-sphere (approximating a 0% chop) and streamlined half-body (which has Cd=0.09 with zero ground clearance and no features). Like the EV1 and the UFE-III, the Insight's rear glass doesn't exactly follow the template. I believe a 35-40% chop is a better approximation of the Insight's shape. That would place it around a Cd of 0.21, i.e. a 0.04 Cd reduction from aeromods other than the tail, and the potential for 0.10 Cd reduction from a complete boat-tail. My tail extension will be cantilevered, not towed, and its underside will depart from that of the template to allow good clearance angles. This departure occupies 17% of the tail extension's area, in rear projection, which could not result in a drag penalty of more than 0.017 Cd. It looks like I'm on track to build a car with a Cd of 0.13-0.15! I will be impressed if I achieve a figure in that range.
 04-17-2010, 10:21 PM #8 (permalink) Master EcoModder   Join Date: Oct 2008 Location: Mid-Atlantic Posts: 491 OurInsight - '06 Honda Insight Thanks: 170 Thanked 69 Times in 44 Posts Got my Hucho! I found interesting his comment that there had been no agreement between CFD simulations and the results achieved by real cars ;-) Last edited by jime57; 04-17-2010 at 10:29 PM..
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grille block

Quote:
 Originally Posted by RobertSmalls What do you make of my grille block, in terms of its contribution to Cd: Insight grille block I will probably also block most of the are to the driver's side of the license plate, leaving <10in˛ of cooling air inlet low in the bumper, and ducted to the front wheel wells. I see in the other thread you translated the above percentages into Cd figures: Code: ```zero------ Cd 0.488 10 %----- Cd 0.375 20 %----- Cd 0.299 30 %----- Cd 0.240 40 %----- Cd 0.194 50 %----- Cd 0.162 60 %----- Cd 0.135 70 %----- Cd 0.125 80 %----- Cd 0.118 90 %----- Cd 0.118 100 %---- Cd 0.115``` It is interesting to compare your table to this one: Specifically the half-sphere (approximating a 0% chop) and streamlined half-body (which has Cd=0.09 with zero ground clearance and no features). Like the EV1 and the UFE-III, the Insight's rear glass doesn't exactly follow the template. I believe a 35-40% chop is a better approximation of the Insight's shape. That would place it around a Cd of 0.21, i.e. a 0.04 Cd reduction from aeromods other than the tail, and the potential for 0.10 Cd reduction from a complete boat-tail. My tail extension will be cantilevered, not towed, and its underside will depart from that of the template to allow good clearance angles. This departure occupies 17% of the tail extension's area, in rear projection, which could not result in a drag penalty of more than 0.017 Cd. It looks like I'm on track to build a car with a Cd of 0.13-0.15! I will be impressed if I achieve a figure in that range.
Robert,if the grille block isn't cooking anything under the hood then I guess you're good to go.
Since the hybrids have expensive MOSFET and IGFET power transistors requiring cooling,I'm at a loss for recommendations with these advanced powertrains.
With respect to the drag reduction contribution from the block, crude logic might suggest that if you've cut airflow by 50%,then maybe that load has been halved.Just a SWAG.
Don Sherman got 0.4 mpg @ 70 mph with complete block to Pinto.2.0 mpg when airdam was added,then after cutting some block away for minimum cooling,gain came in at 1.4 mpg @ 70 mph.
With respect to the tail,I see no reason why you couldn't achieve your 0.13-0.15 numbers.

 04-21-2010, 12:24 AM #10 (permalink) Pokémoderator     Join Date: Dec 2007 Location: Southern California Posts: 5,864 1999 Saturn SW2 - '99 Saturn SW2 Wagon Team Saturn 90 day: 40.49 mpg (US) Thanks: 439 Thanked 529 Times in 356 Posts aerohead - I started with this image : Are the red vertical lines the correct chop-off points? : Orrrrr, the original black vertical hatched lines? CarloSW2 __________________ What's your EPA MPG? Go Here and find out! American Solar Energy Society